Nerve growth factor: the central hub in the development of allergic asthma?

Functional expression of NaV1.7 channels in freshly dispersed mouse bronchial smooth muscle cells

Isolated smooth muscle cells (SMC) from mouse bronchus were studied using the whole-cell patch clamp technique at ~21oC. Stepping from -100 mV to -20 mV evoked inward currents of mean amplitude -275 pA. These inactivated (tau=1.1 ms) and were abolished when external Na+ was substituted with N-Methyl-D-glucamine. In current-voltage protocols, current peaked at -10 mV and reversed between +20 and +30 mV. The V1/2s of activation and inactivation were -25 & -86 mV, respectively. The current was highly sensitive to tetrodotoxin (IC50=1.5 nM) and the NaV1.7 subtype selective blocker, PF-05089771 (IC50=8.6 nM), consistent with NaV1.7 as the underlying pore-forming a subunit. Two NaV1.7-selective antibodies caused membrane-delineated staining of isolated SMC, as did a non-selective pan-NaVantibody. RT-PCR, performed on groups of ~15 isolated SMC, revealed transcripts for NaV1.7 in 7/8 samples. Veratridine (30 mM), a non-selective NaV channel activator, reduced peak current evoked by depolarization but induced a sustained current of 40 pA. Both effects were reversed by tetrodotoxin (100 nM). In tension experiments veratridine (10 mM) induced contractions that were entirely blocked by atropine (1 mM). However, in the presence of atropine, veratridine was able to modulate the pattern of activity induced by a combination of U-46619 (a thromboxane A2 mimetic) & PGE2(prostaglandin E2), by eliminating bursts in favour of sustained phasic contractions. These effects were readily reversed to control-like activity by tetrodotoxin (100 nM). In conclusion, mouse bronchial SMC functionally express NaV1.7 channels that are capable of modulating contractile activity, at least under experimental conditions.

Nerve Growth Factor: A Potential Therapeutic Target for Lung Diseases

The lungs play a very important role in the human respiratory system. However, many factors can destroy the structure of the lung, causing several lung diseases and, often, serious damage to people's health. Nerve growth factor (NGF) is a polypeptide which is widely expressed in lung tissues. Under different microenvironments, NGF participates in the occurrence and development of lung diseases by changing protein expression levels and mediating cell function. In this review, we summarize the functions of NGF as well as some potential underlying mechanisms in pulmonary fibrosis (PF), coronavirus disease 2019 (COVID-19), pulmonary hypertension (PH), asthma, chronic obstructive pulmonary disease (COPD), and lung cancer. Furthermore, we highlight that anti-NGF may be used in future therapeutic strategies.

MicroRNA-98 reduces nerve growth factor expression in nicotine-induced airway remodeling

Evolving evidence suggests that nicotine may contribute to impaired asthma control by stimulating expression of nerve growth factor (NGF), a neurotrophin associated with airway remodeling and airway hyperresponsiveness. We explored the hypothesis that nicotine increases NGF by reducing lung fibroblast (LF) microRNA-98 (miR-98) and PPARγ levels, thus promoting airway remodeling. Levels of NGF, miR-98, PPARγ, fibronectin 1 (FN1), endothelin-1 (EDN1, herein referred to as ET-1), and collagen (COL1A1 and COL3A1) were measured in human LFs isolated from smoking donors, in mouse primary LFs exposed to nicotine (50 μg/ml), and in whole lung homogenates from mice chronically exposed to nicotine (100 μg/ml) in the drinking water. In selected studies, these pathways were manipulated in LFs with miR-98 inhibitor (anti-miR-98), miR-98 overexpression (miR-98 mimic), or the PPARγ agonist rosiglitazone. Compared with unexposed controls, nicotine increased NGF, FN1, ET-1, COL1A1, and COL3A1 expression in human and mouse LFs and mouse lung homogenates. In contrast, nicotine reduced miR-98 levels in LFs in vitro and in lung homogenates in vivo Treatment with anti-miR-98 alone was sufficient to recapitulate increases in NGF, FN1, and ET-1, whereas treatment with a miR-98 mimic significantly suppressed luciferase expression in cells transfected with a luciferase reporter linked to the putative seed sequence in the NGF 3'UTR and also abrogated nicotine-induced increases in NGF, FN1, and ET-1 in LFs. Similarly, rosiglitazone increased miR-98 and reversed nicotine-induced increases in NGF, FN1, and ET-1. Taken together, these findings demonstrate that nicotine-induced increases in NGF and other markers of airway remodeling are negatively regulated by miR-98.

Molecular Targets of Fatty Acid Ethanolamides in Asthma

Asthma is a common allergic pathology of the respiratory tract that requires the study of mechanisms underlying it, due to severe forms of the disease, which are refractory to therapy. The review is devoted to the search for molecular targets of fatty acid ethanolamides in asthma, in particular palmitoylethanolamide (PEA), which has been successfully used in the treatment of chronic inflammatory and neurodegenerative diseases, in the pathogenesis of which the nervous and immune systems are involved. Recently, the potentially important role of neuro-immune interactions in the development of allergic reactions has been established. Many of the clinical symptoms accompanying allergic airway inflammation are the result of the activation of neurons in the airways, so the attention of researchers is currently focused on neuro-immune interactions, which can play an important role in asthma pathophysiology. A growing number of scientific works confirm that the key molecule in the implementation of these inter-systemic interactions is nerve growth factor (NGF). In addition to its classic role in nervous system physiology, NGF is considered as an important factor associated with the pathogenesis of allergic diseases, particularly asthma, by regulating of mast cell differentiation. In this regard, NGF can be one of the targets of PEA in asthma therapy. PEA has a biological effect on the nervous system, and affects the activation and the degranulation of mast cells.

Cellular and molecular mechanisms of asthma and COPD

Asthma and chronic obstructive pulmonary disease (COPD) both cause airway obstruction and are associated with chronic inflammation of the airways. However, the nature and sites of the inflammation differ between these diseases, resulting in different pathology, clinical manifestations and response to therapy. In this review, the inflammatory and cellular mechanisms of asthma and COPD are compared and the differences in inflammatory cells and profile of inflammatory mediators are highlighted. These differences account for the differences in clinical manifestations of asthma and COPD and their response to therapy. Although asthma and COPD are usually distinct, there are some patients who show an overlap of features, which may be explained by the coincidence of two common diseases or distinct phenotypes of each disease. It is important to better understand the underlying cellular and molecular mechanisms of asthma and COPD in order to develop new treatments in areas of unmet need, such as severe asthma, curative therapy for asthma and effective anti-inflammatory treatments for COPD.

Novel transcripts reveal a complex structure of the human TRKA gene and imply the presence of multiple protein isoforms

Background

Tropomyosin-related kinase A (TRKA) is a nerve growth factor (NGF) receptor that belongs to the tyrosine kinase receptor family. It is critical for the correct development of many types of neurons including pain-mediating sensory neurons and also controls proliferation, differentiation and survival of many neuronal and non-neuronal cells. TRKA (also known as NTRK1) gene is a target of alternative splicing which can result in several different protein isoforms. Presently, three human isoforms (TRKAI, TRKAII and TRKAIII) and two rat isoforms (TRKA L0 and TRKA L1) have been described.

Results

We show here that human TRKA gene is overlapped by two genes and spans 67 kb—almost three times the size that has been previously described. Numerous transcription initiation sites from eight different 5′ exons and a sophisticated splicing pattern among exons encoding the extracellular part of TRKA receptor indicate that there might be a large variety of alternative protein isoforms. TrkA genes in rat and mouse appear to be considerably shorter, are not overlapped by other genes and display more straightforward splicing patterns. We describe the expression profile of alternatively spliced TRKA transcripts in different tissues of human, rat and mouse, as well as analyze putative endogenous TRKA protein isoforms in human SH-SY5Y and rat PC12 cells. We also characterize a selection of novel putative protein isoforms by portraying their phosphorylation, glycosylation and intracellular localization patterns. Our findings show that an isoform comprising mainly of TRKA kinase domain is capable of entering the nucleus.

Conclusions

Results obtained in this study refer to the existence of a multitude of TRKA mRNA and protein isoforms, with some putative proteins possessing very distinct properties.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-015-0215-x) contains supplementary material, which is available to authorized users.

Val66Met polymorphism in the BDNF gene in children with bronchial asthma

OBJECTIVES

Bronchial asthma is a chronic respiratory disease characterized by airway inflammation. There is increasing evidence that neurotrophins play an important role in the development and maintenance of neurogenic airway inflammation in chronic allergic diseases.

WORKING HYPOTHESIS

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family and has several important functions in the airways. There are only a few reports on the association between genetic variations in the BDNF gene and various allergic diseases, and the results are generally conflicting. Therefore, we aimed to study the functional polymorphism Val66Met (also called rs6265 or G196A) in the BDNF gene in a group of asthmatic children and healthy controls.

STUDY DESIGN, PATIENT-SELECTION, AND METHODOLOGY

We studied 248 asthmatic patients (aged 12.28 ± 0.24 years) and 249 healthy children (aged 13.14 ± 0.48 years). Analysis of the Val66Met polymorphism of the BDNF gene was performed by polymerase chain reaction (PCR) and PCR products were digested by PmlI.

RESULTS

The prevalence of the Val66Met polymorphisms (Val/Val, Val/Met, and Met/Met) was 61.7%, 33.5%, and 4.8% in asthmatics, respectively, and 47.0%, 51.8%, and 1.2% in healthy subjects, respectively. We observed a significant association of the Met/Met variant genotype with asthmatics (OR = 4.17, 95% CI = 1.16-14.96, P = 0.018). The Val/Met genotype was protective against bronchial asthma (OR = 0.69, 95% CI = 0.48-0.99, P = 0.045), especially in girls (OR = 0.34, 95% CI = 0.20-0.59, P = 0.001).

CONCLUSION

Specific BDNF gene polymorphism may contribute to bronchial asthma susceptibility. Our study suggested the positive association between selected functional BDNF polymorphism (rs6265) and asthma in children.

Lung dendritic cells undergo maturation and polarization towards a T helper type 2-stimulating phenotype in a mouse model of asthma: Role of nerve growth factor

Nerve growth factor (NGF) and dendritic cells (DCs) have been hypothesized to modulate T cell responses in a mouse model of asthma. However, whether NGF plays a role in regulating the maturation and polarization of lung DCs remains unclear. In the present study, the effect of NGF inhibition on the maturation and phenotype of lung DCs was investigated in a mouse model of asthma. BALB/c mice were sensitized and challenged with ovalbumin (OVA), and subsequently received anti-NGF treatment. At 24 h following the last challenge, airway responsiveness and inflammation were examined. The concentrations of NGF, interferon (IFN)-γ and interleukin (IL)-4 were analyzed. In addition, maturation and CD103 expression in the lung DCs were investigated. Anti-NGF treatment was found to significantly reduce airway hyperreactivity and inflammation in asthmatic mice. In addition, a subdued T helper 2 (Th2) response was observed, characterized by the downregulation of IL-4 and the upregulation of IFN-γ. Furthermore, the expression of the DC surface molecules, CD80, CD86 and major histocompatibility complex class II, as well as the proportion of lung CD103⁺ DCs, decreased in the OVA-sensitized and challenged mice. The proportion of lung CD103⁺ DCs also exhibited a positive correlation with the levels of plasma NGF in the mice. These results may provide an explanation for the role of NGF in amplifying the Th2 response in allergic diseases. Therefore, NGF may promote the maturation and polarization towards a Th2-stimulating phenotype of activated DCs, contributing to an amplification of the Th2 response in asthma.

Nerve growth factor reduces amiloride-sensitive Na+ transport in human airway epithelial cells

Nerve growth factor (NGF) is overexpressed in patients with inflammatory lung diseases, including virus infections. Airway surface liquid (ASL), which is regulated by epithelial cell ion transport, is essential for normal lung function. No information is available regarding the effect of NGF on ion transport of airway epithelium. To investigate whether NGF can affect ion transport, human primary air‐interface cultured epithelial cells were placed in Ussing chambers to obtain transepithelial voltage (−7.1 ± 3.4 mV), short‐circuit current (I_sc, 5.9 ± 1.0 μA), and transepithelial resistance (750 Ω·cm²), and to measure responses to ion transport inhibitors. Amiloride (apical, 3.5 × 10⁻⁵ mol/L) decreased I_sc by 55.3%. Apically applied NGF (1 ng/mL) reduced I_sc by 5.3% in 5 min; basolaterally applied NGF had no effect. The response to amiloride was reduced (41.6%) in the presence of NGF. K‐252a (10 nmol/L, apical) did not itself affect Na⁺ transport, but it attenuated the NGF‐induced reduction in Na⁺ transport, indicating the participation of the trkA receptor in the NGF‐induced reduction in Na⁺ transport. PD‐98059 (30 μmol/L, apical and basolateral) did not itself affect Na⁺ transport, but attenuated the NGF‐induced reduction in Na⁺ transport, indicating that trkA activated the Erk 1/2 signaling cascade. NGF stimulated phosphorylation of Erk 1/2 and the β‐subunit of ENaC. K‐252a and PD‐98059 inhibited these responses. NGF had no effect on I_sc in the presence of apical nystatin (50 μmol/L). These results indicate that NGF inhibits Na⁺ transport through a trkA‐Erk 1/2‐activated signaling pathway linked to ENaC phosphorylation.

Nerve growth factor (apical, 1 ng/mL) reduced amiloride‐sensitive Na⁺ transport in human cultured bronchial epithelial cells. We determined that this reduction in Na⁺ transport occurred through NGF‐mediated activation of the trkA receptor and Erk 1/2 signaling cascade to cause phosphorylation of ENaC.

Expression of nerve growth factor and matrix metallopeptidase-9/tissue inhibitor of metalloproteinase-1 in asthmatic patients

OBJECTIVE

The aim of this study was to measure the level of nerve growth factor (NGF) in bronchial specimens from humans and to determine whether it correlated with not only clinical characteristics of asthma such as percent eosinophils, Th2 cytokine levels, and pulmonary function, but also metallopeptidase-9 (MMP-9) and tissue inhibitor of metalloproteinases-1 (TIMP-1).

METHODS

Fifty-three people participated; 42 had asthma. The participants underwent bronchoscopy and the specimens were analyzed. The participants' clinical data including pulmonary function tests were reviewed.

RESULTS

Bronchoalveolar lavage fluid (BALF) from patients with asthma had a significantly higher level of NGF compared with that from participants without asthma. NGF level showed a positive correlation with the percentage of eosinophils in both BALF and serum. The concentration of NGF did not correlate with that of Th2 cytokines interleukin (IL)-4, IL-5, and IL-13 in BALF or parameters of pulmonary function including degree of airway hyperresponsiveness (ARH). The levels of MMP-9 and TIMP-1 in BALF were higher in asthma patients than in participants without asthma. The levels of NGF correlated with TIMP-1 levels but not with MMP-9 in the whole participants.

CONCLUSIONS

This study shows that NGF correlates with levels of eosinophils, a major effector cell in asthma. The high expression of NGF and TIMP-1 in asthma patients and the moderate correlation between NGF and TIMP-1 in the entire group of asthma subjects suggest a possible association between NGF and TIMP-1, which may influence asthma pathogenesis.

Stress induced neuroendocrine-immune plasticity: A role for the spleen in peripheral inflammatory disease and inflammaging?

Research over the past decade has revealed close interaction between the nervous and immune systems in regulation of peripheral inflammation linking psychosocial stress with chronic somatic disease and aging. Moreover emerging data suggests that chronic inflammations lead to a pro-inflammatory status underlying premature aging called inflammaging. In this context, the spleen can be seen as a switch board monitoring peripherally derived neuroendocrine-immune mediators in the blood and keeping up a close communication with the central stress response via its mainly sympathetic innervation. The effect aims at balanced and well-timed stress axis activation and immune adaptation in acute peripheral inflammatory events. Constant adjustment to the needs generated by environmental and endogenous challenges is provided by neuroendocrine-immune plasticity. However, maladaptive plasticity induced e.g., by chronic stress-axis activation and excessive non-neuronal derived neuroendocrine mediators may be at the heart of the observed stress sensitivity promote inflammaging under chronic inflammatory conditions. We here review the role of neurotransmitters, neuropeptides and neurotrophins as stress mediators modulating the immune response in the spleen and their potential role in inflammaging.

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.

Pesticides and asthma

PURPOSE OF REVIEW

Several clinical and epidemiological studies have reported an association between exposure to pesticides, bronchial hyper-reactivity and asthma symptoms. This article reviews the mechanistic evidence lending support to the concept that either acute or chronic low-level inhalation of pesticides may trigger asthma attacks, exacerbate asthma or increase the risk of developing asthma.

RECENT FINDINGS

Pesticide aerosols or gases, like other respiratory irritants, can lead to asthma through interaction with functional irritant receptors in the airway and promoting neurogenic inflammation. Cross-talk between airway nerves and inflammatory cells helps to maintain chronic inflammation that eventually damages the bronchial epithelium. Certain organophosphorus insecticides cause airway hyper-reactivity via a common mechanism of disrupting negative feedback control of cholinergic regulation in the lungs. These pesticides may interact synergistically with allergen sensitization rendering individuals more susceptible for developing asthma.

SUMMARY

Many pesticides are sensitizers or irritants capable of directly damaging the bronchial mucosa, thus making the airway very sensitive to allergens or other stimuli. However, most pesticides are weakly immunogenic so that their potential to sensitize airways in exposed populations is limited. Pesticides may increase the risk of developing asthma, exacerbate a previous asthmatic condition or even trigger asthma attacks by increasing bronchial hyper-responsiveness.

Breast cancer-induced bone remodeling, skeletal pain, and sprouting of sensory nerve fibers

Breast cancer metastasis to bone is frequently accompanied by pain. What remains unclear is why this pain tends to become more severe and difficult to control with disease progression. Here we test the hypothesis that with disease progression, sensory nerve fibers that innervate the breast cancer bearing bone undergo a pathological sprouting and reorganization, which in other nonmalignant pathologies has been shown to generate and maintain chronic pain. Injection of human breast cancer cells (MDA-MB-231-BO) into the femoral intramedullary space of female athymic nude mice induces sprouting of calcitonin gene-related peptide (CGRP(+)) sensory nerve fibers. Nearly all CGRP(+) nerve fibers that undergo sprouting also coexpress tropomyosin receptor kinase A (TrkA(+)) and growth-associated protein-43 (GAP43(+)). This ectopic sprouting occurs in periosteal sensory nerve fibers that are in close proximity to breast cancer cells, tumor-associated stromal cells, and remodeled cortical bone. Therapeutic treatment with an antibody that sequesters nerve growth factor (NGF), administered when the pain and bone remodeling were first observed, blocks this ectopic sprouting and attenuates cancer pain. The present data suggest that the breast cancer cells and tumor-associated stromal cells express and release NGF, which drives bone pain and the pathological reorganization of nearby CGRP(+)/TrkA(+)/GAP43(+) sensory nerve fibers.

PERSPECTIVE

Therapies that block breast cancer pain by reducing the tumor-induced pathological sprouting and reorganization of sensory nerve fibers may provide insight into the evolving mechanisms that drive breast cancer pain and lead to more effective therapies for attenuating this chronic pain state.

Pathological sprouting of adult nociceptors in chronic prostate cancer-induced bone pain

Pain frequently accompanies cancer. What remains unclear is why this pain frequently becomes more severe and difficult to control with disease progression. Here we test the hypothesis that with disease progression, sensory nerve fibers that innervate the tumor-bearing tissue undergo a pathological sprouting and reorganization, which in other nonmalignant pathologies has been shown to generate and maintain chronic pain. Injection of canine prostate cancer cells into mouse bone induces a remarkable sprouting of calcitonin gene-related peptide (CGRP(+)) and neurofilament 200 kDa (NF200(+)) sensory nerve fibers. Nearly all sensory nerve fibers that undergo sprouting also coexpress tropomyosin receptor kinase A (TrkA(+)). This ectopic sprouting occurs in sensory nerve fibers that are in close proximity to colonies of prostate cancer cells, tumor-associated stromal cells and newly formed woven bone, which together form sclerotic lesions that closely mirror the osteoblastic bone lesions induced by metastatic prostate tumors in humans. Preventive treatment with an antibody that sequesters nerve growth factor (NGF), administered when the pain and bone remodeling were first observed, blocks this ectopic sprouting and attenuates cancer pain. Interestingly, reverse transcription PCR analysis indicated that the prostate cancer cells themselves do not express detectable levels of mRNA coding for NGF. This suggests that the tumor-associated stromal cells express and release NGF, which drives the pathological reorganization of nearby TrkA(+) sensory nerve fibers. Therapies that prevent this reorganization of sensory nerve fibers may provide insight into the evolving mechanisms that drive cancer pain and lead to more effective control of this chronic pain state.

Mouse natural killer (NK) cells express the nerve growth factor receptor TrkA, which is dynamically regulated

BACKGROUND

Nerve growth factor (NGF) is a neurotrophin crucial for the development and survival of neurons. It also acts on cells of the immune system which express the NGF receptors TrkA and p75(NTR) and can be produced by them. However, mouse NK cells have not yet been studied in this context.

METHODOLOGY/PRINCIPAL FINDINGS

We used cell culture, flow cytometry, confocal microscopy and ELISA assays to investigate the expression of NGF receptors by NK cells and their secretion of NGF. We show that resting NK cells express TrkA and that the expression is different on NK cell subpopulations defined by the relative presence of CD27 and CD11b. Expression of TrkA is dramatically increased in IL-2-activated NK cells. The p75(NTR) is expressed only on a very low percentage of NK cells. Functionally, NGF moderately inhibits NK cell degranulation, but does not influence proliferation or cytokine production. NK cells do not produce NGF.

CONCLUSIONS/SIGNIFICANCE

We demonstrate for the first time that mouse NK cells express the NGF receptor TrkA and that this expression is dynamically regulated.

Neurotrophins and neurotrophin receptors in pulmonary sarcoidosis - granulomas as a source of expression

Background

Pulmonary sarcoidosis is an inflammatory disease, characterized by an accumulation of CD4⁺ lymphocytes and the formation of non-caseating epithelioid cell granulomas in the lungs. The disease either resolves spontaneously or develops into a chronic disease with fibrosis. The neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) have been suggested to be important mediators of inflammation and mediate tissue remodelling. In support of this, we have recently reported enhanced NGF levels in the airways of patients with pulmonary sarcoidosis. However, less is known about levels of BDNF and NT-3, and moreover, knowledge in the cellular sources of neurotrophins and the distribution of the corresponding neurotrophin receptors in airway tissue in sarcoidosis is lacking.

Methods

The concentrations of NGF, BDNF and NT-3 in bronchoalveolar lavage fluid (BALF) of 41 patients with newly diagnosed pulmonary sarcoidosis and 27 healthy controls were determined with ELISA. The localization of neurotrophins and neurotrophin receptors were examined by immunohistochemistry on transbronchial lung biopsies from sarcoidosis patients.

Results

The sarcoidosis patients showed significantly enhanced NT-3 and NGF levels in BALF, whereas BDNF was undetectable in both patients and controls. NT-3 levels in BALF were found higher in patients with non-Löfgren sarcoidosis as compared to patients with Löfgren's syndrome, and in more advanced disease stage. Epithelioid cells and multinucleated giant cells within the sarcoid granulomas showed marked immunoreactivity for NGF, BDNF and NT-3. Also, immunoreactivity for the neurotrophin receptor TrkA, TrkB and TrkC, was found within the granulomas. In addition, alveolar macrophages showed positive immunoreactivity for NGF, BDNF and NT-3 as well as for TrkA, TrkB and TrkC.

Conclusions

This study provides evidence of enhanced neurotrophin levels locally within the airways of patients with sarcoidosis. Findings suggest that sarcoid granuloma cells and alveolar macrophages are possible cellular sources of, as well as targets for, neurotrophins in the airways of these patients.

The role of neurotrophins in the pathophysiology of allergic rhinitis

PURPOSE OF REVIEW

Allergic rhinitis is characterized by allergic airway inflammation and a hyperresponsiveness to nonspecific stimuli which is partly neuronally controlled. In this regard, neurotrophins are prime candidates as mediators of neuronal and immunological plasticity and they will be the focus of the current review.

RECENT FINDINGS

Neurotrophins including nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are expressed in the nasal mucosa. The majority of NGF expression has been found in eosinophil granulocytes, the glandular apparatus and peripheral nerves. As shown recently, nasal allergen provocation upregulates BDNF expression in nasal mucosa and NGF expression on peripheral nerves and nasal lavage in patients with allergic rhinitis. In this regard, increased BDNF expression positively correlates with the maximum increase in total nasal symptom score. The neurotrophin receptors including pan-neurotrophin receptor p75, tyrosine kinase A (trkA) and trkB are expressed in nasal tissue. TrkA is expressed on endothelial, p75 on peripheral nerves and trkB on nasal mucosa mast cells that decreases after allergen provocation. The expression of these neurotrophin receptors is increased on peripheral blood eosinophils in allergic rhinitis compared with nonatopic controls. Further, BDNF and NGF exert immunomodulatory functions on eosinophils of patients with allergic rhinitis. Finally, eosinophils of patients with allergic rhinitis are capable of BDNF and NGF production.

SUMMARY

Neurotrophins represent prime candidates in upper airway pathophysiology in allergic rhinitis. Research on neurotrophins in allergic rhinitis is thus becoming a progressively more exciting field and may reveal new and promising therapeutic options for the future.

Effects of titanium dioxide nanoparticle exposure on neuroimmune responses in rat airways

Exposure to ambient nanoparticles (defined as particulate matter [PM] having one dimension <100 nm) is associated with increased risk of childhood and adult asthma. Nanomaterials feature a smaller aerodynamic diameter and a higher surface area per unit mass ratio compared to fine or coarse-sized particles, resulting in greater lung deposition efficiency and an increased potential for biological interaction. The neurotrophins nerve growth factor and brain-derived neurotrophic factor are key regulatory elements of neuronal development and responsiveness of airway sensory neurons. Changes in their expression are associated with bronchoconstriction, airway hyperresponsiveness, and airway inflammation. The neurogenic-mediated control of airway responses is a key pathophysiological mechanism of childhood asthma. However, the effects of nanoparticle exposure on neurotrophin-driven airway responses and their potential role as a predisposing factor for developing asthma have not been clearly elucidated. In this study, in vivo inhalation exposure to titanium dioxide nanoparticles (12 mg/m(3); 5.6 h/d for 3 d) produced upregulation of lung neurotrophins in weanling (2-wk-old) and newborn (2-d-old) rats but not in adult (12-wk-old) animals compared to controls. This effect was associated with increased airway responsiveness and upregulation of growth-related oncogene/keratine-derived chemokine (GRO/KC; CXCL1, rat equivalent of human interleukin [IL]-8) in bronchoalveolar lavage fluid. These data show for the first time that exposure to nanoparticulate upregulates the expression of lung neurotrophins in an age-dependent fashion and that this effect is associated with airway hyperresponsiveness and inflammation. These results suggest the presence of a critical window of vulnerability in earlier stages of lung development, which may lead to a higher risk of developing asthma.

Inflammatory Disease Processes and Interactions with Nutrition

Inflammation is a stereotypical physiological response to infections and tissue injury; it initiates pathogen killing as well as tissue repair processes and helps to restore homeostasis at infected or damaged sites. Acute inflammatory reactions are usually self-limiting and resolve rapidly, due to the involvement of negative feedback mechanisms. Thus, regulated inflammatory responses are essential to remain healthy and maintain homeostasis. However, inflammatory responses that fail to regulate themselves can become chronic and contribute to the perpetuation and progression of disease. Characteristics typical of chronic inflammatory responses underlying the pathophysiology of several disorders include loss of barrier function, responsiveness to a normally benign stimulus, infiltration of inflammatory cells into compartments where they are not normally found in such high numbers, and overproduction of oxidants, cytokines, chemokines, eicosanoids and matrix metalloproteinases. The levels of these mediators amplify the inflammatory response, are destructive and contribute to the clinical symptoms. Various dietary components including long chain ω-3 fatty acids, antioxidant vitamins, plant flavonoids, prebiotics and probiotics have the potential to modulate predisposition to chronic inflammatory conditions and may have a role in their therapy. These components act through a variety of mechanisms including decreasing inflammatory mediator production through effects on cell signaling and gene expression (ω-3 fatty acids, vitamin E, plant flavonoids), reducing the production of damaging oxidants (vitamin E and other antioxidants), and promoting gut barrier function and anti-inflammatory responses (prebiotics and probiotics). However, in general really strong evidence of benefit to human health through anti-inflammatory actions is lacking for most of these dietary components. Thus, further studies addressing efficacy in humans linked to studies providing greater understanding of the mechanisms of action involved are required.

The role of neuro-immune cross-talk in the regulation of inflammation and remodelling in asthma

Despite recent advances in the development of anti-asthmatic medication, asthma continues to be a major health problem worldwide. The symptoms of asthmatic patients include wheezing, chest tightness, cough and shortness of breath, which, together with airway hyperresponiveness, previously have been attributed to a dysfunction of airway nerves. However, research in the last two decades identified Th2-sensitization and the subsequent allergic reaction to innocuous environmental antigens as a basic immunological mechanism leading to chronic airway inflammation. Recent evidence suggests that the development of allergic asthma is influenced by events and circumstances in early childhood and even in utero. Allergen, ozone or stress exposure, as well as RSV infection in early life could be able to induce irreversible changes in the developing epithelial-mesenchymal trophic unit of the airways. The co-existence of chronic inflammation and neural dysfunction have recently drawn attention to the involvement of interaction pathways between the nervous and the immune system in the airways. Intensive basic research has accumulated morphological as well as functional evidence for the interaction between nerves and immune cells. Neuropeptides and neurotrophins have come into focus of attention as the key mediators of neuro-immune interactions, which lead to the development of several pharmacological compounds specifically targeting these molecules. This review will integrate our current knowledge on the involvement of neuro-immune pathways in asthma on the cellular and molecular level. It will summarize the results of pharmacological studies addressing the potential of neuropeptides and neurotrophins as novel therapeutic targets in asthma.

Breathtaking TRP channels: TRPA1 and TRPV1 in airway chemosensation and reflex control

New studies have revealed an essential role for TRPA1, a sensory neuronal TRP ion channel, in airway chemosensation and inflammation. TRPA1 is activated by chlorine, reactive oxygen species, and noxious constituents of smoke and smog, initiating irritation and airway reflex responses. Together with TRPV1, the capsaicin receptor, TRPA1 may contribute to chemical hypersensitivity, chronic cough, and airway inflammation in asthma, COPD, and reactive airway dysfunction syndrome.

Nerve growth factor localization in the nasal mucosa of patients with persistent allergic rhinitis

BACKGROUND AND OBJECTIVES

Nerve growth factor (NGF) and NGF receptors have been shown to be expressed by structural and infiltrating inflammatory cells in the human allergic bronchial mucosa and conjunctiva. In the nose, a positive immunostaining for NGF was recently reported in biopsies of subjects undergoing surgery for refractory nasal obstruction. This study was aimed at studying by immunohistochemistry NGF expression and localization in the nasal mucosa from subjects with moderate/severe persistent allergic rhinitis and natural allergen exposure.

METHODS

Immunostaining for NGF, tryptase and eosinophil cationic protein was performed in human nasal turbinate sections of 25 patients affected by persistent allergic rhinitis and sensitization to Dermatophagoides pteronyssinus.

RESULTS

NGF was consistently expressed in the epithelium and in the submucosa of allergic rhinitic subjects, preferentially localized in eosinophils and mast cells. A strong NGF immunostaining was found in mucous cells of the epithelial lining and in the submucosal glands.

CONCLUSIONS

As previously shown for allergic asthma and allergic conjunctivitis, NGF is also detectable in the nasal mucosa of patients with persistent allergic rhinitis. The preferential NGF localization in mucous cells of the epithelial lining and in the submucosal glands suggests a possible role for NGF in modulating secretion in allergic rhinitis and possibly other allergic diseases.

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.

Molecular and cellular mechanisms in the viral exacerbation of asthma

The aetiology of asthma associated with viral infection is complex. The dynamics that contribute to disease pathogenesis are multifactorial and involve overlapping molecular and cellular mechanisms, particularly the immune response to respiratory virus infection or allergen sensitization. This review summarizes the evidence associated with factors that may contribute to the development or exacerbation of asthma including age, host factors, genetic polymorphisms, altered immune responses, and aspects of viral antigen expression. This review also provides an important perspective of key events linked to the development of asthmatic disease and related pulmonary inflammation from human and animal studies, and discusses their relationship as targets for disease intervention strategies.

Modulation of neurotrophin and neurotrophin receptor expression in nasal mucosa after nasal allergen provocation in allergic rhinitis

BACKGROUND

Patients with allergic rhinitis (AR) feature both allergic airway inflammation and a hyperresponsiveness to nonspecific stimuli which is partly neuronally controlled. Still, it is unclear whether or not neurotrophins are involved in airway pathophysiology of AR and in nasobronchial interaction.

METHODS

Nine AR patients with mono-allergy to grass pollen and nine healthy controls underwent nasal allergen provocation (NP). Serum samples, nasal and bronchial biopsies were taken before (T(0)) and 24 h after (T(24)) NP. Pan-neurotrophin receptor p75(NTR), tyrosine kinase A (trkA), trkB, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) were assessed with immunohistochemistry, and NGF and BDNF levels with ELISA.

RESULTS

At T(24), BDNF and NGF were upregulated in nasal mucosa (P < 0.05) and increased in the peripheral blood of AR compared with T(0). The increase in nasal BDNF expression correlated positively with the maximum increase in total nasal symptom score in AR (P = 0.02). p75(NTR) was expressed on peripheral nerves and epithelial layer, trkA on endothelial cells, and trkB on mast cells. trkB + mast cells significantly decreased after NP in AR (P < 0.01). NP did not modulate p75(NTR) and trkA expression in nasal mucosa and had no effect on the expression of neurotrophins and receptors in bronchial mucosa.

CONCLUSION

This study shows that neurotrophins and their receptors are expressed in human airways. Allergic rhinitis was characterized by a modulation of BDNF, NGF, and trkB in nasal mucosa after NP and a correlation of nasal BDNF with the maximal increase of total nasal symptom score. Therefore, our data suggest that neurotrophins participate in upper-airway pathophysiology in AR, whereas their role in nasobronchial interaction remains unclear.

Understanding the mechanisms of viral induced asthma: new therapeutic directions

Asthma is a common and debilitating disease that has substantially increased in prevalence in Western Societies in the last 2 decades. Respiratory tract infections by respiratory syncytial virus (RSV) and rhinovirus (RV) are widely implicated as common causes of the induction and exacerbation of asthma. These infections in early life are associated with the induction of wheeze that may progress to the development of asthma. Infections may also promote airway inflammation and enhance T helper type 2 lymphocyte (Th2 cell) responses that result in exacerbations of established asthma. The mechanisms of how RSV and RV induce and exacerbate asthma are currently being elucidated by clinical studies, in vitro work with human cells and animal models of disease. This research has led to many potential therapeutic strategies and, although none are yet part of clinical practise, they show much promise for the prevention and treatment of viral disease and subsequent asthma.

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.

Neuroimmune crosstalk in asthma: dual role of the neurotrophin receptor p75NTR

BACKGROUND

Neurotrophins have been implicated in the pathogenesis of asthma because of their ability to induce airway inflammation and to promote hyperreactivity of sensory neurons, which reflects an important mechanism in the pathogenesis of airway hyperreactivity. Neurotrophins use a dual-receptor system consisting of Trk-receptor tyrosine kinases and the structurally unrelated p75NTR. Previous studies revealed an important role of p75NTR in the pathogenesis of allergic asthma.

OBJECTIVES

The aim of the study was to investigate the precise mechanisms of neurotrophins in neuroimmune interaction, which can lead to both airway inflammation and sensory nerve hyperreactivity in vivo.

METHODS

Mice selectively expressing p75NTR in immune cells or nerves, respectively, were generated. After sensitization and allergen provocation, hyperreactivity of sensory nerves was tested in response to capsaicin. Airway inflammation was analyzed on the basis of differential cell counts and cytokine levels in bronchoalveolar lavage fluids.

RESULTS

Allergic mice selectively expressing p75NTR in immune cells showed normal inflammation but no sensory nerve hyperreactivity, whereas mice selectively expressing p75NTR in nerve cells had a diminished inflammation and a distinct sensory nerve hyperreactivity.

CONCLUSION

Our data indicate that p75NTR plays a dual role by promoting hyperreactivity of sensory nerves and airway inflammation. Additionally, our study provides experimental evidence that development of sensory nerve hyperreactivity depends on an established airway inflammation in asthma. In contrast, development of airway inflammation seems to be independent from sensory nerve hyperreactivity.

CLINICAL IMPLICATIONS

Because of its dual function, antagonization of p75NTR-mediated signals might be a novel approach in asthma therapy.

Serum neurotrophins—A study on the time course and influencing factors in a large old age sample

The neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are important mediators of brain and neuronal development, the maintenance of homeostatic conditions in the adult nervous system, and the complex interplay of central and peripheral physiological and pathophysiological factors. To date there are few studies examining blood concentrations of neurotrophic factors in large samples of healthy and diseased individuals and no published study specifically addresses peripheral BDNF and NGF levels in late life. Using improved highly sensitive and specific fluorometric two-site enzyme-linked immunosorbent assays we examined BDNF (n=465) and NGF (n=175) serum levels in a large cohort of elderly individuals (age range: 70-103 years). Neither BDNF nor NGF serum levels proved to be normally distributed, indicating that previously published studies with small sample sizes using parametric testing may be misleading. A significant correlation was found between BDNF and platelet count (r=0.344, p<0.01), age and BDNF protein (r=-0.101, p=0.029) and BDNF and NGF serum levels (r=0.152, p=0.04). No other major influencing factors were found including gender, depression, and dementia.

The nerve growth factor and its receptors in airway inflammatory diseases

The nerve growth factor (NGF) belongs to the neurotrophin family and induces its effects through activation of 2 distinct receptor types: the tropomyosin-related kinase A (TrkA) receptor, carrying an intrinsic tyrosine kinase activity in its intracellular domain, and the receptor p75 for neurotrophins (p75NTR), belonging to the death receptor family. Through activation of its TrkA receptor, NGF activates signalling pathways, including phospholipase Cgamma (PLCgamma), phosphatidyl-inositol 3-kinase (PI3K), the small G protein Ras, and mitogen-activated protein kinases (MAPK). Through its p75NTR receptor, NGF activates proapoptotic signalling pathways including the MAPK c-Jun N-terminal kinase (JNK), ceramides, and the small G protein Rac, but also activates pathways promoting cell survival through the transcription factor nuclear factor-kappaB (NF-kappaB). NGF was first described by Rita Levi-Montalcini and collaborators as an important factor involved in nerve differentiation and survival. Another role for NGF has since been established in inflammation, in particular of the airways, with increased NGF levels in chronic inflammatory diseases. In this review, we will first describe NGF structure and synthesis and NGF receptors and their signalling pathways. We will then provide information about NGF in the airways, describing its expression and regulation, as well as pointing out its potential role in inflammation, hyperresponsiveness, and remodelling process observed in airway inflammatory diseases, in particular in asthma.

Clinical relevance of nerve growth factor serum levels in patients with atopic dermatitis and psoriasis

BACKGROUND

Nerve growth factor (NGF) is known to act as a potent mediator in neuroinflammatory processes. Recent studies point to a role of NGF in the skin pathophysiology of atopic dermatitis (AD) and psoriasis. Hereby, NGF was found to interact with the major cellular components (mast cells and eosinophils) of both skin diseases.

OBJECTIVES

In order to investigate NGF serum levels as a possible clinical marker of disease activity and immunological status, we determined serum NGF, eosinophil cationic protein (ECP), total IgE and score of AD (SCORAD) in 57 patients with AD as well as NGF and the psoriasis area and severity index (PASI) in 17 patients with psoriasis. Fifty healthy subjects served as controls.

METHODS

We used a highly sensitive improved fluorometric two-site ELISA system for serum NGF detection. IgE and ECP were measured by CAP-FEIA and radioimmunoassay.

RESULTS

We did not find a significant correlation between NGF and either ECP, total IgE, or severity of disease assessed by SCORAD. Also in patients with psoriasis, there was no significant correlation with disease activity determined by PASI.

CONCLUSION

Even though there is increasing evidence showing NGF to be involved in the local inflammatory pathophysiology of AD within skin lesions, our findings suggest that NGF systemic serum concentration is not a suitable parameter to estimate the clinical or immunological status of AD or psoriasis patients. This result is inconsistent with some previous studies showing a positive correlation of serum NGF with AD severity, which might be, for example, due to the fact that the present results are corrected for unspecific binding.

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.

Nerve growth factor acutely potentiates synaptic transmission in vitro and induces dendritic growth in vivo on adult neurons in airway parasympathetic ganglia

Elevated levels of nerve growth factor (NGF) and NGF-mediated neural plasticity may have a role in airway diseases such as asthma and chronic obstructive pulmonary disease (COPD). Although NGF is known to affect sensory and sympathetic nerves, especially during development, little is known regarding its effect on parasympathetic nerves, especially on adult neurons. The purpose of this study was to analyze the acute and chronic effects of NGF on the electrophysiological and anatomical properties of neurons in airway parasympathetic ganglia from adult guinea pigs. Using single cell recording, direct application of NGF caused a lasting decrease in the cumulative action potential afterhyperpolarization (AHP) and increased the amplitude of vagus nerve-stimulated nicotinic fast excitatory postsynaptic potentials. Neuronal responsiveness to nicotinic receptor stimulation was increased by NGF, which was blocked by the tyrosine kinase inhibitor, K-252a, implicating neurotrophin-specific (Trk) receptors. Neurotrophin-3 and brain-derived neurotrophic factor had no effect on the synaptic potentials, AHP, or nicotinic response; inhibition of cyclooxygenase with indomethacin inhibited the effect of NGF on the cumulative AHP. Forty-eight hours after in vivo application of NGF to the trachealis muscle caused an increase in dendritic length on innervating neurons. These results are the first to demonstrate that NGF increases the excitability of lower airway parasympathetic neurons, primarily through enhanced synaptic efficacy and changes to intrinsic neuron properties. NGF also had dramatic effects on the growth of dendrites in vivo. Such effects may indicate a new role for NGF in the regulation of parasympathetic tone in the diseased or inflamed lower airways.

Allergic bronchial airway inflammation in nerve growth factor (NGF)-deprived rats: evidence suggesting a neuroimmunomodulatory role of NGF

In the present study, ovalbumin-sensitized/challenged rats were characterized by an nerve growth factor (NGF) increase in both serum and bronchial alveolar lavage fluid (BALF), but not in the lung. Exogenous administration of NGF or NGF-neutralizing antibodies did not modify immunoglobulin (IgE) and eosinophil parameters. In control rats, NGF administration did not induce increase of IgE or eosinophils in both BALF and lung. The present findings suggest that at least NGF does not act as a proper proinflammatory factor but most probably as a neuroimmune modulator molecule of the allergic state.

Neuro-immune interaction in allergic asthma: role of neurotrophins

The nature of persistent airway hyperreactivity and chronic inflammation in asthma remains unclear. It has been suggested that bi-directional neuro-immune interaction plays an important role in the pathogenesis of this disease, leading to enhanced airway narrowing after contact with unspecific stimuli, as well as infiltration, activation and degranulation of several immune cell subtypes. Important mediators in neuro-immune cross-talk are neurotrophins, which are produced by cells at the site of inflammation. In addition to modulating the function of several leucocyte subsets, they play an important role in the synthesis of neuropeptides by sensory nerve cells. Neuropeptides have been shown to cause smooth-muscle contraction and, in addition, modulate the production of pro-inflammatory molecules by leucocytes. The aim of the present review is to provide an overview of the molecular mechanisms by which neurotrophins and neuropeptides are involved in neuro-immune cross-talk in allergic asthma.

Multiple role of histamine H1-receptor-PKC-MAPK signalling pathway in histamine-stimulated nerve growth factor synthesis and secretion

Histamine is a potent stimulator of nerve growth factor (NGF) production in the central nerve system and in the periphery as well. In this review, the biochemical mechanisms of histamine-stimulated NGF synthesis and secretion, and interactions between histamine, interleukin-1beta, and interleukin-6 are discussed. The main signalling pathway, involved in the stimulation of NGF production by histamine, includes activation of histamine H(1)-receptor, stimulation of Ca(2+)-dependent protein kinase C and mitogen-activated protein kinase. The same signalling pathway is involved in the interactions between histamine, interleukin-1beta, and interleukin-6, where NGF secretion is amplified. Whereas histamine and interleukin-1beta cause additive stimulatory effect on NGF secretion, interaction between histamine and interleukin-6 causes a long-term synergism. Thus, activation of histamine H(1)-receptor-protein kinase C-mitogen-activated protein kinase signalling pathway plays a crucial role not only in the direct stimulation of NGF secretion by histamine, but also in the indirect stimulation via different types of interactions between histamine, interleukin-1beta, and interleukin-6, which may have important therapeutic implications in modulation of NGF production.