Neurochemical pattern of the complex innervation of neuroepithelial bodies in mouse lungs

The pulmonary neuroepithelial body microenvironment represents an underestimated multimodal component in airway sensory pathways

Exciting new imaging and molecular tools, combined with state-of-the-art genetically modified mouse models, have recently boosted interest in pulmonary (vagal) sensory pathway investigations. In addition to the identification of diverse sensory neuronal subtypes, visualization of intrapulmonary projection patterns attracted renewed attention on morphologically identified sensory receptor end-organs, such as the pulmonary neuroepithelial bodies (NEBs) that have been our area of expertise for the past four decades. The current review aims at providing an overview of the cellular and neuronal components of the pulmonary NEB microenvironment (NEB ME) in mice, underpinning the role of these complexly organized structures in the mechano- and chemosensory potential of airways and lungs. Interestingly, the pulmonary NEB ME additionally harbors different types of stem cells, and emerging evidence suggests that the signal transduction pathways that are active in the NEB ME during lung development and repair also determine the origin of small cell lung carcinoma. Although documented for many years that NEBs appear to be affected in several pulmonary diseases, the current intriguing knowledge on the NEB ME seems to encourage researchers that are new to the field to explore the possibility that these versatile sensor-effector units may be involved in lung pathogenesis or pathobiology.

Advances in co-pathogenesis of the united airway diseases

According to the concept of "united airway diseases", the airway is a single organ in which upper and lower airway diseases are commonly comorbid. A range of inflammatory factors have been found to play an important role in the chain reaction of upper and lower airway diseases. However, the amount of research on this concept remains limited. The underlying mechanism of the relationship between typical diseases of the united airway, such as asthma, allergic rhinitis, and chronic sinusitis, also needs to be further explored. This review highlights the interaction between upper and lower respiratory diseases gathered from epidemiological, histoembryology, neural mechanistic, microbiological, and clinical studies, revealing the relationship between the upper and lower respiratory tracts.

The Dual Role of the Airway Epithelium in Asthma: Active Barrier and Regulator of Inflammation

Chronic airway inflammation is the cornerstone on which bronchial asthma arises, and in turn, chronic inflammation arises from a complex interplay between environmental factors such as allergens and pathogens and immune cells as well as structural cells constituting the airway mucosa. Airway epithelial cells (AECs) are at the center of these processes. On the one hand, they represent the borderline separating the body from its environment in order to keep inner homeostasis. The airway epithelium forms a multi-tiered, self-cleaning barrier that involves an unstirred, discontinuous mucous layer, the dense and rigid mesh of the glycocalyx, and the cellular layer itself, consisting of multiple, densely interconnected cell types. On the other hand, the airway epithelium represents an immunologically highly active tissue once its barrier has been penetrated: AECs play a pivotal role in releasing protective immunoglobulin A. They express a broad spectrum of pattern recognition receptors, enabling them to react to environmental stressors that overcome the mucosal barrier. By releasing alarmins-proinflammatory and regulatory cytokines-AECs play an active role in the formation, strategic orientation, and control of the subsequent defense reaction. Consequently, the airway epithelium is of vital importance to chronic inflammatory diseases, such as asthma.

Distinct roles for SOX2 and SOX21 in differentiation, distribution and maturation of pulmonary neuroendocrine cells

Pulmonary neuroendocrine (NE) cells represent a small population in the airway epithelium, but despite this, hyperplasia of NE cells is associated with several lung diseases, such as congenital diaphragmatic hernia and bronchopulmonary dysplasia. The molecular mechanisms causing the development of NE cell hyperplasia remains poorly understood. Previously, we showed that the SOX21 modulates the SOX2-initiated differentiation of epithelial cells in the airways. Here, we show that precursor NE cells start to develop in the SOX2 + SOX21 + airway region and that SOX21 suppresses the differentiation of airway progenitors to precursor NE cells. During development, clusters of NE cells start to form and NE cells mature by expressing neuropeptide proteins, such as CGRP. Deficiency in SOX2 resulted in decreased clustering, while deficiency in SOX21 increased both the numbers of NE ASCL1 + precursor cells early in development, and the number of mature cell clusters at E18.5. In addition, at the end of gestation (E18.5), a number of NE cells in Sox2 heterozygous mice, did not yet express CGRP suggesting a delay in maturation. In conclusion, SOX2 and SOX21 function in the initiation, migration and maturation of NE cells.

Air Pollutant impacts on the brain and neuroendocrine system with implications for peripheral organs: a perspective

Air pollutants are being increasingly linked to extrapulmonary multi-organ effects. Specifically, recent studies associate air pollutants with brain disorders including psychiatric conditions, neuroinflammation and chronic diseases. Current evidence of the linkages between neuropsychiatric conditions and chronic peripheral immune and metabolic diseases provides insights on the potential role of the neuroendocrine system in mediating neural and systemic effects of inhaled pollutants (reactive particulates and gases). Autonomically-driven stress responses, involving sympathetic-adrenal-medullary and hypothalamus-pituitary-adrenal axes regulate cellular physiological processes through adrenal-derived hormones and diverse receptor systems. Recent experimental evidence demonstrates the contribution of the very stress system responding to non-chemical stressors, in mediating systemic and neural effects of reactive air pollutants. The assessment of how respiratory encounter of air pollutants induce lung and peripheral responses through brain and neuroendocrine system, and how the impairment of these stress pathways could be linked to chronic diseases will improve understanding of the causes of individual variations in susceptibility and the contribution of habituation/learning and resiliency. This review highlights effects of air pollution in the respiratory tract that impact the brain and neuroendocrine system, including the role of autonomic sensory nervous system in triggering neural stress response, the likely contribution of translocated nano particles or metal components, and biological mediators released systemically in causing effects remote to the respiratory tract. The perspective on the use of systems approaches that incorporate multiple chemical and non-chemical stressors, including environmental, physiological and psychosocial, with the assessment of interactive neural mechanisms and peripheral networks are emphasized.

CGRP induces myofibroblast differentiation and the production of extracellular matrix in MRC5s via autocrine and paracrine signalings

There are contradictory views on which calcitonin gene-related peptide (CGRP) causes pulmonary fibrosis. Fibrotic potency of CGRP was tested and compared to that of transforming growth factor-β (TGF-β). Myofibroblast differentiation, cell proliferation, and activations of TGF-β and Wnt pathways were examined for 24, 48, and 72 h in A549 and MRC5 cell lines stimulated with CGRP and TGF-β. CGRP-induced cell proliferation in MRC5s early on while cell proliferation in A549 occurred progressively. CGRP promoted fibroblast-myofibroblast differentiation by inducing the transcription of ACTA2, COL1A1, SMAD2/3, and SMAD4 genes, the production of collagen, fibronectin, α-smooth muscle actin, and activation of TGF-β signaling starting from 24 h. Additionally, TGF-β signaling induced by CGRP decreased the DKK1 level and activated the Wnt signaling in MRC5s. After CGRP stimulation, Wnt7a levels were increased from 24 to 72 h, while Wnt5a levels were elevated at 72 h in MRC5s. CGRP did not induce epithelial-mesenchymal transition in A549s, unlike TGF-β. A comparison of fibrotic potency of CGRP and TGF-β showed that TGF-β is a powerful profibrotic molecule and induces earlier myofibroblast differentiation. Even so, CGRP promotes myofibroblast differentiation and extracellular matrix production by inducing Smad-dependent-TGF-β and Wnt signalings via autocrine and paracrine signalings in MRC5s.

Mechanosensitive Piezo channels mediate the physiological and pathophysiological changes in the respiratory system

Mechanosensitive Piezo ion channels were first reported in 2010 in a mouse neuroblastoma cell line, opening up a new field for studying the composition and function of eukaryotic mechanically activated channels. During the past decade, Piezo ion channels were identified in many species, such as bacteria, Drosophila, and mammals. In mammals, basic life activities, such as the sense of touch, proprioception, hearing, vascular development, and blood pressure regulation, depend on the activation of Piezo ion channels. Cumulative evidence suggests that Piezo ion channels play a major role in lung vascular development and function and diseases like pneumonia, pulmonary hypertension, apnea, and other lung-related diseases. In this review, we focused on studies that reported specific functions of Piezos in tissues and emphasized the physiological and pathological effects of their absence or functional mutations on the respiratory system.

Mapping of the Sensory Innervation of the Mouse Lung by Specific Vagal and Dorsal Root Ganglion Neuronal Subsets

The airways are densely innervated by sensory afferent nerves, whose activation regulates respiration and triggers defensive reflexes (e.g., cough, bronchospasm). Airway innervation is heterogeneous, and distinct afferent subsets have distinct functional responses. However, little is known of the innervation patterns of subsets within the lung. A neuroanatomical map is critical for understanding afferent activation under physiological and pathophysiological conditions. Here, we quantified the innervation of the mouse lung by vagal and dorsal root ganglion (DRG) sensory subsets defined by the expression of Pirt (all afferents), 5HT3 (vagal nodose afferents), Tac1 (tachykinergic afferents), and transient receptor potential vanilloid 1 channel (TRPV1; defensive/nociceptive afferents) using Cre-mediated reporter expression. We found that vagal afferents innervate almost all conducting airways and project into the alveolar region, whereas DRG afferents only innervate large airways. Of the two vagal ganglia, only nodose afferents project into the alveolar region, but both nodose and jugular afferents innervate conducting airways throughout the lung. Many afferents that project into the alveolar region express TRPV1. Few DRG afferents expressed TRPV1. Approximately 25% of blood vessels were innervated by vagal afferents (many were Tac1+). Approximately 10% of blood vessels had DRG afferents (some were Tac1+), but this was restricted to large vessels. Lastly, innervation of neuroepithelial bodies (NEBs) correlated with the cell number within the bodies. In conclusion, functionally distinct sensory subsets have distinct innervation patterns within the conducting airways, alveoli and blood vessels. Physiologic (e.g., stretch) and pathophysiological (e.g., inflammation, edema) stimuli likely vary throughout these regions. Our data provide a neuroanatomical basis for understanding afferent responses in vivo.

Mechanosensitivity of Murine Lung Slowly Adapting Receptors: Minimal Impact of Chemosensory, Serotonergic, and Purinergic Signaling

Murine slowly adapting receptors (SARs) within airway smooth muscle provide volume-related feedback; however, their mechanosensitivity and morphology are incompletely characterized. We explored two aspects of SAR physiology: their inherent static mechanosensitivity and a potential link to pulmonary neuroepithelial bodies (NEBs). SAR mechanosensitivity displays a rate sensitivity linked to speed of inflation; however, to what extent static SAR mechanosensitivity is tuned for the very rapid breathing frequency (B f ) of small mammals (e.g., mouse) is unclear. NEB-associated, morphologically described smooth muscle-associated receptors (SMARs) may be a structural analog for functionally characterized SARs, suggesting functional linkages between SARs and NEBs. We addressed the hypotheses that: (1) rapid murine B f is associated with enhanced in vivo SAR static sensitivity; (2) if SARs and NEBs are functionally linked, stimuli reported to impact NEB function would alter SAR mechanosensitivity. We measured SAR action potential discharge frequency (AP f, action potentials/s) during quasi-static inflation [0-20 cmH2O trans-respiratory pressure (PTR)] in NEB-relevant conditions of hypoxia (FIO2 = 0.1), hypercarbia (FICO2 = 0.1), and pharmacologic intervention (serotonergic 5-HT3 receptor antagonist, Tropisetron, 4.5 mg/kg; P2 purinergic receptor antagonist, Suramin, 50 mg/kg). In all protocols, we obtained: (1) AP f vs. PTR; (2) PTR threshold; and (3) AP f onset at PTR threshold. The murine AP f vs. PTR response comprises high AP f (average maximum AP f: 236.1 ± 11.1 AP/s at 20 cmH2O), a low PTR threshold (mean 2.0 ± 0.1 cmH2O), and a plateau in AP f between 15 and 20 cmH2O. Murine SAR mechanosensitivity (AP f vs. PTR) is up to 60% greater than that reported for larger mammals. Even the maximum difference between intervention and control conditions was minimally impacted by NEB-related alterations: Tropisetron -7.6 ± 1.8% (p = 0.005); Suramin -10.6 ± 1.5% (p = 0.01); hypoxia +9.3 ± 1.9% (p < 0.001); and hypercarbia -6.2 ± 0.9% (p < 0.001). We conclude that the high sensitivity of murine SARs to inflation provides enhanced resolution of operating lung volume, which is aligned with the rapid B f of the mouse. We found minimal evidence supporting a functional link between SARs and NEBs and speculate that the <10% change in SAR mechanosensitivity during altered NEB-related stimuli is not consistent with a meaningful physiologic role.

Identification of lung innervating sensory neurons and their target specificity

Known as the gas exchange organ, the lung is also critical for responding to the aerosol environment in part through interaction with the nervous system. The diversity and specificity of lung innervating neurons remain poorly understood. Here, we interrogated the cell body location and molecular signature and projection pattern of lung innervating sensory neurons. Retrograde tracing from the lung coupled with whole tissue clearing highlighted neurons primarily in the vagal ganglia. Centrally, they project specifically to the nucleus of the solitary tract in the brainstem. Peripherally, they enter the lung alongside branching airways. Labeling of nociceptor Trpv1+ versus peptidergic Tac1+ vagal neurons showed shared and distinct terminal morphology and targeting to airway smooth muscles, vasculature including lymphatics, and alveoli. Notably, a small population of vagal neurons that are Calb1+ preferentially innervate pulmonary neuroendocrine cells, a demonstrated airway sensor population. This atlas of lung innervating neurons serves as a foundation for understanding their function in lung.

Pingchuan formula attenuates airway mucus hypersecretion via regulation of the PNEC-GABA-IL13-Muc5ac axis in asthmatic mice

BACKGROUND/AIMS

Asthma is a common chronic respiratory disease. It has been reported that Pingchuan formula (PCF) can control asthma attacks by reducing airway inflammation, muscle spasm and mucus secretion. However, PCF's mechanism for reducing airway mucus hypersecretion remains unclear. This study aimed to investigate the effect of PCF on airway mucus secretion in asthmatic mice and to explore changes in the PNEC-GABA-IL13-Muc5ac axis.

METHODS

Male Babl/c mice were used to establish the asthma model via sensitisation with OVA. Mice were randomly divided into Normal, OVA, DEX, and PCF groups. After treatment, lung histopathology was observed with H&E and PAS staining. BALF levels of IL-5 and IL-13 were detected using ELISA. The levels of mRNA and protein expression for GAD1, GABAARβ1, GABAARα1 and Muc5ac in the lung tissue were measured by RT-PCR and Western blot assays. PNECs were observed with AgNOR staining.

RESULTS

PCF treatment effectively reduced goblet cell (P < 0.01) and PNEC (P < 0.05) proliferation, lung tissue inflammation and airway mucus hypersecretion. In addition, PCF also markedly downregulated mRNA and protein expression of GAD1, GABAARβ1, GABAARα1 and Muc5ac (P < 0.05, compared with OVA), thus inhibiting the GABA-IL-13 pathway in the lung tissue of asthmatic mice.

CONCLUSION

These findings suggest that PCF controls asthma attacks by reducing airway inflammation and mucus hypersecretion via the PNEC-GABA-IL13-Muc5ac axis.

Molecular identity, anatomy, gene expression and function of neural crest vs. placode-derived nociceptors in the lower airways

The lower airways (larynx to alveoli) are protected by a complex array of neural networks that regulate respiration and airway function. Harmful stimuli trigger defensive responses such as apnea, cough and bronchospasm by activating a subpopulation of sensory afferent nerves (termed nociceptors) which are found throughout the airways. Airway nociceptive fibers are projected from the nodose vagal ganglia, the jugular vagal ganglia and the dorsal root ganglia, which are derived from distinct embryological sources: the former from the epibranchial placodes, the latter two from the neural crest. Embryological source determines nociceptive gene expression of receptors and neurotransmitters and recent evidence suggests that placode- and neural crest-derived nociceptors have distinct stimuli sensitivity, innervation patterns and functions. Improved understanding of the function of each subset in specific reflexes has substantial implications for therapeutic targeting of the neuronal components of airway disease such as asthma, viral infections and chronic obstructive pulmonary disease.

Less Is More: Rare Pulmonary Neuroendocrine Cells Function as Critical Sensors in Lung

Pulmonary neuroendocrine cells (PNECs) are rare airway epithelial cells that also uniquely harbor neuronal and endocrine characteristics. In vitro data indicate that these cells respond to chemical or mechanical stimuli by releasing neuropeptides and neurotransmitters, implicating them as airway sensors. Emerging in vivo data corroborate this role and demonstrate that PNECs are important for lung response to signals, such as allergens. With close proximity to steady-state immune cells and innervating nerves, PNECs, as prototype tissue-resident neuroendocrine cells, are at the center of a neuro-immune module that enables the fundamental ability of an organ to sense and respond to the environment.

Gastrin-releasing peptide induces fibrotic response in MRC5s and proliferation in A549s

Idiopathic pulmonary fibrosis (IPF) is a complex lung disease, whose build-up scar tissue is induced by several molecules. Gastrin-releasing peptide (GRP) is released from pulmonary neuroendocrine cells, alveolar macrophages, and some nerve endings in the lung. A possible role of GRP in IPF is unclear. We aimed to investigate the fibrotic response to GRP, at the cellular level in MRC5 and A549 cell lines. The proliferative and fibrotic effects of GRP on these cells were evaluated by using BrdU, immunoblotting, immunofluorescence and qRT-PCR for molecules associated with myofibroblast differentiation, TGF-β and Wnt signalling. All doses of GRP increased the amount of BrdU incorporation in A549 cells. In contrast, the amount of BrdU increased in MRC5 cells in the first 24 h, though progressively decreased by 72 h. GRP did not stimulate epithelial-mesenchymal transition in A549 cells, rather, it stimulated the differentiation of MRC5 cells into myofibroblasts. Furthermore, GRP induced gene and protein expressions of p-Smad2/3 and Smad4, and reduced the levels of Smad7 in MRC5 cells. In addition, GRP decreased Wnt5a protein levels and stimulated β-catenin activation by increasing Wnt4, Wnt7a and β-catenin protein levels. GRP caused myofibroblast differentiation by inducing TGF-βand Wnt pathways via paracrine and autocrine signalling in MRC5 cells. In conclusion, GRP may lead to pulmonary fibrosis due to its proliferative and fibrotic effects on lung fibroblasts. The abrogation of GRP-mediated signal activation might be considered as a treatment modality for fibrotic lung diseases. Video Abstract.

Neuroendocrine regulation of innate lymphoid cells

The activities of the immune system in repairing tissue injury and combating pathogens were long thought to be independent of the nervous system. However, a major regulatory role of immunomodulatory molecules released locally or systemically by the neuroendocrine system has recently emerged. A number of observations and discoveries support indeed the notion of the nervous system as an immunoregulatory system involved in immune responses. Innate lymphoid cells (ILCs), including natural killer (NK) cells and tissue-resident ILCs, form a family of effector cells present in organs and mucosal barriers. ILCs are involved in the maintenance of tissue integrity and homeostasis. They can also secrete effector cytokines rapidly, and this ability enables them to play early roles in the immune response. ILCs are activated by multiple pathways including epithelial and myeloid cell-derived cytokines. Their functions are also regulated by mediators produced by the nervous system. In particular, the peripheral nervous system, through neurotransmitters and neuropeptides, works in parallel with the hypothalamic-pituitary-adrenal and gonadal axis to modulate inflammatory events and maintain homeostasis. We summarize here recent findings concerning the regulation of ILC activities by neuroendocrine mediators in homeostatic and inflammatory conditions.

Selective activation and proliferation of a quiescent stem cell population in the neuroepithelial body microenvironment

BACKGROUND

The microenvironment (ME) of neuroepithelial bodies (NEBs) harbors densely innervated groups of pulmonary neuroendocrine cells that are covered by Clara-like cells (CLCs) and is believed to be important during development and for adult airway epithelial repair after severe injury. Yet, little is known about its potential stem cell characteristics in healthy postnatal lungs.

METHODS

Transient mild lung inflammation was induced in mice via a single low-dose intratracheal instillation of lipopolysaccharide (LPS). Bronchoalveolar lavage fluid (BALF), collected 16 h after LPS instillation, was used to challenge the NEB ME in ex vivo lung slices of control mice. Proliferating cells in the NEB ME were identified and quantified following simultaneous LPS instillation and BrdU injection.

RESULTS

The applied LPS protocol induced very mild and transient lung injury. Challenge of lung slices with BALF of LPS-treated mice resulted in selective Ca²⁺-mediated activation of CLCs in the NEB ME of control mice. Forty-eight hours after LPS challenge, a remarkably selective and significant increase in the number of divided (BrdU-labeled) cells surrounding NEBs was observed in lung sections of LPS-challenged mice. Proliferating cells were identified as CLCs.

CONCLUSIONS

A highly reproducible and minimally invasive lung inflammation model was validated for inducing selective activation of a quiescent stem cell population in the NEB ME. The model creates new opportunities for unraveling the cellular mechanisms/pathways regulating silencing, activation, proliferation and differentiation of this unique postnatal airway epithelial stem cell population.

Pulmonary neuroendocrine cells amplify allergic asthma responses

Pulmonary neuroendocrine cells (PNECs) are rare airway epithelial cells whose function is poorly understood. Here we show that Ascl1-mutant mice that have no PNECs exhibit severely blunted mucosal type 2 response in models of allergic asthma. PNECs reside in close proximity to group 2 innate lymphoid cells (ILC2s) near airway branch points. PNECs act through calcitonin gene-related peptide (CGRP) to stimulate ILC2s and elicit downstream immune responses. In addition, PNECs act through the neurotransmitter γ-aminobutyric acid (GABA) to induce goblet cell hyperplasia. The instillation of a mixture of CGRP and GABA in Ascl1-mutant airways restores both immune and goblet cell responses. In accordance, lungs from human asthmatics show increased PNECs. These findings demonstrate that the PNEC-ILC2 neuroimmunological modules function at airway branch points to amplify allergic asthma responses.

Serotonergic paraneurones in the female mouse urethral epithelium and their potential role in peripheral sensory information processing

AIM

The mechanisms underlying detection and transmission of sensory signals arising from visceral organs, such as the urethra, are poorly understood. Recently, specialized ACh-expressing cells embedded in the urethral epithelium have been proposed as chemosensory sentinels for detection of bacterial infection. Here, we examined the morphology and potential role in sensory signalling of a different class of specialized cells that express serotonin (5-HT), termed paraneurones.

METHODS

Urethrae, dorsal root ganglia neurones and spinal cords were isolated from adult female mice and used for immunohistochemistry and calcium imaging. Visceromotor reflexes (VMRs) were recorded in vivo.

RESULTS

We identified two morphologically distinct groups of 5-HT+ cells with distinct regional locations: bipolar-like cells predominant in the mid-urethra and multipolar-like cells predominant in the proximal and distal urethra. Sensory nerve fibres positive for calcitonin gene-related peptide, substance P, and TRPV1 were found in close proximity to 5-HT+ paraneurones. In vitro 5-HT (1 μm) stimulation of urethral primary afferent neurones, mimicking 5-HT release from paraneurones, elicited changes in the intracellular calcium concentration ([Ca2+ ]i ) mediated by 5-HT2 and 5-HT3 receptors. Approximately 50% of 5-HT responding cells also responded to capsaicin with changes in the [Ca2+ ]i . In vivo intra-urethral 5-HT application increased VMRs induced by urethral distention and activated pERK in lumbosacral spinal cord neurones.

CONCLUSION

These morphological and functional findings provide insights into a putative paraneurone-neural network within the urethra that utilizes 5-HT signalling, presumably from paraneurones, to modulate primary sensory pathways carrying nociceptive and non-nociceptive (mechano-sensitive) information to the central nervous system.

Morphology of P2X3-immunoreactive nerve endings in the rat tracheal mucosa

Nerve endings with immunoreactivity for the P2X3 purinoreceptor (P2X3) in the rat tracheal mucosa were examined by immunohistochemistry of whole-mount preparations with confocal scanning laser microscopy. P2X3 immunoreactivity was observed in ramified endings distributed in the whole length of the trachea. The myelinated parent axons of P2X3-immunoreactive nerve endings ramified into several branches that extended two-dimensionally in every direction at the interface between the epithelial layer and lamina propria. The axonal branches of P2X3-immunoreactive endings branched off many twigs located just beneath the epithelium, and continued to intraepithelial axon terminals. The axon terminals of P2X3-immunoreactive endings were beaded, rounded, or club-like in shape and terminated between tracheal epithelial cells. Flat axon terminals sometimes partly ensheathed neuroendocrine cells with immunoreactivity for SNAP25 or CGRP. Some axons and axon terminals with P2X3 immunoreactivity were immunoreactive for P2X2, while some terminals were immunoreactive for vGLUT2. Furthermore, a retrograde tracing method using fast blue (FB) revealed that 88.4% of FB-labeled cells with P2X3 immunoreactivity originated from the nodose ganglion. In conclusion, P2X3-immunoreactive nerve endings in the rat tracheal mucosa have unique morphological characteristics, and these endings may be rapidly adapting receptors and/or irritant receptors that are activated by mucosal irritant stimuli.

An immunohistochemical identification key for cell types in adult mouse prostatic and urethral tissue sections

Though many methods can be used to identify cell types contained in complex tissues, most require cell disaggregation and destroy information about where cells reside in relation to their microenvironment. Here, we describe a polytomous key for cell type identification in intact sections of adult mouse prostate and prostatic urethra. The key is organized as a decision tree and initiates with one round of immunostaining for nerve, epithelial, fibromuscular/hematolymphoid, or vascular associated cells. Cell identities are recursively eliminated by subsequent staining events until the remaining pool of potential cell types can be distinguished by direct comparison to other cells. We validated our identification key using wild type adult mouse prostate and urethra tissue sections and it currently resolves sixteen distinct cell populations which include three nerve fiber types as well as four epithelial, five fibromuscular/hematolymphoid, one nerve-associated, and three vascular-associated cell types. We demonstrate two uses of this novel identification methodology. We first used the identification key to characterize prostate stromal cell type changes in response to constitutive phosphatidylinositide-3-kinase activation in prostate epithelium. We then used the key to map cell lineages in a new reporter mouse strain driven by Wnt10aem1(cre/ERT2)Amc. The identification key facilitates rigorous and reproducible cell identification in prostate tissue sections and can be expanded to resolve additional cell types as new antibodies and other resources become available.

Selective gene expression analysis of the neuroepithelial body microenvironment in postnatal lungs with special interest for potential stem cell characteristics

BACKGROUND

The pulmonary neuroepithelial body (NEB) microenvironment (ME) consists of innervated cell clusters that occur sparsely distributed in the airway epithelium, an organization that has so far hampered reliable selective gene expression analysis. Although the NEB ME has been suggested to be important for airway epithelial repair after ablation, little is known about their potential stem cell characteristics in healthy postnatal lungs. Here we report on a large-scale selective gene expression analysis of the NEB ME.

METHODS

A GAD67-GFP mouse model was used that harbors GFP-fluorescent NEBs, allowing quick selection and pooling by laser microdissection (LMD) without further treatment. A panel of stem cell-related PCR arrays was used to selectively compare mRNA expression in the NEB ME to control airway epithelium (CAE). For genes that showed a higher expression in the NEB ME, a ranking was made based on the relative expression level. Single qPCR and immunohistochemistry were used to validate and quantify the PCR array data.

RESULTS

Careful optimization of all protocols appeared to be essential to finally obtain high-quality RNA from pooled LMD samples of NEB ME. About 30% of the more than 600 analyzed genes showed an at least two-fold higher expression compared to CAE. The gene that showed the highest relative expression in the NEB ME, Delta-like ligand 3 (Dll3), was investigated in more detail. Selective Dll3 gene expression in the NEB ME could be quantified via single qPCR experiments, and Dll3 protein expression could be localized specifically to NEB cell surface membranes.

CONCLUSIONS

This study emphasized the importance of good protocols and RNA quality controls because of the, often neglected, fast RNA degradation in postnatal lung samples. It was shown that sufficient amounts of high-quality RNA for reliable complex gene expression analysis can be obtained from pooled LMD-collected NEB ME samples of postnatal lungs. Dll3 expression, which has also been reported to be important in high-grade pulmonary tumor-initiating cells, was used as a proof-of-concept to confirm that the described methodology represents a promising tool for further unraveling the molecular basis of NEB ME physiology in general, and its postnatal stem cell capacities in particular.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Glial Fibrillary Acidic Protein-Expressing Glia in the Mouse Lung

Autonomic nerves regulate important functions in visceral organs, including the lung. The postganglionic portion of these nerves is ensheathed by glial cells known as non-myelinating Schwann cells. In the brain, glia play important functional roles in neurotransmission, neuroinflammation, and maintenance of the blood brain barrier. Similarly, enteric glia are now known to have analogous roles in gastrointestinal neurotransmission, inflammatory response, and barrier formation. In contrast to this, very little is known about the function of glia in other visceral organs. Like the gut, the lung forms a barrier between airborne pathogens and the bloodstream, and autonomic lung innervation is known to affect pulmonary inflammation and lung function. Lung glia are described as non-myelinating Schwann cells but their function is not known, and indeed no transgenic tools have been validated to study them in vivo. The primary goal of this research was, therefore, to investigate the relationship between non-myelinating Schwann cells and pulmonary nerves in the airways and vasculature and to validate existing transgenic mouse tools that would be useful for studying their function. We focused on the glial fibrillary acidic protein promoter, which is a cognate marker of astrocytes that is expressed by enteric glia and non-myelinating Schwann cells. We describe the morphology of non-myelinating Schwann cells in the lung and verify that they express glial fibrillary acidic protein and S100, a classic glial marker. Furthermore, we characterize the relationship of non-myelinating Schwann cells to pulmonary nerves. Finally, we report tools for studying their function, including a commercially available transgenic mouse line.

Formation of a Neurosensory Organ by Epithelial Cell Slithering

Epithelial cells are normally stably anchored, maintaining their relative positions and association with the basement membrane. Developmental rearrangements occur through cell intercalation, and cells can delaminate during epithelial-mesenchymal transitions and metastasis. We mapped the formation of lung neuroepithelial bodies (NEBs), innervated clusters of neuroendocrine/neurosensory cells within the bronchial epithelium, revealing a targeted mode of cell migration that we named "slithering," in which cells transiently lose epithelial character but remain associated with the membrane while traversing neighboring epithelial cells to reach cluster sites. Immunostaining, lineage tracing, clonal analysis, and live imaging showed that NEB progenitors, initially distributed randomly, downregulate adhesion and polarity proteins, crawling over and between neighboring cells to converge at diametrically opposed positions at bronchial branchpoints, where they reestablish epithelial structure and express neuroendocrine genes. There is little accompanying progenitor proliferation or apoptosis. Activation of the slithering program may explain why lung cancers arising from neuroendocrine cells are highly metastatic.

Sensory input to the central nervous system from the lungs and airways: A prominent role for purinergic signalling via P2X2/3 receptors

Specific subpopulations of lung-related primary afferent neurons in dorsal root and vagal sensory ganglia have been reported to express P2X2 and P2X3 receptors both in the neuronal cell bodies and in their peripheral terminals. The afferent innervation of airways and lungs is organised as sensory receptor structures, of which at least seven types with a vagal origin and two with a spinal origin have been reported. In view of the recently suggested therapeutic promise of ATP antagonism - specifically at P2X3 receptor expressing nociceptive fibres - in respiratory disorders, the present work focusses on four distinct populations of pulmonary sensory receptors that have so far been reported to express P2X2/3 receptors. Three of them originate from myelinated nerve fibres that display similar mechanosensor-like morphological and neurochemical characteristics. Two of the latter concern vagal nodose sensory fibres, either related to pulmonary neuroepithelial bodies (NEBs), or giving rise to smooth muscle-associated airway receptors (SMARs); the third gives rise to visceral pleura receptors (VPRs) and most likely arises from dorsal root ganglia. The fourth population concerns C-fibre receptors (CFRs) that also derive from neuronal cell bodies located in vagal nodose ganglia. Although the majority of the airway- and lung-related sensory receptors that express P2X2/3 receptors apparently do not belong to accepted nociceptive populations, these data definitely point out that ATP may be an important player in the physiological transduction of different lung-related afferent signals from the periphery to the CNS. The observed variety within the populations of pulmonary sensory receptors that express P2X2/3 receptors argues for a critical and careful interpretation of the functional data.

Vagal Sensory Neuron Subtypes that Differentially Control Breathing

Breathing is essential for survival and under precise neural control. The vagus nerve is a major conduit between lung and brain required for normal respiration. Here, we identify two populations of mouse vagus nerve afferents (P2ry1, Npy2r), each a few hundred neurons, that exert powerful and opposing effects on breathing. Genetically guided anatomical mapping revealed that these neurons densely innervate the lung and send long-range projections to different brainstem targets. Npy2r neurons are largely slow-conducting C fibers, while P2ry1 neurons are largely fast-conducting A fibers that contact pulmonary endocrine cells (neuroepithelial bodies). Optogenetic stimulation of P2ry1 neurons acutely silences respiration, trapping animals in exhalation, while stimulating Npy2r neurons causes rapid, shallow breathing. Activating P2ry1 neurons did not impact heart rate or gastric pressure, other autonomic functions under vagal control. Thus, the vagus nerve contains intermingled sensory neurons constituting genetically definable labeled lines with different anatomical connections and physiological roles.

Immunohistochemical characterization of the chemosensory pulmonary neuroepithelial bodies in the naked mole-rat reveals a unique adaptive phenotype

The pulmonary neuroepithelial bodies (NEBs) constitute polymodal airway chemosensors for monitoring and signaling ambient gas concentrations (pO₂, pCO₂/H⁺) via complex innervation to the brain stem controlling breathing. NEBs produce the bioactive amine, serotonin (5-HT), and a variety of peptides with multiple effects on lung physiology and other organ systems. NEBs in mammals appear prominent and numerous during fetal and neonatal periods, and decline in the post-natal period suggesting an important role during perinatal adaptation. The naked mole-rat (NMR), Heterocephalus glaber, has adapted to the extreme environmental conditions of living in subterranean burrows in large colonies (up to 300 colony mates). The crowded, unventilated burrows are environments of severe hypoxia and hypercapnia. However, NMRs adjust readily to above ground conditions. The chemosensory NEBs of this species were characterized and compared to those of the conventional Wistar rat (WR) to identify similarities and differences that could explain the NMR’s adaptability to environments. A multilabel immunohistochemical analysis combined with confocal microscopy revealed that the expression patterns of amine, peptide, neuroendocrine, innervation markers and chemosensor component proteins in NEBs of NMR were similar to that of WR. However, we found the following differences: 1) NEBs in both neonatal and adult NMR lungs were significantly larger and more numerous as compared to WR; 2) NEBs in NMR had a more variable compact cell organization and exhibited significant differences in the expression of adhesion proteins; 3) NMR NEBs showed a significantly greater ratio of 5-HT positive cells with an abundance of 5-HT; 4) NEBs in NMR expressed the proliferating cell nuclear antigen (PCNA) and the neurogenic gene (MASH1) indicating active proliferation and a state of persistent differentiation. Taken together our findings suggest that NEBs in lungs of NMR are in a hyperactive, functional and developmental state, reminiscent of a persistent fetal state that extends postnatally.

Functional expression of the multimodal extracellular calcium-sensing receptor in pulmonary neuroendocrine cells

The Ca(2+)-sensing receptor (CaSR) is the master regulator of whole-body extracellular free ionized [Ca(2+)]o. In addition to sensing [Ca(2+)]o, CaSR integrates inputs from a variety of different physiological stimuli. The CaSR is also expressed in many regions outside the [Ca(2+)]o homeostatic system, including the fetal lung where it plays a crucial role in lung development. Here, we show that neuroepithelial bodies (NEBs) of the postnatal mouse lung express a functional CaSR. NEBs are densely innervated groups of neuroendocrine epithelial cells in the lung representing complex sensory receptors in the airways and exhibiting stem cell characteristics. qRT-PCR performed on laser microdissected samples from GAD67-GFP mouse lung cryosections revealed exclusive expression of the CaSR in the NEB microenvironment. CaSR immunoreactivity was present at NEB cells from postnatal day 14 onwards. Confocal imaging of lung slices revealed that NEB cells responded to an increase of [Ca(2+)]o with a rise in intracellular Ca(2+) ([Ca(2+)]i); an effect mimicked by several membrane-impermeant CaSR agonists (e.g. the calcimimetic R-568) and that was blocked by the calcilytic Calhex-231. Block of TRPC channels attenuated the CaSR-dependent increases in [Ca(2+)]i, suggesting that Ca(2+) influx through TRPC channels contributes to the total [Ca(2+)]i signal evoked by the CaSR in NEBs. CaSR also regulated baseline [Ca(2+)]i in NEBs and, through paracrine signaling from Clara-like cells, coordinated intercellular communication in the NEB microenvironment. These data suggest that the NEB CaSR integrates multiple signals converging on this complex chemosensory unit, and is a key regulator of this intrapulmonary airway stem cell niche.

Mechanisms of respiratory innervation during embryonic development

During embryogenesis, the development of the respiratory tract is closely associated with the formation of an extensive neuronal network. While the topic of respiratory innervation is not new, and similar articles were published previously, recent studies using animal models and genetic tools are breathing new life into the field. In this review, we focus on signaling mechanisms that underlie innervation of the embryonic respiratory tract.

GABAergic signaling in the pulmonary neuroepithelial body microenvironment: functional imaging in GAD67-GFP mice

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates, but has also been reported in multiple cell types outside the CNS. A GABAergic system has been proposed in neuroepithelial bodies (NEBs) in monkey lungs. Pulmonary NEBs are known as complex intraepithelial sensory airway receptors and are part of the NEB microenvironment. Aim of the present study was to unravel a GABAergic signaling system in the NEB microenvironment in mouse lungs, enabling the use of genetically modified animals for future functional studies. Immunostaining of mouse lungs revealed that glutamic acid decarboxylase 65/67 (GAD65/67), a rate-limiting enzyme in the biosynthesis of GABA, and the vesicular GABA transporter (VGAT) were exclusively expressed in NEB cells. In GAD67-green fluorescent protein (GFP) knock-in mice, all pulmonary NEBs appeared to express GFP. For confocal live cell imaging, ex vivo vibratome lung slices of GAD67-GFP mice can be directly loaded with fluorescent functional probes, e.g. a red-fluorescent calcium dye, without the necessity of time-consuming prior live visualization of NEBs. RT-PCR of the NEB microenvironment obtained by laser microdissection revealed the presence of both GABAA and GABAB (R1 and R2) receptors, which was confirmed by immunostaining. In conclusion, the present study not only revealed the presence of a GABAergic signaling pathway, but also the very selective expression of GFP in pulmonary NEBs in a GAD67-GFP mouse model. Different proof of concept experiments have clearly shown that adoption of the GAD67-GFP mouse model will certainly boost future functional imaging and gene expression analysis of the mouse NEB microenvironment.

Molecular signaling and targets from itch: lessons for cough

Itch is described as an unpleasant sensation that elicits the desire to scratch, which results in the removal of the irritant from the skin. The cough reflex also results from irritation, with the purpose of removing said irritant from the airway. Could cough then be similar to itch? Anatomically, both pathways are mediated by small-diameter sensory fibers. These cough and itch sensory fibers release neuropeptides upon activation, which leads to inflammation of the nerves. Both cough and itch also involve mast cells and their mediators, which are released upon degranulation. This common inflammation and interaction with mast cells are involved in the development of chronic conditions of itch and cough. In this review, we examine the anatomy and molecular mechanisms of itch and compare them to known mechanisms for cough. Highlighting the common aspects of itch and cough could lead to new thoughts and perspectives in both fields.

Distribution of acetylcholine and catecholamines in fish gills and their potential roles in the hypoxic ventilatory response

Carotid body glomus cells in mammals contain a plethora of different neurochemicals. Several hypotheses exist to explain their roles in oxygen-chemosensing. In the present study we assessed the distribution of serotonin, acetylcholine and catecholamines in the gills of trout (Oncorhynchus mykiss) and goldfish (Carassius auratus) using immunohistochemistry, and an activity-dependent dye, Texas Red hydrazide (TXR). In fish the putative oxygen sensing cells are neuroepithelial cells (NECs) and the focus in recent studies has been on the role of serotonin in oxygen chemoreception. The NECs of trout and goldfish contain serotonin, but, in contrast to the glomus cells of mammals, not acetylcholine or catecholamines. Acetylcholine was expressed in chain and proximal neurons and in extrinsic nerve bundles in the filaments. The serotonergic NECs did not label with the HNK-1 antibody suggesting that if they are derived from the neural crest, they are no longer proliferative or migrating. Furthermore, we predicted that if serotonergic NECs were chemosensory, they would increase their activity during hypoxia (endocytose TXR), but following 30 min of hypoxic exposure (45 Torr), serotonergic NECs did not take up TXR. Based on these and previous findings we propose several possible models outlining the ways in which serotonin and acetylcholine could participate in oxygen chemoreception in completing the afferent sensory pathway.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Recent advances and contraversies on the role of pulmonary neuroepithelial bodies as airway sensors

Pulmonary neuroepithelial bodies are polymodal sensors widely distributed within the airway mucosa of mammals and other species. Neuroepithelial body cells store and most likely release serotonin and peptides as transmitters. Neuroepithelial bodies have a complex innervation that includes vagal sensory afferent fibers and dorsal root ganglion fibers. Neuroepithelial body cells respond to a number of intraluminal airway stimuli, including hypoxia, hypercarbia, and mechanical stretch. This article reviews recent findings in the cellular and molecular biology of neuroepithelial body cells and their potential role as airway sensors involved in the control of respiration, particularly during the perinatal period. Alternate hypotheses and areas of controversy regarding potential function as mechanosensory receptors involved in pulmonary reflexes are discussed.

Activation of mouse bronchopulmonary C-fibres by serotonin and allergen-ovalbumin challenge

Abstract  The effect of serotonin on capsaicin-sensitive vagal C-fibre afferent nerves was evaluated in an ex vivo vagally innervated mouse lung preparation. Action potentials arising from receptive fields in the lungs were recorded with an extracellular electrode positioned in the nodose/jugular ganglion. Among the 62 capsaicin-sensitive C-fibres studied (conduction velocity ∼0.5 m s(-1)), 71% were of the nodose phenotype and 29% of the jugular phenotype. The nodose C-fibres responded strongly to serotonin and this effect was blocked with the 5-HT3-receptor antagonist ondansetron. Using single cell RT-PCR, we noted that the vast majority of nodose neurons retrogradely labelled from the lung, expressed 5-HT3 receptor mRNA. The jugular C-fibres also responded strongly to serotonin with action potential discharge, but this effect was not inhibited by ondansetron. Lung-specific jugular neurons did not express 5-HT3 receptor mRNA but frequently expressed 5-HT1 or 5-HT4 receptor mRNA. Mast cells are the major source of serotonin in healthy murine airways. Ovalbumin-induced mast cell activation in actively sensitized lungs caused action potential discharge in jugular but not nodose C-fibres. The data show that vagal C-fibres in the respiratory tract of the mouse are strongly activated by serotonin. Depending on the C-fibre subtype both 5-HT3 and non-5-HT3 mechanisms are involved.

NOX2 (gp91phox) is a predominant O2 sensor in a human airway chemoreceptor cell line: biochemical, molecular, and electrophysiological evidence

Pulmonary neuroepithelial bodies (NEBs), composed of clusters of amine [serotonin (5-HT)] and peptide-producing cells, are widely distributed within the airway mucosa of human and animal lungs. NEBs are thought to function as airway O(2)-sensors, since they are extensively innervated and release 5-HT upon hypoxia exposure. The small cell lung carcinoma cell line (H146) provides a useful model for native NEBs, since they contain (and secrete) 5-HT and share the expression of a membrane-delimited O(2) sensor [classical NADPH oxidase (NOX2) coupled to an O(2)-sensitive K(+) channel]. In addition, both native NEBs and H146 cells express different NADPH oxidase homologs (NOX1, NOX4) and its subunits together with a variety of O(2)-sensitive voltage-dependent K(+) channel proteins (K(v)) and tandem pore acid-sensing K(+) channels (TASK). Here we used H146 cells to investigate the role and interactions of various NADPH oxidase components in O(2)-sensing using a combination of coimmunoprecipitation, Western blot analysis (quantum dot labeling), and electrophysiology (patchclamp, amperometry) methods. Coimmunoprecipitation studies demonstrated formation of molecular complexes between NOX2 and K(v)3.3 and K(v)4.3 ion channels but not with TASK1 ion channels, while NOX4 associated with TASK1 but not with K(v) channel proteins. Downregulation of mRNA for NOX2, but not for NOX4, suppressed hypoxia-sensitive outward current and significantly reduced hypoxia -induced 5-HT release. Collectively, our studies suggest that NOX2/K(v) complexes are the predominant O(2) sensor in H146 cells and, by inference, in native NEBs. Present findings favor a NEB cell-specific plasma membrane model of O(2)-sensing and suggest that unique NOX/K(+) channel combinations may serve diverse physiological functions.

Neuroepithelial bodies as mechanotransducers in the intrapulmonary airway epithelium: involvement of TRPC5

In rodent lungs, a major part of the myelinated vagal airway afferents selectively contacts pulmonary neuroepithelial bodies (NEBs). Because most myelinated vagal airway afferents concern physiologically characterized mechanoreceptors, the present study aimed at unraveling the potential involvement of NEB cells in transducing mechanosensory information from the airways to the central nervous system. Physiological studies were performed using confocal Ca(2+) imaging of airway epithelium in murine lung slices. Mechanical stimulation by short-term application of a mild hypoosmotic solution (230 mosmol) resulted in a selective, fast, reversible, and reproducible Ca(2+) rise in NEB cells. Other airway epithelial cells could only be activated using more severe hypoosmotic stimuli (< 200 mosmol). NEB cells selectively expressed the Ca(2+)-permeable osmo- and mechanosensitive transient receptor potential canonical channel 5 (TRPC5) in their apical membranes, whereas immunoreactivity for TRP vanilloid-4 and TRP melastatin-3 was abundant in virtually all other airway epithelial cells. Hypoosmotic activation of NEB cells was prevented by GsMTx-4, an inhibitor of mechanosensitive ion channels, and by SKF96365, an inhibitor of TRPC channels. Short application of gadolinium, reported to activate TRPC5 channels, evoked a transient Ca(2+) rise in NEB cells. Osmomechanical activation of NEB cells gave rise to a typical delayed activation of Clara-like cells due to the release of ATP from NEB cells. Because ATP may activate the NEB-associated P2X(2/3) ATP receptor expressing myelinated vagal afferents, the current observations strongly suggest that pulmonary NEB cells are fully equipped to initiate mechanosensory signal transduction to the central nervous system via a purinergic signaling pathway.

Perspectives of purinergic signaling in stem cell differentiation and tissue regeneration

Replacement of lost or dysfunctional tissues by stem cells has recently raised many investigations on therapeutic applications. Purinergic signaling has been shown to regulate proliferation, differentiation, cell death, and successful engraftment of stem cells originated from diverse origins. Adenosine triphosphate release occurs in a controlled way by exocytosis, transporters, and lysosomes or in large amounts from damaged cells, which is then subsequently degraded into adenosine. Paracrine and autocrine mechanisms induced by immune responses present critical factors for the success of stem cell therapy. While P1 receptors generally exert beneficial effects including anti-inflammatory activity, P2 receptor-mediated actions depend on the subtype of stimulated receptors and localization of tissue repair. Pro-inflammatory actions and excitatory tissue damages mainly result from P2X7 receptor activation, while other purinergic receptor subtypes participate in proliferation and differentiation, thereby providing adequate niches for stem cell engraftment and novel mechanisms for cell therapy and endogenous tissue repair. Therapeutic applications based on regulation of purinergic signaling are foreseen for kidney and heart muscle regeneration, Clara-like cell replacement for pulmonary and bronchial epithelial cells as well as for induction of neurogenesis in case of neurodegenerative diseases.

CGRP induction in cystic fibrosis airways alters the submucosal gland progenitor cell niche in mice

In cystic fibrosis (CF), a lack of functional CF transmembrane conductance regulator (CFTR) chloride channels causes defective secretion by submucosal glands (SMGs), leading to persistent bacterial infection that damages airways and necessitates tissue repair. SMGs are also important niches for slow-cycling progenitor cells (SCPCs) in the proximal airways, which may be involved in disease-related airway repair. Here, we report that calcitonin gene-related peptide (CGRP) activates CFTR-dependent SMG secretions and that this signaling pathway is hyperactivated in CF human, pig, ferret, and mouse SMGs. Since CGRP-expressing neuroendocrine cells reside in bronchiolar SCPC niches, we hypothesized that the glandular SCPC niche may be dysfunctional in CF. Consistent with this hypothesis, CFTR-deficient mice failed to maintain glandular SCPCs following airway injury. In wild-type mice, CGRP levels increased following airway injury and functioned as an injury-induced mitogen that stimulated SMG progenitor cell proliferation in vivo and altered the proliferative potential of airway progenitors in vitro. Components of the receptor for CGRP (RAMP1 and CLR) were expressed in a very small subset of SCPCs, suggesting that CGRP indirectly stimulates SCPC proliferation in a non-cell-autonomous manner. These findings demonstrate that CGRP-dependent pathways for CFTR activation are abnormally upregulated in CF SMGs and that this sustained mitogenic signal alters properties of the SMG progenitor cell niche in CF airways. This discovery may have important implications for injury/repair mechanisms in the CF airway.

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.

Neural crest cell origin and signals for intrinsic neurogenesis in the mammalian respiratory tract

Our study investigates the innervation of the respiratory tract during mouse embryonic development, with a focus on the identification of cell origin and essential developmental signals for the resident, or intrinsic, neurons. Using lineage tracing, we show that these intrinsic neurons are exclusively derived from neural crest cells, and cluster to form ganglia that reside in the dorsal trachea and medial bronchi with diminishing frequency. Comparisons of intrinsic neurogenesis between wild-type, glial cell-derived neurotrophic factor (GDNF)(-/-), neurturin(-/-), and tyrosine kinase receptor Ret(-/-) embryos, in combination with lung organ cultures, identified that Ret signaling, redundantly activated by GDNF family members, is required for intrinsic neurogenesis in the trachea and primary bronchi. In contrast, Ret deficiency exerts no effect on the innervation of the rest of the respiratory tract, suggesting that innervation by neurons whose cell bodies are located outside of the lung (so-called extrinsic neurons) is independent of Ret signaling. Furthermore, although the trachea, the esophagus, and their intrinsic neurons share foregut endoderm and a neural crest cell origin, respectively, the signals required for their intrinsic neurogenesis are divergent. Together, our results not only establish the neural crest lineage of intrinsic neurons in the respiratory tract, but also identify regional differences in the abundance and developmental signals of intrinsic neurons along the respiratory tract and in the esophagus.

Extending the knowledge in histochemistry and cell biology

Central to modern Histochemistry and Cell Biology stands the need for visualization of cellular and molecular processes. In the past several years, a variety of techniques has been achieved bridging traditional light microscopy, fluorescence microscopy and electron microscopy with powerful software-based post-processing and computer modeling. Researchers now have various tools available to investigate problems of interest from bird's- up to worm's-eye of view, focusing on tissues, cells, proteins or finally single molecules. Applications of new approaches in combination with well-established traditional techniques of mRNA, DNA or protein analysis have led to enlightening and prudent studies which have paved the way toward a better understanding of not only physiological but also pathological processes in the field of cell biology. This review is intended to summarize articles standing for the progress made in "histo-biochemical" techniques and their manifold applications.

Stabilized beta-catenin in lung epithelial cells changes cell fate and leads to tracheal and bronchial polyposis

The precise mechanisms by which beta-catenin controls morphogenesis and cell differentiation remain largely unknown. Using embryonic lung development as a model, we deleted exon 3 of beta-catenin via Nkx2.1-cre in the Catnb[+/lox(ex3)] mice and studied its impact on epithelial morphogenesis. Robust selective accumulation of truncated, stabilized beta-catenin was found in Nkx2.1-cre;Catnb[+/lox(ex3)] lungs that were associated with the formation of polyp-like structures in the trachea and main-stem bronchi. Characterization of polyps suggests that accumulated beta-catenin impacts epithelial morphogenesis in at least two ways. "Intracellular" accumulation of beta-catenin blocked differentiation of spatially-appropriate airway epithelial cell types, Clara cells, ciliated cells and basal cells, and activated UCHL1, a marker for pulmonary neuroendocrine cells. There was also evidence for a "paracrine" impact of beta-catenin accumulation, potentially mediated via activation of Bmp4 that inhibited Clara and ciliated, but not basal cell differentiation. Thus, excess beta-catenin can alter cell fate determination by both direct and paracrine mechanisms.

Purinergic signaling in the pulmonary neuroepithelial body microenvironment unraveled by live cell imaging

Pulmonary neuroepithelial bodies (NEBs) are densely innervated groups of complex sensory airway receptors involved in the regulation of breathing. Together with their surrounding Clara-like cells, they exhibit stem cell potential through their capacity to regenerate depopulated areas of the epithelium following lung injury. We have employed confocal live cell imaging microscopy and novel electrophysiological techniques in a new ex vivo lung slice model to unravel potential purinergic signaling pathways within the NEB microenvironment. Quinacrine histochemistry indicated high amounts of vesicular ATP in NEB cells. Using a "reporter-patching" method adapted to create a uniquely sensitive and selective biosensor for the direct detection of ATP release from NEBs ex vivo, we demonstrated quantal ATP release from NEBs following their depolarization. Enhancing enzymatic extracellular ATP hydrolysis or inhibiting P2 receptors confirmed the central role of ATP in paracrine interactions between NEB cells and Clara-like cells. Combined calcium imaging, pharmacology, and immunohistochemistry showed that ligand-binding to functional P2Y(2) receptors underpins the activation of Clara-like cells. Hence, NEB cells communicate with their cellular neighbors in the NEB microenvironment by releasing ATP, which rapidly evokes purinergic activation of surrounding Clara-like cells. Besides ATP acting on the P2X(3) receptor expressing vagal sensory nerve terminals between NEB cells, local paracrine purinergic signaling within this potential stem cell niche may be important to both normal airway function, airway epithelial regeneration after injury, and/or the pathogenesis of small cell lung carcinomas.