Airway Chemoreceptors in Vertebrates

Giacomo Zaccone

• Éditeur: CRC Press
• Année: 2019
• ISBN:
  9780429526701
• Édition: 1
• Sujet: Biowissenschaften
• Sous-sujet: Botanik

This book provides a comprehensive and up-to-date account of the information available on the morphological, physiological and evolutionary aspects of specialized cells distributed within the epithelia of the airways in the vertebrates. A lot of work has been done on the cell and molecular biology of these cells which are regarded as as oxygen receptor neuroepithelial cells. These chemoreceptors which were conserved throughout evolution have neuroendocrine functions carrying their signals to the central nervous system. The chemoreceptor cells are sensors which detect the signal changes in the external and internal environments, and play a key role in the survival of various species. Studies addressed to the chemoreceptor cell systems in the airways are of great importance for investigating their response to changes in the oxygen and carbon dioxide concentrations in the environment since the future of the planet earth is being threatened by global warming and climate change. Praise for the book: ...This volume would be of special interest to researchers who are curious about the evolution of vertebrate respiratory control in general and the regulation of ventilation in nonmammalian vertebrates in particular. -Wayne L. Silver, Wake Forest University, in The Quarterly Review of Biology, Volume 85, Number 2

Pulmonary Neuroepithelial Cells in Mammals: Structure, Molecular Markers, Ontogeny and Functions

11. Diverse and Complex Airway Receptors in Rodent Lungs

Inge Brouns, Isabel Pintelon, Ian de Proost, Jean-Pierre Timmermans and Dirk Adriaensen

12. Oxygen Sensing in Mammalian Pulmonary Neuroepithelial Bodies

E. Cutz, W.X. Fu, H. Yeger J. Pan and C.A. Nurse

13. Precursors and Stem Cells of the Pulmonary Neuroendocrine Cell System in the Developing Mammalian Lung

H. Yeger, J. Pan and E. Cutz

14. Pulmonary Neuroepithelial Bodies as Hypothetical Immunomodulators: Some New Findings and a Review of the Literature

Alfons T.L. Van Loomel, Tania Bollé and Peter W. Hellings

15. Neuroepithelial Bodies and Carotid Bodies: A Comparative Discussion of Pulmonary and Arterial Chemoreceptors

Alfons T.L. Van Lommel

11

Diverse and Complex Airway Receptors in Rodent Lungs

Inge Brouns, Isabel Pintelon, Ian De Proost, Jean-Pierre Timmermans and Dirk Adriaensen*

Abstract

Nowadays, it is clear that pulmonary neuroepithelial bodies (NEBs) are organized as integrated receptor complexes. The groups of pulmonary neuroendocrine cells that are able to store and release transmitters upon appropriate stimulation are, apart from their thin apical processes, crowned by specialized Clara-like cells in most mammalian species. The profuse contacts of many diffrent populations of sensory and motor nerve’fibers with NEB cells strongly suggests that NEBs are able to transduce sensory information and conduct it to the CNS, while the activity of NEB cells can be modulated via autocrine (NEB cells), paracrine (Clara-like cells) and neurocrine (innervation) pathways.

This chapter summarizes our current knowledge of the origin, neurochemical coding and morphology of the selective innervation of pulmonary NEBs in rodents (rats/mice) against a background of ultrastructural information, thereby providing supporting evidence for NEBs being diverse and complex airway receptors with the capacity of chemo- and/or mechanoreceptors.

Introduction

Recent functional morphological data about the innervation of pulmonary neuroepithelial bodies (NEBs) in rodents suggest that NEBs represent an extensive population of very complex intraepithelial receptors. Although the physiological significance of the complex innervation pattern of NEBs is still an enigma, their connections with sensory nerve terminals, and therefore the sensory nature of NEBs, is now beyond dispute. A few years ago, pulmonary NEBs were added to the list of presumed electrophysiologically identified afferent receptors in the lower airways, which until then included slowly adapting stretch receptors (SARs), rapidly adapting stretch receptors (RARs) and C-fiber receptors only (Widdicombe, 2001).

As early as 1949 (Fröhlich), a (chemo)receptor function was suggested for NEBs because of their close association with nerve terminals. At the end of the 20th century, a possible dual role for NEBs in healthy lungs was proposed (Sorokin and Hoyt, 1990, 1993): (1) during early stages of lung organogenesis, pulmonary neuroendocrine cells (PNECs) acting via their amine and peptide transmitters would function as local modulators of lung growth and differentiation; (2) later in fetal life and post-natally, PNECs and, particularly, innervated NEBs would play a role as airway chemoreceptors. Recently, some of the functional facets of the pulmonary neuroendocrine system have been reviewed extensively (Adriaensen et al., 2003, 2006; Linnoila, 2006), including a potential novel role of PNECs/NEBs as guardians of lung stem cell niches (Linnoila, 2006).

Because of the presumed receptor function of NEBs, the current chapter will summarize our present knowledge on the nervous connections of pulmonary NEBs in rats and mice. The chemical coding, exact location and origin of the nerve terminals in contact with pulmonary NEBs will be outlined. A short overview of literature data on the ultrastructural characteristics of pulmonary NEBs in rodents is intended to help to put the data in their correct perspective. Finally, the functional implications of our recent findings, and the findings of others, on the possible working mechanisms of pulmonary NEBs in rodents will be evaluated.

General Morphological Features and Ultrastructural Characteristics of Pulmonary Neuroepithelial Bodies in Rats and Mice

More than 30 years ago, the presence of pulmonary NEBs (Lauweryns et al., 1972) in rats (Cutz et al., 1974) and mice (Hung et al., 1973) was reported. The rather recent discovery of NEBs in the pulmonary epithelium (in comparison to other specific pulmonary cell types) may be explained by the fact that they are highly scattered and, in many species, are not easily detected without specific staining.

Early observations of NEBs in rodents were based on the ultrastructural characteristics of NEB cells (Hung et al., 1973; Cutz et al., 1974; Jeffery and Reid, 1975), or on silver staining (Wasano, 1977; Hung, 1984) and formaldehyde-induced fluorescence (Hage, 1976). Fortunately, advances in the methods for immunolabeling substances in tissue sections in the early 1980s resulted in powerful tools for those interested in the pulmonary diffuse neuroendocrine system (Lauweryns et al., 1982) and have greatly increased our knowledge of the distribution and “chemical coding” of PNECs/NEBs.

As in most other species, NEBs in mice and rats are found in bronchi, bronchioles and respiratory areas. Whereas NEBs in mice were reported to be preferentially located at intrapulmonary airway bifurcations (Wasano, 1977; Wasano and Yamamoto, 1981), such a favourite position is less apparent for NEBs in (newborn) rats (Carabba et al., 1985; Gomez-Pascual et al., 1990; De Proost et al., 2007). The total number of pulmonary NEBs in different rat strains has been estimated to vary between 3000 and 4000 (Van Genechten et al., 2004).

Ultrastructurally, pulmonary NEBs in rodent lungs occur as small, well-defined clusters of PNECs in which all constituent cells rest on the basement membrane. The apical poles of PNECs are covered by a unicellular layer of flattened non-ciliated epithelial cells, the so-called Clara-like cells. Only a few narrow pores between these cells allow direct communication between the NEB cells and the airway lumen, via slender apical processes (Hung and Loosli, 1974; Van Lommel and Lauweryns, 1993).

The most important ultrastructural feature for the indisputable identification of PNECs is the demonstration of dense-cored vesicles (DCVs), which are secretory granules that consist of an electron-dense core, surrounded by a limiting membrane that is separated from the core by a clear space. Transmission electron microscopical studies confirmed the presence of DCVs in the subnuclear cytoplasm of PNECs in rats (Van Lommel and Lauweryns, 1993) and mice (Hung and Loosli, 1974) and were even able to provide indirect evidence for the release of the content of DCVs by exocytosis (Van Lommel and Lauweryns, 1993). The released transmitters may bind to structures in very close proximity to NEBs, e.g., smooth muscle cells and NEB-associated nerve terminals, or may be taken up in the bloodstream by fenestrated capillaries that are sometimes found less than 1 μm from the base of NEBs (Van Lommel and Lauweryns, 1993).

Secretory Products and Cytoplasmic Contents of Pulmonary Neuroendocrine Cells in Rodent Lungs

After routine fixation and light microscopical staining, only very few NEBs can be recognized in the brightfield microscope, and none of them unambiguously. Numerous methods have, however, been developed to selectively identify PNECs and NEB cells. For a historical overview of the most relevant approaches to examine PNECs/NEBs, we would like to refer to other reviews (Scheuermann, 1987; Sorokin and Hoyt, 1989; Adriaensen and Scheuermann, 1993; Adriaensen et al., 2003). Nowadays, immunohistochemical methods using fluorescent labels, visualized by fluorescence microscopy or confocal microscopy, are widely used to detect pulmonary NEBs. While immunohistochemistry can determine which transmitters are stored in NEB cells, at least some of the detected proteins can also serve to specifically “mark” NEBs when multiple immunohistochemical stainings are performed.

One of the routinely used NEB marker molecules is protein gene-product 9.5 (PGP9.5), a pan-neuronal and neuroendocrine marker of the carboxyl-terminal ubiquitin hydrolase family (Thompson et al., 1983; Wilkinson et al., 1989). Pulmonary NEBs in different rat (Lauweryns and Van Ranst, 1988b; Van Genechten et al., 2004) and mouse strains (Lauweryns and Van Ranst, 1988b) could selectively be detected (Figures 3a, 4, 6a). Also, neuron-specific enolase (NSE), a glycolytic enzyme localized primarily to the neuronal cytoplasm, can be used to detect NEB cells in rodents (Sheppard et al., 1982; Cole et al., 1982). Aromatic L-amino acid decarboxylase, an enzyme that catalyzes the decarboxylation of all L-aromatic amino acids, has been reported in NEBs of mice and rats (Lauweryns and Van Ranst, 1988a). Recently, synaptic vesicle protein 2 (SV2) was demonstrated to be a pan-neural marker for NEB cells in mice (Pan et al., 2006).

Antibodies against the calcium-binding protein calbindin D28k (CB) can be used to label all cells in rat NEBs (Figure 2a) (Brouns et al., 2000), whereas in mice only a minor part of the NEB cells can be labeled with this marker (Brouns et al., 2009).

Positive staining with antibodies against vesicular acetylcholine transporter (VAChT), a protein present in the membrane of cholinergic secretory vesicles, suggested that NEB cells in rodents may have the ability to store and release acetylcholine (Brouns et al., 2002b).

In rabbits, the presence of serotonin (5-hydroxytryptamine; 5-HT) and the release of 5-HT after stimulation of NEB cells has been described using different experimental set-ups. Regardless of some sparse reports (Luts et al., 1991; Verástegui et al., 1997a), it appears to be rather difficult to detect 5-HT in rodent NEB cells due to their low 5-HT concentrations (own unpublished observations; Cutz et al., 1974; Wasano, 1977; Gomez-Pascual et al., 1990).

In contrast to humans and primates, where gastrin-releasing peptide (GRP) is a major neuropeptide produced by PNECs and NEBs (Linnoila, 2006), rodent PNECs and NEBs store and release calcitonin gene-related peptide (CGRP) as the most prominent transmitter (Uddman et al., 1985; Cadieux et al., 1986; Lauweryns and Van Ranst, 1987; Luts et al., 1991; Shimosegawa and Said, 1991a; Verástegui et al., 1997a). In rats, calcitonin has also been observed in almost all NEB cells (Gosney and Sissons, 1985; Luts et al., 1991), sometimes even completely colocalized with CGRP (Shimosegawa and Said, 1991a), whereas in mice calcitonin was only observed in some NEB cells (Luts et al., 1991). Helodermin, a member of the vasoactive intestinal polypeptide (VIP) family, is found in some cells in rodent NEBs (Luts et al., 1991, 1994).

While even recent reviews do not take the purine transmitters of NEBs into account, quinacrine histochemistry (Olson et al., 1976; Crowe and Burnstock, 1981) has unambiguously revealed the accumulation of quinacrine in rodent pulmonary NEBs, strongly suggesting that ATP is accumulated in secretory vesicles of NEBs (Brouns et al., 2000, 2009).

Selective Innervation of Neuroepithelial Bodies

Long before the existence of pulmonary NEBs was known, several authors described intraepithelial varicose nerve terminals that were concentrated in groups, irregularly distributed along the airways of different animal species (Berkley, 1894; Larsell, 1921; Larsell and Dow, 1933; Elftman, 1943). Since that time, many researchers have observed an unquestionable innervation of NEBs in both light and electron microscopical investigations. For visualization of nerve fibers that contact pulmonary NEBs, an unambiguous and simultaneous identification of PNECs and nerves is, however, essential (for methodological overview, see Adriaensen et al., 2003).

Observations in Rodents at the Electron Microscopical Level

Most of the earlier information about the innervation of NEBs has been obtained using transmission electron microscopy (TEM). Different types of morphologically characterized nerve terminals have been described in contact with NEBs in mice (Hung et al., 1973; Hung and Loosli, 1974; Wasano, 1977; Wasano and Yamamoto, 1981) and rats (Jeffery and Reid, 1973; Cutz et al., 1974; Hung, 1984; Carabba et al., 1985; Lauweryns and Van Ranst, 1987; Van Lommel and Lauweryns, 1993).

The most often reported type of innervation of NEBs in rodents consists of nerve fibers that are observed to penetrate the basement membrane as unmyelinated processes that widen and enter into the intercellular space between the NEB cells. The terminals of these nerve fibers are packed with numerous mitochondria, suggesting, on morphological grounds, that these are afferent nerve endings. Locally, the nerve endings are seen to form synapses with NEB cells (Wasano, 1977; Wasano and Yamamoto, 1981; Van Lommel and Lauweryns, 1993). At the level of these asymmetric synaptic contacts, DCVs accumulate near electron-dense cone-shaped thickenings of the surface membrane of the NEB cells. This type of synapse is indicative for signals passing from NEB cell to nerve ending, implying afferent signaling to the central nervous system (CNS). Frequently, these nerve endings can be found between the apex of NEB cells and the covering Clara-like cells (Van Lommel and Lauweryns, 1993). No direct contacts between the nerve endings in NEBs and the airway lumen, via the pores between the Clara-like cells that overlay the NEB cells, were ever observed. In favorable sections, nerve endings were seen to form “loops” over some NEB cells (Van Lommel and Lauweryns, 1993). The intraepithelial nerve terminals were reported to disappear after infranodosal, but not supranodosal, vagotomy (Van Lommel and Lauweryns, 1993).

Much more rarely, intraepithelial nerve endings with small clear cholinergic-type synaptic vesicles could be observed (Van Lommel and Lauweryns, 1993). These morphological efferent-like nerve endings were regarded as axon-collaterals of the intraepithelial (sensory) nerve endings (Scheuermann et al., 1989; Van Lommel and Lauweryns, 1993). The two kinds of synaptic regions could be seen side by side in a single nerve terminal and were referred to as reciprocal synapses (for review see Scheuermann, 1987).

Without a doubt, TEM has offered a good morphological characterization of the direct innervation of pulmonary NEBs. However, because o...

Table des matières

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Neurosecretory Epithelial Cells (NEC’s) in the Airways and Carotid Labyrinth of Aquatic Vertebrates: Morphology, Distribution, Innervation and Function
• Neurosecretory Cells (NEC’s) in the Lung of Amphibians and Accessory Respiratory Organs of the Air-breathing Fishes and in Amphibian Carotid Labyrinth: Structural Morphology and Function
• Neuroepithelial Bodies(NEB’s) in the Lung of Reptiles: Structural Morphology, Immunohistochemistry and Function
• Pulmonary Neuroepithelial Cells in Mammals: Structure, Molecular Markers, Ontogeny and Functions
• Solitary Chemosensory Cells in the Airways of Mammals: Distribution, Immunocytochemistry, Fine Structure and Function
• Index