Role of tachykinin NK3 receptors in the release and effects of nerve growth factor in human isolated bronchi

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.

Brain-derived neurotrophic factor enhances histamine-induced airway responses and changes levels of exhaled nitric oxide in guinea pigs in vivo

The neurotrophin brain-derived neurotrophic factor (BDNF) occurs in elevated levels during airway inflammation, including asthma and hypoxic lung injury, and has been suggested to be associated with airway hyperresponsiveness in these conditions. The aim of the present study was to examine whether airway responses to histamine challenge and levels of exhaled nitric oxide (NO) in vivo might be altered upon BDNF treatment. Pulmonary resistance, lung compliance, insufflation pressure, and levels of exhaled NO were measured in anaesthetized guinea pigs exposed to BDNF prior to challenge with histamine and with intact or inhibited endogenous NO production. BDNF pretreatment significantly enhanced histamine-evoked increase in pulmonary resistance and insufflation pressure, as well as the decrease in lung compliance. BDNF markedly accentuated the reduction in exhaled NO following histamine challenge. In animals with inhibited endogenous NO production BDNF induced a significantly earlier histamine-evoked increase in airway responses. The present data show that BDNF can induce an augmentation of histamine-evoked airway responses and reduce levels of NO in exhaled air in vivo. Endogenous NO seems to exert a braking action on BDNF-induced enhancement of airway responses and a reduced ability to release NO may be one mechanism for increased airway response during elevated BDNF levels. Taken together this indicates that BDNF may be of importance for airway hyperresponsiveness in vivo. The interaction between BDNF and airway NO formation, and its relation to airway responses, merit further investigation.

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.

Expression and coupling of neurokinin receptor subtypes to inositol phosphate and calcium signaling pathways in human airway smooth muscle cells

Neuropeptide tachykinins (substance P, neurokinin A, and neurokinin B) are present in peripheral terminals of sensory nerve fibers within the respiratory tract and cause airway contractile responses and hyperresponsiveness in humans and most mammalian species. Three subtypes of neurokinin receptors (NK1R, NK2R, and NK3R) classically couple to Gq protein-mediated inositol 1,4,5-trisphosphate (IP3) synthesis and liberation of intracellular Ca2+, which initiates contraction, but their expression and calcium signaling mechanisms are incompletely understood in airway smooth muscle. All three subtypes were identified in native and cultured human airway smooth muscle (HASM) and were subsequently overexpressed in HASM cells using a human immunodeficiency virus-1-based lentivirus transduction system. Specific NKR agonists {NK1R, [Sar9,Met(O2)11]-substance P; NK2R, [beta-Ala8]-neurokinin A(4-10); NK3R, senktide} stimulated inositol phosphate synthesis and increased intracellular Ca2+ concentration ([Ca2+]i) in native HASM cells and in HASM cells transfected with each NKR subtype. These effects were blocked by NKR-selective antagonists (NK1R, L-732138; NK2R, GR-159897; NK3R, SB-222200). The initial transient and sustained phases of increased [Ca2+]i were predominantly inhibited by the IP3 receptor antagonist 2-aminoethoxydiphenyl borate (2-APB) or the store-operated Ca2+ channel antagonist SKF-96365, respectively. These results show that all three subtypes of NKRs are expressed in native HASM cells and that IP3 levels are the primary mediators of NKR-stimulated initial [Ca2+]i increases, whereas store-operated Ca2+ channels mediate the sustained phase of the [Ca2+]i increase.

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.