Elevated neurotrophin and neurotrophin receptor expression in spontaneously hypertensive rat lungs

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.

[The expression and role of nerve growth factor (NGF) in pulmonary hypertension]

Pulmonary hypertension is a severe multifactorial disease of the pulmonary circulation characterized by a progressive elevation in mean pulmonary arterial pressure (PAPm), leading to right ventricular failure and the death of the patient. Current therapies slow the progression of the disease but do not offer a cure. Nerve growth factor NGF is a growth factor playing a significant role in the pathophysiology of pulmonary hypertension, particularly in pulmonary arterial hyperreactivity, and the remodelling and inflammation of the pulmonary vasculature. Thus, targeting NGF may offer new therapeutic strategies in the treatment of this disease.

The Role of Neurotrophins in Inflammation and Allergy

Allergic inflammation is the result of a specific pattern of cellular and humoral responses leading to the activation of the innate and adaptive immune system, which, in turn, results in physiological and structural changes affecting target tissues such as the airways and the skin. Eosinophil activation and the production of soluble mediators such as IgE antibodies are pivotal features in the pathophysiology of allergic diseases. In the past few years, however, convincing evidence has shown that neurons and other neurosensory structures are not only a target of the inflammatory process but also participate in the regulation of immune responses by actively releasing soluble mediators. The main products of these activated sensory neurons are a family of protein growth factors called neurotrophins. They were first isolated in the central nervous system and identified as important factors for the survival and differentiation of neurons during fetal and postnatal development as well as neuronal maintenance later in life. Four members of this family have been identified and well defined: nerve growth factor, brain-derived neurotrophic factor, neurotrophin 3, and neurotrophin 4/5. Neurotrophins play a critical role in the bidirectional signaling mechanisms between immune cells and the neurosensory network structures in the airways and the skin. Pruritus and airway hyperresponsiveness, two major features of atopic dermatitis and asthma, respectively, are associated with the disruption of the neurosensory network activities. In this chapter, we provide a comprehensive description of the neuroimmune interactions underlying the pathophysiological mechanisms of allergic and inflammatory diseases.

Role of Nerve Growth Factor in Development and Persistence of Experimental Pulmonary Hypertension

RATIONALE

Pulmonary hypertension (PH) is characterized by a progressive elevation in mean pulmonary arterial pressure, often leading to right ventricular failure and death. Growth factors play significant roles in the pathogenesis of PH, and their targeting may therefore offer novel therapeutic strategies in this disease.

OBJECTIVES

To evaluate the nerve growth factor (NGF) as a potential new target in PH.

METHODS

Expression and/or activation of NGF and its receptors were evaluated in rat experimental PH induced by chronic hypoxia or monocrotaline and in human PH (idiopathic or associated with chronic obstructive pulmonary disease). Effects of exogenous NGF were evaluated ex vivo on pulmonary arterial inflammation and contraction, and in vitro on pulmonary vascular cell proliferation, migration, and cytokine secretion. Effects of NGF inhibition were evaluated in vivo with anti-NGF blocking antibodies administered both in rat chronic hypoxia- and monocrotaline-induced PH.

MEASUREMENTS AND MAIN RESULTS

Our results show increased expression of NGF and/or increased expression/activation of its receptors in experimental and human PH. Ex vivo/in vitro, we found out that NGF promotes pulmonary vascular cell proliferation and migration, pulmonary arterial hyperreactivity, and secretion of proinflammatory cytokines. In vivo, we demonstrated that anti-NGF blocking antibodies prevent and reverse PH in rats through significant reduction of pulmonary arterial inflammation, hyperreactivity, and remodeling.

CONCLUSIONS

This study highlights the critical role of NGF in PH. Because of the recent development of anti-NGF blocking antibodies as a possible new pain treatment, such a therapeutic strategy of NGF inhibition may be of interest in PH.

Neurotrophins induce nitric oxide generation in human pulmonary artery endothelial cells

AIMS

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

METHODS AND RESULTS

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

CONCLUSION

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

Neurotrophic tyrosine kinase receptor B/neurotrophin 4 signaling axis is perturbed in clinical and experimental pulmonary fibrosis

The neurotrophins (NTs) are emerging as exciting new participants in normal lung physiology, as well as in several pathological processes in diseased lungs. In this study, the increased expression of NT4/5 and of its cognate receptor, the neurotrophic tyrosine kinase receptor Type 2 (TrkB), was observed in human lungs explanted from patients with idiopathic pulmonary fibrosis (IPF), and in lungs from mice with bleomycin-induced pulmonary fibrosis. The expression of NT4/5 and TrkB localized to hyperplastic alveolar Type II cells (ATII) and fibroblastic foci in affected lungs. Increased concentrations of NT4/5 and TrkB were evident in ATII isolated from the lungs of bleomycin-treated mice. Primary ATII were shown to secrete NT4/5 into the cell culture medium. The profibrotic cytokine transforming growth factor-β1, stimulated TrkB, but not NT4/5 gene expression, suggesting that perturbed profibrotic growth factor signaling in affected lungs may drive the expression of TrkB. NT4/5 enhanced the proliferation of ATII through a TrkB/extracellular-regulated kinase/protein kinase B pathway, and could also drive the proliferation of primary human and murine lung fibroblasts, through TrkB-dependent and protein kinase B-dependent pathways. Taken together, these data suggest that a dysregulated TrkB/NT4/5 axis may contribute to several of the pathological lesions associated with pulmonary fibrosis, including ATII hyperplasia and the proliferation of fibroblasts, and we would add IPF to the list of disorders, such as pain and cancer, for which therapeutic targeting of the TrkB/neurotrophin axis has been proposed for further investigation.

Neurotrophins in lung health and disease

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

TrkB is necessary for the normal development of the lung

Normal development of the lung requires coordinated activation of cascades of signaling pathways initiated by growth factors signaling through their receptors. TrkB and its ligands, brain-derived neurotrophic factor (BDNF) and neurotrophin-4, belong to the neurotrophin family of growth factors, which are expressed in a large variety of non-neuronal tissues including the lung. Aberrant neurotrophin signaling underlies the pathogenesis of several lung-related pathologies, including asthma and lung cancer, however, little is known about the role of neurotrophins in the embryonic development of the lung. To fill this gap in knowledge, we analyzed the pattern of TrkB expression in the murine lung and we observed that TrkB is expressed in alveolar macrophages, type II pneumocytes, neuroepithelial bodies and nerves. Analysis of the structure of lung from mice deficient in TrkB revealed that absence of TrkB signaling results in thinner bronchial epithelium and apparent larger air space, and, more importantly, lack of neuroepithelial bodies, an important reduction in the density of nerve fibres in the bronchial smooth muscle, submucous plexus in bronchioles, and pulmonary artery walls. These findings suggest TrkB is essential for the normal development of the lung and the nervous system in the lung.

p75 neurotrophin receptor regulates agonist-induced pulmonary vasoconstriction

A member of the TNF receptor family, the p75 neurotrophin receptor (p75(NTR)) has been previously shown to play a role in the regulation of fibrin deposition in the lung. However, the role of p75(NTR) in the regulation of pulmonary vascular tone in the lung is unknown. In the present study, we evaluated the expression of p75(NTR) in mouse pulmonary arteries and the putative role of p75(NTR) in modulating pulmonary vascular tone and agonist responsiveness using wild-type (WT) and p75(NTR) knockout (p75(-/-)) mice. Our data indicated that p75(NTR) is expressed in both smooth muscle and endothelial cells within the pulmonary vascular wall in WT mice. Pulmonary artery rings from p75(-/-) mice exhibited significantly elevated active tension due to endothelin-1-mediated Ca(2+) influx. Furthermore, the contraction due to capacitative Ca(2+) entry (CCE) in response to phenylephrine-mediated active depletion of intracellular Ca(2+) stores was significantly enhanced compared with WT rings. The contraction due to CCE induced by passive store depletion, however, was comparable between WT and p75(-/-) rings. Active tension induced by serotonin, U-46619 (a thromboxane A(2) analog), thrombin, 4-aminopyridine (a K(+) channel blocker), and high extracellular K(+) in p75(-/-) rings was similar to that in WT rings. Deletion of p75(NTR) did not alter pulmonary vasodilation to sodium nitroprusside (a nitric oxide donor). These data suggest that intact p75(NTR) signaling may play a role in modulating pulmonary vasoconstriction induced by endothelin-1 and by active store depletion.