Ultrastructure of nerves and associated cells in bronchiolar epithelium of the mouse lung

doi:10.1016/S0022-5320(73)90019-1

Journal of Ultrastructure Research

Volume 43, Issues 5–6, June 1973, Pages 426-437

https://doi.org/10.1016/S0022-5320(73)90019-1 Get rights and content

The nerves and their associated cells in the bronchiolar epithelium of the mouse lung were studied with the electron microscope. Single unmyelinated axons, some of which are enlarged and contain many small mitochondria, are closely associated with groups of specialized epithelial cells. These cells are identified by having numerous dense-cored granules in the basal cytoplasm. Portions of the apical surfaces of these cells are exposed to the bronchiolar lumen. The perinuclear cytoplasm contains bundles of branching microfilaments. Many of these cells bear a single cilium on their lateral surfaces adjacent to the bronchiolar lumen. These nerve-epithelial cell complexes probably function as sensory receptor units in the bronchioles.

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Mitochondria have been shown to increase ROS production in response to hypoxia, neurotrophins, tumor necrosis factor-α and Toll-like receptors (Bell et al., 2007; Corda et al., 2001; Pehar et al., 2007; West et al., 2011). Mitochondrial dysfunction may play a substantial role in signaling in sensory afferent function because mitochondria are densely packed within afferent peripheral terminals (Hung et al., 1973; von During and Andres, 1988v). We have shown that mitochondrial dysfunction caused by the electron transfer complex III inhibitor antimycin A evoked acute bronchopulmonary C-fiber activation via the downstream gating of transient receptor potential ankyrin 1 (TRPA1) and TRPV1 (Nesuashvili et al., 2013; Stanford et al., 2019).
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Although activation of these nociceptive sensory nerves initiates protective reflexes, aberrant activation is thought to play a primary role in the chronic initiation of uncontrolled defensive reflexes, sensations and behaviors in inflammatory diseases including asthma, irritable bowel disease, gastroesophageal reflux disorder and colitis. Sensory nerve terminals are densely packed with mitochondria (Hung et al., 1973; von During and Andres, 1988). In addition to the production of ATP through oxidative phosphorylation, mitochondria contribute to cellular signaling processes via many mechanisms including the production of reactive oxygen species (ROS) and the buffering of cytosolic Ca2+ (Gunter et al., 2004; Stowe and Camara, 2009).
Inflammation causes activation of nociceptive sensory nerves, resulting in debilitating sensations and reflexes. Inflammation also induces mitochondrial dysfunction through multiple mechanisms. Sensory nerve terminals are densely packed with mitochondria, suggesting that mitochondrial signaling may play a role in inflammation-induced nociception. We have previously shown that agents that induce mitochondrial dysfunction, such as antimycin A, activate a subset of nociceptive vagal sensory nerves that express transient receptor potential (TRP) channels ankyrin 1 (A1) and vanilloid 1 (V1). However, the mechanisms underlying these responses are incompletely understood. Here, we studied the contribution of TRPA1, TRPV1 and reactive oxygen species (ROS) to antimycin A-induced vagal sensory nerve activation in dissociated neurons and at the sensory terminals of bronchopulmonary C-fibers. Nociceptive neurons were defined chemically and genetically. Antimycin A-evoked activation of vagal nociceptors in a Fura2 Ca²⁺ assay correlated with TRPV1 responses compared to TRPA1 responses. Nociceptor activation was dependent on both TRP channels, with TRPV1 predominating in a majority of responding nociceptors and TRPA1 predominating only in nociceptors with the greatest responses. Surprisingly, both TRPA1 and TRPV1 were activated by H₂O₂ when expressed in HEK293. Nevertheless, targeting ROS had no effect of antimycin A-evoked TRPV1 activation in either HEK293 or vagal neurons. In contrast, targeting ROS inhibited antimycin A-evoked TRPA1 activation in HEK293, vagal neurons and bronchopulmonary C-fibers, and a ROS-insensitive TRPA1 mutant was completely insensitive to antimycin A. We therefore conclude that mitochondrial dysfunction activates vagal nociceptors by ROS-dependent (TRPA1) and ROS-independent (TRPV1) mechanisms.
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Citation Excerpt :
However it is becoming increasingly clear that mitochondrial ROS play critical roles in physiological signaling downstream of multiple pathways [123], including hypoxia [124,125], TNFα [126], neurotrophins via p75NTR [127], Toll-like Receptors [128,129] and TGFβ [130]. Importantly, afferent terminals innervating the peripheral tissue, including the airways, are densely packed with mitochondria [131–133]. Thus both mitochondria and afferent receptors are co-localized.
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Citation Excerpt :
Higher concentrations of H2O2 (120 mM) activate bronchopulmonary C-fibers in the rat via a combination of TRPA1 and P2X channels [50]. The peripheral nerve terminals of vagal airway sensory fibers are densely packed with mitochondria [51–53]. Mitochondria can also be observed in axonal terminals of vagal neurons in long-term culture [54].
Excessive activation of the cough reflex is a major clinical problem in respiratory diseases. The cough reflex is triggered by activation of nociceptive sensory nerve terminals innervating the airways by noxious stimuli. Oxidative stress is a noxious stimuli associated with inhalation of pollutants and inflammatory airway disease. Here, we discuss recent findings that oxidative stress, in particular downstream of mitochondrial dysfunction, evokes increased electrical activity in airway nociceptive sensory nerves. Mechanisms include activation of transient receptor potential (TRP) channels and protein kinase C. Such mechanisms may contribute to excessive cough reflexes in respiratory diseases.
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Citation Excerpt :
Vagal sensory fibers innervate the majority of the viscera with free nerve terminals. Electron microscopy of these 1–2 micro-thick fiber terminals are densely packed with mitochondria, but lack overt ER-like structures (Hung et al., 1973; Kikuchi, 1976; Neuhuber, 1987; von During and Andres, 1988). Despite these reports, we found evidence of a functional ER at vagal nerve growth cone/neurites.
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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.
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: This work was supported in part by the Environmental Protection Agency, APO 1075; the Council for Tobacco Research—USA; the Howard Hughes Employees Give Once Club; and the Hastings Foundation Fund of the University of Southern California.

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Ultrastructure of nerves and associated cells in bronchiolar epithelium of the mouse lung

J. Ultrastruct. Res.

J. Ultrastruct. Res.

Life Sci.

J. Ultrastruct. Res.

J. Ultrastruct. Res.

Lancet

J. Ultrastruct. Res.

Acta Anat.

Cancer

J. Cell Biol.

J. Electron Microsc.

Science

Experimentia

Anat. Rec.

Anat. Rec.

Amer. J. Ant.

Frankfurt. Z. Pathol.

Brain

Lab. Invest.

J. Pharmacol. Exp. Ther.

Gann