Gel-forming mucins form distinct morphologic structures in airways

Titaminates, a Biocompatible Titanium-Organic Hybrid Film, Support the Growth and Function of Conjunctival Epithelial Cells

PURPOSE

To find an ideal biosurface to regenerate conjunctival epithelium.

DESIGN

Experimental study.

METHODS

Pieces of human cadaveric conjunctival epithelium removed of connective tissues were randomly placed on 8 different biosurfaces to grow primary conjunctival epithelial cells for 9 to 14 days until the fastest growing cultures covered 80% to 90% of the film surface. Main outcome measures were growth, proliferation capacity, startification, mucin 5AC (MUC5AC) secretion, and compositional balance in cell types.

RESULTS

All titanium-based films (titaminates) supported the growth, stratification, proliferation, and MUC5AC secretion function of human primary conjunctival epithelial cells (HCjECs), but none of the zinc-based films grew cells at all. Although HCjECs' ability to proliferate, stratify, and secrete MUC5AC per unit cell area was similar across all titaminates, titanium dioxide (TiO2) and glycine titaminates (TiG) outperformed TiD and TiGD in terms of outgrowth, recapitulation of native conjunctival epithelium, and compositional balance in cell types. When goblet cells (a subtype of HCjECs) were predominantly grown on these films by using a different culture medium, the results were similar in all aspects.

CONCLUSIONS

Given the body-friendly nature and clinically proven safety and efficacy of titanium, titaminates TiO2 and TiG have potential as an alternative bioactive substrate for conjunctival epithelium regeneration.

MUC5AC filaments illuminate the structural diversification of respiratory and intestinal mucins

Secreted mucins are multimegadalton glycoprotein polymers that share the function of protecting mucosal tissues but diversified for activities in different organs of the body. Structural studies of secreted mucins are complicated by the enormous sizes, flexibility, and complex supramolecular assembly modes of these glycoproteins. The two major respiratory mucins are MUC5AC and MUC5B. Here, we present structures of a large amino-terminal segment of MUC5AC in the form of helical filaments. These filaments differ from filamentous and tubular structures observed previously for the intestinal mucin MUC2 and the partial mucin homolog VWF. Nevertheless, the MUC5AC helical filaments support the proposed mechanism, based on MUC2 and VWF, for how noncovalent interactions between mucin monomers guide disulfide crosslinking to form polymers. The high-resolution MUC5AC structures show how local and limited changes in amino acid sequence can profoundly affect higher-order assembly while preserving the overall folds and polymerization activity of mucin glycoproteins. Differences in supramolecular assembly are likely to be functionally significant considering the divergence of mechanical properties and physiological requirements between respiratory and intestinal mucins. Determining the high-resolution structures of respiratory mucins provides a foundation for understanding the mechanisms by which they clean and protect the lungs. Moreover, the MUC5AC structure enables visualization of the sites of human amino acid sequence variation and disease-associated mutations.

The oxygen level in air directs airway epithelial cell differentiation by controlling mitochondrial citrate export

Oxygen controls most metazoan metabolism, yet in mammals, tissue O2 levels vary widely. While extensive research has explored cellular responses to hypoxia, understanding how cells respond to physiologically high O2 levels remains uncertain. To address this problem, we investigated respiratory epithelia as their contact with air exposes them to some of the highest O2 levels in the body. We asked how the O2 level in air controls differentiation of airway basal stem cells into the ciliated epithelial cells essential for clearing airborne pathogens from the lung. Through a metabolomics screen and 13C tracing on primary cultures of human airway basal cells, we found that the O2 level in air directs ciliated cell differentiation by increasing mitochondrial citrate export. Unexpectedly, disrupting mitochondrial citrate export elicited hypoxia transcriptional responses independently of HIF1α stabilization and at O2 levels that would be hyperoxic for most tissues. These findings identify mitochondrial citrate export as a cellular mechanism for responding to physiologically high O2 levels.

Study repair function of mucin-2 on the tight junction protein of uterine epithelial cells under bacterial endotoxins

To analysis repair function of mucin-2(MUC2) and glycoprotein particles on the tight junction protein of uterus under bacterial endotoxins. In this experiment, we showed that the thicker mucus layer of the uterus is used to prevent the translocation of endotoxin at 21d postdelivery. When endotoxin acts on the uterus to thin its mucous layer, the cells in the lamina propria of the uterus secrete a large number of glycoprotein particles at 27d postdelivery. Due to a significantly decrease in the expression of glycosyltransferase, the glycoprotein particles are incompletely glycosylation MUC2, which can interact with the cell membrane and are released in large quantities in the form of exocytosis. These glycoprotein particles can significantly repair tight junction proteins in the inter-cellular space and significantly increase the expression of Claudin-1, JAM (Junction adhesion molecule-A), E-cadherin, ZO-1(Zonula occludens-1) and desmosome proteins after endotoxin treatment. The results of the present study show that endotoxins can thin the uterine mucus layer and accelerate the release of incompletely glycosylated MUC2 from lamina propria cells. In inter-cellular spaces, MUC2 can increase its expression levels and distribution area to repair the tight junction structure of cells with larger gaps. Further strengthening of the barrier prevents endotoxin translocation by repairing the tight junction structure of uterine epithelial cells.

Breaking Down the Bottlebrush: Atomically Detailed Structural Dynamics of Mucins

Mucins, the biomolecular components of mucus, are glycoproteins that form a thick physical barrier at all tissue-air interfaces, forming a first line of defense against pathogens. Structural features of mucins and their interactions with other biomolecules remain largely unexplored due to the challenges associated with their high-resolution characterization. Combining limited mass spectrometry glycomics and protein sequencing data, we present all-atom, explicitly solvated molecular dynamics simulations of a major respiratory mucin, MUC5B. We detail key forces and degrees of freedom imposed by the extensive O-glycosylation, which imbue the canonically observed bottlebrush-like structures to these otherwise intrinsically disordered protein backbones. We compare our simulation results to static structures observed in recent scanning tunneling microscopy experiments as well as other published experimental efforts. Our work represents the demonstration of a workflow applied to a mucin example, which we hope will be employed by other groups to investigate the dynamics and interactions of other mucins, which can inform on structural details currently inaccessible to experimental techniques.

Scalable Extraction of Airway Mucins from Porcine Trachea

Mucins are a major component of the innate defense system in the airways and their biological functions are important to consider in pulmonary disease research. However, the available mucus models for basic research relevant to the lung can be difficult to acquire in sufficient quantity to conduct such studies. Here, we present a new strategy to isolate airway mucins from pig trachea at the milligram to gram scale for use in pulmonary disease research. Using this protocol, we were able to isolate mucins with minimal DNA contamination consisting of ~70% by weight protein. Compared to porcine gastric mucins extracted with the same procedure, the porcine tracheal extract possessed significantly greater O-linked glycoprotein (mucin) content. Particle tracking microrheology was used to evaluate the biophysical properties of porcine trachea mucins. We found porcine tracheal mucins formed a much tighter mesh network and possessed a significantly greater microviscosity compared to lab extracted porcine gastric mucins. In comparison to mucus harvested from human airway tissue cultures, we found porcine tracheal mucins also possessed a greater microviscosity suggesting these mucins can form into a gel-like material at physiological total solids concentrations. These studies establish an accessible means to isolate airway mucins from porcine trachea at large scale for use in pulmonary disease research.

Mucociliary clearance is impaired in small airways of cystic fibrosis pigs

Cystic fibrosis (CF) is a genetic disorder characterized by recurrent airway infections, inflammation, impaired mucociliary clearance, and progressive decline in lung function. The disease may start in the small airways; however, this is difficult to prove due to the limited accessibility of the small airways with the current single-photon mucociliary clearance assay. Here, we developed a dynamic positron emission tomography assay with high spatial and temporal resolution. We tested that mucociliary clearance is abnormal in the small airways of newborn cystic fibrosis pigs. Clearance of [68Ga]-tagged macroaggregated albumin from small airways started immediately after delivery and continued for the duration of the study. Initial clearance was fast but slowed down a few minutes after delivery. Cystic fibrosis pigs' small airways cleared significantly less than non-CF pigs' small airways (non-CF 25.1 ± 3.1% vs. CF 14.6 ± 0.1%). Stimulation of the cystic fibrosis airways with the purinergic secretagogue uridine-5'-triphosphate (UTP) further impaired clearance (non-CF with UTP 20.9 ± 0.3% vs. CF with UTP 13.0 ± 1.8%). None of the cystic fibrosis pigs treated with UTP (n = 6) cleared more than 20% of the delivered dose. These data indicate that mucociliary clearance in the small airways is fast and can easily be missed if the assay is not sensitive enough. The data also indicate that mucociliary clearance is impaired in the small airways of cystic fibrosis pigs. This defect is exacerbated by stimulation of mucus secretions with purinergic agonists.NEW & NOTEWORTHY We developed a novel positron emission tomography scan assay with unprecedented temporal and spatial resolution to measure mucociliary clearance in the small airways. We proved a long-standing but unproven assertion that mucociliary clearance is inherently abnormal in the small airways of newborn cystic fibrosis piglets that are otherwise free of infection or inflammation. This technique can be easily extended to other airway diseases such as asthma, idiopathic pulmonary fibrosis, or chronic obstructive pulmonary disease.

Development and Initial Characterization of Pigs with DNAI1 Mutations and Primary Ciliary Dyskinesia

Mutations in more than 50 different genes cause primary ciliary dyskinesia (PCD) by disrupting the activity of motile cilia that facilitate mucociliary transport (MCT). Knowledge of PCD has come from studies identifying disease-causing mutations, characterizing structural cilia abnormalities, finding genotype-phenotype relationships, and studying the cell biology of cilia. Despite these important findings, we still lack effective treatments and people with PCD have significant pulmonary impairment. As with many other diseases, a better understanding of pathogenic mechanisms may lead to effective treatments. To pursue disease mechanisms, we used CRISPR-Cas9 to develop a PCD pig with a disrupted DNAI1 gene. PCD pig airway cilia lacked the outer dynein arm and had impaired beating. MCT was impaired under both baseline conditions and after cholinergic stimulation in PCD pigs. Neonatal PCD pigs developed neonatal respiratory distress with evidence of atelectasis, air trapping, and airway mucus obstruction. Despite airway mucus accumulation, lung bacterial counts were similar between neonatal wild-type and PCD pigs. Sinonasal disease was present in all neonatal PCD pigs. Older PCD pigs developed worsening airway mucus obstruction, inflammation, and bacterial infection. This pig model closely mimics the disease phenotype seen in people with PCD and can be used to better understand the pathophysiology of PCD airway disease.

Disease-specific transcriptional programs govern airway goblet cell metaplasia

Hypersecretion of airway mucus caused by goblet cell metaplasia is a characteristic of chronic pulmonary inflammatory diseases including asthma, cystic fibrosis (CF), and chronic obstructive pulmonary disease (COPD). Goblet cells originate from airway progenitor club cells. However, the molecular mechanisms and features of goblet cell metaplasia in lung disease are poorly understood. Herein, public single-cell RNA sequencing datasets of human lungs were reanalyzed to explore the transitional phase as club cells differentiate into goblet cells in asthma, CF, and COPD. We found that changes in club and goblet cells during pathogenesis and cellular transition were associated with signalling pathways related to immune response, oxidative stress, and apoptosis. Moreover, other key drivers of goblet cell specification appeared to be pathologically specific, with interleukin (IL)-13 and hypoxia inducible factor 1 (HIF-1)-induced genetic changes in asthma, cystic fibrosis transmembrane conductance regulator (CFTR) mutation being present in CF, and interactions with CD8+ T cells, mitophagy, and mitochondria-induced apoptosis in COPD. In conclusion, this study revealed the similarities and differences in goblet cell metaplasia in asthma, CF, and COPD at the transcriptome level, thereby providing insights into possible novel therapeutic approaches for these diseases.

Swine models in translational research and medicine

Swine are increasingly studied as animal models of human disease. The anatomy, size, longevity, physiology, immune system, and metabolism of swine are more like humans than traditional rodent models. In addition, the size of swine is preferred for surgical placement and testing of medical devices destined for humans. These features make swine useful for biomedical, pharmacological, and toxicological research. With recent advances in gene-editing technologies, genetic modifications can readily and efficiently be made in swine to study genetic disorders. In addition, gene-edited swine tissues are necessary for studies testing and validating xenotransplantation into humans to meet the critical shortfall of viable organs versus need. Underlying all of these biomedical applications, the knowledge of husbandry, background diseases and lesions, and biosecurity needs are important for productive, efficient, and reproducible research when using swine as a human disease model for basic research, preclinical testing, and translational studies.

Imaging assessment of conjunctival goblet cells in dry eye disease

Dry eye disease (DED) is a widespread, multifactorial, and chronic disorder of the ocular surface with disruption of tear film homeostasis as its core trait. Conjunctival goblet cells (CGCs) are specialised secretory cells found in the conjunctival epithelium that participate in tear film formation by secreting mucin. Changes in both the structure and function of CGCs are hallmarks of DED, and imaging assessment of CGCs is important for the diagnosis, classification, and severity evaluation of DED. Existing imaging methods include conjunctival biopsy, conjunctival impression cytology and in vivo confocal microscopy, which can be used to assess the morphology, distribution, and density of the CGCs. Recently, moxifloxacin-based fluorescence microscopy has emerged as a novel technique that enables efficient, non-invasive and in vivo imaging of CGCs. This article presents a comprehensive overview of both the structure and function of CGCs and their alterations in the context of DED, as well as current methods of CGCs imaging assessment. Additionally, potential directions for the visual evaluation of CGCs are discussed.

Mucus production, host-microbiome interactions, hormone sensitivity, and innate immune responses modeled in human cervix chips

Modulation of the cervix by steroid hormones and commensal microbiome play a central role in the health of the female reproductive tract. Here we describe organ-on-a-chip (Organ Chip) models that recreate the human cervical epithelial-stromal interface with a functional epithelial barrier and production of mucus with biochemical and hormone-responsive properties similar to living cervix. When Cervix Chips are populated with optimal healthy versus dysbiotic microbial communities (dominated by Lactobacillus crispatus and Gardnerella vaginalis, respectively), significant differences in tissue innate immune responses, barrier function, cell viability, proteome, and mucus composition are observed that are similar to those seen in vivo. Thus, human Cervix Organ Chips represent physiologically relevant in vitro models to study cervix physiology and host-microbiome interactions, and hence may be used as a preclinical testbed for development of therapeutic interventions to enhance women's health.

Mucociliary Clearance is Impaired in Small Airways of Cystic Fibrosis Pigs

Rationale

Cystic fibrosis is a genetic disorder characterized by recurrent airway infections, inflammation, and progressive decline in lung function. Autopsy and spirometry data suggest that cystic fibrosis may start in the small airways which, due to the fractal nature of the airways, account for most of the airway tree surface area. However, they are not easily accessible for testing.

Objectives

Here, we tested the hypothesis that mucociliary clearance is abnormal in the small airways of newborn cystic fibrosis pigs.

Methods

Current mucociliary clearance assays are limited therefore we developed a dynamic positron emission tomography scan assay with high spatial and temporal resolution. Each study was accompanied by a high-resolution computed tomography scan that helped identify the thin outer region of the lung that contained small airways.

Measurements and Main Results

Clearance of aerosolized [ 68 Ga]macro aggregated albumin from distal airways occurred within minutes after delivery and followed a two-phase process. In cystic fibrosis pigs, both early and late clearance rates were slower. Stimulation of the cystic fibrosis airways with the purinergic agonist UTP further impaired late clearance. Only 1 cystic fibrosis pig treated with UTP out of 6 cleared more than 20% of the delivered dose.

Conclusions

These data indicate that mucociliary transport in the small airways is fast and can easily be missed if the acquisition is not fast enough. The data also indicate that mucociliary transport is impaired in small airways of cystic fibrosis pigs. This defect is exacerbated by stimulation of mucus secretions with purinergic agonists.

Hyposecretion of cervical MUC5B is related to preterm birth in pregnant women after cervical excisional surgery

PROBLEM

Excisional surgery for cervical intraepithelial neoplasia is a risk factor for preterm birth in subsequent pregnancies. However, the underlying mechanisms of this association remain unclear. We previously showed that cervical MUC5B, a mucin protein, may be a barrier to ascending pathogens during pregnancy. We thus hypothesized that hyposecretion of cervical MUC5B is associated with preterm birth after cervical excisional surgery.

METHOD OF STUDY

This prospective nested case-control study (Study 1) included pregnant women who had previously undergone cervical excisional surgery across 11 hospitals. We used proteomics to compare cervicovaginal fluid at 18-22 weeks of gestation between the preterm and term birth groups. In another case-control analysis (Study 2), we compared MUC5B expression in nonpregnant uterine tissues between 15 women with a history of cervical excisional surgery and 26 women without a history of cervical surgery.

RESULTS

The abundance of MUC5B in cervicovaginal fluid was significantly decreased in the preterm birth group (fold change = 0.41, p = .035). Among the 480 quantified proteins, MUC5B had the second highest positive correlation with gestational age at delivery in the combined preterm and term groups. The cervicovaginal microbiome composition was not significantly different between the two groups. Cervical length was not correlated with gestational age at delivery (r = 0.18, p = .079). Histologically, the MUC5B-positive area in the nonpregnant cervix was significantly decreased in women with a history of cervical excisional surgery (0.85-fold, p = .048). The distribution of MUC5B-positive areas in the cervical tissues of 26 women without a history of cervical excisional surgery differed across individuals.

CONCLUSIONS

This study suggests that the primary mechanism by which cervical excisional surgery causes preterm birth is the hyposecretion of MUC5B due to loss of the cervical glands.

Mucus Structure, Viscoelastic Properties, and Composition in Chronic Respiratory Diseases

The respiratory mucus, a viscoelastic gel, effectuates a primary line of the airway defense when operated by the mucociliary clearance. In chronic respiratory diseases (CRDs), such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF), the mucus is overproduced and its solid content augments, changing its structure and viscoelastic properties and determining a derangement of essential defense mechanisms against opportunistic microbial (virus and bacteria) pathogens. This ensues in damaging of the airways, leading to a vicious cycle of obstruction and infection responsible for the harsh clinical evolution of these CRDs. Here, we review the essential features of normal and pathological mucus (i.e., sputum in CF, COPD, and asthma), i.e., mucin content, structure (mesh size), micro/macro-rheology, pH, and osmotic pressure, ending with the awareness that sputum biomarkers (mucins, inflammatory proteins and peptides, and metabolites) might serve to indicate acute exacerbation and response to therapies. There are some indications that old and novel treatments may change the structure, viscoelastic properties, and biomarker content of sputum; however, a wealth of work is still needed to embrace these measures as correlates of disease severity in association with (or even as substitutes of) pulmonary functional tests.

INSIGHTS INTO THE ORIGINS OF CYSTIC FIBROSIS LUNG DISEASE

In this paper, I will discuss recent studies using a cystic fibrosis pig model to better understand the origins of cystic fibrosis lung disease. Specifically, I will review our work investigating how loss of the cystic fibrosis transmembrane conductance regulator function (CFTR) impairs mucociliary transport in the cystic fibrosis airway. These studies reveal new insights into the early, underlying mechanisms of cystic fibrosis lung disease and could lead to novel therapeutic interventions.

Impact of lipopolysaccharide-induced acute lung injury in aged mice

Study Aim: As the geriatric population rapidly expands, there has been a concurrent increase in elderly admissions to intensive care units (ICUs). Acute lung injury (ALI) is a prevalent reason for these admissions and carries poorer survival rates for the aged population compared to younger counterparts. The aging lung is subject to physiological, cellular, and immunological changes. However, our understanding of how aging impacts the clinical progression of ALI is limited. This study explored the effect of aging using a murine model of ALI. Methods: Female C57BL/6J mice, aged 7-8 wk (young) and 18 months (aged), were divided into four groups: young controls, aged controls, young with ALI (YL), and aged with ALI (AL). ALI was induced via intratracheal administration of lipopolysaccharide (LPS, 0.5 mg/kg). The animals were euthanized 72 h after LPS exposure. Results: The AL group exhibited a significantly increased wet/dry ratio compared to the other three groups, including the YL group. The bronchoalveolar lavage (BAL) fluid in the AL group had more cells overall, including more neutrophils, than the other groups. Inflammatory cytokines in BAL fluid showed similar trends. Histological analyses demonstrated more severe lung injury and fibrosis in the AL group than in the other groups. Increased transcription of senescence-associated secretory phenotype markers, including PAI-1 and MUC5B, was more prominent in the AL group than in the other groups. This trend was also observed in BAL samples from humans with pneumonia. Conclusions: Aging may amplify lung damage and inflammatory responses in ALI. This suggests that physicians should exercise increased caution in the clinical management of aged patients with ALI.

Goblet cell interactions reorient bundled mucus strands for efficient airway clearance

The respiratory tract of larger animals is cleared by sweeping bundled strands along the airway surface. These bundled strands can be millimetric in length and consist of MUC5B mucin. They are produced by submucosal glands, and upon emerging from these glands, the long axis of the bundled strands is oriented along the cilia-mediated flow toward the oral cavity. However, after release, the bundled strands are found to have turned orthogonal to the flow, which maximizes their clearance potential. How this unexpected reorientation is accomplished is presently not well understood. Recent experiments suggest that the reorientation process involves bundled strands sticking to MUC5AC mucus threads, which are tethered to the goblet cells. Such goblet cells are present in small numbers throughout the airway epithelium. Here, we develop a minimal model for reorientation of bundled mucus strands through adhesive interactions with surface goblet cells. Our simulations reveal that goblet cell interactions can reorient the bundled strands within 10 mm of release-making reorientation on the length scale of the tracheal tube feasible-and can stabilize the orthogonal orientation. Our model also reproduces other experimental observations such as strong velocity fluctuations and significant slow-down of the bundled strand with respect to the cilia-mediated flow. We further provide insight into the strand turning mechanism by examining the effect of strand shape on the impulse exerted by a single goblet cell. We conclude that goblet cell-mediated reorientation is a viable route for bundled strand reorientation, which should be further validated in future experiment.

Immunomodulatory effects of inactivated Ligilactobacillus salivarius CECT 9609 on respiratory epithelial cells

The microbiota in humans and animals play crucial roles in defense against pathogens and offer a promising natural source for immunomodulatory products. However, the development of physiologically relevant model systems and protocols for testing such products remains challenging. In this study, we present an experimental condition where various natural products derived from the registered lactic acid bacteria Ligilactobacillus salivarius CECT 9609, known for their immunomodulatory activity, were tested. These products included live and inactivated bacteria, as well as fermentation products at different concentrations and culture times. Using our established model system, we observed no morphological changes in the airway epithelium upon exposure to Pasteurella multocida, a common respiratory pathogen. However, early molecular changes associated with the innate immune response were detected through transcript analysis. By employing diverse methodologies ranging from microscopy to next-generation sequencing (NGS), we characterized the interaction of these natural products with the airway epithelium and their potential beneficial effects in the presence of P. multocida infection. In particular, our discovery highlights that among all Ligilactobacillus salivarius CECT 9609 products tested, only inactivated cells preserve the conformation and morphology of respiratory epithelial cells, while also reversing or altering the natural immune responses triggered by Pasteurella multocida. These findings lay the groundwork for further exploration into the protective role of these bacteria and their derivatives.

Pseudomonas aeruginosa type IV pili actively induce mucus contraction to form biofilms in tissue-engineered human airways

The opportunistic pathogen Pseudomonas aeruginosa causes antibiotic-recalcitrant pneumonia by forming biofilms in the respiratory tract. Despite extensive in vitro experimentation, how P. aeruginosa forms biofilms at the airway mucosa is unresolved. To investigate the process of biofilm formation in realistic conditions, we developed AirGels: 3D, optically accessible tissue-engineered human lung models that emulate the airway mucosal environment. AirGels recapitulate important factors that mediate host-pathogen interactions including mucus secretion, flow and air-liquid interface (ALI), while accommodating high-resolution live microscopy. With AirGels, we investigated the contributions of mucus to P. aeruginosa biofilm biogenesis in in vivo-like conditions. We found that P. aeruginosa forms mucus-associated biofilms within hours by contracting luminal mucus early during colonization. Mucus contractions facilitate aggregation, thereby nucleating biofilms. We show that P. aeruginosa actively contracts mucus using retractile filaments called type IV pili. Our results therefore suggest that, while protecting epithelia, mucus constitutes a breeding ground for biofilms.

Air-Liquid interface cultures to model drug delivery through the mucociliary epithelial barrier

Epithelial cells from mucociliary portions of the airways can be readily grown and expanded in vitro. When grown on a porous membrane at an air-liquid interface (ALI) the cells form a confluent, electrically resistive barrier separating the apical and basolateral compartments. ALI cultures replicate key morphological, molecular and functional features of the in vivo epithelium, including mucus secretion and mucociliary transport. Apical secretions contain secreted gel-forming mucins, shed cell-associated tethered mucins, and hundreds of additional molecules involved in host defense and homeostasis. The respiratory epithelial cell ALI model is a time-proven workhorse that has been employed in various studies elucidating the structure and function of the mucociliary apparatus and disease pathogenesis. It serves as a critical milestone test for small molecule and genetic therapies targeting airway diseases. To fully exploit the potential of this important tool, numerous technical variables must be thoughtfully considered and carefully executed.

Immunohistochemical detection of MUC5AC and MUC5B mucins in ferrets

OBJECTIVE

Cystic fibrosis (CF) is a genetic condition that causes abnormal mucus secretions in affected organs. MUC5AC and MUC5B are gel-forming mucins and frequent targets for investigations in CF tissues. Our objective was to qualify MUC5AC and MUC5B immunohistochemical techniques to provide a useful tool to identify, localize and interpret mucin expression in ferret tissues.

RESULTS

MUC5AC and MUC5B mucins were detected most commonly in large airways and least in small airways, consistent with reported goblet cell density in airway surface epithelia. We evaluated whether staining method affected the detection of goblet cell mucins in serial sections of bronchial surface epithelia. Significant differences between stains were not observed suggesting common co-expression MUC5AC and MUC5B proteins in goblet cells of airway surface epithelia. Gallbladder and stomach tissues are reported to have differential mucin enrichment, so we tested these tissues in wildtype ferrets. Stomach tissues were enriched in MUC5AC and gallbladder tissues enriched in MUC5B, mucin enrichment similar to human tissues. Mucin immunostaining techniques were further qualified for specificity using lung tissue from recently generated MUC5AC-/- and MUC5B-/- ferrets. Qualified techniques for MUC5AC and MUC5B immunohistochemistry will be useful tools for mucin tissue studies in CF and other ferret models.

Gene Therapy for Cystic Fibrosis: Recent Advances and Future Prospects

Gene replacement therapies are novel therapeutic approaches that seek to tackle hereditary diseases caused by a congenital deficiency in a particular gene, when a functional copy of a gene can be delivered to the cells and tissues using various delivery systems. To do this, viral particles carrying a functional copy of the gene of interest and various nonviral gene delivery systems, including liposomes, nanoparticles, etc., can be used. In this review, we discuss the state of current knowledge regarding the molecular mechanisms and types of genetic mutations that lead to cystic fibrosis and highlight recent developments in gene therapy that can be leveraged to correct these mutations and to restore the physiological function of the carrier protein transporting sodium and chlorine ions in the airway epithelial cells. Restoration of carrier protein expression could lead to the normalization of ion and water transport across the membrane and induce a decrease in the viscosity of airway surface fluid, which is one of the pathological manifestations of this disease. This review also summarizes recently published preclinical and clinical data for various gene therapies to allow one to make some conclusions about future prospects for gene therapy in cystic fibrosis treatment.

Mucin networks: Dynamic structural assemblies controlling mucus function

Contrary to first appearances, mucus structural biology is not an oxymoron. Though mucus hydrogels derive their characteristics largely from intrinsically disordered, heavily glycosylated polypeptide segments, the secreted mucin glycoproteins that constitute mucus undergo an orderly assembly process controlled by folded domains at their termini. Recent structural studies revealed how mucin complexes promote disulphide-mediated polymerization to produce the mucus gel scaffold. Additional protein-protein and protein-glycan interactions likely tune the mesoscale properties, stability, and activities of mucins. Evidence is emerging that even intrinsically disordered glycosylated segments have specific structural roles in the production and properties of mucus. Though soft-matter biophysical approaches to understanding mucus remain highly relevant, high-resolution structural studies of mucins and other mucus components are providing new perspectives on these vital, protective hydrogels.

Mucus aberrant properties in CF: Insights from cells and animal models

Cystic fibrosis (CF), an autosomal genetic disorder caused by the dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) protein, is characterized by mucus accumulation in the lungs, the intestinal tract, and the pancreatic ducts. Mucins are high-molecular-weight glycoproteins that govern the biochemical and biophysical properties of mucus. In the CF lung, increased mucus viscoelasticity is associated with decreased mucociliary clearance and defects in host defense mechanisms. The link between defective ion channel and abnormal mucus properties has been investigated in studies involving cell and animal models. In this review article, we discuss recent progress toward understanding the different regions and cells that express CFTR in the airways and how mucus is produced and cleared from the lungs. In addition, we reflect on animal models that provided insights into the organization and the role of the mucin network and how mucus and antimicrobial activities act in concert to protect the lungs from invading pathogens.

The Pathology of Asthma: What Is Obstructing Our View?

Despite the advent of sophisticated and efficient new biologics to treat inflammation in asthma, the disease persists. Even following treatment, many patients still experience the well-known symptoms of wheezing, shortness of breath, and coughing. What are we missing? Here we examine the evidence that mucus plugs contribute to a substantial portion of disease, not only by physically obstructing the airways but also by perpetuating inflammation. In this way, mucus plugs may act as an immunogenic stimulus even in the absence of allergen or with the use of current therapeutics. The alterations of several parameters of mucus biology, driven by type 2 inflammation, result in sticky and tenacious sputum, which represents a potent threat, first due to the difficulties in expectoration and second by acting as a platform for viral, bacterial, or fungal colonization that allows exacerbations. Therefore, in this way, mucus plugs are an overlooked but critical feature of asthmatic airway disease.

Mucins MUC5AC and MUC5B Are Variably Packaged in the Same and in Separate Secretory Granules

Rationale: MUC5AC (mucin 5AC, oligomeric gel-forming) and MUC5B (mucin 5B, oligomeric gel-forming) are the predominant secreted polymeric mucins in mammalian airways. They contribute differently to the pathogenesis of various muco-obstructive and interstitial lung diseases, and their genes are separately regulated, but whether they are packaged together or in separate secretory granules is not known. Objectives: To determine the packaging of MUC5AC and MUC5B within individual secretory granules in mouse and human airways under varying conditions of inflammation and along the proximal-distal axis. Methods: Lung tissue was obtained from mice stimulated to upregulate mucin production by the cytokines IL-1β and IL-13 or by porcine pancreatic elastase. Human lung tissue was obtained from donated normal lungs, biopsy samples of transplanted lungs, and explanted lungs from subjects with chronic obstructive pulmonary disease. MUC5AC and MUC5B were labeled with antibodies from different animal species or, in mice only, by transgenic chimeric mucin-fluorescent proteins and imaged using widefield deconvolution or Airyscan fluorescence microscopy. Measurements and Main Results: In both mouse and human airways, most secretory granules contained both mucins interdigitating within the granules. Smaller numbers of granules contained MUC5B alone, and even fewer contained MUC5AC alone. Conclusions: MUC5AC and MUC5B are variably stored both in the same and in separate secretory granules of both mice and humans. The high fraction of granules containing both mucins under a variety of conditions makes it unlikely that their secretion can be differentially controlled as a therapeutic strategy. This work also advances knowledge of the packaging of mucins within secretory granules to understand mechanisms of epithelial stress in the pathogenesis of chronic lung diseases.

Physiology and pathophysiology of human airway mucus

The mucus clearance system is the dominant mechanical host defense system of the human lung. Mucus is cleared from the lung by cilia and airflow, including both two-phase gas-liquid pumping and cough-dependent mechanisms, and mucus transport rates are heavily dependent on mucus concentration. Importantly, mucus transport rates are accurately predicted by the gel-on-brush model of the mucociliary apparatus from the relative osmotic moduli of the mucus and periciliary-glycocalyceal (PCL-G) layers. The fluid available to hydrate mucus is generated by transepithelial fluid transport. Feedback interactions between mucus concentrations and cilia beating, via purinergic signaling, coordinate Na+ absorptive vs Cl- secretory rates to maintain mucus hydration in health. In disease, mucus becomes hyperconcentrated (dehydrated). Multiple mechanisms derange the ion transport pathways that normally hydrate mucus in muco-obstructive lung diseases, e.g., cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD), non-CF bronchiectasis (NCFB), and primary ciliary dyskinesia (PCD). A key step in muco-obstructive disease pathogenesis is the osmotic compression of the mucus layer onto the airway surface with the formation of adherent mucus plaques and plugs, particularly in distal airways. Mucus plaques create locally hypoxic conditions and produce airflow obstruction, inflammation, infection, and, ultimately, airway wall damage. Therapies to clear adherent mucus with hydrating and mucolytic agents are rational, and strategies to develop these agents are reviewed.

Pulmonary neuroendocrine cells sense succinate to stimulate myoepithelial cell contraction

Pulmonary neuroendocrine cells (PNECs) are rare airway cells with potential sensory capacity linked to vagal neurons and immune cells. How PNECs sense and respond to external stimuli remains poorly understood. We discovered PNECs located within pig and human submucosal glands, a tissue that produces much of the mucus that defends the lung. These PNECs sense succinate, an inflammatory molecule in liquid lining the airway surface. The results indicate that succinate migrates down the submucosal gland duct to the acinus, where it triggers apical succinate receptors, causing PNECs to release ATP. The short-range ATP signal stimulates the contraction of myoepithelial cells wrapped tightly around the submucosal glands. Succinate-triggered gland contraction may complement the action of neurotransmitters that induce mucus release but not gland contraction to promote mucus ejection onto the airway surface. These findings identify a local circuit in which rare PNECs within submucosal glands sense an environmental cue to orchestrate the function of airway glands.

Tromethamine improves mucociliary clearance in cystic fibrosis pigs

In cystic fibrosis (CF), the loss of cystic fibrosis transmembrane conductance regulator (CFTR) mediated Cl-  and HCO3 -  secretion across the epithelium acidifies the airway surface liquid (ASL). Acidic ASL alters two key host defense mechanisms: Rapid ASL bacterial killing and mucociliary transport (MCT). Aerosolized tromethamine (Tham) increases ASL pH and restores the ability of ASL to rapidly kill bacteria in CF pigs. In CF pigs, clearance of insufflated microdisks is interrupted due to abnormal mucus causing microdisks to abruptly recoil. Aerosolizing a reducing agent to break disulfide bonds that link mucins improves MCT. Here, we are interested in restoring MCT in CF by aerosolizing Tham, a buffer with a pH of 8.4. Because Tham is hypertonic to serum, we use an acidified formulation as a control. We measure MCT by tracking the caudal movement of individual tantalum microdisks with serial chest computed tomography scans. Alkaline Tham improves microdisk clearance to within the range of that seen in non-CF pigs. It also partially reverses MCT defects, including reduced microdisk recoil and elapse time until they start moving after methacholine stimulation in CF pig airways. The effect is not due to hypertonicity, as it is not seen with acidified Tham or hypertonic saline. This finding indicates acidic ASL impairs CF MCT and suggests that alkalinization of ASL pH with inhaled Tham may improve CF airway disease.

Influence of SARS-CoV-2 on airway mucus production: A review and proposed model

Coronavirus disease 2019 (COVID-19) is a worldwide pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that has affected millions of lives. Individuals who survive severe COVID-19 can experience sustained respiratory symptoms that persist for months after initial infection. In other airway diseases, abnormal airway mucus contributes to sustained airway symptoms. However, the impact of SARS-CoV-2 on airway mucus has received limited attention. In the current review, we assess literature describing the impact of SARS-CoV-2 on airway pathophysiology with specific emphasis on mucus production. Accumulating evidence suggests that the 2 major secreted airway mucin glycoproteins, MUC5AC and MUC5B, are abnormal in some patients with COVID-19. Aberrations in MUC5AC or MUC5B in response to SARS-CoV-2 infection are likely due to inflammation, though the responsible mechanisms have yet to be determined. Thus, we also provide a proposed model highlighting mechanisms that can contribute to acute and sustained mucus abnormalities in SARS-CoV-2, with an emphasis on inflammatory cells and mediators, including mast cells and histamine. Last, we bring to light the challenges of studying abnormal mucus production in SARS-CoV-2 infections and discuss the strengths and limitations of model systems commonly used to study COVID-19. The evidence to date suggests that ferrets, nonhuman primates, and cats may have advantages over other models to investigate mucus in COVID-19.

Muc5b Contributes to Mucus Abnormality in Rat Models of Cystic Fibrosis

Cystic fibrosis (CF) airway disease is characterized by excessive and accumulative mucus in the airways. Mucociliary clearance becomes defective as mucus secretions become hyperconcentrated and viscosity increases. The CFTR-knockout (KO) rat has been previously shown to progressively develop delayed mucociliary transport, secondary to increased viscoelasticity of airway secretions. The humanized-G551D CFTR rat model has demonstrated that abnormal mucociliary clearance and hyperviscosity is reversed by ivacaftor treatment. In this study, we sought to identify the components of mucus that changes as the rat ages to contribute to these abnormalities. We found that Muc5b concentrations, and to a lesser extent Muc5ac, in the airway were increased in the KO rat compared to WT, and that Muc5b concentration was directly related to the viscosity of the mucus. Additionally, we found that methacholine administration to the airway exacerbates these characteristics of disease in the KO, but not WT rat trachea. Lastly we determined that at 6 months of age, CF rats had mucus that was adherent to the airway epithelium, a process that is reversed by ivacaftor therapy in the hG551D rat. Overall, these data indicate that accumulation of Muc5b initiates the muco-obstructive process in the CF lung prior to infection.

Mucus concentration-dependent biophysical abnormalities unify submucosal gland and superficial airway dysfunction in cystic fibrosis

Cystic fibrosis (CF) is characterized by abnormal transepithelial ion transport. However, a description of CF lung disease pathophysiology unifying superficial epithelial and submucosal gland (SMG) dysfunctions has remained elusive. We hypothesized that biophysical abnormalities associated with CF mucus hyperconcentration provide a unifying mechanism. Studies of the anion secretion-inhibited pig airway model of CF revealed elevated SMG mucus concentrations, osmotic pressures, and SMG mucus accumulation. Human airway studies revealed hyperconcentrated CF SMG mucus with raised osmotic pressures and cohesive forces predicted to limit SMG mucus secretion/release. Using proline-rich protein 4 (PRR4) as a biomarker of SMG secretion, CF sputum proteomics analyses revealed markedly lower PRR4 levels compared to healthy and bronchiectasis controls, consistent with a failure of CF SMGs to secrete mucus onto airway surfaces. Raised mucus osmotic/cohesive forces, reflecting mucus hyperconcentration, provide a unifying mechanism that describes disease-initiating mucus accumulation on airway surfaces and in SMGs of the CF lung.

Lysophosphatidylserine Induces MUC5AC Production via the Feedforward Regulation of the TACE-EGFR-ERK Pathway in Airway Epithelial Cells in a Receptor-Independent Manner

Lysophosphatidylserine (LysoPS) is an amphipathic lysophospholipid that mediates a broad spectrum of inflammatory responses through a poorly characterized mechanism. Because LysoPS levels can rise in a variety of pathological conditions, we sought to investigate LysoPS's potential role in airway epithelial cells that actively participate in lung homeostasis. Here, we report a previously unappreciated function of LysoPS in production of a mucin component, MUC5AC, in the airway epithelial cells. LysoPS stimulated lung epithelial cells to produce MUC5AC via signaling pathways involving TACE, EGFR, and ERK. Specifically, LysoPS- dependent biphasic activation of ERK resulted in TGF-α secretion and strong EGFR phosphorylation leading to MUC5AC production. Collectively, LysoPS induces the expression of MUC5AC via a feedback loop composed of proligand synthesis and its proteolysis by TACE and following autocrine EGFR activation. To our surprise, we were not able to find a role of GPCRs and TLR2, known LyoPS receptors in LysoPS-induced MUC5AC production in airway epithelial cells, suggesting a potential receptor-independent action of LysoPS during inflammation. This study provides new insight into the potential function and mechanism of LysoPS as an emerging lipid mediator in airway inflammation.

Elastic mucus strands impair mucociliary clearance in cystic fibrosis pigs

SignificanceIn many lung diseases, increased amounts of and/or abnormal mucus impair mucociliary clearance, a key defense against inhaled and aspirated material. Submucosal glands lining cartilaginous airways secrete mucus strands that are pulled by cilia until they break free from the duct and sweep upward toward the larynx, carrying particulates. In cystic fibrosis (CF) pigs, progressive clearance of insufflated microdisks was repeatedly interrupted as microdisks abruptly recoiled. Aerosolizing a reducing agent to break disulfide bonds linking mucins ruptured mucus strands, freeing them from submucosal gland ducts and allowing cilia to propel them up the airways. These findings highlight the abnormally increased elasticity of CF mucus and suggest that agents that break disulfide bonds might have value in lung diseases with increased mucus.

Respiratory MUC5B disproportion is involved in severe community-acquired pneumonia

BACKGROUND

Mucus production is a process involved in the pathogenesis of Community-acquired pneumonia (CAP). The study is to determine Mucin 5B (MUC5B) protein concentration and its proportion in the bronchoalveolar lavage fluid (BALF) of CAP patients and evaluate its value to help assess disease severity.

METHODS

A total of 118 patients were enrolled in this cross-sectional study, including 45 with severe CAP (SCAP) and 73 with non-severe CAP (NSCAP). MUC5B concentration in BALF were determined by immunoblotting analysis. Total protein concentration of BALF was detected by Pierce BCA kit. Cytokines IL6, IL10, IFNγ, IL13, and IL17 in BALF were measured using commercial enzyme-linked immunosorbent assay (ELISA). Spearman's correlation analysis was applied to evaluate the relationships between MUC5B concentration or MUC5B/total protein ratio and the CURB-65 score, as well as cytokines. Logistic regression analysis was used to identify the independent factors associated with severe CAP. Receiver operating characteristic (ROC) curve was used to evaluate the assessment value of MUC5B/total protein ratio and other indexes for CAP severity.

RESULTS

MUC5B concentration in the BALF of NSCAP group was higher than that in SCAP group [NSCAP 13.56 µg/ml (IQR 5.92-25.79) vs. SCAP 8.20 µg/ml (IQR 4.97-14.03), p = 0.011]. The total protein concentration in the BALF of NSCAP group was lower than that in SCAP group [NSCAP 0.38 mg/ml (IQR 0.15-1.10) vs. SCAP 0.68 mg/ml (IQR 0.46-1.69), p = 0.002]. The MUC5B/total protein ratio was remarkably higher in NSCAP group than that in SCAP groups [NSCAP 3.66% (IQR 1.50-5.56%) vs. SCAP 1.38% (IQR 0.73-1.76%), p < 0.001]. MUC5B/total protein ratio was negatively correlated with total protein concentration (r_s = - 0.576, p < 0.001), IL6 (r_s = - 0.312, p = 0.001), IL10 (r_s = - 0.228, p = 0.013), IL13 (r_s = - 0.183, p = 0.048), IL17 (r_s = - 0.282, p = 0.002) and CURB-65 score (r_s = - 0.239, p = 0.009). Logistic regression identified that MUC5B/total protein ratio, IL6 level and CURB-65 score as independent variables related to CAP severity. ROC curve demonstrated best assessment value of MUC5B/total protein ratio for SCAP (AUC 0.803, p < 0.001), with a sensitivity of 88.9% and a specificity of 64.4%.

CONCLUSIONS

Respiratory MUC5B disproportion is related to CAP severity. MUC5B/total protein ratio may serve as an assessment marker and a potential therapeutic target for severe CAP.

Methods of Sputum and Mucus Assessment for Muco-Obstructive Lung Diseases in 2022: Time to “Unplug” from Our Daily Routine!

Obstructive lung diseases, such as chronic obstructive pulmonary disease, asthma, or non-cystic fibrosis bronchiectasis, share some major pathophysiological features: small airway involvement, dysregulation of adaptive and innate pulmonary immune homeostasis, mucus hyperproduction, and/or hyperconcentration. Mucus regulation is particularly valuable from a therapeutic perspective given it contributes to airflow obstruction, symptom intensity, disease severity, and to some extent, disease prognosis in these diseases. It is therefore crucial to understand the mucus constitution of our patients, its behavior in a stable state and during exacerbation, and its regulatory mechanisms. These are all elements representing potential therapeutic targets, especially in the era of biologics. Here, we first briefly discuss the composition and characteristics of sputum. We focus on mucus and mucins, and then elaborate on the different sample collection procedures and how their quality is ensured. We then give an overview of the different direct analytical techniques available in both clinical routine and more experimental settings, giving their advantages and limitations. We also report on indirect mucus assessment procedures (questionnaires, high-resolution computed tomography scanning of the chest, lung function tests). Finally, we consider ways of integrating these techniques with current and future therapeutic options. Cystic fibrosis will not be discussed given its monogenic nature.

Reconstituted basement membrane enables airway epithelium modeling and nanoparticle toxicity testing

Basement membrane (BM) acts as a sheet-like extracellular matrix to support and promote the formation of epithelial and endothelial cell layers. The in vitro reconstruction of the BM is however not easy due to its ultrathin membrane features. This difficulty is overcome by self-assembling type IV collagen and laminin in the porous areas of a monolayer of crosslinked gelatin nanofibers deposited on a honeycomb microframe. Herein, a method is presented to generate airway epithelium by using such an artificial basement membrane (ABM) and human-induced pluripotent stem cells (hiPSCs). Bipolar primordial lung progenitors are firstly induced from hiPSCs and then replated on the ABM for differentiation toward matured airway epithelium under submerged and air-liquid interface culture conditions. As a result, a pseudostratified airway epithelium consisting of several cell types is achieved, showing remarkable apical secretion of MUC5AC proteins and clear advantages over other types of substrates. As a proof of concept, the derived epithelium is used for toxicity test of cadmium telluride (CdTe) nanoparticles (NPs), demonstrating the applicability of ABM-based assays involving hiPSC-derived epithelial cells-based assays.

Cellular and molecular architecture of submucosal glands in wild-type and cystic fibrosis pigs

Submucosal glands (SMGs) protect lungs but can also contribute to disease. For example, in cystic fibrosis (CF), SMGs produce abnormal mucus that disrupts mucociliary transport. CF is an ion transport disease, yet knowledge of the ion transporters expressed by SMG acini, which produce mucus, and SMG ducts that carry it to the airway lumen is limited. Therefore, we isolated SMGs from newborn pigs and used single-cell messenger RNA sequencing, immunohistochemistry, and in situ hybridization to identify cell types, gene expression, and spatial distribution. Cell types and transcript levels were the same in non-CF and CF SMGs, suggesting that loss of epithelial anion secretion rather than an intrinsic cell defect causes CF mucus abnormalities. Gene signatures of acinar mucous and acinar serous cells revealed specialized functions in producing mucins and antimicrobials, respectively. However, surprisingly, these two cell types expressed the same ion transporters and neurohumoral receptors, suggesting the importance of balancing mucin and liquid secretion to produce optimal mucus properties. SMG duct cell transcripts suggest that they secrete HCO₃^- and Cl^-, and thus have some similarity to pancreatic ducts that are also defective in CF. These and additional findings suggest the functions of the SMG acinus and duct and provide a baseline for understanding how environmental and genetic challenges impact their contribution to lung disease.

Distinct Secretion of MUC5AC and MUC5B in Upper and Lower Chronic Airway Diseases

The human airway is protected by a defensive mucus barrier. The most prominent components of mucus are the mucin glycoproteins MUC5AC and MUC5B. They are produced by goblet cells and submucosal gland cells in the upper and lower airways. Hyperplasia of these cells and hypersecretion of MUC5AC and MUC5B characterize chronic inflammatory diseases of the upper and lower airways. Recent studies have revealed that MUC5AC and MUC5B are expressed at specific sites in the respiratory tract through different molecular mechanisms and have distinct functions. Morphometric and histochemical studies have also examined the roles of goblet cells, submucosal gland cells, MUC5AC, and MUC5B in different chronic airway diseases individually. The individual study of goblet cells, submucosal gland cells, MUC5AC, and MUC5B in airway diseases would be helpful for precisely diagnosing chronic inflammatory diseases of the airway and establishing optimal treatments. This review focuses on the distinct secretion of MUC5AC and MUC5B and their producing cells in chronic inflammatory diseases of the upper and lower airway.

A disorder clinically resembling cystic fibrosis caused by biallelic variants in the AGR2 gene

Purpose

We sought to describe a disorder clinically mimicking cystic fibrosis (CF) and to elucidate its genetic cause.

Methods

Exome/genome sequencing and human phenotype ontology data of nearly 40 000 patients from our Bio/Databank were analysed. RNA sequencing of samples from the nasal mucosa from patients, carriers and controls followed by transcriptome analysis was performed.

Results

We identified 13 patients from 9 families with a CF-like phenotype consisting of recurrent lower respiratory infections (13/13), failure to thrive (13/13) and chronic diarrhoea (8/13), with high morbidity and mortality. All patients had biallelic variants in AGR2, (1) two splice-site variants, (2) gene deletion and (3) three missense variants. We confirmed aberrant AGR2 transcripts caused by an intronic variant and complete absence of AGR2 transcripts caused by the large gene deletion, resulting in loss of function (LoF). Furthermore, transcriptome analysis identified significant downregulation of components of the mucociliary machinery (intraciliary transport, cilium organisation), as well as upregulation of immune processes.

Conclusion

We describe a previously unrecognised autosomal recessive disorder caused by AGR2 variants. AGR2-related disease should be considered as a differential diagnosis in patients presenting a CF-like phenotype. This has implications for the molecular diagnosis and management of these patients. AGR2 LoF is likely the disease mechanism, with consequent impairment of the mucociliary defence machinery. Future studies should aim to establish a better understanding of the disease pathophysiology and to identify potential drug targets.

Mucus threads from surface goblet cells clear particles from the airways

Background

The mucociliary clearance system driven by beating cilia protects the airways from inhaled microbes and particles. Large particles are cleared by mucus bundles made in submucosal glands by parallel linear polymers of the MUC5B mucins. However, the structural organization and function of the mucus generated in surface goblet cells are poorly understood.

Methods

The origin and characteristics of different mucus structures were studied on live tissue explants from newborn wild-type (WT), cystic fibrosis transmembrane conductance regulator (CFTR) deficient (CF) piglets and weaned pig airways using video microscopy, Airyscan imaging and electron microscopy. Bronchoscopy was performed in juvenile pigs in vivo.

Results

We have identified a distinct mucus formation secreted from the surface goblet cells with a diameter less than two micrometer. This type of mucus was named mucus threads. With time mucus threads gathered into larger mucus assemblies, efficiently collecting particles. The previously observed Alcian blue stained mucus bundles were around 10 times thicker than the threads. Together the mucus bundles, mucus assemblies and mucus threads cleared the pig trachea from particles.

Conclusions

These results demonstrate that normal airway mucus is more complex and has a more variable structural organization and function than was previously understood. These observations emphasize the importance of studying young objects to understand the function of a non-compromised lung.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-021-01898-3.

Enhanced microscopic dynamics in mucus gels under a mechanical load in the linear viscoelastic regime

Mucus is a biological gel covering the surface of several tissues and ensuring key biological functions, including as a protective barrier against dehydration, pathogen penetration, or gastric acids. Mucus biological functioning requires a finely tuned balance between solid-like and fluid-like mechanical response, ensured by reversible bonds between mucins, the glycoproteins that form the gel. In living organisms, mucus is subject to various kinds of mechanical stresses, e.g., due to osmosis, bacterial penetration, coughing, and gastric peristalsis. However, our knowledge of the effects of stress on mucus is still rudimentary and mostly limited to macroscopic rheological measurements, with no insight into the relevant microscopic mechanisms. Here, we run mechanical tests simultaneously to measurements of the microscopic dynamics of pig gastric mucus. Strikingly, we find that a modest shear stress, within the macroscopic rheological linear regime, dramatically enhances mucus reorganization at the microscopic level, as signaled by a transient acceleration of the microscopic dynamics, by up to 2 orders of magnitude. We rationalize these findings by proposing a simple, yet general, model for the dynamics of physical gels under strain and validate its assumptions through numerical simulations of spring networks. These results shed light on the rearrangement dynamics of mucus at the microscopic scale, with potential implications in phenomena ranging from mucus clearance to bacterial and drug penetration.

Abnormal Airway Mucus Secretion Induced by Virus Infection

The airway mucus barrier is a primary defensive layer at the airway surface. Mucins are the major structural components of airway mucus that protect the respiratory tract. Respiratory viruses invade human airways and often induce abnormal mucin overproduction and airway mucus secretion, leading to airway obstruction and disease. The mechanism underlying the virus-induced abnormal airway mucus secretion has not been fully studied so far. Understanding the mechanisms by which viruses induce airway mucus hypersecretion may open new avenues to treatment. In this article, we elaborate the clinical and experimental evidence that respiratory viruses cause abnormal airway mucus secretion, review the underlying mechanisms, and also discuss the current research advance as well as potential strategies to treat the abnormal airway mucus secretion caused by SARS-CoV-2.

Early Lung Disease Exhibits Bacterial-Dependent and -Independent Abnormalities in Cystic Fibrosis Pigs

RATIONALE

While it is clear that cystic fibrosis airway disease begins at a very young age, the early and subsequent steps in disease pathogenesis and the relative contribution of infection, mucus, and inflammation are not well understood.

OBJECTIVES

As one approach to assessing the early contribution of infection, we tested the hypothesis that early and continuous antibiotics would decrease the airway bacterial burden. We thought that, if it does, it might reveal aspects of the disease that are more or less sensitive to decreasing infection.

METHODS

Three groups of pigs were studied from birth until ~3 weeks of age: 1) wild-type, 2) cystic fibrosis, and 3) cystic fibrosis pigs treated continuously with broad-spectrum antibiotics from birth until study completion. Disease was assessed with chest computed tomography, histopathology, microbiology, and bronchoalveolar lavage.

MEASUREMENTS AND MAIN RESULTS

Disease was present by 3 weeks of age in cystic fibrosis pigs. Continuous antibiotics from birth improved chest computed tomography imaging abnormalities and airway mucus accumulation, but not airway inflammation in the cystic fibrosis pig model. However, reducing bacterial infection did not improve two disease features already present at birth in cystic fibrosis pigs, air trapping and submucosal gland duct plugging. In the cystic fibrosis sinuses, antibiotics did not prevent the development of infection, disease, or the number of bacteria but did alter the bacterial species.

CONCLUSIONS

These findings suggest that cystic fibrosis airway disease begins immediately following birth, and that early and continuous antibiotics impact some, but not all, aspects of CF lung disease development.

Mucus, Microbiomes and Pulmonary Disease

The respiratory tract harbors a stable and diverse microbial population within an extracellular mucus layer. Mucus provides a formidable defense against infection and maintaining healthy mucus is essential to normal pulmonary physiology, promoting immune tolerance and facilitating a healthy, commensal lung microbiome that can be altered in association with chronic respiratory disease. How one maintains a specialized (healthy) microbiome that resists significant fluctuation remains unknown, although smoking, diet, antimicrobial therapy, and infection have all been observed to influence microbial lung homeostasis. In this review, we outline the specific role of polymerizing mucin, a key functional component of the mucus layer that changes during pulmonary disease. We discuss strategies by which mucin feed and spatial orientation directly influence microbial behavior and highlight how a compromised mucus layer gives rise to inflammation and microbial dysbiosis. This emerging field of respiratory research provides fresh opportunities to examine mucus, and its function as predictors of infection risk or disease progression and severity across a range of chronic pulmonary disease states and consider new perspectives in the development of mucolytic treatments.

Proteases, Mucus, and Mucosal Immunity in Chronic Lung Disease

Dysregulated protease activity has long been implicated in the pathogenesis of chronic lung diseases and especially in conditions that display mucus obstruction, such as chronic obstructive pulmonary disease, cystic fibrosis, and non-cystic fibrosis bronchiectasis. However, our appreciation of the roles of proteases in various aspects of such diseases continues to grow. Patients with muco-obstructive lung disease experience progressive spirals of inflammation, mucostasis, airway infection and lung function decline. Some therapies exist for the treatment of these symptoms, but they are unable to halt disease progression and patients may benefit from novel adjunct therapies. In this review, we highlight how proteases act as multifunctional enzymes that are vital for normal airway homeostasis but, when their activity becomes immoderate, also directly contribute to airway dysfunction, and impair the processes that could resolve disease. We focus on how proteases regulate the state of mucus at the airway surface, impair mucociliary clearance and ultimately, promote mucostasis. We discuss how, in parallel, proteases are able to promote an inflammatory environment in the airways by mediating proinflammatory signalling, compromising host defence mechanisms and perpetuating their own proteolytic activity causing structural lung damage. Finally, we discuss some possible reasons for the clinical inefficacy of protease inhibitors to date and propose that, especially in a combination therapy approach, proteases represent attractive therapeutic targets for muco-obstructive lung diseases.

Role for Mucin-5AC in Upper and Lower Airway Pathogenesis in Mice

Mucin-5AC (MUC5AC) is a major secreted mucin in pathogenic airways. To determine its role in mucus-related airway disorders, Muc5ac-deficient (Muc5ac^-/-) and wild-type (Muc5ac^+/+) mice were compared in bleomycin-induced pulmonary fibrosis, respiratory syncytial virus (RSV) disease, and ozone toxicity. Significantly greater inflammation and fibrosis by bleomycin were developed in Muc5ac^-/- lungs compared to Muc5ac^+/+ lungs. More severe mucous cell metaplasia in fibrotic Muc5ac^-/- lungs coincided with bronchial Muc2, Muc4, and Muc5b overexpression. Airway RSV replication was higher in Muc5ac^-/- than in Muc5ac^+/+ during early infection. RSV-caused pulmonary epithelial death, bronchial smooth muscle thickening, and syncytia formation were more severe in Muc5ac^-/- compared to Muc5ac^+/+. Nasal septal damage and subepithelial mucoserous gland enrichment by RSV were greater in Muc5ac^-/- than in Muc5ac^+/+. Ozone exposure developed more severe nasal airway injury accompanying submucosal gland hyperplasia and pulmonary proliferation in Muc5ac^-/- than in Muc5ac^+/+. Ozone caused periodic acid-Schiff-positive secretion only in Muc5ac^-/- nasal airways. Lung E-cadherin level was relatively lower in Muc5ac^-/- than in Muc5ac^+/+ basally and after bleomycin, RSV, and ozone exposure. Results indicate that MUC5AC is an essential mucosal component in acute phase airway injury protection. Subepithelial gland hyperplasia and adaptive increase of other epithelial mucins may compensate airway defense in Muc5ac^-/- mice.