Immunomodulatory functions of the diffuse neuroendocrine system: implications for bronchopulmonary dysplasia

Neuropeptides or their receptors in pathogenesis of lung diseases and therapeutic potentials

There are complex interactions between the immune system and the nervous system in the lung. The nervous system perceives environmental stimuli and transmits these signals to immune cells via neurotransmitters, which is essential for effective immunity and environmental balance. Neuropeptides are important neurotransmitters in the lung, where they regulate immune responses through direct and indirect mechanisms, affecting the occurrence and development of lung diseases. In this review, we emphasize the role of neuropeptides in the pathogeneis of lung diseases and their potential therapeutic value for lung diseases.

Pulmonary neuroendocrine cells: crucial players in respiratory function and airway-nerve communication

Pulmonary neuroendocrine cells (PNECs) are unique airway epithelial cells that blend neuronal and endocrine functions, acting as key sensors in the lung. They respond to environmental stimuli like allergens by releasing neuropeptides and neurotransmitters. PNECs stand out as the only lung epithelial cells innervated by neurons, suggesting a significant role in airway-nerve communication via direct neural pathways and hormone release. Pathological conditions such as asthma are linked to increased PNECs counts and elevated calcitonin gene-related peptide (CGRP) production, which may affect neuroprotection and brain function. CGRP is also associated with neurodegenerative diseases, including Parkinson's and Alzheimer's, potentially due to its influence on inflammation and cholinergic activity. Despite their low numbers, PNECs are crucial for a wide range of functions, highlighting the importance of further research. Advances in technology for producing and culturing human PNECs enable the exploration of new mechanisms and cell-specific responses to targeted therapies for PNEC-focused treatments.

Upregulation of neuropeptides and obstructive airway disorder in infancy: A review with focus on post-RSV wheezing and NEHI

Obstructive airway disorders, common in infancy and early childhood, include some entities that are recognized to have neuro immune mediators as their underlying pathogenetic mechanisms. The best characterized example amongst post-viral wheezing phenotypes is the disorder that follows respiratory syncytial virus (RSV) infection and leads to intermittent, long-term wheezing. The underlying mechanisms of the airway reactivity related to RSV infection have been extensively studies and are associated with dysregulation of the nonadrenergic-noncholinergic (NANC) system, via upregulation of neurotransmitters, typically Substance P. Neuroendocrine hyperplasia of infancy (NEHI), while a less common entity, is a disorder characterized by more severe and long-term obstructive airway disease. NEHI is pathophysiologically characterized by abundance of neuroendocrine cells in the airways containing the neuroimmune mediator bombesin, the release of which is presumed to be the driver of the persistent small airway obstruction and functional air-trapping. Here we review the NANC and neuroendocrine cells, the neurotransmitter systems and their studied roles in pulmonary diseases with a focus on their role in lung development, and subsequent various pediatric lung diseases. We focus on the juxtaposition of the separate neuroimmune mechanisms underlying the pathogenesis of post-RSV recurrent wheezing and NEHI's persistent small airway obstruction. We finally propose a unifying concept of neuropeptides in obstructive disorders that may encompass these two entities and possibly others.

Lung innervation in the eye of a cytokine storm: neuroimmune interactions and COVID-19

COVID-19 is an infectious disease caused by the coronavirus SARS-CoV-2, which was first reported in Wuhan, China, in December 2019 and has caused a global pandemic. Acute respiratory distress syndrome (ARDS) is a common feature of severe forms of COVID-19 and can lead to respiratory failure, especially in older individuals. The increasing recognition of the neurotropic potential of SARS-CoV-2 has sparked interest in the role of the nervous system in respiratory failure in people with COVID-19. However, the neuroimmune interactions in the lung in the context of ARDS are poorly understood. In this Perspectives article, we propose the concept of the neuroimmune unit as a critical determinant of lung function in the context of COVID-19, inflammatory conditions and ageing, focusing particularly on the involvement of the vagus nerve. We discuss approaches such as neurostimulation and pharmacological neuromodulation to reduce tissue inflammation with the aim of preventing respiratory failure.

Acute respiratory distress syndrome is a common occurrence in COVID-19, an infectious disease caused by the coronavirus SARS-CoV-2. In this article, the authors consider how lung innervation might crosstalk with the immune system to modulate lung function and influence outcomes in COVID-19.

Epithelial cell plasticity defines heterogeneity in lung cancer

Lung cancer is the leading cause of cancer death for both men and women and accounts for almost 18.4% of all deaths due to cancer worldwide, with the global incidence increasing by approximately 0.5% per year. Lung cancer is regarded as a devastating type of cancer owing to its high prevalence, reduction in the health-related quality of life, frequently delayed diagnosis, low response rate, high toxicity, and resistance to available therapeutic options. The highly heterogeneous nature of this cancer with a proximal-to-distal distribution throughout the respiratory tract dramatically affects its diagnostic and therapeutic management. The diverse composition and plasticity of lung epithelial cells across the respiratory tract are regarded as significant factors underlying lung cancer heterogeneity. Therefore, definitions of the cells of origin for different types of lung cancer are urgently needed to understand lung cancer biology and to achieve early diagnosis and develop cell-targeted therapies. In the present review, we will discuss the current understanding of the cellular and molecular alterations in distinct lung epithelial cells that result in each type of lung cancer.

Pulmonary neuroendocrine cells function as airway sensors to control lung immune response

The lung is constantly exposed to environmental atmospheric cues. How it senses and responds to these cues is poorly defined. Here, we show that Roundabout receptor (Robo) genes are expressed in pulmonary neuroendocrine cells (PNECs), a rare, innervated epithelial population. Robo inactivation in mouse lung results in an inability of PNECs to cluster into sensory organoids and triggers increased neuropeptide production upon exposure to air. Excess neuropeptides lead to an increase in immune infiltrates, which in turn remodel the matrix and irreversibly simplify the alveoli. We demonstrate in vivo that PNECs act as precise airway sensors that elicit immune responses via neuropeptides. These findings suggest that the PNEC and neuropeptide abnormalities documented in a wide array of pulmonary diseases may profoundly affect symptoms and progression.

Biomarkers for Bronchopulmonary Dysplasia in the Preterm Infant

Bronchopulmonary dysplasia (BPD) is a chronic inflammatory lung disease of very-low-birth-weight (VLBW) preterm infants, associated with arrested lung development and a need for supplemental oxygen. Over the past few decades, the incidence of BPD has significantly raised as a result of improved survival of VLBW infants requiring mechanical ventilation. While early disease detection is critical to prevent chronic lung remodeling and complications later in life, BPD is often difficult to diagnose and prevent due to the lack of good biomarkers for identification of infants at risk, and overlapping symptoms with other diseases, such as pulmonary hypertension (PH). Due to the current lack of effective treatment available for BPD and PH, research is currently focused on primary prevention strategies, and identification of biomarkers for early diagnosis, that could also represent potential therapeutic targets. In addition, novel histopathological, biochemical, and molecular factors have been identified in the lung tissue and in biological fluids of BPD and PH patients that could associate with the disease phenotype. In this review, we provide an overview of biomarkers for pediatric BPD and PH that have been identified in clinical studies using various biological fluids. We also present a brief summary of the information available on current strategies and guidelines to prevent and diagnose BPD and PH, as well as their pathophysiology, risk factors, and experimental therapies currently available.

Insights into bombesin receptors and ligands: Highlighting recent advances

This following article is written for Prof. Abba Kastin's Festschrift, to add to the tribute to his important role in the advancement of the role of peptides in physiological, as well as pathophysiological processes. There have been many advances during the 35 years of his prominent role in the Peptide field, not only as editor of the journal Peptides, but also as a scientific investigator and editor of two volumes of the Handbook of Biological Active Peptides [146,147]. Similar to the advances with many different peptides, during this 35 year period, there have been much progress made in the understanding of the pharmacology, cell biology and the role of (bombesin) Bn receptors and their ligands in various disease states, since the original isolation of bombesin from skin of the European frog Bombina bombina in 1970 [76]. This paper will briefly review some of these advances over the time period of Prof. Kastin 35 years in the peptide field concentrating on the advances since 2007 when many of the results from earlier studies were summarized [128,129]. It is appropriate to do this because there have been 280 articles published in Peptides during this time on bombesin-related peptides and it accounts for almost 5% of all publications. Furthermore, 22 Bn publications we have been involved in have been published in either Peptides [14,39,55,58,81,92,93,119,152,216,225,226,231,280,302,309,355,361,362] or in Prof. Kastin's Handbook of Biological Active Peptides [137,138,331].

Hyperplasia of pulmonary neuroendocrine cells in infancy and childhood

Pulmonary neuroendocrine cells (PNEC) are widely distributed throughout the airway mucosa of mammalian lung as solitary cells and as distinctive innervated clusters, neuroepithelial bodies (NEB). These cells differentiate early during lung development and are more prominent in fetal/neonatal lungs compared to adults. PNEC/NEB cells produce biogenic amine (serotonin) and a variety of peptides (i.e., bombesin) involved in regulation of lung function. During the perinatal period, NEB are thought to function as airway O(2)/CO(2) sensors. Increased numbers of PNEC/NEBs have been observed in a variety of perinatal and postnatal lung disorders. Recent advances in cellular and molecular biology of these cells, as they relate to perinatal and postnatal lung disorders associated with PNEC/NEB cell hyperplasia are reviewed and their possible role in pulmonary pathobiology discussed (WC 125).

Radiation-induced lung injury is mitigated by blockade of gastrin-releasing peptide

Gastrin-releasing peptide (GRP), secreted by pulmonary neuroendocrine cells, mediates oxidant-induced lung injury in animal models. Considering that GRP blockade abrogates pulmonary inflammation and fibrosis in hyperoxic baboons, we hypothesized that ionizing radiation triggers GRP secretion, contributing to inflammatory and fibrotic phases of radiation-induced lung injury (RiLI). Using C57BL/6 mouse model of pulmonary fibrosis developing ≥20 weeks after high-dose thoracic radiation (15 Gy), we injected small molecule 77427 i.p. approximately 1 hour after radiation then twice weekly for up to 20 weeks. Sham controls were anesthetized and placed in the irradiator without radiation. Lung paraffin sections were immunostained and quantitative image analyses performed. Mice exposed to radiation plus PBS had increased interstitial CD68(+) macrophages 4 weeks after radiation and pulmonary neuroendocrine cells hyperplasia 6 weeks after radiation. Ten weeks later radiation plus PBS controls had significantly increased pSmad2/3(+) nuclei/cm(2). GRP blockade with 77427 treatment diminished CD68(+), GRP(+), and pSmad2/3(+) cells. Finally, interstitial fibrosis was evident 20 weeks after radiation by immunostaining for α-smooth muscle actin and collagen deposition. Treatment with 77427 abrogated interstitial α-smooth muscle actin and collagen. Sham mice given 77427 did not differ significantly from PBS controls. Our data are the first to show that GRP blockade decreases inflammatory and fibrotic responses to radiation in mice. GRP blockade is a novel radiation fibrosis mitigating agent that could be clinically useful in humans exposed to radiation therapeutically or unintentionally.

Genome-wide transcriptional profiling reveals connective tissue mast cell accumulation in bronchopulmonary dysplasia

RATIONALE

Bronchopulmonary dysplasia (BPD) is a major complication of premature birth. Risk factors for BPD are complex and include prenatal infection and O(2) toxicity. BPD pathology is equally complex and characterized by inflammation and dysmorphic airspaces and vasculature. Due to the limited availability of clinical samples, an understanding of the molecular pathogenesis of this disease and its causal mechanisms and associated biomarkers is limited.

OBJECTIVES

Apply genome-wide expression profiling to define pathways affected in BPD lungs.

METHODS

Lung tissue was obtained at autopsy from 11 BPD cases and 17 age-matched control subjects without BPD. RNA isolated from these tissue samples was interrogated using microarrays. Standard gene selection and pathway analysis methods were applied to the data set. Abnormal expression patterns were validated by quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry.

MEASUREMENTS AND MAIN RESULTS

We identified 159 genes differentially expressed in BPD tissues. Pathway analysis indicated previously appreciated (e.g., DNA damage regulation of cell cycle) as well as novel (e.g., B-cell development) biological functions were affected. Three of the five most highly induced genes were mast cell (MC)-specific markers. We confirmed an increased accumulation of connective tissue MC(TC) (chymase expressing) mast cells in BPD tissues. Increased expression of MC(TC) markers was also demonstrated in an animal model of BPD-like pathology.

CONCLUSIONS

We present a unique genome-wide expression data set from human BPD lung tissue. Our data provide information on gene expression patterns associated with BPD and facilitated the discovery that MC(TC) accumulation is a prominent feature of this disease. These observations have significant clinical and mechanistic implications.

Pulmonary neuroepithelial bodies as airway sensors: putative role in the generation of dyspnea

The neuroepithelial bodies (NEB) of the intrapulmonary airways (AW) are multimodal AW sensors responding to a variety of stimuli including hypoxia, hypercarbia, and mechanical stretch. NEBs are richly innervated by a diverse population of mostly vagal afferent nerve fibers and owing to their early developmental maturation may be especially important during the perinatal period. This article reviews recent findings of NEB functional morphology and innervation, and postulates a role in the generation of dyspnea. This is based on their potential for transduction of dyspneic stimuli and findings of NEB cell abnormalities in a number of pulmonary disorders presenting with this symptom.

Glucocorticoid actions on airway epithelial responses in immunity: functional outcomes and molecular targets

Research on the biology of airway epithelium in the last decades has progressively uncovered the many roles of this cell type during the immune response. Far from the early view of the epithelial layer simply as a passive barrier, the airway epithelium is now considered a central player in mucosal immunity, providing innate mechanisms of first-line host defense as well as facilitating adaptive immune responses. Alterations of the epithelial phenotype are primarily involved in the pathogenesis of allergic airways disease, particularly in severe asthma. Appreciation of the epithelium as target of glucocorticoid therapy has also grown, because of studies defining the pathways and mediators affected by glucocorticoids, and studies illustrating the relevance of the control of the response from epithelium in the overall efficacy of topical and systemic therapy with glucocorticoids. Studies of the mechanism of action of glucocorticoids within the biology of the immune response of the epithelium have uncovered mechanisms of gene regulation involving both transcriptional and posttranscriptional events. The view of epithelium as therapeutic target therefore has plenty of room to evolve, as new knowledge on the role of epithelium in immunity is established and novel pathways mediating glucocorticoid regulation are elucidated.

Pulmonary neuroendocrine cell system in pediatric lung disease-recent advances

The airway epithelium of human and animal lungs contains highly specialized pulmonary neuroendocrine cells (PNEC), distributed as solitary cells and as innervated clusters, neuroepithelial bodies (NEB). The designation "PNEC system" stems from the expression of both neural and endocrine cell phenotypes, including the synthesis and release of amine (serotonin, 5-HT) and a variety of neuropeptides (that is, bombesin). The role and function of PNEC in the lung have remained a subject of speculation for many years. During the last decade, studies using modern techniques of cellular and molecular biology revealed a complex functional role for PNEC, beginning during the early stages of lung development as modulators of fetal lung growth and differentiation and at the time of birth as airway O2 sensors involved in neonatal adaptation. Postnatally and beyond, PNEC/NEB are providers of a lung stem cell niche that is important in airway epithelial regeneration and lung carcinogenesis. The focus of this review is to present and discuss recent findings pertaining to the responses of PNEC to intrauterine environmental stimuli, ontogeny and molecular regulation of PNEC differentiation, innervation of NEB, and their role as airway chemoreceptors, including mechanisms of O2 sensing and chemotransmission of hypoxia stimulus. Abnormalities of PNEC/NEB have been reported in a variety of pediatric pulmonary disorders but the clinical significance or the mechanisms involved are unknown. The discussion on the possible role of PNEC/NEB in the pathogenesis and pathobiology of pediatric lung diseases includes congenital lung disorders, bronchopulmonary dysplasia, disorders of respiratory control, neuroendocrine hyperplasia of infancy, cystic fibrosis, bronchial asthma, and pulmonary hypertension.

Bombesin-like peptides modulate alveolarization and angiogenesis in bronchopulmonary dysplasia

RATIONALE

The incidence of bronchopulmonary dysplasia (BPD), a chronic lung disease of newborns, is paradoxically rising despite medical advances. We demonstrated elevated bombesin-like peptide levels in infants that later developed BPD. In the 140-day hyperoxic baboon model of BPD, anti-bombesin antibody 2A11 abrogated lung injury.

OBJECTIVES

To test the hypothesis that bombesin-like peptides mediate BPD in extremely premature baboons (born at Gestational Day 125 and given oxygen pro re nata [PRN], called the 125-day PRN model), similar to "modern-day BPD."

METHODS

The 125-day animals were treated with 2A11 on Postnatal Day 1 (P1), P3, and P6. On P14 and P21, lungs were inflation-fixed for histopathologic analyses of alveolarization. Regulation of angiogenesis by bombesin was evaluated using cultured pulmonary microvascular endothelial cells.

MEASUREMENTS AND MAIN RESULTS

In 125-day PRN animals, urine bombesin-like peptide levels at P2-3 are directly correlated with impaired lung function at P14. Gastrin-releasing peptide (the major pulmonary bombesin-like peptide) mRNA was elevated eightfold at P1 and remained high thereafter. At P14, 2A11 reduced alveolar wall thickness and increased the percentage of secondary septa containing endothelial cells. At P21, 2A11-treated 125-day PRN animals had improved alveolarization according to mean linear intercepts and number of branch points per millimeter squared. Bombesin promoted tubulogenesis of cultured pulmonary microvascular endothelial cells, but cocultured fetal lung mesenchymal cells abrogated this effect.

CONCLUSIONS

Early bombesin-like peptide overproduction in 125-day PRN animals predicted alveolarization defects weeks later. Bombesin-like peptide blockade improved septation, with the greatest effects at P21. This could have implications for preventing BPD in premature infants.

Progress in Discovery and Evaluation of Treatments to Prevent Bronchopulmonary Dysplasia

BACKGROUND

Recent improvements in the survival of extremely preterm infants have been accompanied by evolution in the pathogenesis and histopathology of bronchopulmonary dysplasia (BPD). Although oxygen and barotrauma-induced injury remain important contributing factors, pulmonary developmental arrest appears to play an equally important causal role in prolonged respiratory illness, especially among the most immature surviving preterm newborns. To date, clinical trials have failed to demonstrate a substantial benefit of a single treatment or preventive strategy for BPD.

OBJECTIVES

To evaluate the current evidence in favor of treatments that might prevent BPD.

METHODS

Review of clinical studies of preventive treatment strategies for BPD.

RESULTS

High frequency oscillatory ventilation, permissive hypercapnea, and inhaled nitric oxide might offer benefit to infants at risk of BPD. These and other potential preventive therapies for BPD, such as superoxide dismutase, inositol, and alpha(1)-proteinase inhibitor, deserve further study.

CONCLUSIONS

Although some current treatments offer promise, no preventive therapy for BPD has proven safe and effective, except for intramuscular vitamin A. Additional studies of respiratory technologies, management strategies, and protective molecules are needed.

Functional facets of the pulmonary neuroendocrine system

Pulmonary neuroendocrine cells (PNECs) have been around for 60 years in the scientific literature, although phylogenetically they are ancient. Their traditionally ascribed functions include chemoreception and regulation of lung maturation and growth. There is recent evidence that neuroendocrine (NE) differentiation in the lung is regulated by genes and pathways that are conserved in the development of the nervous system from Drosophila to humans (such as achaete-scute homolog-1), or implicated in the carcinogenesis of the nervous or NE system (such as the retinoblastoma tumor suppressor gene). In addition, complex neural networks are in place to regulate chemosensory and other functions. Even solitary PNECs appear to be innervated. For the first time ever, we have mouse models for lung NE carcinomas, including the most common and virulent small cell lung carcinoma. Moreover, PNECs may be important for inflammatory responses, and pivotal for lung stem cell niches. These discoveries signify an exciting new era for PNECs and are likely to have therapeutic and diagnostic applications.

Bombesin inhibits alveolarization and promotes pulmonary fibrosis in newborn mice

RATIONALE

Bombesin-like peptides promote fetal lung development. Normally, levels of mammalian bombesin (gastrin-releasing peptide [GRP]) drop postnatally, but these levels are elevated in newborns that develop bronchopulmonary dysplasia (BPD), a chronic lung disease characterized by arrested alveolarization. In premature baboons with BPD, antibombesin antibodies reduce lung injury and promote alveolarization.

OBJECTIVES

The present study tests whether exogenous bombesin or GRP given perinatally alters alveolar development in newborn mice.

METHODS

Mice were given peptides intraperitoneally twice daily on Postnatal Days 1-3. On Day 14 lungs were inflation-fixed for histopathologic analyses of alveolarization.

MEASUREMENTS AND MAIN RESULTS

Bombesin had multiple effects on Day 14 lung, when alveolarization was about half complete. First, bombesin induced alveolar myofibroblast proliferation and increased alveolar wall thickness compared with saline-treated control animals. Second, bombesin diminished alveolarization in C57BL/6 (but not Swiss-Webster) mice. We used receptor-null mice to explore which receptors might mediate these effects. Compared with wild-type littermates, bombesin-treated GRP receptor (GRPR)-null mice had increased interstitial fibrosis but reduced defects in alveolarization. Neuromedin B (NMB) receptor-null and bombesin receptor subtype 3-null mice had the same responses as their wild-type littermates. GRP had the same effects as bombesin, whereas neither NMB nor a synthetic bombesin receptor type 3 ligand had any effect. All effects of GRP were abrogated in GRPR-null mice.

CONCLUSIONS

Bombesin/GRP can induce features of BPD, including interstitial fibrosis and diminished alveolarization. GRPR appears to mediate all effects of GRP, but only part of the bombesin effect on alveolarization, suggesting that novel receptors may mediate some effects of bombesin in newborn lung.