Differential effects of claudin-3 and claudin-4 on alveolar epithelial barrier function

Disruption of pulmonary microvascular endothelial barrier by dysregulated claudin-8 and claudin-4: uncovered mechanisms in porcine reproductive and respiratory syndrome virus infection

The pulmonary endothelium is a dynamic and metabolically active monolayer of endothelial cells. Dysfunction of the pulmonary endothelial barrier plays a crucial role in the acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), frequently observed in the context of viral pneumonia. Dysregulation of tight junction proteins can lead to the disruption of the endothelial barrier and subsequent leakage. Here, the highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) served as an ideal model for studying ALI and ARDS. The alveolar lavage fluid of pigs infected with HP-PRRSV, and the supernatant of HP-PRRSV infected pulmonary alveolar macrophages were respectively collected to treat the pulmonary microvascular endothelial cells (PMVECs) in Transwell culture system to explore the mechanism of pulmonary microvascular endothelial barrier leakage caused by viral infection. Cytokine screening, addition and blocking experiments revealed that proinflammatory cytokines IL-1β and TNF-α, secreted by HP-PRRSV-infected macrophages, disrupt the pulmonary microvascular endothelial barrier by downregulating claudin-8 and upregulating claudin-4 synergistically. Additionally, three transcription factors interleukin enhancer binding factor 2 (ILF2), general transcription factor III C subunit 2 (GTF3C2), and thyroid hormone receptor-associated protein 3 (THRAP3), were identified to accumulate in the nucleus of PMVECs, regulating the transcription of claudin-8 and claudin-4. Meanwhile, the upregulation of ssc-miR-185 was found to suppress claudin-8 expression via post-transcriptional inhibition. This study not only reveals the molecular mechanisms by which HP-PRRSV infection causes endothelial barrier leakage in acute lung injury, but also provides novel insights into the function and regulation of tight junctions in vascular homeostasis.

The role of ZEB1 in regulating tight junctions in antrochoanal polyp

Background

Antrochoanal polyp (ACP) is a benign nasal mass of unknown etiology. Tight junctions (TJs) are essential to the epithelial barrier that protects the body from external damage. However, the phenotype of tight junction in ACP is currently unclear.

Methods

The samples were collected from 20 controls, 37 patients with ACP and 45 patients with chronic rhinosinusitis with nasal polyp (CRSwNP). Quantitative Real-Time PCR (qRT-PCR) and immunofluorescence staining (IF) were performed to analyze the expressions of TJs markers (ZO-1, claudin-3 and occludin) and ZEB1. hNEpCs were transfected with ZEB1 small interfering RNA (si-ZEB1) or ZEB1 over-expression plasmid (OE-ZEB1). qRT-PCR and Western blotting were used to determine the levels of TJs-related markers. Primary human nasal epithelial cells (hNECs) were stimulated with IL-17A and si-ZEB1, and the expression of epithelial barrier markers were measured by qRT-PCR and Western blotting.

Results

Compared to the control group, ACP group showed a significant downregulation in both mRNA and protein levels of ZO-1, occludin, and claudin-3. Furthermore, disease severity correlates positively with the degree of disruption of tight junctions. In addition, higher expression levels of ZEB1, IL-17A, and IFN-γ were observed in the ACP group compared to controls. Overexpression of ZEB1 in hNEpCs led to impairments in the levels of ZO-1, occludin, and claudin-3, while silencing of ZEB1 expression was found to enhance the barrier function of epithelial cells. Finally, IL-17 stimulation of hNECs impaired the expression of TJs-associated molecules (ZO-1, occludin, and claudin-3), which was effectively reversed by the IL-17A + si-ZEB1 group.

Conclusions

The tight junctions in ACP were extremely damaged and were correlated with the severity of the disease. ZEB1 was involved in the pathogenesis of ACP mediated by IL-17A through regulating tight junctions.

Claudin Barriers on the Brink: How Conflicting Tissue and Cellular Priorities Drive IBD Pathogenesis

Inflammatory bowel diseases (IBDs) are characterized by acute or chronic recurring inflammation of the intestinal mucosa, often with increasing severity over time. Life-long morbidities and diminishing quality of life for IBD patients compel a search for a better understanding of the molecular contributors to disease progression. One unifying feature of IBDs is the failure of the gut to form an effective barrier, a core role for intercellular complexes called tight junctions. In this review, the claudin family of tight junction proteins are discussed as they are a fundamental component of intestinal barriers. Importantly, claudin expression and/or protein localization is altered in IBD, leading to the supposition that intestinal barrier dysfunction exacerbates immune hyperactivity and disease. Claudins are a large family of transmembrane structural proteins that constrain the passage of ions, water, or substances between cells. However, growing evidence suggests non-canonical claudin functions during mucosal homeostasis and healing after injury. Therefore, whether claudins participate in adaptive or pathological IBD responses remains an open question. By reviewing current studies, the possibility is assessed that with claudins, a jack-of-all-trades is master of none. Potentially, a robust claudin barrier and wound restitution involve conflicting biophysical phenomena, exposing barrier vulnerabilities and a tissue-wide frailty during healing in IBD.

A Monoclonal Human Alveolar Epithelial Cell Line ("Arlo") with Pronounced Barrier Function for Studying Drug Permeability and Viral Infections

In the development of orally inhaled drug products preclinical animal models regularly fail to predict pharmacological as well as toxicological responses in humans. Models based on human cells and tissues are potential alternatives to animal experimentation allowing for the isolation of essential processes of human biology and making them accessible in vitro. Here, the generation of a novel monoclonal cell line "Arlo," derived from the polyclonal human alveolar epithelium lentivirus immortalized cell line hAELVi via single-cell printing, and its characterization as a model for the human alveolar epithelium as well as a building block for future complex in vitro models is described. "Arlo" is systematically compared in vitro to primary human alveolar epithelial cells (hAEpCs) as well as to the polyclonal hAELVi cell line. "Arlo" cells show enhanced barrier properties with high transepithelial electrical resistance (TEER) of ≈3000 Ω cm2 and a potential difference (PD) of ≈30 mV under air-liquid interface (ALI) conditions, that can be modulated. The cells grow in a polarized monolayer and express genes relevant to barrier integrity as well as homeostasis as is observed in hAEpCs. Successful productive infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a proof-of-principle study offers an additional, attractive application of "Arlo" beyond biopharmaceutical experimentation.

Acute arsenic exposure exacerbates lipopolysaccharide-induced lung injury possibly by compromising the integrity of the lung epithelial barrier in rats

Inhalation of large amounts of arsenic can damage the respiratory tract and may exacerbate the development of bacterial pneumonia, but the exact mechanism remains unclear. In this study, male Wistar rats were randomly divided into control, arsenic trioxide (16.0 μg/kg ATO), lipopolysaccharide (0.5 mg/kg LPS), and ATO combined with LPS (16.0 μg/kg ATO + 0.5 mg/kg LPS) groups. Blood and lung tissue samples were collected from each group 12 h after exposure. The results showed that exposure to ATO or LPS alone produced different effects on leukocytes and inflammatory factors, while combined exposure significantly increased serum interleukin-6, interleukin-10, lung water content, lung lavage fluid protein, and p38 protein phosphorylation levels. Alveolar interstitial thickening, alveolar membrane edema, alveolar type I and II cell matrix vacuolization, and nuclear pyknosis were observed in rats exposed to either ATO or LPS. More severe ultrastructural changes were found in the combined exposure group, and chromatin splitting was observed in alveolar type I cells. Lanthanum nitrate particles leaked from the alveolar vascular lumen in the ATO-exposed group, whereas in the combined exposure group, Evans Blue levels were increased and lanthanum nitrate particles were present in the lung parenchyma. Claudin-3 protein expression increased and claudin-4 expression decreased after ATO or LPS exposure, while claudin-18 expression was unchanged. The changes in claudin-3 and claudin-4 protein expression were further exacerbated by combined exposure. In conclusion, these results suggest that inhalation of ATO may exacerbate the development of bacterial pneumonia and that common mechanisms may exist to synergistically disrupt epithelial barrier integrity.

The role and mechanism of claudins in cancer

Claudins are a tetraspan membrane protein multigene family that plays a structural and functional role in constructing tight junctions. Claudins perform crucial roles in maintaining cell polarity in epithelial and endothelial cell sheets and controlling paracellular permeability. In the last two decades, increasing evidence indicates that claudin proteins play a major role in controlling paracellular permeability and signaling inside cells. Several types of claudins are dysregulated in various cancers. Depending on where the tumor originated, claudin overexpression or underexpression has been shown to regulate cell proliferation, cell growth, metabolism, metastasis and cell stemness. Epithelial-to-mesenchymal transition is one of the most important functions of claudin proteins in disease progression. However, the exact molecular mechanisms and signaling pathways that explain why claudin proteins are so important to tumorigenesis and progression have not been determined. In addition, claudins are currently being investigated as possible diagnostic and treatment targets. Here, we discuss how claudin-related signaling pathways affect tumorigenesis, tumor progression, and treatment sensitivity.

Microbiota Transplantation in Day-Old Broiler Chickens Ameliorates Necrotic Enteritis via Modulation of the Intestinal Microbiota and Host Immune Responses

A microbiota transplant (MT) originating from mature adult chicken ceca and propagated in bioreactors was administered to day-old broiler chicks to ascertain the degree to which, and how, the MT affects Clostridium perfringens (Cp)-incited necrotic enteritis (NE). Using a stress predisposition model of NE, birds administered the MT and challenged with Cp showed fewer necrotic lesions, and exhibited a substantially higher α- and β-diversity of bacteria in their jejunum and ceca. Birds challenged with Cp and not administered the MT showed decreased Lactobacillus and increased Clostridium sensu strico 1 in the jejunum. In ceca, Megamonas, a genus containing butyrate-producing bacteria, was only present in birds administered the MT, and densities of this genus were increased in birds challenged with Cp. Metabolite profiles in cecal digesta were altered in birds administered the MT and challenged with the pathogen; 59 metabolites were differentially abundant following MT treatment, and the relative levels of short chain fatty acids, butyrate, valerate, and propionate, were decreased in birds with NE. Birds administered the MT and challenged with Cp showed evidence of enhanced restoration of intestinal barrier functions, including elevated mRNA of MUC2B, MUC13, and TJP1. Likewise, birds administered the MT exhibited higher mRNA of IL2, IL17A, and IL22 at 2-days post-inoculation with Cp, indicating that these birds were better immunologically equipped to respond to pathogen challenge. Collectively, study findings demonstrated that administering a MT containing a diverse mixture of microorganisms to day-old birds ameliorated NE in broilers by increasing bacterial diversity and promoting positive immune responses.

Impact of endometrial claudin-3 deletion on murine implantation, decidualization and embryo development

The composition of cell contacts in the endometrium plays an important role in the process of embryo implantation and the establishment of pregnancy. In previous studies, we showed an induction of the tight junction protein claudin-3 in the developing decidua from 6.5 dpc onwards. To evaluate the role if this specific claudin-3 distribution, we here evaluated the effect of an endometrial claudin-3 deletion in implantation and embryo development in claudin-3 knockout mice. Claudin-3 KO mice were fertile but revealed a slightly reduced amount of implantation sites as well as of litter size. Though implantation sites showed morphologically regularly developed embryos and deciduas, depth of ectoplacental cone invasion was reduced in tendency compared to controls. The weight of the implantation sites on 6.5 and 8.5 dpc as well as the weight of the embryos on 17.5 dpc, but not of the placentas, was significantly reduced in claudin-3 KO mice due to a maternal effect. This could be due to an impairment of decidualization as substantiated by a downregulation of the transcription of various decidua-associated genes in the early implantation sites of claudin-3 KO mice. The fact that claudin-3 KO mice are nevertheless fertile possibly may be compensated by the presence of other claudins like claudin-4 and claudin-10.

Probiotics, prebiotics, and synbiotics to prevent or combat air pollution consequences: The gut-lung axis

Air pollution exposure is a public health emergency, which attributes globally to an estimated seven million deaths on a yearly basis We are all exposed to air pollutants, varying from ambient air pollution hanging over cities to dust inside the home. It is a mixture of airborne particulate matter and gases that can be subdivided into three categories based on particle diameter. The smallest category called PM_0.1 is the most abundant. A fraction of the particles included in this category might enter the blood stream spreading to other parts of the body. As air pollutants can enter the body via the lungs and gut, growing evidence links its exposure to gastrointestinal and respiratory impairments and diseases, like asthma, rhinitis, respiratory tract infections, Crohn's disease, ulcerative colitis, and abdominal pain. It has become evident that there exists a crosstalk between the respiratory and gastrointestinal tracts, commonly referred to as the gut-lung axis. Via microbial secretions, metabolites, immune mediators and lipid profiles, these two separate organ systems can influence each other. Well-known immunomodulators and gut health stimulators are probiotics, prebiotics, together called synbiotics. They might combat air pollution-induced systemic inflammation and oxidative stress by optimizing the microbiota composition and microbial metabolites, thereby stimulating anti-inflammatory pathways and strengthening mucosal and epithelial barriers. Although clinical studies investigating the role of probiotics, prebiotics, and synbiotics in an air pollution setting are lacking, these interventions show promising health promoting effects by affecting the gastrointestinal- and respiratory tract. This review summarizes the current data on how air pollution can affect the gut-lung axis and might impact gut and lung health. It will further elaborate on the potential role of probiotics, prebiotics and synbiotics on the gut-lung axis, and gut and lung health.

Nrf2 Activation Protects Against Organic Dust and Hydrogen Sulfide Exposure Induced Epithelial Barrier Loss and K. pneumoniae Invasion

Agriculture workers report various respiratory symptoms owing to occupational exposure to organic dust (OD) and various gases. Previously, we demonstrated that pre-exposure to hydrogen sulfide (H₂S) alters the host response to OD and induces oxidative stress. Nrf2 is a master-regulator of host antioxidant response and exposures to toxicants is known to reduce Nrf2 activity. The OD exposure-induced lung inflammation is known to increase susceptibility to a secondary microbial infection. We tested the hypothesis that repeated exposure to OD or H₂S leads to loss of Nrf2, loss of epithelial cell integrity and that activation of Nrf2 rescues this epithelial barrier dysfunction. Primary normal human bronchial epithelial (NHBE) cells or mouse precision cut-lung slices (PCLS) were treated with media, swine confinement facility organic dust extract (ODE) or H₂S or ODE+H₂S for one or five days. Cells were also pretreated with vehicle control (DMSO) or RTA-408, a Nrf2 activator. Acute exposure to H₂S and ODE+H₂S altered the cell morphology, decreased the viability as per the MTT assay, and reduced the Nrf2 expression as well as increased the keap1 levels in NHBE cells. Repeated exposure to ODE or H₂S or ODE+H₂S induced oxidative stress and cytokine production, decreased tight junction protein occludin and cytoskeletal protein ezrin expression, disrupted epithelial integrity and resulted in increased Klebsiella pneumoniae invasion. RTA-408 (pharmacological activator of Nrf2) activated Nrf2 by decreasing keap1 levels and reduced ODE+H₂S-induced changes including reversing loss of barrier integrity, inflammatory cytokine production and microbial invasion in PCLS but not in NHBE cell model. We conclude that Nrf2 activation has a partial protective function against ODE and H₂S.

Exercise in hypobaric hypoxia increases markers of intestinal injury and symptoms of gastrointestinal distress

NEW FINDING

What is the central question of this study? What is the effect of hypobaric hypoxia on markers of exercise-induced intestinal injury and symptoms of GI distress? What is the main finding and its importance? Exercise performed at 4300 m of simulated altitude increased I-FABP, CLDN-3, and LBP which together suggest that exercise-induced intestinal injury may be aggravated by concurrent hypoxic exposure. Increases in I-FABP, LBP, CLDN-3 were correlated to exercise-induced GI symptoms, providing some evidence of a link between intestinal barrier injury and symptoms of GI distress.

ABSTRACT

We sought to determine the effect of exercise in hypobaric hypoxia on markers of intestinal injury and gastrointestinal (GI) symptoms. Using a randomized and counterbalanced design, 9 males completed two experimental trials: one at local altitude of 1585 m (NORM) and one at 4300 m of simulated hypobaric hypoxia (HYP). Participants performed 60-minutes of cycling at a workload that elicited 65% of their NORM VO₂ max. GI symptoms were assessed before and every 15-minutes during exercise. Pre- and post-exercise blood samples were assessed for intestinal fatty acid binding protein (I-FABP), claudin-3 (CLDN-3), and lipopolysaccharide binding protein (LBP). All participants reported at least one GI symptom in HYP compared to just 1 participant in NORM. I-FABP significantly increased from pre- to post-exercise in HYP (708±191 to 1215±518 pg mL^-1 ; p = 0.011, d = 1.10) but not NORM (759±224 to 828±288 pg mL^-1 ; p>0.99, d = 0.27). CLDN-3 significantly increased from pre- to post-exercise in HYP (13.8±0.9 to 15.3±1.2 ng mL^-1 ; p = 0.003, d = 1.19) but not NORM (13.7±1.8 to 14.2±1.6 ng mL^-1 ; p = .435, d = 0.45). LBP significantly increased from pre- to post-exercise in HYP (10.8±1.2 to 13.9±2.8 μg mL^-1 ; p = 0.006, d = 1.12) but not NORM (11.3±1.1 to 11.7±0.9 μg mL^-1 ; p>0.99, d = 0.32). I-FABP (d = 0.85), CLDN-3 (d = 0.95), and LBP (d = 0.69) were all significantly higher post-exercise in HYP compared to NORM (p≤0.05). Overall GI discomfort was significantly correlated to ΔI-FABP (r = 0.71), ΔCLDN-3 (r = 0.70), and ΔLBP (r = 0.86). These data indicate that cycling exercise performed in hypobaric hypoxia can cause intestinal injury, which might cause some commonly reported GI symptoms. This article is protected by copyright. All rights reserved.

Gene Therapy for Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a devastating clinical syndrome that leads to acute respiratory failure and accounts for over 70,000 deaths per year in the United States alone, even prior to the COVID-19 pandemic. While its molecular details have been teased apart and its pathophysiology largely established over the past 30 years, relatively few pharmacological advances in treatment have been made based on this knowledge. Indeed, mortality remains very close to what it was 30 years ago. As an alternative to traditional pharmacological approaches, gene therapy offers a highly controlled and targeted strategy to treat the disease at the molecular level. Although there is no single gene or combination of genes responsible for ARDS, there are a number of genes that can be targeted for upregulation or downregulation that could alleviate many of the symptoms and address the underlying mechanisms of this syndrome. This review will focus on the pathophysiology of ARDS and how gene therapy has been used for prevention and treatment. Strategies for gene delivery to the lung, such as barriers encountered during gene transfer, specific classes of genes that have been targeted, and the outcomes of these approaches on ARDS pathogenesis and resolution will be discussed.

Induction of the NEK family of kinases in the lungs of mice subjected to cecal ligation and puncture model of sepsis

Endothelial barrier dysfunction (EBD) is the hallmark of Acute Respiratory Distress Syndrome (ARDS), a potentially lethal respiratory disorder associated with the COVID-19 - related deaths. Herein, we employed a cecal ligation and puncture (CLP) murine model of sepsis, to evaluate the effects of sepsis-induced EBD in the expression of the never in mitosis A (NIMA)-related kinases (NEKs). Members of that family of kinases regulate the activity and expression of the tumor suppressor P53, previously shown to modulate the actin cytoskeleton remodeling. Our results introduce the induction of NEK2, NEK3, NEK4, NEK7, and NEK9 in a CLP model of sepsis. Hence, we suggest that NEKs are involved in inflammatory processes and are holding the potential to serve as novel therapeutic targets for pathologies related to EBD, including ARDS and sepsis. Further studies will delineate the underlying molecular events and their interrelations with P53.

Increased In Vitro Intercellular Barrier Function of Lung Epithelial Cells Using Adipose-Derived Mesenchymal Stem/Stromal Cells

With the emergence of coronavirus disease-2019, researchers have gained interest in the therapeutic efficacy of mesenchymal stem/stromal cells (MSCs) in acute respiratory distress syndrome; however, the mechanisms of the therapeutic effects of MSCs are unclear. We have previously reported that adipose-derived MSCs (AD-MSCs) strengthen the barrier function of the pulmonary vessels in scaffold-based bioengineered rat lungs. In this study, we evaluated whether AD-MSCs could enhance the intercellular barrier function of lung epithelial cells in vitro using a transwell coculture system. Transepithelial electrical resistance (TEER) measurements revealed that the peak TEER value was significantly higher in the AD-MSC coculture group than in the AD-MSC non-coculture group. Similarly, the permeability coefficient was significantly decreased in the AD-MSC coculture group compared to that in the AD-MSC non-coculture group. Immunostaining of insert membranes showed that zonula occuldens-1 expression was significantly high at cell junctions in the AD-MSC coculture group. Moreover, cell junction-related gene profiling showed that the expression of some claudin genes, including claudin-4, was upregulated in the AD-MSC coculture group. Taken together, these results showed that AD-MSCs enhanced the barrier function between lung epithelial cells, suggesting that both direct adhesion and indirect paracrine effects strengthened the barrier function of lung alveolar epithelium in vitro.

Effects of ozone and particulate matter on airway epithelial barrier structure and function: a review of in vitro and in vivo studies

The airway epithelium represents a crucial line of defense against the spread of inhaled pathogens. As the epithelium is the first part of the body to be exposed to the inhaled environment, it must act as both a barrier to and sentinel against any inhaled agents. Despite its vital role in limiting the spread of inhaled pathogens, the airway epithelium is also regularly exposed to air pollutants which disrupt its normal function. Here we review the current understanding of the structure and composition of the airway epithelial barrier, as well as the impact of inhaled pollutants, including the reactive gas ozone and particulate matter, on epithelial function. We discuss the current in vitro, rodent model, and human exposure findings surrounding the impact of various inhaled pollutants on epithelial barrier function, mucus production, and mucociliary clearance. Detailed information on how inhaled pollutants impact epithelial structure and function will further our understanding of the adverse health effects of air pollution exposure.

Characteristics of Passive Solute Transport across Primary Rat Alveolar Epithelial Cell Monolayers

Primary rat alveolar epithelial cell monolayers (RAECM) were grown without (type I cell-like phenotype, RAECM-I) or with (type II cell-like phenotype, RAECM-II) keratinocyte growth factor to assess passive transport of 11 hydrophilic solutes. We estimated apparent permeability (Papp) in the absence/presence of calcium chelator EGTA to determine the effects of perturbing tight junctions on "equivalent" pores. Papp across RAECM-I and -II in the absence of EGTA are similar and decrease as solute size increases. We modeled Papp of the hydrophilic solutes across RAECM-I/-II as taking place via heterogeneous populations of equivalent pores comprised of small (0.41/0.32 nm radius) and large (9.88/11.56 nm radius) pores, respectively. Total equivalent pore area is dominated by small equivalent pores (99.92-99.97%). The number of small and large equivalent pores in RAECM-I was 8.55 and 1.29 times greater, respectively, than those in RAECM-II. With EGTA, the large pore radius in RAECM-I/-II increased by 1.58/4.34 times and the small equivalent pore radius increased by 1.84/1.90 times, respectively. These results indicate that passive diffusion of hydrophilic solutes across an alveolar epithelium occurs via small and large equivalent pores, reflecting interactions of transmembrane proteins expressed in intercellular tight junctions of alveolar epithelial cells.

Differential effects of intense exercise and pollution on the airways in a murine model

Background

Exercise-induced bronchoconstriction (EIB) is a transient airway narrowing, occurring during or shortly after intensive exercise. It is highly prevalent in non-asthmatic outdoor endurance athletes suggesting an important contribution of air pollution in the development of EIB. Therefore, more research is necessary to investigate the combination of exercise and pollutants on the airways.

Methods

Balbc/ByJ mice were intranasally challenged 5 days a week for 3 weeks with saline or 0.2 mg/ml diesel exhaust particles (DEP), prior to a daily incremental running session or non-exercise session. Once a week, the early ventilatory response was measured and lung function was determined at day 24. Airway inflammation and cytokine levels were evaluated in bronchoalveolar lavage fluid. Furthermore, innate lymphoid cells, dendritic cells and tight junction mRNA expression were determined in lung tissue.

Results

Submaximal exercise resulted in acute alterations of the breathing pattern and significantly improved FEV_0.1 at day 24. DEP exposure induced neutrophilic airway inflammation, accompanied with increased percentages of CD11b⁺ DC in lung tissue and pro-inflammatory cytokines, such as IL-13, MCP-1, GM-CSF and KC. Occludin and claudin-1(Cldn-1) expression were respectively increased and decreased by DEP exposure. Whereas, exercise increased Cldn-3 and Cldn-18 expression. Combining exercise and DEP exposure resulted in significantly increased SP-D levels in the airways.

Conclusion

DEP exposure induced typical airway neutrophilia, DC recruitment and pro-inflammatory cytokine production. Whereas, intensive exercise induced changes of the breathing pattern. The combination of both triggers resulted in a dysregulation of tight junction expression, suggesting that intensive exercise in polluted environments can induce important changes in the airway physiology and integrity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-021-00401-6.

Nanotopography Enhances Dynamic Remodeling of Tight Junction Proteins through Cytosolic Liquid Complexes

Nanotopographic materials provide special biophysical stimuli that can regulate epithelial tight junctions and their barrier function. Through the use of total internal reflection fluorescence microscopy of live cells, we demonstrated that contact of synthetic surfaces with defined nanotopography at the apical surface of epithelial monolayers increased paracellular permeability of macromolecules. To monitor changes in tight junction morphology in live cells, we fluorescently tagged the scaffold protein zonula occludens-1 (ZO-1) through CRISPR/Cas9-based gene editing to enable live cell tracking of ZO-1 expressed at physiologic levels. Contact between cells and nanostructured surfaces destabilized junction-associated ZO-1 and promoted its arrangement into highly dynamic liquid cytosolic complexes with a 1-5 μm diameter. Junction-associated ZO-1 rapidly remodeled, and we observed the direct transformation of cytosolic complexes into junction-like structures. Claudin-family tight junction transmembrane proteins and F-actin also were associated with these ZO-1 containing cytosolic complexes. These data suggest that these cytosolic structures are important intermediates formed in response to nanotopographic cues that facilitate rapid tight junction remodeling in order to regulate paracellular permeability.

Nanotopography Enhances Dynamic Remodeling of Tight Junction Proteins through Cytosolic Liquid Complexes

Nanotopographic materials provide special biophysical stimuli that can regulate epithelial tight junctions and their barrier function. Through the use of total internal reflection fluorescence microscopy of live cells, we demonstrated that contact of synthetic surfaces with defined nanotopography at the apical surface of epithelial monolayers increased paracellular permeability of macromolecules. To monitor changes in tight junction morphology in live cells, we fluorescently tagged the scaffold protein zonula occludens-1 (ZO-1) through CRISPR/Cas9-based gene editing to enable live cell tracking of ZO-1 expressed at physiologic levels. Contact between cells and nanostructured surfaces destabilized junction-associated ZO-1 and promoted its arrangement into highly dynamic liquid cytosolic complexes with a 1-5 μm diameter. Junction-associated ZO-1 rapidly remodeled, and we observed the direct transformation of cytosolic complexes into junction-like structures. Claudin-family tight junction transmembrane proteins and F-actin also were associated with these ZO-1 containing cytosolic complexes. These data suggest that these cytosolic structures are important intermediates formed in response to nanotopographic cues that facilitate rapid tight junction remodeling in order to regulate paracellular permeability.

Interleukin-13 Alters Tight Junction Proteins Expression Thereby Compromising Barrier Function and Dampens Rhinovirus Induced Immune Responses in Nasal Epithelium

Tight junctions (TJs) are intercellular structures which are essential for epithelial barrier function and play an important role in antimicrobial defense. Epithelium dysfunction and type-2-skewed inflammation are two main pathological phenomena of chronic rhinosinusitis with nasal polyps (CRSwNP). However, the effect of pro-inflammatory type-2 cytokine IL-13 on TJs in CRSwNP is poorly understood. Nasal biopsies of CRSwNP patients and in vitro IL-13-matured human nasal epithelial cells (hNECs) were used to analyze epithelial markers and TJ proteins. Epithelium permeability, transepithelial electrical resistance (TEER), expression of TJs were quantified for IL-13-matured hNECs and that with RV infection. The expression of occludin, claudin-3, and ZO-1 were significantly decreased in CRSwNP biopsies and in hNECs after IL-13 treatment. IL-13 treatment increased epithelium permeability, decreased TEER and altered hNECs composition resulting in lesser ciliated cells and mucus over-secretion. Interestingly, claudin-3 is selectively expressed on ciliated cells. While RV infection induced minimal changes to TJs, the IL-13-matured hNECs has reduced capacity for upregulation of IFN-λ1 and CXCL10 but further increased the expression of TSLP upon RV infection. These findings suggested that IL-13-mediated dysfunction of TJs and compromised epithelial barrier. IL-13-induced cilia loss conferred lowered viral replication and impaired antiviral responses of nasal epithelium against RV infection.

An Improved Vascularized, Dual-Channel Microphysiological System Facilitates Modeling of Proximal Tubular Solute Secretion

A vascularized human proximal tubule model in a dual-channel microphysiological system (VPT-MPS) was developed, representing an advance over previous, single-cell-type kidney microphysiological systems. Human proximal tubule epithelial cells (PTECs) and human umbilical vein endothelial cells (HUVECs) were cocultured in side-by-side channels. Over 24 h of coculturing, PTECs maintained polarized expression of Na+/K+ ATPase, tight junctions (ZO-1), and OAT1. HUVECs showed the absence of ZO-1 but expressed endothelial cell marker (CD-31). In time-lapse imaging studies, fluorescein isothiocyanate (FITC)-dextran passed freely from the HUVEC vessel into the supporting extracellular matrix, confirming the leakiness of the endothelium (at 80 min, matrix/intravessel fluorescence ratio = 0.2). Dextran-associated fluorescence accumulated in the matrix adjacent to the basolateral aspect of the PTEC tubule with minimal passage of the compound into the tubule lumen observed (at 80 min, tubule lumen/matrix fluorescence ratio = 0.01). This demonstrates that the proximal tubule compartment is the rate-limiting step in the secretion of compounds in VPT-MPS. In kinetic studies with radiolabeled markers, p-aminohippuric acid (PAH) exhibited greater output into the tubule lumen than did paracellular markers mannitol and FITC-dextran (tubule outflow/vessel outflow concentration ratio of 7.7% vs 0.5 and 0.4%, respectively). A trend toward reduced PAH secretion by 45% was observed upon coadministration of probenecid. This signifies functional expression of renal transporters in PTECs that normally mediate the renal secretion of PAH. The VPT-MPS holds the promise of providing an in vitro platform for evaluating the renal secretion of new drug candidates and investigating the dysregulation of tubular drug secretion in chronic kidney disease.

Host responses to Clostridium perfringens challenge in a chicken model of chronic stress

Background

This study utilized a chicken model of chronic physiological stress mediated by corticosterone (CORT) administration to ascertain how various host metrics are altered upon challenge with Clostridium perfringens. Necrotic enteritis (NE) is a disease of the small intestine of chickens incited by C. perfringens, which can result in elevated morbidity and mortality. The objective of the current study was to investigate how physiological stress alters host responses and predisposes birds to subclinical NE.

Results

Birds administered CORT exhibited higher densities of C. perfringens in their intestine, and this corresponded to altered production of intestinal mucus. Characterization of mucus showed that C. perfringens treatment altered the relative abundance of five glycans. Birds inoculated with C. perfringens did not exhibit evidence of acute morbidity. However, histopathologic changes were observed in the small intestine of infected birds. Birds administered CORT showed altered gene expression of tight junction proteins (i.e. CLDN3 and CLDN5) and toll-like receptors (i.e. TLR2 and TLR15) in the small intestine. Moreover, birds administered CORT exhibited increased expression of IL2 and G-CSF in the spleen, and IL1β, IL2, IL18, IFNγ, and IL6 in the thymus. Body weight gain was impaired only in birds that were administered CORT and challenged with C. perfringens.

Conclusion

CORT administration modulated a number of host functions, which corresponded to increased densities of C. perfringens in the small intestine and weight gain impairment in chickens. Importantly, results implicate physiological stress as an important predisposing factor to NE, which emphasizes the importance of managing stress to optimize chicken health.

Tight junctions in the development of asthma, chronic rhinosinusitis, atopic dermatitis, eosinophilic esophagitis, and inflammatory bowel diseases

This review focuses on recent developments related to asthma, chronic rhinosinusitis, atopic dermatitis (AD), eosinophilic esophagitis, and inflammatory bowel diseases (IBD), with a particular focus on tight junctions (TJs) and their role in the pathogenetic mechanisms of these diseases. Lung, skin, and intestinal surfaces are lined by epithelial cells that interact with environmental factors and immune cells. Therefore, together with the cellular immune system, the epithelium performs a pivotal role as the first line physical barrier against external antigens. Paracellular space is almost exclusively sealed by TJs and is maintained by complex protein-protein interactions. Thus, TJ dysfunction increases paracellular permeability, resulting in enhanced flux across TJs. Epithelial TJ dysfunction also causes immune cell activation and contributes to the pathogenesis of chronic lung, skin, and intestinal inflammation. Characterization of TJ protein alteration is one of the key factors for enhancing our understanding of allergic diseases as well as IBDs. Furthermore, TJ-based epithelial disturbance can promote immune cell behaviors, such as those in dendritic cells, Th2 cells, Th17 cells, and innate lymphoid cells (ILCs), thereby offering new insights into TJ-based targets. The purpose of this review is to illustrate how TJ dysfunction can lead to the disruption of the immune homeostasis in barrier tissues and subsequent inflammation. This review also highlights the various TJ barrier dysfunctions across different organ sites, which would help to develop future drugs to target allergic diseases and IBD.

The IL1β-HER2-CLDN18/CLDN4 axis mediates lung barrier damage in ARDS

Objective: The high mortality rate associated with acute respiratory distress syndrome (ARDS) is a major challenge for intensive care units. In the present study, we applied bioinformatics and animal models to identify core genes and potential corresponding pathways in ARDS.

Results: Using bioinformatics analysis, IL-1β was identified as the core gene of ARDS. Cell experiments showed that up-regulation of IL-1β downregulates claudin18 to promote lung barrier function damage by regulating the IL-1β-HER2/HER3 axis, further promoting the development of ARDS. This was validated in the animal models.

Conclusion: IL-1β promotes the development of ARDS by regulating the IL-1β-HER2/HER3 axis. These findings deepen the understanding of the pathological mechanisms of ARDS.

Methods: Transcription data sets related to ARDS were subjected to differential expression gene analysis, functional enrichment analysis, and receiver operating characteristic curve analysis and, so as to identify core genes in ARDS. Cell experiments were used to further explore the effects of core genes on lung barrier function damage. Animal models were applied to validate the effects of core gene in mediating biological signal pathways in ARDS.

MyD88 regulates a prolonged adaptation response to environmental dust exposure-induced lung disease

BACKGROUND

Environmental organic dust exposures enriched in Toll-like receptor (TLR) agonists can reduce allergic asthma development but are associated with occupational asthma and chronic bronchitis. The TLR adaptor protein myeloid differentiation factor88 (MyD88) is fundamental in regulating acute inflammatory responses to organic dust extract (ODE), yet its role in repetitive exposures is unknown and could inform future strategies.

METHODS

Wild-type (WT) and MyD88 knockout (KO) mice were exposed intranasally to ODE or saline daily for 3 weeks (repetitive exposure). Repetitively exposed animals were also subsequently rested with no treatments for 4 weeks followed by single rechallenge with saline/ODE.

RESULTS

Repetitive ODE exposure induced neutrophil influx and release of pro-inflammatory cytokines and chemokines were profoundly reduced in MyD88 KO mice. In comparison, ODE-induced cellular aggregates, B cells, mast cell infiltrates and serum IgE levels remained elevated in KO mice and mucous cell metaplasia was increased. Expression of ODE-induced tight junction protein(s) was also MyD88-dependent. Following recovery and then rechallenge with ODE, inflammatory mediators, but not neutrophil influx, was reduced in WT mice pretreated with ODE coincident with increased expression of IL-33 and IL-10, suggesting an adaptation response. Repetitively exposed MyD88 KO mice lacked inflammatory responsiveness upon ODE rechallenge.

CONCLUSIONS

MyD88 is essential in mediating the classic airway inflammatory response to repetitive ODE, but targeting MyD88 does not reduce mucous cell metaplasia, lymphocyte influx, or IgE responsiveness. TLR-enriched dust exposures induce a prolonged adaptation response that is largely MyD88-independent. These findings demonstrate the complex role of MyD88-dependent signaling during acute vs. chronic organic dust exposures.

Targeting the IL-22/IL-22BP axis enhances tight junctions and reduces inflammation during influenza infection

The seasonal burden of influenza coupled with the pandemic outbreaks of more pathogenic strains underscore a critical need to understand the pathophysiology of influenza injury in the lung. Interleukin-22 (IL-22) is a promising cytokine that is critical in protecting the lung during infection. This cytokine is strongly regulated by the soluble receptor IL-22-binding protein (IL-22BP), which is constitutively expressed in the lungs where it inhibits IL-22 activity. The IL-22/IL-22BP axis is thought to prevent chronic exposure of epithelial cells to IL-22. However, the importance of this axis is not understood during an infection such as influenza. Here we demonstrate through the use of IL-22BP-knockout mice (il-22ra2-/-) that a pro-IL-22 environment reduces pulmonary inflammation during H1N1 (PR8/34 H1N1) infection and protects the lung by promoting tight junction formation. We confirmed these results in normal human bronchial epithelial cells in vitro demonstrating improved membrane resistance and induction of the tight junction proteins Cldn4, Tjp1, and Tjp2. Importantly, we show that administering recombinant IL-22 in vivo reduces inflammation and fluid leak into the lung. Taken together, our results demonstrate the IL-22/IL-22BP axis is a potential targetable pathway for reducing influenza-induced pneumonia.

Engineering drug delivery systems to overcome mucosal barriers for immunotherapy and vaccination

Mucosal surfaces protect our bodies from pathogens and external irritants using a system of biological barriers. Overcoming these barriers is a significant drug delivery challenge, particularly for immunotherapies that aim to modulate the local immune response. Reaching local lymphoid tissues and draining lymph nodes (LNs) requires crossing the mucus mesh, mucosal epithelium, and either targeting M cells covering lymphoid tissues or utilizing lymphatic transport that shuttles molecules and particulates from the periphery to the LN. We first highlight the barrier properties of mucus and mucosal epithelium, and the function of the mucosal immune system. We then dive into existing drug delivery technologies that have been engineered to overcome each of these barriers. We particularly focus on novel strategies for targeting lymphoid tissues, which has been shown to enhance immunotherapies and vaccinations, via directly targeting LNs, lymphatic vessels, and M cells that transport samples of mucosal content to the lymphoid tissues.

Tandem mass tag (TMT) proteomic analysis of fetal lungs revealed differential expression of tight junction proteins in a rat model of congenital diaphragmatic hernia

OBJECTIVE

Congenital diaphragmatic hernia (CDH) is a common and often lethal birth defect characterized by congenital lung malformation, which severely affects neonate prognosis and mortality. This study aimed to investigate differences in protein expression in order to elucidate the mechanism of CDH-associated pulmonary hypoplasia during the early stage of lung development using tandem mass tag (TMT) quantitative proteomics.

METHODS

Nitrofen was administered orally to establish a rat CDH model, and pathological changes were evaluated through hematoxylin-eosin (H&E), PCNA, and Ki67 staining at the pseudoglandular stage. Fetal lungs were then collected, pooled before TMT labeling, and subjected to mass spectrometry. Immunohistochemistry (IHC), Western blotting, and Q-PCR were used to further validate the candidate proteins.

RESULTS

A total of 79 differentially expressed proteins (DEPs) were identified when CDH and control lungs were compared, and further bioinformatics analysis showed that these proteins play important roles in tight-junctions, phospholipase D signaling, and the HIF-1 signaling pathway. Three differentially expressed proteins, Cldn3, Magi1, and Myh9 are involved in the tight-junction pathway (P < 0.05), and their differential expressions were confirmed by IHC, Western blotting, and Q-PCR.

CONCLUSION

These findings indicate that alterations of tight-junction protein expression may play an important role in the pathogenesis of abnormal lung development in CDH. Further studies are warranted to verify the mechanism by which these tight-junction proteins influence the pathogenesis of CDH-associated pulmonary hypoplasia.

Effects of cigarette smoke on barrier function and tight junction proteins in the bronchial epithelium: protective role of cathelicidin LL-37

Background

Airway epithelial barrier function is maintained by the formation of tight junctions (TJs) and adherens junctions (AJs). Inhalation of cigarette smoke causes airway epithelial barrier dysfunction and may contribute to the pathogenesis of chronic lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). We assessed the effects of cigarette smoke on barrier function and expression of multiple TJ and AJ proteins in the bronchial epithelium. We also examined whether treatment with glucocorticosteroids (GCSs), long-acting β₂-agonists (LABAs), and human cathelicidin LL-37 can protect against cigarette smoke extract (CSE)-induced barrier dysfunction.

Methods

Calu-3 cells cultured at the air-liquid interface were pretreated with or without GCSs, LABAs, GCSs plus LABAs, or LL-37, and subsequently exposed to CSE. Barrier function was assessed by transepithelial electronic resistance (TEER) measurements. Gene and protein expression levels of TJ and AJ proteins were analyzed by quantitative PCR and western blotting, respectively. Immunofluorescence staining of TJ and AJ proteins was performed.

Results

CSE decreased TEER and increased permeability in a concentration-dependent manner. CSE suppressed gene expression of claudin-1, claudin-3, claudin-4, claudin-7, claudin-15, occludin, E-cadherin, junctional adhesion molecule-A (JAM-A) and zonula occludens-1 (ZO-1) within 12 h post-CSE exposure, while suppressed protein expression levels of occludin at 12 h. CSE-treated cells exhibited discontinuous or attenuated immunostaining for claudin-1, claudin-3, claudin-4, occludin, ZO-1, and E-cadherin compared with untreated cells. GCS treatment partially restored CSE-induced TEER reduction, while LABA treatment had no effect. GCS and LABA combination treatment had no additive effect on CSE-induced TEER reduction and gene suppression of TJ and AJ proteins. Human cathelicidin LL-37 counteracted CSE-induced TEER reduction and prevented disruption of occludin and ZO-1. LL-37 also attenuated CSE-induced decreases in gene and protein expression levels of occludin.

Conclusions

CSE caused airway epithelial barrier dysfunction and simultaneously downregulated multiple TJ and AJ proteins. GCS and LABA combination treatment had no additive effect on CSE-induced TEER reduction. LL-37 counteracted CSE-induced TEER reduction and prevented disruption of occludin and ZO-1. Use of LL-37 to counteract airway epithelial barrier dysfunction may have significant benefits for respiratory diseases such as asthma and COPD.

P2X7 Receptor Indirectly Regulates the JAM-A Protein Content via Modulation of GSK-3β

The alveolar epithelial cells represent an important part of the alveolar barrier, which is maintained by tight junction proteins, particularly JAM-A, occludin, and claudin-18, which regulate paracellular permeability. In this study, we report on a strong increase in epithelial JAM-A expression in P2X7 receptor knockout mice when compared to the wildtype. Precision-cut lung slices of wildtype and knockout lungs and immortal epithelial lung E10 cells were treated with bleomycin, the P2X7 receptor inhibitor oxATP, and the agonist BzATP, respectively, to evaluate early changes in JAM-A expression. Biochemical and immunohistochemical data showed evidence for P2X7 receptor-dependent JAM-A expression in vitro. Inhibition of the P2X7 receptor using oxATP increased JAM-A, whereas activation of the receptor decreased the JAM-A protein level. In order to examine the role of GSK-3β in the expression of JAM-A in alveolar epithelial cells, we used lithium chloride for GSK-3β inhibiting experiments, which showed a modulating effect on bleomycin-induced alterations in JAM-A levels. Our data suggest that an increased constitutive JAM-A protein level in P2X7 receptor knockout mice may have a protective effect against bleomycin-induced lung injury. Bleomycin-treated precision-cut lung slices from P2X7 receptor knockout mice responded with a lower increase in mRNA expression of JAM-A than bleomycin-treated precision-cut lung slices from wildtype mice.

Parasympathectomy increases resting secretion of the submandibular gland in minipigs in the long term

Parasympathectomy leads to retrogressive alteration and dysfunction of the submandibular gland (SMG) within 1 month, but its long-term effect is unclear. Excessive secretion is observed in half of the patients 4-6 months after SMG transplantation, which completely denervates the gland. Here, we investigated the long-term effect of parasympathectomy on the secretion of SMGs in minipigs. The results showed that the resting salivary secretion of SMGs decreased by 82.9% of that in control at 2 months after denervation, but increased by 156% at 6 months. Although experiencing an atrophic period, the denervated glands regained their normal morphology by 6 months. The expression of the function-related proteins, including muscarinic acetylcholine receptor (mAChR) 3, aquaporin 5 (AQP5), tight junction protein claudin-3, and claudin-4 was decreased at 2 months after denervation. Meanwhile, the protein expression of stem cell markers, including sex-determining region Y-box 2 and octamer-binding transcription factor 4, and the number of Ki67⁺ cells were significantly increased. However, at 6 months after denervation, the expression of mAChR3, AQP5, claudin-1, claudin-3, and claudin-4 was significantly raised, and the membrane distribution of these proteins was increased accordingly. The autonomic axonal area of the glands was reduced at 2 months after denervation but returned to the control level at 6 months, suggesting that reinnervation took place in the long term. In summary, parasympathectomy increases resting secretion of the SMGs in the long term with a possible mechanism involving improved transepithelial fluid transport. This finding may provide a new strategy for xerostomia treatment.

Knee Joint Tissues Effectively Separate Mixed Sized Molecules Delivered in a Single Bolus to the Heart

The role of molecular size selectivity in the onset and progression of osteoarthritis (OA), a degenerative disease of the musculoskeletal system and the most common cause of disability in aging adults, is unknown. Here we delivered a mixture of Texas-red (70 kDa), and Rhodamine-green (10 kDa) tagged, dextrans of neutral charge in a single bolus via heart injection to middle aged (8-10 months) and aged (17-19 months) Dunkin-Hartley Guinea pigs, a natural model for OA. We quantified tracer transport in serial-sectioned, cryofixed block specimens after five minutes' circulation. A remarkable separation of the molecules was observed in serial fluorescent images of whole joint sections. The larger, 70 kDa red tracer was abundant in the marrow cavity albeit less prevalent or absent in the bone, cartilage, meniscus and other tissues of the joint. Tissues of the meniscus, ligament, and tendon exhibited abundant 10 kDa tracer; volumes of tissue containing this molecular tracer were significantly lower in older than in younger animals. Surprisingly, muscle fiber bundles exhibited little fluorescence, while their bounding fasciae fluoresced either red or green. Small caliber channels through the articular cartilage appeared to show a degree of green fluorescence not observed in the surrounding cartilage matrix. This study opens up new avenues for study of musculoskeletal physiology in health and disease as well as new strategies for drug delivery.

Paracellular transport in the collecting duct

PURPOSE OF REVIEW

The paracellular pathway through the tight junction provides an important route for chloride reabsorption in the collecting duct of the kidney. This review describes recent findings of how defects in paracellular chloride permeation pathway may cause kidney diseases and how such a pathway may be regulated to maintain normal chloride homeostasis.

RECENT FINDINGS

The tight junction in the collecting duct expresses two important claudin genes - claudin-4 and claudin-8. Transgenic knockout of either claudin gene causes hypotension, hypochloremia, and metabolic alkalosis in experimental animals. The claudin-4 mediated chloride permeability can be regulated by a protease endogenously expressed by the collecting duct cell - channel-activating protease 1. Channel-activating protease 1 regulates the intercellular interaction of claudin-4 and its membrane stability. Kelch-like 3, previously identified as a causal gene for Gordon's syndrome, also known as pseudohypoaldosteronism II, directly interacts with claudin-8 and regulates its ubiquitination and degradation. The dominant pseudohypoaldosteronism-II mutation (R528H) in Kelch-like 3 abolishes claudin-8 binding, ubiquitination, and degradation.

SUMMARY

The paracellular chloride permeation pathway in the kidney is an important but understudied area in nephrology. It plays vital roles in renal salt handling and regulation of extracellular fluid volume and blood pressure. Two claudin proteins, claudin-4 and claudin-8, contribute to the function of this paracellular pathway. Deletion of either claudin protein from the collecting duct causes renal chloride reabsorption defects and low blood pressure. Claudins can be regulated on posttranslational levels by several mechanisms involving protease and ubiquitin ligase. Deregulation of claudins may cause human hypertension as exemplified in the Gordon's syndrome.

Cytokine-Ion Channel Interactions in Pulmonary Inflammation

The lungs conceptually represent a sponge that is interposed in series in the bodies’ systemic circulation to take up oxygen and eliminate carbon dioxide. As such, it matches the huge surface areas of the alveolar epithelium to the pulmonary blood capillaries. The lung’s constant exposure to the exterior necessitates a competent immune system, as evidenced by the association of clinical immunodeficiencies with pulmonary infections. From the in utero to the postnatal and adult situation, there is an inherent vital need to manage alveolar fluid reabsorption, be it postnatally, or in case of hydrostatic or permeability edema. Whereas a wealth of literature exists on the physiological basis of fluid and solute reabsorption by ion channels and water pores, only sparse knowledge is available so far on pathological situations, such as in microbial infection, acute lung injury or acute respiratory distress syndrome, and in the pulmonary reimplantation response in transplanted lungs. The aim of this review is to discuss alveolar liquid clearance in a selection of lung injury models, thereby especially focusing on cytokines and mediators that modulate ion channels. Inflammation is characterized by complex and probably time-dependent co-signaling, interactions between the involved cell types, as well as by cell demise and barrier dysfunction, which may not uniquely determine a clinical picture. This review, therefore, aims to give integrative thoughts and wants to foster the unraveling of unmet needs in future research.

New insights into the mechanisms of pulmonary edema in acute lung injury

Appearance of alveolar protein-rich edema is an early event in the development of acute respiratory distress syndrome (ARDS). Alveolar edema in ARDS results from a significant increase in the permeability of the alveolar epithelial barrier, and represents one of the main factors that contribute to the hypoxemia in these patients. Damage of the alveolar epithelium is considered a major mechanism responsible for the increased pulmonary permeability, which results in edema fluid containing high concentrations of extravasated macromolecules in the alveoli. The breakdown of the alveolar-epithelial barrier is a consequence of multiple factors that include dysregulated inflammation, intense leukocyte infiltration, activation of pro-coagulant processes, cell death and mechanical stretch. The disruption of tight junction (TJ) complexes at the lateral contact of epithelial cells, the loss of contact between epithelial cells and extracellular matrix (ECM), and relevant changes in the communication between epithelial and immune cells, are deleterious alterations that mediate the disruption of the alveolar epithelial barrier and thereby the formation of lung edema in ARDS.

Claudin-3 Inhibits Lung Squamous Cell Carcinoma Cell Epithelial-mesenchymal Transition and Invasion via Suppression of the Wnt/β-catenin Signaling Pathway

Altered expression of claudin-3 (CLDN3), a key cytoskeletal structural protein of the tight junctions in the epithelium, is associated with the development and metastasis of various human cancers. CLDN3 expression has been shown to be significantly associated with the prognosis of lung squamous cell carcinoma (SqCC). This study investigated the role of CLDN3 in inhibiting lung SqCC cell migration and invasion as well as the underlying molecular mechanisms. The CLDN3 levels were assessed between 20 paired lung SqCC tissues and adjacent normal tissues using quantitative real-time polymerase chain reaction (qRT-PCR) and western blot. The ectopic CLDN3 overexpression or knockdown was generated by using a plasmid carrying CLDN3 cDNA or shRNA, respectively. CLDN3 expression was significantly reduced in lung SqCC tissues vs. the adjacent normal tissues. The ectopic CLDN3 overexpression markedly inhibited the migration, invasion, and epithelial-mesenchymal transition (EMT) of lung cancer H520 cells, whereas CLDN3 knockdown had an inverse effect on SK-MES-1 cells. However, cell viability and plate colony formation assays showed that both CLDN3 knockdown and overexpression did not affect SqCC cell proliferation. Both tissue and cell data revealed that CLDN3 expression was significantly associated with the expression of the EMT biomarkers E-cadherin and Vimentin. Furthermore, CLDN3-modulated EMT and expression of the EMT markers were through regulation of the Wnt/β-catenin signaling pathway. In conclusion, this study identified reduced CLDN3 expression in lung SqCC tissues, which was associated with the progression and metastasis of lung SqCC and was attributed to EMT by activation of the Wnt pathway. Thus, CLDN3 could be further evaluated as a novel biomarker for predicting the prognosis of lung SqCC and as a target for the treatment of lung SqCC in the future.

Hyperoxia induces paracellular leak and alters claudin expression by neonatal alveolar epithelial cells

BACKGROUND

Premature neonates frequently require oxygen supplementation as a therapeutic intervention that, while necessary, also exposes the lung to significant oxidant stress. We hypothesized that hyperoxia has a deleterious effect on alveolar epithelial barrier function rendering the neonatal lung susceptible to injury and/or bronchopulmonary dysplasia (BPD).

MATERIALS AND METHODS

We examined the effects of exposure to 85% oxygen on neonatal rat alveolar barrier function in vitro and in vivo. Whole lung was measured using wet-to-dry weight ratios and bronchoalveolar lavage protein content and cultured primary neonatal alveolar epithelial cells (AECs) were measured using transepithelial electrical resistance (TEER) and paracellular flux measurements. Expression of claudin-family tight junction proteins, E-cadherin and the Snail transcription factor SNAI1 were measured by Q-PCR, immunoblot and confocal immunofluorescence microscopy.

RESULTS

Cultured neonatal AECs exposed to 85% oxygen showed impaired barrier function. This oxygen-induced increase in paracellular leak was associated with altered claudin expression, where claudin-3 and -18 were downregulated at both the mRNA and protein level. Claudin-4 and -5 mRNA were also decreased, although protein expression of these claudins was largely maintained. Lung alveolarization and barrier function in vivo were impaired in response to hyperoxia. Oxygen exposure also significantly decreased E-cadherin expression and induced expression of the SNAI1 transcription factor in vivo and in vitro.

CONCLUSIONS

These data support a model in which hyperoxia has a direct impact on alveolar tight and adherens junctions to impair barrier function. Strategies to antagonize the effects of high oxygen on alveolar junctions may potentially reverse this deleterious effect.

Potential contribution of alveolar epithelial type I cells to pulmonary fibrosis

Pulmonary fibrosis (PF) is characterized by inflammation and fibrosis of the interstitium and destruction of alveolar histoarchitecture ultimately leading to a fatal impairment of lung function. Different concepts describe either a dominant role of inflammatory pathways or a disturbed remodeling of resident cells of the lung parenchyma during fibrogenesis. Further, a combination of both the mechanisms has been postulated. The present review emphasizes the particular involvement of alveolar epithelial type I cells in all these processes, their contribution to innate immune/inflammatory functions and maintenance of proper alveolar barrier functions. Amongst the different inflammatory and repair events the purinergic receptor P2X7, an ATP-gated cationic channel that regulates not only apoptosis, necrosis, autophagy, and NLPR3 inflammosome activation, but also the turnover of diverse tight junction (TJ) and water channel proteins, seems to be essential for the stability of alveolar barrier integrity and for the interaction with protective factors during lung injury.

Protective effects of chebulic acid on alveolar epithelial damage induced by urban particulate matter

BACKGROUND

Chebulic acid (CA) isolated from T. chebula, which has been reported for treating asthma, as a potent anti-oxidant resources. Exposure to ambient urban particulate matter (UPM) considered as a risk for cardiopulmonary vascular dysfunction. To investigate the protective effect of CA against UPM-mediated collapse of the pulmonary alveolar epithelial (PAE) cell (NCI-H441), barrier integrity parameters, and their elements were evaluated in PAE.

METHODS

CA was acquired from the laboratory previous reports. UPM was obtained from the National Institutes of Standards and Technology, and these were collected in St. Louis, MO, over a 24-month period and used as a standard reference. To confirm the protection of PAE barrier integrity, paracellular permeability and the junctional molecules were estimated with determination of transepithelial electrical resistance, Western Blotting, RT-PCR, and fluorescent staining.

RESULTS

UPM aggravated the generation of reactive oxygen species (ROS) in PAE and also decreased mRNA and protein levels of junction molecules and barrier integrity in NCI-H441. However, CA repressed the ROS in PAE, also improved barrier integrity by protecting the junctional parameters in NCI-H411.

CONCLUSIONS

These data showed that CA resulted in decreased UPM-induced ROS formation, and the protected the integrity of the tight junctions against UPM exposure to PAE barrier.

Intestinal epithelial claudins: expression and regulation in homeostasis and inflammation

The intestinal epithelium forms a highly dynamic and selective barrier that controls absorption of fluid and solutes while restricting pathogen access to underlying tissues. Barrier properties are achieved by intercellular junctions that include an apical tight junction (TJ) and subjacent adherens junctions and desmosomes. The TJ tetraspan claudin proteins form pores between epithelial cells to control paracellular fluid and ion movement. In addition to regulation of barrier function, claudin family members control epithelial homeostasis and are expressed in a spatiotemporal manner in the intestinal crypt-luminal axis. This delicate balance of physiologic differential claudin protein expression is altered during mucosal inflammation. Inflammatory mediators influence transcriptional regulation, as well as endocytic trafficking, targeting, and retention of claudins in the TJ. Increased expression of intestinal epithelial claudin-1, -2, and -18 with downregulation of claudin-3, -4, -5, -7, -8, and -12 has been observed in intestinal inflammatory disorders. Such changes in claudin proteins modify the epithelial barrier function in addition to influencing epithelial and mucosal homeostasis. An improved understanding of the regulatory mechanisms that control epithelial claudin proteins will provide strategies to strengthen the epithelial barrier function and restore mucosal homeostasis in inflammatory disorders.

Tight junctions in pulmonary epithelia during lung inflammation

Inflammatory lung diseases like asthma bronchiale, chronic obstructive pulmonary disease and allergic airway inflammation are widespread public diseases that constitute an enormous burden to the health systems. Mainly classified as inflammatory diseases, the treatment focuses on strategies interfering with local inflammatory responses by the immune system. Inflammatory lung diseases predispose patients to severe lung failures like alveolar oedema, respiratory distress syndrome and acute lung injury. These life-threatening syndromes are caused by increased permeability of the alveolar and airway epithelium and exudate formation. However, the mechanism underlying epithelium barrier breakdown in the lung during inflammation is elusive. This review emphasises the role of the tight junction of the airway epithelium as the predominating structure conferring epithelial tightness and preventing exudate formation and the impact of inflammatory perturbations on their function.

Active and passive involvement of claudins in the pathophysiology of intestinal inflammatory diseases

Intestinal inflammatory diseases, four of which are discussed here, are associated with alterations of claudins. In ulcerative colitis, diarrhea and antigen entry into the mucosa occurs. Claudin-2 is upregulated but data on other claudins are still limited or vary (e.g., claudin-1 and -4). Apart from that, tight junction changes contribute to diarrhea via a leak flux mechanism, while protection against antigen entry disappears behind epithelial gross lesions (erosions) and apoptotic foci. Crohn's disease is additionally characterized by a claudin-5 and claudin-8 reduction which plays an active role in antigen uptake already before gross lesions appear. In microscopic colitis (MC), upregulation of claudin-2 expression is weak and a reduction in claudin-4 may be only passively involved, while sodium malabsorption represents the main diarrheal mechanism. However, claudin-5 is removed from MC tight junctions which may be an active trigger for inflammation through antigen uptake along the so-called leaky gut concept. In celiac disease, primary barrier defects are discussed in the context of candidate genes as PARD3 which regulate cell polarity and tight junctions. The loss of claudin-5 allows small antigens to invade, while the reductions in others like claudin-3 are rather passive events. Taken together, the specific role of single tight junction proteins for the onset and perpetuation of inflammation and the recovery from these diseases is far from being fully understood and is clearly dependent on the stage of the disease, the background of the other tight junction components, the transport activity of the mucosa, and the presence of other barrier features like gross lesions, an orchestral interplay which is discussed in this article.

An Optimised Human Cell Culture Model for Alveolar Epithelial Transport

Robust and reproducible in vitro models are required for investigating the pathways involved in fluid homeostasis in the human alveolar epithelium. We performed functional and phenotypic characterisation of ion transport in the human pulmonary epithelial cell lines NCI-H441 and A549 to determine their similarity to primary human alveolar type II cells. NCI-H441 cells exhibited high expression of junctional proteins ZO-1, and E-cadherin, seal-forming claudin-3, -4, -5 and Na⁺-K⁺-ATPase while A549 cells exhibited high expression of pore-forming claudin-2. Consistent with this phenotype NCI-H441, but not A549, cells formed a functional barrier with active ion transport characterised by higher electrical resistance (529 ± 178 Ω cm² vs 28 ± 4 Ω cm²), lower paracellular permeability ((176 ± 42) ×10⁻⁸ cm/s vs (738 ± 190) ×10⁻⁸ cm/s) and higher transepithelial potential difference (11.9 ± 4 mV vs 0 mV). Phenotypic and functional properties of NCI-H441 cells were tuned by varying cell seeding density and supplement concentrations. The cells formed a polarised monolayer typical of in vivo epithelium at seeding densities of 100,000 cells per 12-well insert while higher densities resulted in multiple cell layers. Dexamethasone and insulin-transferrin-selenium supplements were required for the development of high levels of electrical resistance, potential difference and expression of claudin-3 and Na⁺-K⁺-ATPase. Treatment of NCI-H441 cells with inhibitors and agonists of sodium and chloride channels indicated sodium absorption through ENaC under baseline and forskolin-stimulated conditions. Chloride transport was not sensitive to inhibitors of the cystic fibrosis transmembrane conductance regulator (CFTR) under either condition. Channels inhibited by 5-nitro-1-(3-phenylpropylamino) benzoic acid (NPPB) contributed to chloride secretion following forskolin stimulation, but not at baseline. These data precisely define experimental conditions for the application of NCI-H441 cells as a model for investigating ion and water transport in the human alveolar epithelium and also identify the pathways of sodium and chloride transport.

Protective effect of bone marrow derived mesenchymal stem cells in lipopolysaccharide-induced acute lung injury mediated by claudin-4 in a rat model

Our study aims to investigate the effects of bone marrow derived mesenchymal stem cells (BM-MSCs) in lipopolysaccharide (LPS)-induced acute lung injury (ALI) as well as the underlying mechanism. In our study, Wistar rats were randomly divided into four groups: control group; ALI group; ALI+MSCs group and ALI+MSCs claudin-4 siRNA group. MRC-5 and BEAS-2B cell lines were used for in vitro assay. Flow cytometry, western blot, hematoxylin and eosin (H&E) staining, CCK-8 assay, enzyme-linked immunosorbent assay (ELISA) were involved to measure the pathological changes in lung tissues. Results showed that in vivo MSCs administration significantly attenuated pulmonary edema (wet/dry ratio), inflammation cytokines levels (TGF-α), pathological alternations and cell apoptosis which were mediated by claudin-4 in LPS-induced acute lung injury in rats. In vitro experiment showed that hypoxia could induce the expression of claudin-4 in MSCs, and MSCs treatment showed significantly enhanced cell viability (by CCK-8 assay) and reduced cell apoptosis. In conclusion, the present study demonstrated that BM-MSCs can protect against LPS-induced ALI in vivo and in vitro, at least partly mediated by claudin-4.

Regulation of claudin/zonula occludens-1 complexes by hetero-claudin interactions

Claudins are tetraspan transmembrane tight-junction proteins that regulate epithelial barriers. In the distal airspaces of the lung, alveolar epithelial tight junctions are crucial to regulate airspace fluid. Chronic alcohol abuse weakens alveolar tight junctions, priming the lung for acute respiratory distress syndrome, a frequently lethal condition caused by airspace flooding. Here we demonstrate that in response to alcohol, increased claudin-5 paradoxically accompanies an increase in paracellular leak and rearrangement of alveolar tight junctions. Claudin-5 is necessary and sufficient to diminish alveolar epithelial barrier function by impairing the ability of claudin-18 to interact with a scaffold protein, zonula occludens 1 (ZO-1), demonstrating that one claudin affects the ability of another claudin to interact with the tight-junction scaffold. Critically, a claudin-5 peptide mimetic reverses the deleterious effects of alcohol on alveolar barrier function. Thus, claudin controlled claudin-scaffold protein interactions are a novel target to regulate tight-junction permeability.

Chloride secretion by renal collecting ducts

PURPOSE OF REVIEW

Renal collecting ducts maintain NaCl homeostasis by fine-tuning urinary excretion to balance dietary salt intake. This review focuses on recent studies on transcellular Cl secretion by collecting ducts, its regulation and its role in cyst growth in autosomal dominant polycystic kidney disease (ADPKD).

RECENT FINDINGS

Lumens of nonperfused rat medullary collecting ducts collapse in control media but expand with fluid following treatment with cAMP, demonstrating the capacity for both salt absorption and secretion. Recently, inhibition of apical epithelial Na channels (ENaC) unmasked Cl secretion in perfused mouse cortical collecting ducts (CCDs), involving Cl uptake by basolateral NKCC1 and efflux through apical Cl channels. AVP, the key hormone for osmoregulation, promotes cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl secretion. In addition, prostaglandin E2 stimulates Cl secretion through both CFTR and Ca-activated Cl channels.

SUMMARY

Renal Cl secretion has been commonly overlooked because of the overwhelming capacity for the nephron to reabsorb NaCl from the glomerular filtrate. In ADPKD, Cl secretion plays a central role in the accumulation of cyst fluid and the remarkable size of the cystic kidneys. Investigation of renal Cl secretion may provide a better understanding of NaCl homeostasis and identify new approaches to reduce cyst growth in PKD.

Caveolin-1 regulates the expression of tight junction proteins during hyperoxia-induced pulmonary epithelial barrier breakdown

BACKGROUND

Bronchopulmonary dysplasia (BPD) is a common complication in preterm infants that involves the downregulation of tight junction (TJ) proteins. However, the mechanism underlying downregulation of the expression of TJ proteins during at the early stages of hyperoxia-induced BPD remains to be understood. Here, we aimed to identify the role of caveolin-1 (Cav-1) in hyperoxia-induced pulmonary epithelial barrier breakdown.

METHODS

First, we established an in vitro pulmonary epithelial barrier models using primary type II alveolar epithelial cells (AEC-II) from newborn rats. AEC-II was assigned to the hyperoxic (85 % O2/5 % CO2) or normoxic (21 % O2/5 % CO2) groups. Second, AEC-II was transfected with Cav-1-siRNA to downregulate Cav-1 under normoxic exposure. Third, AEC-II was transfected with a cDNA encoding Cav-1 to upregulate Cav-1 expression under hyperoxic exposure. Then, expression levels of Cav-1 and TJ proteins were examined by immunofluorescence staining, reverse transcription-polymerase chain reaction, and Western blotting. The TJ structures visualized using a transmission electron microscope, and transepithelial resistance and apparent permeability coefficient of fluorescein isothiocyanate-dextran, which are indicators of barrier function, were measured.

RESULTS

Our data showed that exposure to hyperoxia disrupted the structure and function of the pulmonary epithelial barrier and decreased the ZO-1, occludin, claudin-4, and Cav-1 expression levels. Moreover, Cav-1 knockdown attenuated the expression of the other three genes and disrupted pulmonary epithelial barrier structure and function under normoxic exposure. However, Cav-1 upregulation markedly antagonized the hyperoxia-induced pulmonary epithelial barrier destruction and TJ protein loss.

CONCLUSIONS

This is the first study to present evidence illustrating the novel role of Cav-1 downregulation-mediated TJ protein loss in pulmonary epithelial barrier destruction during BPD.

Looking into the future: Using induced pluripotent stem cells to build two and three dimensional ocular tissue for cell therapy and disease modeling

Retinal degenerative diseases are the leading cause of irreversible vision loss in developed countries. In many cases the diseases originate in the homeostatic unit in the back of the eye that contains the retina, retinal pigment epithelium (RPE) and the choriocapillaris. RPE is a central and a critical component of this homeostatic unit, maintaining photoreceptor function and survival on the apical side and choriocapillaris health on the basal side. In diseases like age-related macular degeneration (AMD), it is thought that RPE dysfunctions cause disease-initiating events and as the RPE degenerates photoreceptors begin to die and patients start loosing vision. Patient-specific induced pluripotent stem (iPS) cell-derived RPE provides direct access to a patient's genetics and allow the possibility of identifying the initiating events of RPE-associated degenerative diseases. Furthermore, iPS cell-derived RPE cells are being tested as a potential cell replacement in disease stages with RPE atrophy. In this article we summarize the recent progress in the field of iPS cell-derived RPE "disease modeling" and cell therapies and also discuss the possibilities of developing a model of the entire homeostatic unit to aid in studying disease processes in the future. This article is part of a Special Issue entitled SI: PSC and the brain.