Baseline Goblet Cell Mucin Secretion in the Airways Exceeds Stimulated Secretion over Extended Time Periods, and Is Sensitive to Shear Stress and Intracellular Mucin Stores

Emerging cell and molecular targets for treating mucus hypersecretion in asthma

Mucus provides a protective barrier that is crucial for host defense in the lungs. However, excessive or abnormal mucus can have pathophysiological consequences in many pulmonary diseases, including asthma. Patients with asthma are treated with agents that relax airway smooth muscle and reduce airway inflammation, but responses are often inadequate. In part, this is due to the inability of existing therapeutic agents to directly target mucus. Accordingly, there is a critical need to better understand how mucus hypersecretion and airway plugging are affected by the epithelial cells that synthesize, secrete, and transport mucus components. This review highlights recent advances in the biology of mucin glycoproteins with a specific focus on MUC5AC and MUC5B, the chief macromolecular components of airway mucus. An improved mechanistic understanding of key steps in mucin production and secretion will help reveal novel potential therapeutic strategies.

mTOR signaling regulates aberrant epithelial cell proliferative and migratory behaviors characteristic of airway mucous metaplasia in asthma

In asthma, the airway epithelium is hyperplastic, hypertrophied, and lined with numerous large MUC5AC-containing goblet cells (GC). Furthermore, the normal epithelial architecture is disorganized with numerous, what we here describe as, ectopic goblet cells (eGC) deep within the thickened epithelial layer disconnected from the lumenal surface. mTOR is a highly conserved pathway that regulates cell size and proliferation. We hypothesized that the balance between mTOR and autophagy signaling regulates key features of the asthma epithelial layer. Airway histological sections from subjects with asthma had increased frequency of eGC and increased levels of mTOR phosphorylation target-Ribosomal S6. Using human airway epithelial cells (hAECs) with IL-13 stimulation and timed withdrawal to stimulate resolution, we found that multiple key downstream phosphorylation targets downstream from the mTOR complex were increased during early IL-13-mediated mucous metaplasia, and then significantly declined during resolution. The IL-13-mediated changes in mTOR signaling were paralleled by morphologic changes with airway epithelial hypertrophy, hyperplasia, and frequency of eGC. We then examined the relationship between mTOR and autophagy using mice deficient in autophagy protein Atg16L1. Despite having increased cytoplasmic mucins, mouse AECs from Atg16L1 deficient mice had no significant difference in mTOR downstream signaling. mTOR inhibition with rapamycin led to a loss of IL-13-mediated epithelial hypertrophy, hyperplasia, ectopic GC distribution, and reduction in cytoplasmic MUC5AC levels. mTOR inhibition was also associated with a reduction in aberrant IL-13-mediated hAEC proliferation and migration. Our findings demonstrate that mTOR signaling is associated with mucous metaplasia and is crucial to the disorganized airway epithelial structure and function characteristic of muco-obstructive airway diseases such as asthma.

Graphical Abstract Key Concepts

The airway epithelium in asthma is disorganized and characterized by cellular proliferation, aberrant migration, and goblet cell mucous metaplasia.mTOR signaling is a dynamic process during IL-13-mediated mucous metaplasia, increasing with IL-13 stimulation and declining during resolution.mTOR signaling is strongly increased in the asthmatic airway epithelium.mTOR signaling is associated with the development of key features of the metaplastic airway epithelium including cell proliferation and ectopic distribution of goblet cells and aberrant cellular migration.Inhibition of mTOR leads to decreased epithelial hypertrophy, reduced ectopic goblet cells, and cellular migration.

Mechanism Underlying the Regulation of Mucin Secretion in the Uterus during Pregnancy

The function of endometrial epithelial cells is to secrete various substances that are rich in growth factors and nutrients. These substances support both embryo implantation and its subsequent development into a fetus. A vast number of mucins are expressed in endometrial epithelial cells, and they play an important role in regulating the processes of embryo implantation, pregnancy, and parturition. Previous studies have shown that mucin forms a mucus layer covering endometrial epithelial cells, which helps resist damage from foreign bacteria and their toxins. Therefore, this article aims to investigate the location of mucins in the endometrium, the mechanism of mucin secretion by the endometrium, and the regulation of mucins in the uterine epithelium by reproductive hormones, as well as the role of mucins in the protection of the epithelium's structure. This research aims to provide a foundational understanding for future studies on the role and mechanism of endometrial mucins throughout the pregnancy cycle.

Tetraspanin-8 sequesters syntaxin-2 to control biphasic release propensity of mucin granules

Agonist-mediated stimulated pathway of mucin and insulin release are biphasic in which rapid fusion of pre-docked granules is followed by slow docking and fusion of granules from the reserve pool. Here, based on a cell-culture system, we show that plasma membrane-located tetraspanin-8 sequesters syntaxin-2 to control mucin release. Tetraspanin-8 affects fusion of granules during the second phase of stimulated mucin release. The tetraspanin-8/syntaxin-2 complex does not contain VAMP-8, which functions with syntaxin-2 to mediate granule fusion. We suggest that by sequestering syntaxin-2, tetraspanin-8 prevents docking of granules from the reserve pool. In the absence of tetraspanin-8, more syntaxin-2 is available for docking and fusion of granules and thus doubles the quantities of mucins secreted. This principle also applies to insulin release and we suggest a cell type specific Tetraspanin/Syntaxin combination is a general mechanism regulating the fusion of dense core granules.

Intestinal mucus components and secretion mechanisms: what we do and do not know

Damage to the colon mucus barrier, the first line of defense against microorganisms, is an important determinant of intestinal diseases such as inflammatory bowel disease and colorectal cancer, and disorder in extraintestinal organs. The mucus layer has attracted the attention of the scientific community in recent years, and with the discovery of new mucosal components, it has become increasingly clear that the mucosal barrier is a complex system composed of many components. Moreover, certain components are jointly involved in regulating the structure and function of the mucus barrier. Therefore, a comprehensive and systematic understanding of the functional components of the mucus layer is clearly warranted. In this review, we summarize the various functional components of the mucus layer identified thus far and describe their unique roles in shaping mucosal structure and function. Furthermore, we detail the mechanisms underlying mucus secretion, including baseline and stimulated secretion. In our opinion, baseline secretion can be categorized into spontaneous Ca²⁺ oscillation-mediated slow and continuous secretion and stimulated secretion, which is mediated by massive Ca²⁺ influx induced by exogenous stimuli. This review extends the current understanding of the intestinal mucus barrier, with an emphasis on host defense strategies based on fortification of the mucus layer.

Oral exposure to Ag or TiO2 nanoparticles perturbed gut transcriptome and microbiota in a mouse model of ulcerative colitis

Silver (nAg) and titanium dioxide (nTiO₂) nanoparticles improve texture, flavour or anti-microbial properties of various food products and packaging materials. Despite their increased oral exposure, their potential toxicities in the dysfunctional intestine are unclear. Here, the effects of ingested nAg or nTiO₂ on inflamed colon were revealed in a mouse model of chemical-induced acute ulcerative colitis. Mice (eight/group) were exposed to nAg or nTiO₂ by oral gavage for 10 consecutive days. We characterized disease phenotypes, histology, and alterations in colonic transcriptome (RNA sequencing) and gut microbiome (16S sequencing). Oral exposure to nAg caused only minor changes in phenotypic hallmarks of colitic mice but induced extensive responses in gene expression enriching processes of apoptotic cell death and RNA metabolism. Instead, ingested nTiO₂ yielded shorter colon, aggravated epithelial hyperplasia and deeper infiltration of inflammatory cells. Both nanoparticles significantly changed the gut microbiota composition, resulting in loss of diversity and increase of potential pathobionts. They also increased colonic mucus and abundance of Akkermansia muciniphila. Overall, nAg and nTiO₂ induce dissimilar immunotoxicological changes at the molecular and microbiome level in the context of colon inflammation. The results provide valuable information for evaluation of utilizing metallic nanoparticles in food products for the vulnerable population.

The Mechanism of Lipopolysaccharide’s Effect on Secretion of Endometrial Mucins in Female Mice during Pregnancy

The main toxic component of endotoxins released from the death or dissolution of Gram-negative bacteria is lipopolysaccharide (LPS), which exists widely in the natural environment, and a large amount of endotoxin can significantly inhibit the reproductive performance of animals. A previous study showed that endotoxins mainly damaged the physiological function of mucins in the endometrium, but the mechanism is not clear. In this study, the PI3K/Akt signaling pathway was not activated, and the NF-κB signaling pathway was inhibited by LPS treatment; the expression of occludin and E-cadherin proteins were decreased and ZO-1 protein expression was increased, because LPS can lead to the mucous layer becoming thinner, so that the embryonic survival rate is significantly reduced in early pregnancy. In middle and late pregnancy, LPS translocated to the epithelial cells of the uterus and the expression of claudin-1, JAMA, and E-cadherin proteins were decreased; at this time, a large number of glycosaminoglycan particles were secreted by endometrial gland cells through the PI3K/Akt/NF-κB signaling pathway that was activated after LPS treatment, However, there was no significant difference between the survival rates of fetal mice in the LPS (+) and LPS (-) groups. Glycosaminoglycan particles and mucins are secreted by gland cells, which can protect and maintain the pregnancy in the middle and late gestational periods.

Development and evaluation of a bovine lung-on-chip (bLOC) to study bovine respiratory diseases

Purpose

Current air-liquid interface (ALI) models of bovine proximal airways have their limitations. They do not simulate blood flow necessary to mimic systemic drug administration, and repeated sampling requires multiple, independent cultures. A bovine lung-on-chip (bLOC) would overcome these limitations, providing a convenient and cost-effective model for pharmacokinetic or pathogenicity studies.

Methods

Bovine pulmonary arterial endothelial cells seeded into the endothelial channel of an Emulate Lung-Chip were interfaced with bovine bronchial epithelial cells in the epithelial channel. Cells were cultured at ALI for up to 21 days. Differentiation was assessed by mucin quantification, phase-contrast light microscopy and immunofluorescence of cell-specific markers in fixed cultures. Barrier integrity was determined by FITC-labelled dextran 3-5 kDa permeability. To evaluate the model, endothelial-epithelial transport of the antibiotic drug, danofloxacin, was followed using liquid chromatography-mass spectrometry, with the aim of replicating data previously determined in vivo.

Results

bLOC cultures secreted quantifiable mucins, whilst cilia formation was evident in the epithelial channel. Barrier integrity of the model was demonstrated by resistance to FITC-Dextran 3-5 kDa permeation. Bronchial epithelial and endothelial cell-specific markers were observed. Close to plasma, representative PK data for danofloxacin was observed in the endothelial channel; however, danofloxacin in the epithelial channel was mostly below the limit of quantification.

Conclusion

A co-culture model of the bovine proximal airway was successfully generated, with potential to replace in vivo experimentation. With further optimisation and characterisation, the bLOC may be suitable to perform drug pharmacokinetic studies for bovine respiratory disease (BRD), and other applications.

Azithromycin inhibits mucin secretion, mucous metaplasia, airway inflammation and airways hyperresponsiveness in mice exposed to house dust mite extract

Excessive production, secretion and retention of abnormal mucus is a pathologic feature of many obstructive airways diseases including asthma, chronic obstructive pulmonary disease, cystic fibrosis and bronchiectasis. Azithromycin is an antibiotic that also possesses immunomodulatory and mucoregulatory activities, which may contribute to the clinical effectiveness of azithromycin in these obstructive airway diseases. The current study investigated these non-antibiotic activities of azithromycin (or saline) in mice exposed daily to intranasal house dust mite (HDM) extract (or SHAM inoculation) for 10 days. HDM-exposed mice exhibited airways hyperresponsiveness to aerosolised methacholine, a pronounced mixed eosinophilic and neutrophilic inflammatory response, increased airway smooth muscle (ASM) thickness and elevated levels of epithelial mucin staining (compared to SHAM mice). Azithromycin (50 mg/kg s.c., 2 h prior to each HDM exposure) significantly attenuated HDM-induced airways hyperresponsiveness to methacholine, airways inflammation (bronchoalveolar lavage eosinophil and neutrophils numbers, and cytokine/chemokine levels), and epithelial mucin staining (mucous metaplasia) (P<0.05, 2-way ANOVA). Isolated tracheal segments of HDM-exposed mice secreted Muc5ac and Muc5b (above baseline levels) in response to exogenous ATP. Moreover, ATP-induced secretion of mucins was significantly attenuated in segments obtained from azithromycin-treated, HDM-exposed mice (P<0.05, 2-way ANOVA). In additional ex vivo studies, ATP-induced secretion of Muc5ac from HDM-exposed tracheal segments was inhibited by in vitro exposure to azithromycin. In vitro azithromycin also inhibited ATP-induced secretion of Muc5ac and Muc5b in tracheal segments from IL-13-exposed mice. In summary, azithromycin inhibited ATP-induced mucin secretion and airways inflammation in HDM-exposed mice, both of which are likely to contribute to suppression of airways hyperresponsiveness.

Inhibition of calcium-triggered secretion by hydrocarbon-stapled peptides

Membrane fusion triggered by Ca²⁺ is orchestrated by a conserved set of proteins to mediate synaptic neurotransmitter release, mucin secretion and other regulated exocytic processes^1–4. For neurotransmitter release, the Ca²⁺ sensitivity is introduced by interactions between the Ca²⁺ sensor synaptotagmin and the SNARE complex⁵, and sequence conservation and functional studies suggest that this mechanism is also conserved for mucin secretion⁶. Disruption of Ca²⁺-triggered membrane fusion by a pharmacological agent would have therapeutic value for mucus hypersecretion as it is the major cause of airway obstruction in the pathophysiology of respiratory viral infection, asthma, chronic obstructive pulmonary disease and cystic fibrosis^7–11. Here we designed a hydrocarbon-stapled peptide that specifically disrupts Ca²⁺-triggered membrane fusion by interfering with the so-called primary interface between the neuronal SNARE complex and the Ca²⁺-binding C2B domain of synaptotagmin-1. In reconstituted systems with these neuronal synaptic proteins or with their airway homologues syntaxin-3, SNAP-23, VAMP8, synaptotagmin-2, along with Munc13-2 and Munc18-2, the stapled peptide strongly suppressed Ca²⁺-triggered fusion at physiological Ca²⁺ concentrations. Conjugation of cell-penetrating peptides to the stapled peptide resulted in efficient delivery into cultured human airway epithelial cells and mouse airway epithelium, where it markedly and specifically reduced stimulated mucin secretion in both systems, and substantially attenuated mucus occlusion of mouse airways. Taken together, peptides that disrupt Ca²⁺-triggered membrane fusion may enable the therapeutic modulation of mucin secretory pathways.

Peptides that disrupt Ca²⁺-triggered membrane fusion may enable the therapeutic modulation of mucin secretory pathways.

Reversing chemorefraction in colorectal cancer cells by controlling mucin secretion

Fifteen percent of colorectal cancer (CRC) cells exhibit a mucin hypersecretory phenotype, which is suggested to provide resistance to immune surveillance and chemotherapy. We now formally show that CRC cells build a barrier to chemotherapeutics by increasing mucins’ secretion. We show that low levels of KChIP3, a negative regulator of mucin secretion (Cantero-Recasens et al., 2018), is a risk factor for CRC patients’ relapse in a subset of untreated tumours. Our results also reveal that cells depleted of KChIP3 are four times more resistant (measured as cell viability and DNA damage) to chemotherapeutics 5-fluorouracil + irinotecan (5-FU+iri.) compared to control cells, whereas KChIP3-overexpressing cells are 10 times more sensitive to killing by chemotherapeutics. A similar increase in tumour cell death is observed upon chemical inhibition of mucin secretion by the sodium/calcium exchanger (NCX) blockers (Mitrovic et al., 2013). Finally, sensitivity of CRC patient-derived organoids to 5-FU+iri. increases 40-fold upon mucin secretion inhibition. Reducing mucin secretion thus provides a means to control chemoresistance of mucinous CRC cells and other mucinous tumours.

Adhesive-Based Fabrication Technique for Culture of Lung Airway Epithelial Cells with Applications in Cell Patterning and Microfluidics

This work describes a versatile and cost-effective cell culture method for micropatterning and growing adherent cells on porous membranes using pressure-sensitive double-sided adhesives. This technique also allows cell culture using conventional methods and their easy integration into microfluidic chip devices. Adhesives can be used to form different patterns of cultured cells, which can be used for cell proliferation and wound-healing models. To demonstrate the viability of our system, we evaluate the toxicity effect of five different adhesives on two distinct airway epithelial cell lines and show functional applications for cell patterning and microfluidic cell culture chip fabrication. We developed a sandwiched microfluidic device that enabled us to culture cells in a submerged condition and transformed it into a dynamic platform when required. The viability of cells and their inflammatory responses to IL-1β stimulation were investigated. Our technique is applicable for conventional culturing of cells, widely available in biomedical research labs, while enabling the introduction of perfusion for an advanced dynamic cell culture model when needed.

Bordetella Adenylate Cyclase Toxin Elicits Airway Mucin Secretion through Activation of the cAMP Response Element Binding Protein

The mucus layer protects airway epithelia from damage by noxious agents. Intriguingly, Bordetella pertussis bacteria provoke massive mucus production by nasopharyngeal epithelia during the initial coryza-like catarrhal stage of human pertussis and the pathogen transmits in mucus-containing aerosol droplets expelled by sneezing and post-nasal drip-triggered cough. We investigated the role of the cAMP-elevating adenylate cyclase (CyaA) and pertussis (PT) toxins in the upregulation of mucin production in B. pertussis-infected airway epithelia. Using human pseudostratified airway epithelial cell layers cultured at air–liquid interface (ALI), we show that purified CyaA and PT toxins (100 ng/mL) can trigger production of the major airway mucins Muc5AC and Muc5B. Upregulation of mucin secretion involved activation of the cAMP response element binding protein (CREB) and was blocked by the 666-15-Calbiochem inhibitor of CREB-mediated gene transcription. Intriguingly, a B. pertussis mutant strain secreting only active PT and producing the enzymatically inactive CyaA-AC^– toxoid failed to trigger any important mucus production in infected epithelial cell layers in vitro or in vivo in the tracheal epithelia of intranasally infected mice. In contrast, the PT^– toxoid-producing B. pertussis mutant secreting the active CyaA toxin elicited a comparable mucin production as infection of epithelial cell layers or tracheal epithelia of infected mice by the wild-type B. pertussis secreting both PT and CyaA toxins. Hence, the cAMP-elevating activity of B. pertussis-secreted CyaA was alone sufficient for activation of mucin production through a CREB-dependent mechanism in B. pertussis-infected airway epithelia in vivo.

In Vitro Measurements of Cellular Forces and their Importance in the Lung-From the Sub- to the Multicellular Scale

Throughout life, the body is subjected to various mechanical forces on the organ, tissue, and cellular level. Mechanical stimuli are essential for organ development and function. One organ whose function depends on the tightly connected interplay between mechanical cell properties, biochemical signaling, and external forces is the lung. However, altered mechanical properties or excessive mechanical forces can also drive the onset and progression of severe pulmonary diseases. Characterizing the mechanical properties and forces that affect cell and tissue function is therefore necessary for understanding physiological and pathophysiological mechanisms. In recent years, multiple methods have been developed for cellular force measurements at multiple length scales, from subcellular forces to measuring the collective behavior of heterogeneous cellular networks. In this short review, we give a brief overview of the mechanical forces at play on the cellular level in the lung. We then focus on the technological aspects of measuring cellular forces at many length scales. We describe tools with a subcellular resolution and elaborate measurement techniques for collective multicellular units. Many of the technologies described are by no means restricted to lung research and have already been applied successfully to cells from various other tissues. However, integrating the knowledge gained from these multi-scale measurements in a unifying framework is still a major future challenge.

Autophagy of mucin granules contributes to resolution of airway mucous metaplasia

Exacerbations of muco-obstructive airway diseases such as COPD and asthma are associated with epithelial changes termed mucous metaplasia (MM). Many molecular pathways triggering MM have been identified; however, the factors that regulate resolution are less well understood. We hypothesized that the autophagy pathway is required for resolution of MM by eliminating excess non-secreted intracellular mucin granules. We found increased intracellular levels of mucins Muc5ac and Muc5b in mice deficient in autophagy regulatory protein, Atg16L1, and that this difference was not due to defects in the known baseline or stimulated mucin secretion pathways. Instead, we found that, in mucous secretory cells, Lc3/Lamp1 vesicles colocalized with mucin granules particularly adjacent to the nucleus, suggesting that some granules were being eliminated in the autophagy pathway rather than secreted. Using a mouse model of MM resolution, we found increased lysosomal proteolytic activity that peaked in the days after mucin production began to decline. In purified lysosomal fractions, Atg16L1-deficient mice had reduced proteolytic degradation of Lc3 and Sqstm1 and persistent accumulation of mucin granules associated with impaired resolution of mucous metaplasia. In normal and COPD derived human airway epithelial cells (AECs), activation of autophagy by mTOR inhibition led to a reduction of intracellular mucin granules in AECs. Our findings indicate that during peak and resolution phases of MM, autophagy activity rather than secretion is required for elimination of some remaining mucin granules. Manipulation of autophagy activation offers a therapeutic target to speed resolution of MM in airway disease exacerbations.

Inhibitory Effect of Paquinimod on a Murine Model of Neutrophilic Asthma Induced by Ovalbumin with Complete Freund's Adjuvant

Background

Quinoline-3-carboxamides have been used to treat autoimmune/inflammatory diseases in humans because they inhibit the functions of S100 calcium-binding protein A9 (S100A9), which participates in the development of neutrophilic inflammation in asthmatics and in an animal model of neutrophilic asthma. However, the therapeutic effects of these chemicals have not been evaluated in asthma. The purpose of this study was to evaluate the effect of paquinimod, one of the quinoline-3-carboxamides, on a murine model of neutrophilic asthma.

Methods

Paquinimod was orally administered to 6-week-old C57BL/6 mice sensitized and challenged with ovalbumin (OVA)/complete Freund's adjuvant (CFA) and OVA. Lung inflammation and remodeling were evaluated using bronchoalveolar lavage (BAL) and histologic findings including goblet cell count. S100A9, caspase-1, IL-1β, MPO, IL-17, IFN-γ, and TNF-α were measured in lung lysates using western blotting.

Results

Paquinimod restored the enhancement of airway resistance and the increases in numbers of neutrophils and macrophages of BAL fluids and those of goblet cells in OVA/CFA mice toward the levels of sham-treated mice in a dose-dependent manner (0.1, 1, 10, and 25 mg/kg/day, p.o.). Concomitantly, p20 activated caspase-1, IL-1β, IL-17, TNF-α, and IFN-γ levels were markedly attenuated.

Conclusion

These data indicate that paquinimod effectively inhibits neutrophilic inflammation and remodeling in the murine model of neutrophilic asthma, possibly via downregulation of IL-17, IFN-γ, and IL-1β.

Potentiating TMEM16A does not stimulate airway mucus secretion or bronchial and pulmonary arterial smooth muscle contraction

The calcium-activated chloride channel (CaCC) TMEM16A enables chloride secretion across several transporting epithelia, including in the airways. Additional roles for TMEM16A have been proposed, which include regulating mucus production and secretion and stimulating smooth muscle contraction. The aim of the present study was to test whether the pharmacological regulation of TMEM16A channel function, could affect any of these proposed biological roles in the airways. In vitro, neither a potent and selective TMEM16A potentiator (ETX001) nor the potent TMEM16A inhibitor (Ani9) influenced either baseline mucin release or goblet cell numbers in well-differentiated primary human bronchial epithelial (HBE) cells. In vivo, a TMEM16A potentiator was without effect on goblet cell emptying in an IL-13 stimulated goblet cell metaplasia model. Using freshly isolated human bronchi and pulmonary arteries, neither ETX001 or Ani9 had any effect on the contractile or relaxant responses of the tissues. In vivo, ETX001 also failed to influence either lung or cardiovascular function when delivered directly into the airways of telemetered rats. Together, these studies do not support a role for TMEM16A in the regulation of goblet cell numbers or baseline mucin release, or on the regulation of airway or pulmonary artery smooth muscle contraction.

Effect of dietary supplementation with yeast cell wall extracts on performance and gut response in broiler chickens

Background

The dietary supplementation of yeast cell wall extracts (YCW) has been found to reduce pathogenic bacteria load, promote immunoglobulin production, prevent diseases by pro-inflammatory responses, and alter gut microbiota composition. This study evaluated growth and slaughter results, health, gut morphology, immune status and gut transcriptome of 576 male chickens fed two diets, i.e. C (control) or Y (with 250-500 g/t of YCW fractions according to the growth period). At 21 and 42 d the jejunum of 12 chickens per diet were sampled and stained with hematoxylin/eosin for morphometric evaluation, with Alcian-PAS for goblet cells, and antibodies against CD3+ intraepithelial T-cells and CD45+ intraepithelial leukocytes. The jejunum sampled at 42 d were also used for whole-transcriptome profiling.

Results

Dietary YCW supplementation did not affect final live weight, whereas it decreased feed intake (114 to 111 g/d; P ≤ 0.10) and improved feed conversion (1.74 to 1.70; P ≤ 0.01). Regarding the gut, YCW supplementation tended to increase villi height (P = 0.07); it also increased the number of goblet cells and reduced the density of CD45+ cells compared to diet C (P < 0.001). In the gut transcriptome, four genes were expressed more in broilers fed diet Y compared to diet C, i.e. cytochrome P450, family 2, subfamily C, polypeptide 23b (CYP2C23B), tetratricopeptide repeat domain 9 (TTC9), basic helix-loop-helix family member e41 (BHLHE41), and the metalloreductase STEAP4. Only one gene set (HES_PATHWAY) was significantly enriched among the transcripts more expressed in broilers fed diet Y. However, a total of 41 gene sets were significantly over-represented among genes up-regulated in control broilers. Notably, several enriched gene sets are implicated in immune functions and related to NF-κB signaling, apoptosis, and interferon signals.

Conclusions

The dietary YCW supplementation improved broiler growth performance, increased gut glycoconjugate secretion and reduced the inflammatory status together with differences in the gut transcriptome, which can be considered useful to improve animal welfare and health under the challenging conditions of intensive rearing systems in broiler chickens.

Airway Mucin Secretion

Exocytosis of secreted mucins is the final step in their intracellular processing, resulting in their release into the airway lumen to interact with water and ions to form mucus. Mucins are secreted at a low baseline rate and a high stimulated rate, and both rates are regulated by second messengers acting on components of the exocytic machinery. The principal physiologic function of the low baseline rate is to support steady-state mucociliary clearance of inhaled particles and pathogens that enter the airways during normal breathing. Even in the setting of mucin hyperproduction, baseline secretion generally does not induce mucus occlusion. The principal physiologic function of the high stimulated rate of secretion from both submucosal glands and surface goblet cells in proximal airways appears to be to sweep away larger particles, whereas in distal airways it appears to act in concert with mucin hyperproduction to induce mucus occlusion to trap migrating helminths. Pathophysiologically, stimulated mucin secretion in the setting of mucin hyperproduction from allergic or other types of airway inflammation in the absence of helminth infection causes airflow obstruction and infection. Molecular components of the mucin exocytic machinery are increasingly being identified, and surprisingly, many components are not shared between baseline and stimulated machines. The physiologic significance of the presence of two distinct molecular machines is not yet known, such as whether these interact selectively with secretory granules of different sizes or contents. A full understanding of the mechanism and regulation of airway mucin secretion will provide further insight into pathophysiologic processes and may identify therapeutic strategies to alleviate obstructive airway diseases.

Multistep Fluidic Control Network toward the Automated Generation of Organ-on-a-Chip

Organ-on-a-chip, which mimics physiological functions of organs, is a potential tool for drug development and precision medicine. This chip, accompanied by a suitable culture environment and appropriate culture procedure, allows cells to form functional tissues that can be used in drug tests. Due to difficulties in the maintenance of cells and the complex nature of the tissue development process, it is essential to develop an automated culture platform to avoid contamination and reduce operational errors during long-term tissue culture. In this study, we developed a semiautomatic culture platform that integrates with a multistep fluidic control network, which allows multiple culture steps to be controlled and meets the requirement of the air-liquid interface (ALI), while maintaining a dynamic flow onto the cells. The culture platform was assembled with a culture chip, a reservoir, a miniaturized peristaltic pump, and a fluidic control base to connect each component and to operate the multiple culture steps. To demonstrate the capability of the culture platform, we have successfully controlled the multiple cell culture steps by switching the operation modes, allowing (1) cell proliferation under a liquid-liquid interface, (2) medium change from proliferation medium to differentiation medium, (3) cell differentiation under ALI conditions, and (4) repeated mucus washing. The dynamics and ALI culture conditions can simulate a physiological environment that is capable of maintaining and enabling cell differentiation for tissue-specific functions. The results demonstrate that bronchial tissue develops in the culture chip after 4 weeks of tissue culture. A versatile combination of culture steps makes the tissue culture platform suitable as an in vitro organ-on-a-chip culture model, especially for the tissues that involve the ALI culture, such as lung and skin. This platform, with multilogic control procedures, holds promise for enabling the long-term cultivation of differentiated tissues for advanced pharmacological and toxicological applications.

Inhalation exposure to multi-walled carbon nanotubes alters the pulmonary allergic response of mice to house dust mite allergen

Background: Increasing evidence from rodent studies indicates that inhaled multi-walled carbon nanotubes (MWCNTs) have harmful effects on the lungs. In this study, we examined the effects of inhalation exposure to MWCNTs on allergen-induced airway inflammation and fibrosis. We hypothesized that inhalation pre-exposure to MWCNTs would render mice susceptible to developing allergic lung disease induced by house dust mite (HDM) allergen. Methods: Male B6C3F1/N mice were exposed by whole-body inhalation for 6 h a day, 5 d a week, for 30 d to air control or 0.06, 0.2, and 0.6 mg/m³ of MWCNTs. The exposure atmospheres were agglomerates (1.4-1.8 µm) composed of MWCNTs (average diameter 16 nm; average length 2.4 µm; 0.52% Ni). Mice then received 25 µg of HDM extract by intranasal instillation 6 times over 3 weeks. Necropsy was performed at 3 and 30 d after the final HDM dose to collect serum, bronchoalveolar lavage fluid (BALF), and lung tissue for histopathology. Results: MWCNT exposure at the highest dose inhibited HDM-induced serum IgE levels, IL-13 protein levels in BALF, and airway mucus production. However, perivascular and peribronchiolar inflammatory lesions were observed in the lungs of mice at 3 d with MWCNT and HDM, but not MWCNT or HDM alone. Moreover, combined HDM and MWCNT exposure increased airway fibrosis in the lungs of mice. Conclusions: Inhalation pre-exposure to MWCNTs inhibited HDM-induced TH2 immune responses, yet this combined exposure resulted in vascular inflammation and airway fibrosis, indicating that MWCNT pre-exposure alters the immune response to allergens.

Biological Effects of Thermal Water-Associated Hydrogen Sulfide on Human Airways and Associated Immune Cells: Implications for Respiratory Diseases

Natural mineral (thermal) waters have been used for centuries as treatment for various diseases. However, the scientific background of such therapeutic action is mostly empiric and based on knowledge acquired over time. Among the various types of natural mineral waters, sulfurous thermal waters (STWs) are the most common type in the center of Portugal. STWs are characterized by high pH, poor mineralization, and the presence of several ions and salts, such as bicarbonate, sodium, fluoride, silica, and carbonate. Furthermore, these waters are indicated as a good option for the treatment of various illnesses, namely respiratory diseases (e.g., allergic rhinitis, asthma, and chronic obstructive pulmonary disease). From the sulfide species present in these waters, hydrogen sulfide (H₂S) stands out due to its abundance. In healthy conditions, H₂S-related enzymes (e.g., cystathionine β-synthase and cystathionine γ-lyase) are expressed in human lungs, where they have mucolytic, antioxidant, anti-inflammatory, and antibacterial roles, thus contributing to airway epithelium homeostasis. These roles occur mainly through S-sulfhydration, a post-translational modification through which H₂S is able to change the activity of several targets, such as ion channels, second messengers, proteins, among others. However, in respiratory diseases the metabolism of H₂S is altered, which seems to contribute somehow to the respiratory deterioration. Moreover, H₂S has been regarded as a good biomarker of airway dysfunction and severity, and can be measured in serum, sputum, and exhaled air. Hence, in this review we will recapitulate the effects of STWs on lung epithelial-immune crosstalk through the action of its main component, H₂S.

Role of Spdef in the Regulation of Muc5b Expression in the Airways of Naive and Mucoobstructed Mice

Understanding how expression of airway secretory mucins MUC5B and MUC5AC is regulated in health and disease is important to elucidating the pathogenesis of mucoobstructive respiratory diseases. The transcription factor SPDEF (sterile α-motif pointed domain epithelial specific transcription factor) is a key regulator of MUC5AC, but its role in regulating MUC5B in health and in mucoobstructive lung diseases is unknown. Characterization of Spdef-deficient mice upper and lower airways demonstrated region-specific, Spdef-dependent regulation of basal Muc5b expression. Neonatal Spdef-deficient mice exhibited reductions in BAL Muc5ac and Muc5b. Adult Spdef-deficient mice partially phenocopied Muc5b-deficient mice as they exhibited reduced Muc5b in nasopharyngeal and airway epithelia but not in olfactory Bowman glands, 75% incidence of nasopharyngeal hair/mucus plugs, and mild bacterial otitis media, without defective mucociliary clearance in the nasopharynx. In contrast, tracheal mucociliary clearance was reduced in Spdef-deficient mice in the absence of lung disease. To evaluate the role of Spdef in the development and persistence of Muc5b-predominant mucoobstructive lung disease, Spdef-deficient mice were crossed with Scnn1b-transgenic (Scnn1b-Tg) mice, which exhibit airway surface dehydration-induced airway mucus obstruction and inflammation. Spdef-deficient Scnn1b-Tg mice exhibited reduced Muc5ac, but not Muc5b, expression and BAL content. Airway mucus obstruction was not decreased in Spdef-deficient Scnn1b-Tg mice, consistent with Muc5b-dominant Scnn1b disease, but increased airway neutrophilia was observed compared with Spdef-sufficient Scnn1b-Tg mice. Collectively, these results indicate that Spdef regulates baseline Muc5b expression in respiratory epithelia but does not contribute to Muc5b regulation in a mouse model of Muc5b-predominant mucus obstruction caused by airway dehydration.

Different Munc18 proteins mediate baseline and stimulated airway mucin secretion

Airway mucin secretion is necessary for ciliary clearance of inhaled particles and pathogens but can be detrimental in pathologies such as asthma and cystic fibrosis. Exocytosis in mammals requires a Munc18 scaffolding protein, and airway secretory cells express all 3 Munc18 isoforms. Using conditional airway epithelial cell-deletant mice, we found that Munc18a has the major role in baseline mucin secretion, Munc18b has the major role in stimulated mucin secretion, and Munc18c does not function in mucin secretion. In an allergic asthma model, Munc18b deletion reduced airway mucus occlusion and airflow resistance. In a cystic fibrosis model, Munc18b deletion reduced airway mucus occlusion and emphysema. Munc18b deficiency in the airway epithelium did not result in any abnormalities of lung structure, particle clearance, inflammation, or bacterial infection. Our results show that regulated secretion in a polarized epithelial cell may involve more than one exocytic machine at the apical plasma membrane and that the protective roles of mucin secretion can be preserved while therapeutically targeting its pathologic roles.

HSP90 inhibitor geldanamycin reverts IL-13- and IL-17-induced airway goblet cell metaplasia

Goblet cell metaplasia, a disabling hallmark of chronic lung disease, lacks curative treatments at present. To identify novel therapeutic targets for goblet cell metaplasia, we studied the transcriptional response profile of IL-13-exposed primary human airway epithelia in vitro and asthmatic airway epithelia in vivo. A perturbation-response profile connectivity approach identified geldanamycin, an inhibitor of heat shock protein 90 (HSP90) as a candidate therapeutic target. Our experiments confirmed that geldanamycin and other HSP90 inhibitors prevented IL-13-induced goblet cell metaplasia in vitro and in vivo. Geldanamycin also reverted established goblet cell metaplasia. Geldanamycin did not induce goblet cell death, nor did it solely block mucin synthesis or IL-13 receptor-proximal signaling. Geldanamycin affected the transcriptome of airway cells when exposed to IL-13, but not when exposed to vehicle. We hypothesized that the mechanism of action probably involves TGF-β, ERBB, or EHF, which would predict that geldanamycin would also revert IL-17-induced goblet cell metaplasia, a prediction confirmed by our experiments. Our findings suggest that persistent airway goblet cell metaplasia requires HSP90 activity and that HSP90 inhibitors will revert goblet cell metaplasia, despite active upstream inflammatory signaling. Moreover, HSP90 inhibitors may be a therapeutic option for airway diseases with goblet cell metaplasia of unknown mechanism.

MyD88 controls airway epithelial Muc5ac expression during TLR activation conditions from agricultural organic dust exposure

Inflammation from airborne microbes can overwhelm compensatory mucociliary clearance mechanisms, leading to mucous cell metaplasia. Toll-like receptor (TLR) activation via myeloid differentiation factor 88 (MyD88) signaling is central to pathogen responses. We have previously shown that agricultural organic dust extract (ODE), with abundant microbial component diversity, activates TLR-induced airway inflammation. With the use of an established model, C57BL/6J wild-type (WT) and global MyD88 knockout (KO) mice were treated with intranasal inhalation of ODE or saline, daily for 1 wk. ODE primarily increased mucin (Muc)5ac levels relative to Muc5b. Compared with ODE-challenged WT mice, ODE-challenged, MyD88-deficient mice demonstrated significantly increased Muc5ac immunostaining, protein levels by immunoblot, and expression by quantitative PCR. The enhanced Muc5ac levels in MyD88-deficient mice were not explained by differences in the differentiation program of airway secretory cells in naïve mice. Increased Muc5ac levels in MyD88-deficient mice were also not explained by augmented inflammation, IL-17A, or neutrophil elastase levels. Furthermore, the enhanced airway mucins in the MyD88-deficient mice were not due to defective secretion, as the mucin secretory capacity of MyD88-KO mice remained intact. Finally, ODE-induced Muc5ac levels were enhanced in MyD88-deficient airway epithelial cells in vitro. In conclusion, MyD88 deficiency enhances airway mucous cell metaplasia under environments with high TLR activation.

Inflammation-induced upregulation of P2X4 expression augments mucin secretion in airway epithelia

Mucus clearance provides an essential innate defense mechanism to keep the airways and lungs free of particles and pathogens. Baseline and stimulated mucin secretion from secretory airway epithelial cells need to be tightly regulated to prevent mucus hypersecretion and mucus plugging of the airways. It is well established that extracellular ATP is a potent stimulus for regulated mucus secretion. Previous studies revealed that ATP acts via metabotropic P2Y₂ purinoreceptors on goblet cells. Extracellular ATP, however, is also a potent agonist for ionotropic P2X purinoreceptors. Expression of several P2X isoforms has been reported in airways, but cell type-specific expression and the function thereof remained elusive. With this study, we now provide evidence that P2X₄ is the predominant P2X isoform expressed in secretory airway epithelial cells. After IL-13 treatment of either human primary tracheal epithelial cells or mice, P2X₄ expression is upregulated in vitro and in vivo under conditions of chronic inflammation, mucous metaplasia, and hyperplasia. Upregulation of P2X₄ is strongest in MUC5AC-positive goblet cells. Moreover, activation of P2X₄ by extracellular ATP augments intracellular Ca²⁺ signals and mucin secretion, whereas Ca²⁺ signals and mucin secretion are dampened by inhibition of P2X₄ receptors. These data provide new insights into the purinergic regulation of mucin secretion and add to the emerging picture that P2X receptors modulate exocytosis of large secretory organelles and secretion of macromolecular vesicle cargo.

Glycogen depletion can increase the specificity of mucin detection in airway tissues

Objective

Mucin is an important parameter for detection and assessment in studies of airway disease including asthma and cystic fibrosis. Histochemical techniques are often used to evaluate mucin in tissues sections. Periodic acid Schiff (PAS) is a common technique to detect neutral mucins in tissue, but this technique also detects other tissue components including cellular glycogen. We tested whether depletion of glycogen, a common cellular constituent, could impact the detection of mucin in the surface epithelium of the trachea.

Results

Normal tissues stained by PAS had significantly more staining than serial sections of glycogen-depleted tissue with PAS staining (i.e. dPAS technique) based on both quantitative analysis and semiquantitative scores. Most of the excess stain by the PAS technique was detected in ciliated cells adjacent to goblet cells. We also compared normal tissues using the Alcian blue technique, which does not have reported glycogen staining, with the dPAS technique. These groups had similar amounts of staining consistent with a high degree of mucin specificity. Our results suggest that when using PAS techniques to stain airways, the dPAS approach is preferred as it enhances the specificity for airway mucin.

KChIP3 coupled to Ca2+ oscillations exerts a tonic brake on baseline mucin release in the colon

Regulated mucin secretion from specialized goblet cells by exogenous agonist-dependent (stimulated) and -independent (baseline) manner is essential for the function of the epithelial lining. Over extended periods, baseline release of mucin can exceed quantities released by stimulated secretion, yet its regulation remains poorly characterized. We have discovered that ryanodine receptor-dependent intracellular Ca²⁺ oscillations effect the dissociation of the Ca²⁺-binding protein, KChIP3, encoded by KCNIP3 gene, from mature mucin-filled secretory granules, allowing for their exocytosis. Increased Ca²⁺ oscillations, or depleting KChIP3, lead to mucin hypersecretion in a human differentiated colonic cell line, an effect reproduced in the colon of Kcnip3^-/- mice. Conversely, overexpressing KChIP3 or abrogating its Ca²⁺-sensing ability, increases KChIP3 association with granules, and inhibits baseline secretion. KChIP3 therefore emerges as the high-affinity Ca²⁺ sensor that negatively regulates baseline mucin secretion. We suggest KChIP3 marks mature, primed mucin granules, and functions as a Ca²⁺ oscillation-dependent brake to control baseline secretion.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Role of mucins in lung homeostasis: regulated expression and biosynthesis in health and disease

In humans and mice, the first line of innate defense against inhaled pathogens and particles in the respiratory tract is airway mucus. The primary solid components of the mucus layer are the mucins MUC5AC and MUC5B, polymeric glycoproteins whose changes in abundance and structure can dramatically affect airway defense. Accordingly, MUC5AC/Muc5ac and MUC5B/Muc5b are tightly regulated at a transcriptional level by tissue-specific transcription factors in homeostasis and in response to injurious and inflammatory triggers. In addition to modulated levels of mucin gene transcription, translational and post-translational biosynthetic processes also exert significant influence upon mucin function. Mucins are massive macromolecules with numerous functional domains that contribute to their structural composition and biophysical properties. Single MUC5AC and MUC5B apoproteins have molecular masses of >400 kDa, and von Willebrand factor D-like as well as other cysteine-rich domain segments contribute to mucin polymerization and flexibility, thus increasing apoprotein length and complexity. Additional domains serve as sites for O-glycosylation, which increase further mucin mass several-fold. Glycosylation is a defining process for mucins that is specific with respect to additions of glycans to mucin apoprotein backbones, and glycan additions influence the physical properties of the mucins via structural modifications as well as charge interactions. Ultimately, through their tight regulation and complex assembly, airway mucins follow the biological rule of 'form fits function' in that their structural organization influences their role in lung homeostatic mechanisms.

Early-life exposure to three size-fractionated ultrafine and fine atmospheric particulates in Beijing exacerbates asthma development in mature mice

BACKGROUND

Epidemiological studies have suggested that elevated levels of air pollution contribute to an increased incidence or severity of asthma. Although late-onset adult asthma seems to be more attributable to environmental risk factors, limited data is available on the impact of early-life exposure to size-fractionated ambient particulate matter (PM) on asthma in adults. We aimed to determine the effect on the development and exacerbation of asthma in the adult after the mice were exposed as juveniles to three size-fractionated ambient particulates collected from Beijing.

METHODS

The three size-fractionated ambient particulates were collected from urban Beijing in winter, heavily affected by traffic and coal-fired emissions. The typical morphological and major chemical components of the PM were characterized first. Oxidative stress and expression of DNA methyltransferases (DNMTs) were then examined in vitro and in the lungs of mouse pups 48 h after exposure to PM by oropharyngeal aspiration. When the exposed and control juvenile mice matured to adulthood, an antigen-induced asthma model was established and relevant bio-indices were assessed.

RESULTS

PM with different granularities can induce oxidative stress; in particular, F1, with the smallest size (< 0.49 μm), decreased the mRNA expression of DNMTs in vitro and in vivo the most significantly. In an asthma model of adult mice, previous exposure as juveniles to size-fractionated PM caused increased peribronchiolar inflammation, increased airway mucus secretion, and increased production of Th2 cytokines and chemokines. In general, F1 and F2 (aerodynamic diameter < 0.95 μm) particulates affected murine adult asthma development more seriously than F3 (0.95-1.5 μm). Moreover, F1 led to airway inflammation in the form of both increased neutrophils and eosinophils in BALF. The activation of the TGF-β1/Smad2 and Smad3/Stat3 signaling pathways leading to airway fibrosis was more profoundly induced by F1.

CONCLUSION

This study demonstrated that exposure to ambient PM in juvenile mice enhanced adult asthma development, as shown by increased Th2 responses, which might be associated with the persistent effects resulting from the oxidative stress and decreased gene expression of DNMTs induced by PM exposure. The observed differences between the effects of three size-fractionated particulates were attributed to particle sizes and chemical constituents, including heavy metals and also PAHs, since the amounts of PAH associated with more severe toxicity were enriched equivalently in the F1 and F2 fractions. Relative to the often mentioned PM2.5, PM with an aerodynamic diameter smaller than 0.95 μm had a more aggravating effect on asthma development.

Mucin Production and Hydration Responses to Mucopurulent Materials in Normal versus Cystic Fibrosis Airway Epithelia

RATIONALE

Cystic fibrosis (CF) airways disease produces a mucoobstructive lung phenotype characterized by airways mucus plugging, epithelial mucous cell metaplasia/hyperplasia, chronic infection, and inflammation. Simultaneous biochemical and functional in vivo studies of mucin synthesis and secretion from CF airways are not available. In vitro translational models may quantitate differential CF versus normal mucin and fluid secretory responses to infectious/inflammatory stimuli.

OBJECTIVES

We tested the hypothesis that CF airways exhibit defective epithelial fluid, but not mucin, secretory responses to bacterial/inflammatory host products.

METHODS

Well-differentiated primary human bronchial epithelial cultures were exposed to supernatant from mucopurulent material (SMM) from human CF airways as a test of bacterial/inflammatory host product stimulus. Human bronchial epithelia (HBE) with normal CF transmembrane conductance regulator function were compared with ΔF508/ΔF508 CF HBE.

MEASUREMENTS AND MAIN RESULTS

Acute (up to 60 min) SMM exposure promoted mucin secretion, but mucins were degraded by the proteolytic enzymes present in SMM. Chronic SMM exposure induced upregulation of mucin synthesis and storage and generated absolute increases in basal and stimulated mucin release in normal and CF cultures. These responses were similar in normal and CF cultures. In contrast, SMM produced a coordinated CF transmembrane conductance regulator-mediated Cl- secretory response in normal HBE, but not in CF HBE. The absence of the fluid secretory response in CF produced quantitatively more dehydrated mucus.

CONCLUSIONS

Our study reveals the interplay between regulation of mucin and fluid secretion rates in inflamed versus noninflamed conditions and why a hyperconcentrated mucus is produced in CF airways.

Modeling and Simulation of Mucus Flow in Human Bronchial Epithelial Cell Cultures - Part I: Idealized Axisymmetric Swirling Flow

A multi-mode nonlinear constitutive model for mucus is constructed directly from micro- and macro-rheology experimental data on cell culture mucus, and a numerical algorithm is developed for the culture geometry and idealized cilia driving conditions. This study investigates the roles that mucus rheology, wall effects, and HBE culture geometry play in the development of flow profiles and the shape of the air-mucus interface. Simulations show that viscoelasticity captures normal stress generation in shear leading to a peak in the air-mucus interface at the middle of the culture and a depression at the walls. Linear and nonlinear viscoelastic regimes can be observed in cultures by varying the hurricane radius and mean rotational velocity. The advection-diffusion of a drug concentration dropped at the surface of the mucus flow is simulated as a function of Peclet number.

In the lungs, the airway surface liquid protects the airway epithelium from inhaled pathogens and particulates. It is well known that failure to properly clear mucus from the airways leads to chronic, even fatal, lung infections. To date, there is no validated constitutive model capable of recapitulating mucus rheology under diverse, physiological stress and deformation conditions. This gap has hindered studies into the causal relationship between mucus rheology and mucociliary clearance. Our modeling-experimental approach fulfills several purposes: to implement linear and nonlinear constitutive modeling of mucus from micro- and macro-rheology, to test constitutive modeling in an independent experimental system, to build a coarse-grained model of the PCL-mucus boundary condition, to measure and understand modifications in mucociliary transport during and after deposition of a controlled drug concentration, to measure and simulate both the flow and stress fields throughout the mucus layer, and to measure and simulate how the advection profiles in the culture couple with diffusion of particulates landing on the air-mucus interface. These results lay the groundwork for extension of the code to three dimensions and more realistic metachronal wave boundary conditions, both in cell cultures and in airways.

Biosynthesis of the polymeric gel-forming mucin MUC5B

MUC5B is a major polymeric mucin in the airway mucus gel and is an essential component of innate defense of the respiratory epithelium. Knowledge of the synthesis and intracellular processing of MUC5B is incomplete. We investigated the molecular details of MUC5B assembly in primary human bronchial epithelial cells (HBECs) grown at an air-liquid interface (ALI). Electrophoretic and centrifugal separations of intracellular forms of MUC5B probed with antibodies specific for non-O-glycosylated and O-glycosylated forms of the mucin identified three major intracellular populations of MUC5B (non-O-glycosylated monomer and dimer, and O-glycosylated polymers). Biophysical analysis of recombinant MUC5B COOH-terminus (CT5B; D4-B-C-CK) expressed in 293-EBNA cells showed that MUC5B dimerizes by disulfide linkage. Pulse-chase studies in the HBEC ALI cultures showed that non-O-glycosylated MUC5B was synthesized within 20 min of metabolic labeling and O-glycosylated, polymeric mucin within 2 h. Radiolabeled O-glycosylated mucin polymers were secreted within 2 h and the majority were released by 48 h. These data indicate that MUC5B follows a similar assembly to the related glycoprotein, von Willebrand factor (vWF); however, unlike vWF the MUC5B polypeptide shows no evidence of major proteolytic processing of D-domains during the production of the mature secreted polymeric mucin in normal and cystic fibrosis (CF) primary bronchial epithelial cells. In contrast, MUC5B D-domains were modified by neutrophil elastase, a protease commonly found in CF sputum, demonstrating that proteolytic degradation of MUC5B is an extracellular event in CF sputum. These results define the pathway for synthesis of MUC5B in primary human goblet cells.

Goblet cells of the conjunctiva: A review of recent findings

Goblet cells within the conjunctival epithelium are specialized cells that secrete mucins onto the surface of the eye. Recent research has demonstrated new characteristics of the cells, including factors influencing their differentiation, their gene products and their functions at the ocular surface. The following review summarizes the newly discovered aspects of the role of Spdef, a member of the Ets transcription factor family in conjunctival goblet cell differentiation, the newly discovered goblet cell products including claudin2, the Wnt inhibitor Frzb, and the transmembrane mucin Muc16. The current concepts of conjunctival goblet cell function, including debris removal and immune surveillance are reviewed, as are changes in the goblet cell population in ocular surface diseases. Major remaining questions regarding conjunctival cell biology are discussed.