Pulmonary neuroendocrine cells in species at high altitude

Airway stem cells sense hypoxia and differentiate into protective solitary neuroendocrine cells

Neuroendocrine (NE) cells are epithelial cells that possess many of the characteristics of neurons, including the presence of secretory vesicles and the ability to sense environmental stimuli. The normal physiologic functions of solitary airway NE cells remain a mystery. We show that mouse and human airway basal stem cells sense hypoxia. Hypoxia triggers the direct differentiation of these stem cells into solitary NE cells. Ablation of these solitary NE cells during hypoxia results in increased epithelial injury, whereas the administration of the NE cell peptide CGRP rescues this excess damage. Thus, we identify stem cells that directly sense hypoxia and respond by differentiating into solitary NE cells that secrete a protective peptide that mitigates hypoxic injury.

HIF Oxygen Sensing Pathways in Lung Biology

Cellular responses to oxygen fluctuations are largely mediated by hypoxia-inducible factors (HIFs). Upon inhalation, the first organ inspired oxygen comes into contact with is the lungs, but the understanding of the pulmonary HIF oxygen-sensing pathway is still limited. In this review we will focus on the role of HIF1α and HIF2α isoforms in lung responses to oxygen insufficiency. In particular, we will discuss novel findings regarding their role in the biology of smooth muscle cells and endothelial cells in the context of hypoxia-induced pulmonary vasoconstriction. Moreover, we will also discuss recent studies into HIF-dependent responses in the airway epithelium, which have been even less studied than the HIF-dependent vascular responses in the lungs. In summary, we will review the biological functions executed by HIF1 or HIF2 in the pulmonary vessels and epithelium to control lung responses to oxygen fluctuations as well as their pathological consequences in the hypoxic lung.

Hyperplasia and hypertrophy of pulmonary neuroepithelial bodies, presumed airway hypoxia sensors, in hypoxia-inducible factor prolyl hydroxylase-deficient mice

Pulmonary neuroepithelial bodies (NEBs), presumed polymodal airway sensors, consist of innervated clusters of amine (serotonin) and peptide-producing cells. While NEB responses to acute hypoxia are mediated by a membrane-bound O2 sensor complex, responses to sustained and/or chronic hypoxia involve a prolyl hydroxylase (PHD)-hypoxia-inducible factor-dependent mechanism. We have previously reported hyperplasia of NEBs in the lungs of Phd1-/- mice associated with enhanced serotonin secretion. Here we use a novel multilabel immunofluorescence method to assess NEB distribution, frequency, and size, together with the number and size of NEB cell nuclei, and to colocalize multiple cytoplasmic and nuclear epitopes in the lungs of Phd1-/-, Phd2+/-, and Phd3-/- mice and compare them with wild-type controls. To define the mechanisms of NEB cell hyperplasia, we used antibodies against Mash1 and Prox1 (neurogenic genes involved in NEB cell differentiation/maturation), hypoxia-inducible factor-1alpha, and the cell proliferation marker Ki67. Morphometric analysis of (% total lung area) immunostaining for synaptophysin (% synaptophysin), a cytoplasmic marker of NEB cells, was significantly increased in Phd1-/- and Phd3-/- mice compared to wild-type mice. In addition, NEB size and the number and size of NEB nuclei were also significantly increased, indicating that deficiency of Phds is associated with striking hyperplasia and hypertrophy of NEBs. In Phd2+/- mice, while mean % synaptophysin was comparable to wild-type controls, the NEB size was moderately increased, suggesting an effect even in heterozygotes. NEBs in all Phd-deficient mice showed increased expression of Mash1, Prox1, Ki67, and hypoxia-inducible factor-1alpha, in keeping with enhanced differentiation from precursor cells and a minor component of cell proliferation. Since the loss of PHD activity mimics chronic hypoxia, our data provide critical information on the potential role of PHDs in the pathobiology and mechanisms of NEB cell hyperplasia that is relevant to a number of pediatric lung disorders.

Pulmonary endocrine cells in hypoplastic lungs due to foetal urinary tract obstruction: a microscopic immunohistochemical study

METHODS

We performed a urinary tract obstruction (UTO) surgical procedure at 93-107 days' gestation in lambs to investigate the relationship between pulmonary hypoplasia and the appearance of pulmonary endocrine cells by quantitative analysis of respiratory tract cells using light microscopic immunohistochemistry.

RESULTS

UTO produced a significant reduction in lung weight, lung/body weight ratio, air capacity, air capacity/body weight ratio (p < 0.01) and radial alveolar count (p < 0.05), which indicated the presence of lung hypoplasia. These foetuses also showed a significant increase in the number of neuron-specific enolase (NSE)-positive pulmonary endocrine cells, expressed as the number of NSE-positive cells per bronchus (p < 0.01) or bronchiole (p < 0.05), the number of NSE-positive cells per unit perimeter of bronchus or bronchiole (p < 0.01), and the number of NSE-positive cells per unit bronchial or bronchiolar surface area (p < 0.01).

CONCLUSION

These results suggest that UTO significantly retards and modifies the structural growth and functional development of pulmonary endocrine cells in NSE expression. We speculate that pulmonary endocrine cells and their mediators may play a role in the problems associated with UTO during intrauterine life.

Quantitative study of pulmonary endocrine cells in fetal, postnatal and adult sheep

Using light microscopic immunohistochemistry, neuron-specific enolase (NSE)-positive endocrine cells were quantitatively analyzed in the sheep lung during different stages of development from the canalicular stages to adulthood. In all stages, NSE-positive endocrine cells were usually located in the bronchi and bronchioles as solitary cells, although a few NSE-positive cell clusters, the so-called neuroepithelial bodies, were found in some places. The number of NSE-positive endocrine cells decreased with advanced stages of gestation. In the late alveolar stage, the number of NSE-positive endocrine cells reached its bottom during the fetal period. There was a gradual upturn after birth. The overall pattern of growth and differentiation of the endocrine cells is most likely species-related and depends on the state of airway development; the number of the endocrine cells of almost all animals, excluding the sheep, in relation to the size of the lung reaches a peak in the late fetal and early neonatal periods and decreases shortly thereafter. NSE-positive endocrine cells were also predominantly located in the large airways during the early stage of development (canalicular stage), and were found more frequently in the small peripheral airways towards the term. These results show the number of NSE-positive endocrine cells in the sheep to be different from that seen in other species.

Amniotic sac infection syndrome features fetal lung neuroendocrine cell hyperfunction

Neuroendocrine cells (NEC) are abundant in fetal and neonatal lungs, but reduced in infants with hyaline membrane disease. Perinatal neuroendocrine cell hyperplasia (NCH) has been reported in the hypoplastic lung in diaphragmatic hernia, bronchopulmonary dysplasia, and Wilson-Mikity syndrome. Since we are unaware of any reports on NCH in fetal inflammatory conditions, this report addresses the NEC in fetuses with congenital pneumonia. Twenty-one fetuses/neonates with congenital pneumonia, autopsied between 1995 and 2001, were compared to 21 fetuses without a congenital infection matched for gestational age. Lung sections were immunostained for chromogranin, bombesin, calcitonin, and synaptophysin. Proportions of immunopositive cells lining 20 consecutive bronchioles calculated from digital images were significantly higher in the study than the control group for chromogranin (1.8 vs. 0.8%, P = 2.4 E-06), calcitonin (1.2 vs. 0.7%, P = 0.005), and bombesin (1.1 vs. 0.7%, P = 0.005). There was no difference in synaptophysin (11.7% vs. 12.6%, P = 0.07). The absence of significant differences in the synaptophysin ratio excludes simple NCH in the study group. The synchronous increase in three neurohormones is indicative of NEC hyperfunction, due to either altered enzymatic inactivation by neutral endopeptidase, known to be reduced in adult lung inflammation, or by an increase in expression of the neurohormone genes. These data indicate that NEC hyperfunction may be responsible for the deranged fetal/neonatal lung function and circulatory adaptation, and contribute to the lethality of the amniotic sac infection syndrome.

Differentiation and proliferation of pulmonary neuroendocrine cells

In this review article the morphological profiles of pulmonary neuroendocrine cells (PNEC) in experimental animals and humans are described. Although the mechanisms of differentiation and proliferation of neuroendocrine cells in the airway epithelium remain to be solved, several experimental studies using explant culture and cell culture systems of fetal animal lungs have been performed to clarify fundamental phenomena associated with neuroendocrine differentiation and proliferation. Experimental animal studies using chronic hypoxia, toxic substances and carcinogens have succeeded in inducing alterations in PNEC systems, and these studies have elucidated the reactions of PNEC in cell injury and inflammation, and functional aspects of PNEC in disease conditions. Human pulmonary neuroendocrine tumors include various histological subtypes, and show divergent morphological and biological varieties. Molecular abnormalities of small cell carcinoma, the most aggressive subtype of pulmonary neuroendocrine tumors, have been extensively studied, but the mechanism of neuroendocrine differentiation of this tumor is still largely unknown. PNEC share common phenotypes with neuronal cells, and developmental studies have begun contributed evidence that similar transcriptional networks, including active and repressive basic helix-loop-helix (bHLH) factors, function in the differentiation of both PNEC and neuronal cells. Such a bHLH network may also play a central role in determining cell differentiation in lung carcinomas. Further studies of the neuronal bHLH network, its regulatory system and related signal transduction pathways, will be required for understanding the mechanisms of neuroendocrine differentiation and proliferation in normal and pathological lung conditions.

Sensory CGRP depletion by capsaicin exacerbates hypoxia-induced pulmonary hypertension in rats

Pulmonary hypertension is a debilitating disease that occurs among infants and adults. One of many etiologies is airway hypoxia. We previously demonstrated a role of endogenous calcitonin gene-related peptide (CGRP), a potent vasodilator, in ameliorating the pulmonary vascular pressor response to chronic hypoxia and related changes in the lungs and heart. This study evaluates the role of endogenous sensory CGRP in hypoxic pulmonary hypertension and examines the intrinsic neural microcircuitry. Rats were pretreated with capsaicin i.p. to deplete pulmonary sensory C-fiber stores of CGRP and substance P and placed in hypobaric hypoxia (10% O2, 16 days) or normoxia together with sham controls. Hypoxia increased pulmonary artery pressure, right-ventricular weight, arterial medial thickness, elasticized capillaries, endothelial cell density, lung water and hematocrit in control rats. Capsaicin augmented pulmonary artery pressure and right-ventricular hypertrophy in hypoxia, and medial thickness and endothelial cell density both in normoxia and hypoxia. Because of the limited effects on these parameters by substance P and other capsaicin-sensitive lung agents, our results suggest that sensory CGRP deficit severely exacerbates pathological signs of hypoxic pulmonary hypertension. A neural microcircuitry consistent with an axon reflex pathway is outlined histochemically. We conclude that endogenous CGRP modulates pulmonary vascular tone in hypoxic pulmonary hypertension which requires intact primary sensory fibers.

Pulmonary Neuroendocrine Cells and Lung Development

Pulmonary neuroendocrine cells produce bioactive peptides such as gastrin-releasing peptide (GRP) at high levels in developing fetal lung. The role of GRP and other peptides in promoting branching morphogenesis, cell proliferation, and cell differentiation during lung organogenesis is reviewed. Possible roles for bioactive peptides derived from these cells in the pathophysiology of perinatal lung disorders are discussed.