Absence of cilia and basal bodies with predominance of brush cells in the respiratory mucosa from a patient with immotile cilia syndrome

Intestinal Tuft Cells: Morphology, Function, and Implications for Human Health

Tuft cells are a rare and morphologically distinct chemosensory cell type found throughout many organs, including the gastrointestinal tract. These cells were identified by their unique morphologies distinguished by large apical protrusions. Ultrastructural data have begun to describe the molecular underpinnings of their cytoskeletal features, and tuft cell-enriched cytoskeletal proteins have been identified, although the connection of tuft cell morphology to tuft cell functionality has not yet been established. Furthermore, tuft cells display variations in function and identity between and within tissues, leading to the delineation of distinct tuft cell populations. As a chemosensory cell type, they display receptors that are responsive to ligands specific for their environment. While many studies have demonstrated the tuft cell response to protists and helminths in the intestine, recent research has highlighted other roles of tuft cells as well as implicated tuft cells in other disease processes including inflammation, cancer, and viral infections. Here, we review the literature on the cytoskeletal structure of tuft cells. Additionally, we focus on new research discussing tuft cell lineage, ligand-receptor interactions, tuft cell tropism, and the role of tuft cells in intestinal disease. Finally, we discuss the implication of tuft cell-targeted therapies in human health and how the morphology of tuft cells may contribute to their functionality.

Human airway tuft cells influence the mucociliary clearance through cholinergic signalling

BACKGROUND

Airway tuft cells, formerly called brush cells have long been described only morphologically in human airways. More recent RNAseq studies described a chemosensory cell population, which includes tuft cells, by a distinct gene transcription signature. Yet, until which level in the tracheobronchial tree in native human airway epithelium tuft cells occur and if they function as regulators of innate immunity, e.g., by regulating mucociliary clearance, remained largely elusive.

METHODS

We performed immunohistochemistry, RT-PCR and immunoblotting analyses for various tuft cell markers to confirm the presence of this cell type in human tracheal samples. Immunohistochemistry was conducted to study the distribution of tuft cells along the intrapulmonary airways in humans. We assessed the influence of bitter substances and the taste transduction pathway on mucociliary clearance in mouse and human tracheal samples by measuring particle transport speed.

RESULTS

Tuft cells identified by the expression of their well-established marker POU class 2 homeobox 3 (POU2F3) were present from the trachea to the bronchioles. We identified choline acetyltransferase in POU2F3 expressing cells as well as the transient receptor potential melastatin 5 (TRPM5) channel in a small population of tracheal epithelial cells with morphological appearance of tuft cells. Application of bitter substances, such as denatonium, led to an increase in mucociliary clearance in human tracheal preparations. This was dependent on activation of the TRPM5 channel and involved cholinergic and nitric oxide signalling, indicating a functional role for human tuft cells in the regulation of mucociliary clearance.

CONCLUSIONS

We were able to detect tuft cells in the tracheobronchial tree down to the level of the bronchioles. Moreover, taste transduction and cholinergic signalling occur in the same cells and regulate mucociliary clearance. Thus, tuft cells are potentially involved in the regulation of innate immunity in human airways.

An update on the biological characteristics and functions of tuft cells in the gut

The intestine is a powerful digestive system and one of the most sophisticated immunological organs. Evidence shows that tuft cells (TCs), a kind of epithelial cell with distinct morphological characteristics, play a significant role in various physiological processes. TCs can be broadly categorized into different subtypes depending on different molecular criteria. In this review, we discuss its biological properties and role in maintaining homeostasis in the gastrointestinal tract. We also emphasize its relevance to the immune system and highlight its powerful influence on intestinal diseases, including inflammations and tumors. In addition, we provide fresh insights into future clinical diagnostic and therapeutic strategies related to TCs.

The Immune Function of Tuft Cells at Gut Mucosal Surfaces and Beyond

Tuft cells were first discovered in epithelial barriers decades ago, but their function remained unclear until recently. In the last 2 years, a series of studies has provided important advances that link tuft cells to infectious diseases and the host immune responses. Broadly, a model has emerged in which tuft cells use chemosensing to monitor their surroundings and translate environmental signals into effector functions that regulate immune responses in the underlying tissue. In this article, we review the current understanding of tuft cell immune function in the intestines, airways, and thymus. In particular, we discuss the role of tuft cells in type 2 immunity, norovirus infection, and thymocyte development. Despite recent advances, many fundamental questions about the function of tuft cells in immunity remain to be answered.

Tuft cells: From the mucosa to the thymus

Tuft cells are epithelial chemosensory cells with unique morphological and molecular characteristics, the most noticeable of which is a tuft of long and thick microvilli on their apical side, as well as expression of a very distinct set of genes, including genes encoding various members of the taste transduction machinery and pro-inflammatory cyclooxygenases. Initially discovered in rat trachea, tuft cells were gradually identified in various mucosal tissues, and later also in non-mucosal tissues, most recent of which is the thymus. Although tuft cells were discovered more than 60 years ago, their functions in the various tissues remained a mystery until recent years. Today, tuft cells are thought to function as sensors of various types of chemical signals, to which they respond by secretion of diverse biological mediators such as IL25 or acetylcholine. Intestinal tuft cells were also shown to mediate type 2 immunity against parasites. Here, we review the current knowledge on tuft cell characteristics, development and heterogeneity, discuss their potential functions and explore the possible implications and significance of their discovery in the thymus.

The development and functions of multiciliated epithelia

Multiciliated cells are epithelial cells that are in contact with bodily fluids and are required for the proper function of major organs including the brain, the respiratory system and the reproductive tracts. Their multiple motile cilia beat unidirectionally to remove particles of external origin from their surface and/or drive cells or fluids into the lumen of the organs. Multiciliated cells in the brain are produced once, almost exclusively during embryonic development, whereas in respiratory tracts and oviducts they regenerate throughout life. In this Review, we provide a cell-to-organ overview of multiciliated cells and highlight recent studies that have greatly increased our understanding of the mechanisms driving the development and function of these cells in vertebrates. We discuss cell fate determination and differentiation of multiciliated cells, and provide a comprehensive account of their locations and functions in mammals.

The intestinal epithelium tuft cells: specification and function

The intestinal epithelium, composed of at least seven differentiated cell types, represents an extraordinary model to understand the details of multi-lineage differentiation, a question that is highly relevant in developmental biology as well as for clinical applications. This review focuses on intestinal epithelial tuft cells that have been acknowledged as a separate entity for more than 60 years but whose function remains a mystery. We discuss what is currently known about the molecular basis of tuft cell fate and differentiation and why elucidating tuft cell function has been so difficult. Finally, we summarize the current hypotheses on their potential involvement in diseases of the gastro-intestinal tract.

Brush cells in the human duodenojejunal junction: an ultrastructural study

Brush cells have been identified in the respiratory and gastrointestinal tract mucosa of many mammalian species. In humans they are found in the respiratory tract and the gastrointestinal apparatus, in both the stomach and the gallbladder. The function of brush cells is unknown, and most morphological data have been obtained in rodents. To extend our knowledge of human brush cells, we performed an ultrastructural investigation of human small intestine brush cells. Six brush cells identified in five out of more than 300 small intestine biopsies performed for gastrointestinal tract disorders were examined by transmission electron microscopy. Five brush cells were located on the surface epithelium and one in a crypt. The five surface brush cells were characterized by a narrow apical pole from which emerged microvilli that were longer and thicker than those of enterocytes. The filamentous core extended far into the cell body without forming the terminal web. Caveolae were abundant. Filaments were in the form of microfilaments and intermediate filaments. Cytoplasmic projections containing filaments were found on the basolateral surface of brush cells. In a single cell, axons containing vesicles and dense core granules were in close contact both with the basal and the lateral surface of the cell. The crypt brush cell appeared less mature. We concluded that human small intestine brush cells share a similar ultrastructural biology with those of other mammals. They are polarized and well-differentiated cells endowed with a distinctive cytoskeleton. The observation of nerve fibres closely associated with brush cells, never previously described in humans, lends support to the hypothesis of a receptor role for these cells.

A new fate for old cells: brush cells and related elements

Over the past 50 years, hundreds of studies have described those cells that are characterized by a brush of rigid apical microvilli with long rootlets, and which are found in the digestive and respiratory apparatuses. These cells have been given names such as brush cells, tuft cells, fibrillovesicular cells, multivesicular cells and caveolated cells. More recently, it has been realized that all these elements may represent a single cell type, probably with a chemosensory role, even if other functions (e.g. secretory or absorptive) seem to be possible. Very recent developments have permitted a partial definition of the chemical code characterizing these elements, revealing the presence of molecules involved in chemoreceptorial cell signalling. A molecular cascade, similar to those characterizing the gustatory epithelium, seems to be present in these elements. These new data suggest that these elements can be considered solitary chemosensory cells with the presence of the apical 'brush' as an inconsistent feature. They seem to comprise a diffuse chemosensory system that covers large areas (probably the whole digestive and respiratory apparatuses) with analogies to chemosensory systems described in aquatic vertebrates.

Comparison of ciliary wave disorders measured by image analysis and electron microscopy

CONCLUSION

We have developed a simple, reliable method for the simultaneous determination of the ciliary wave disorder (CWD) and ciliary beat frequency (CBF) of actively beating cilia.

OBJECTIVE

The CBF and the directions of beating cilia are two important components of mucociliary transport. Although lots of studies have been performed on the measurement of the CBF, there have been few studies on the direction of cilia, with the exception of those using electron microscopy (EM). EM takes too long to determine the directions of cilia, and it cannot determine the direction of actively beating cilia. The aim of this study was to develop an image analysis (IA) system to conveniently determine the wave directions of multiple actively beating cilia as well as the CBF.

MATERIAL AND METHODS

Sphenoid sinus mucosae obtained from 10 patients undergoing pituitary tumor removal via a trans-septal trans-sphenoidal approach were divided into two 4 x 4 mm2-sized pieces. One piece was studied using IA, the other with EM. Using IA, ciliary wave directions were determined from 5 20 x 20 microm2 regions of interest and the mean of 5 consecutive values was regarded as the CWD of each sample. The CBF was also measured. CWD was also measured using EM.

RESULTS

The average number of cilia analyzed by EM was 102.50 (range 48-136). The mean CWDs determined using IA and EM were 28.25+/-4.84 degrees and 23.59+/-8.16 degrees, respectively. There was a significant correlation between the CWDs determined using these two methods (Spearman's correlation coefficient =0.648; p =0.043). The mean CBF of sphenoid mucosa was 10.50+/-2.20 Hz.

The mysterious pulmonary brush cell: a cell in search of a function

Brush cells, also termed tuft, caveolated, multivesicular, and fibrillovesicular cells, are part of the epithelial layer in the gastrointestinal and respiratory tracts. The cells are characterized by the presence of a tuft of blunt, squat microvilli (approximately 120-140/cell) on the cell surface. The microvilli contain filaments that stretch into the underlying cytoplasm. They have a distinctive pear shape with a wide base and a narrow microvillous apex. The function of the pulmonary brush cell is obscure. For this reason, a working group convened on August 23, 2004, in Bethesda, Maryland, to review the physiologic role of the brush (microvillous) cell in normal airways and alveoli and in respiratory diseases involving the alveolar region (e.g., emphysema and fibrosis) and airway disease characterized by either excessive or insufficient amounts of airway fluid (e.g., cystic fibrosis, chronic bronchitis, and exercise-induced asthma). The group formulated several suggestions for future investigation. For example, it would be useful to have a panel of specific markers for the brush cell and in this way separate these cells for culture and more direct examination of their function (e.g., microarray analysis and proteomics). Using quantitative analysis, it was suggested to examine the number and location of the cells in disease models. Understanding the function of these cells in alveoli and airways may provide clues to the pathogenesis of several disease states (e.g., cystic fibrosis and fibrosis) as well as a key for new therapeutic modalities.

Ciliary assessment in bronchiectasis

Bronchiectasis is a common condition among the Oriental population and affected patients suffer from chronic sputum production punctuated by recurrent infective exacerbations. Cilia are minute structures present on the surface of respiratory and other epithelial cells that beat continuously to maintain a sterile mucosal surface in the respiratory tract. Patients with primary ciliary dyskinesia could potentially develop recurrent sinotrachrobronchitis, bronchiectasis, serous otitis media, hydrocephalus, and male infertility. The assessment of cilia has, however, received little attention until recently and generally involves elaborate methods that require complex and expensive technology. This brief article discusses application of the saccharine test, light microscopy assessment of ciliary beat, and transmission electron microscopy assessment of the ultrastructure of cilia. The rationale and indications for ciliary assessment are also listed along with illustrations showing ciliary structure, equipment required for sampling and assessment of cilia, and transmission electron micrographs of ciliary ultrastructural abnormalities.

Severe bronchiectasis in patients with "cystlike" structures within the ciliary shafts

Cilia are minute hairlike structures which sweep the respiratory tract clean by continuous beating. Ciliary dysfunction can occur which leads to development of bronchiectasis. We describe the clinical and investigation profiles of four patients who had early-onset severe and otherwise idiopathic bronchiectasis associated with a previously undescribed ciliary defect in which numerous cystlike structures are present within the ciliary shafts. The patients had otherwise normal ciliary microtubular ultrastructure and central microtubular orientation. Our newly described ciliary abnormality appears to be a primary defect and more systematic studies should be performed to evaluate this further.

Aplasia of respiratory tract cilia

We report on ciliary aplasia of the respiratory tract, a rare disorder of the mucociliary apparatus, that is insufficiently recognized as a distinct entity. A culture method for ciliogenesis was developed by our laboratory and offers the advantage of studying cilia free of secondary changes associated with infection. Three cases of primary ciliary aplasia were documented histologically in direct biopsy specimens and also in biopsy specimens cultured specifically for ciliogenesis. Primary ciliary aplasia should be differentiated from secondary ciliary aplasia in which basal bodies are present and ciliogenesis takes place in specific culture. Only hereditary ciliary abnormalities are expressed in cell cultures. We critically review the cases of ciliary aplasia reported to date.

Nine Japanese patients with immotile-dyskinetic cilia syndrome: an ultrastructural study using tannic acid-containing fixation

Respiratory cilia and sperm flagella of nine Japanese patients with immotile-dyskinetic cilia syndrome were studied ultrastructurally by using a tannic acid-containing fixative. Respiratory cilia from two female patients with Kartagener's syndrome and one male patient with situs inversus and sinobronchitis were completely immotile and lacked both dynein arms. However, approximately 30% of the spermatozoa from the male patient were weakly motile. In four patients with immotile cilia syndrome without Kartagener's triad, immotile respiratory cilia generally lacked the inner dynein arms. Two clinically unusual cases, an 11-year-old boy and a 29-year-old woman with prolonged saccharin test, recurrent bronchitis, and bronchiectasia, possessed motile respiratory cilia. Ultrastructurally, both dynein arms were normal, but numerous defective central pairs (more than 50% and 70%, respectively) were seen, and the defect in the second case was similar to the transposition of microtubules reported by Sturgess et al (N Engl J Med 303:318-322, 1980). However, defects in the first case were unique and may be congenital. We propose a new type of dyskinetic cilia syndrome with defective central pairs. Additionally, nasal cilia from a 35-year-old man with immotile cilia syndrome contained excess large singlets within ciliary axonemes consisting of 17 protofilaments.

Brush cells in the human fetal trachea

We performed a morphologic examination of human fetal lung tissue, using scanning and transmission electron microscopy, in order to establish the presence of brush cells in extrapulmonary and intrapulmonary airways, and developing acinar epithelium. Brush cells, characterized by a border of regular straight microvilli containing a filamentous core, were observed within the tracheal epithelium of a 19-20 week gestational age fetus. These cells constituted 0.5% of the total epithelial cell population. Brush cells were not seen within the bronchial, bronchiolar or developing acinar epithelium. Our study shows that brush cells occur infrequently but normally in the developing tracheal epithelium of the second trimester fetus.

Pathophysiology of the ciliated epithelium of the respiratory mucosa in humans. Disorders of ciliogenesis

If we want to interpret morphological alterations of the ciliated epithelium of the respiratory tract, we have to consider the physiological differentiation of ciliated cells and cilia. For this, we suggest a system in which disorders are distinguished at three distinct arrest levels of the ciliary development: (1) Disorders in the production of procentrioles (secondary centrioles) lead to partial or total aplasia of the ciliary apparatus. (2) Disorders in the development of kinetosomes from secondary centrioles result in a reduced number of cilia and malformations of ciliary components. (3) Disorders in the outgrowth of cilia result in morphological abnormalities of the peripheral ciliary shaft (i.e., immotile cilia syndrome). We suppose that at all three arrest levels hereditary factors may be important, whereas at levels 2 and 3 exogenous influences may cause these alterations too. Correlation between morphology and clinical status is made difficult by conditions of auxiliary mechanisms, like cough, which compensate the mucociliary dysfunctions. For the purpose of morphometric evaluation of the biopsies we established a method of light microscopical semiquantitative analysis of brush biopsies. The regenerative power of the ciliary apparatus with respect to these frequently observed abnormalities is discussed.

Congenital ciliary aplasia in two siblings. A primitive disregulation of ciliogenesis?

Congenital ciliary aplasia was demonstrated in two siblings with clinical history of primary ciliary dyskinesia. Ultrastructural histochemistry of successive bronchial biopsies revealed the predominance of immature mucous cells and the total absence of ciliated or preciliated cells in the respiratory epithelium. This original disorder may represent a unique variant of primary ciliary dyskinesia with primitive disregulation of ciliogenesis.

Ultrastructural diagnosis in the immotile cilia syndrome

This article reports on the experience with 38 biopsies from nasal mucosa, submitted with the question of immotile cilia syndrome. Fixation in glutaraldehyde with MgSO4 is preferable. At least 50 cilia should be scrutinized in the electron microscope, and dynein arms, radial spokes, sheaths, nexin links and orientation should be tabulated. Six cases displayed virtual absence of inner and outer dynein arms. The orientation of these cilia was random. Two of the patients had situs inversus. In biopsies considered not to represent the immotile cilia syndrome, about four inner and seven outer dynein arms were found per cilium.

Dyskinetic cilia syndrome: clinical, radiographic and scintigraphic findings

The clinical, electron microscopic and radiographic data of 9 patients with dyskinetic cilia syndrome (DCS) are presented. Scintigraphic evaluation of mucociliary dynamics in six patients showed evidence of dyskinesia. Ventilation and perfusion studies were performed to evaluate obstructive lung disease. Retrospectively, bronchiectasis could be detected in 77% of the patients by analysis of the chest radiograph and lung scintigraphy, and bronchography potentially avoided in the seven patients who underwent this procedure.