Coupling between apical and paracellular transport processes

Engineered nanomaterials induce alterations in biological barriers: focus on paracellular permeability

Engineered nanoparticles (ENPs) are widely used in medical diagnosis and treatment, as food additives and as energy materials. ENPs may exert adverse or beneficial effects on the human body, which may be linked to interactions with biological barriers. In this review, the authors summarize the influences of four typical metal/metal oxide nanomaterials (Ag, TiO₂, Au, ZnO nanoparticles) on the paracellular permeability of biological barriers. Disruptions on tight junctions, adhesion junctions, gap junctions and desmosomes via complex signaling pathways, such as the MAPK, PKC and ROCK signaling pathways, affect paracellular permeability. Reactive oxygen species and cytokines underlie the mechanism of ENP-triggered alterations in paracellular permeability. This review provides the information necessary for the cautious application of nanoparticles in medicine and life sciences in the future.

Role of Barrier Integrity and Dysfunctions in Maintaining the Healthy Gut and Their Health Outcomes

Mucosal surface layers are the critical borders throughout epithelial membranes. These epithelial cells segregate luminal material from external environments. However, mucosal linings are also accountable for absorbing nutrients and requiring specific barrier permeability. These functional acts positioned the mucosal epithelium at the epicenter of communications concerning the mucosal immune coordination and foreign materials, such as dietary antigens and microbial metabolites. Current innovations have revealed that external stimuli can trigger several mechanisms regulated by intestinal mucosal barrier system. Crucial constituents of this epithelial boundary are physical intercellular structures known as tight junctions (TJs). TJs are composed of different types transmembrane proteins linked with cytoplasmic adaptors which helps in attachment to the adjacent cells. Disruption of this barrier has direct influence on healthy or diseased condition, as barrier dysfunctions have been interrelated with the initiation of inflammation, and pathogenic effects following metabolic complications. In this review we focus and overview the TJs structure, function and the diseases which are able to influence TJs during onset of disease. We also highlighted and discuss the role of phytochemicals evidenced to enhance the membrane permeability and integrity through restoring TJs levels.

An Experimental Workflow for Studying Barrier Integrity, Permeability, and Tight Junction Composition and Localization in a Single Endothelial Cell Monolayer: Proof of Concept

Endothelial and epithelial barrier function is crucial for the maintenance of physiological processes. The barrier paracellular permeability depends on the composition and spatial distribution of the cell-to-cell tight junctions (TJ). Here, we provide an experimental workflow that yields several layers of physiological data in the setting of a single endothelial cell monolayer. Human umbilical vein endothelial cells were grown on Transwell filters. Transendothelial electrical resistance (TER) and 10 kDa FITC dextran flux were measured using Alanyl-Glutamine (AlaGln) as a paracellular barrier modulator. Single monolayers were immunolabelled for Zonula Occludens-1 (ZO-1) and Claudin-5 (CLDN5) and used for automated immunofluorescence imaging. Finally, the same monolayers were used for single molecule localization microscopy (SMLM) of ZO-1 and CLDN5 at the nanoscale for spatial clustering analysis. The TER increased and the paracellular dextran flux decreased after the application of AlaGln and these functional changes of the monolayer were mediated by an increase in the ZO-1 and CLDN5 abundance in the cell–cell interface. At the nanoscale level, the functional and protein abundance data were accompanied by non-random increased clustering of CLDN5. Our experimental workflow provides multiple data from a single monolayer and has wide applicability in the setting of paracellular studies in endothelia and epithelia.

Functions of EpCAM in physiological processes and diseases (Review)

EpCAM (epithelial cell adhesion molecule) is a type I transmembrane glycoprotein, which was originally identified as a tumor-associated antigen due to its high expression level in rapidly growing epithelial tumors. Germ line mutations of the human EpCAM gene have been indicated as the cause of congenital tufting enteropathy. Previous studies based on cell models have revealed that EpCAM contributes to various biological processes including cell adhesion, signaling, migration and proliferation. Due to the previous lack of genetic animal models, the in vivo functions of EpCAM remain largely unknown. However, EpCAM genetic animal models have recently been generated, and are useful for understanding the functions of EpCAM. The authors here briefly review the functions and mechanisms of EpCAM in physiological processes and different diseases.

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.

Stress induces endotoxemia and low-grade inflammation by increasing barrier permeability

Chronic non-communicable diseases (NCDs) are the leading causes of work absence, disability, and mortality worldwide. Most of these diseases are associated with low-grade inflammation. Here, we hypothesize that stresses (defined as homeostatic disturbances) can induce low-grade inflammation by increasing the availability of water, sodium, and energy-rich substances to meet the increased metabolic demand induced by the stressor. One way of triggering low-grade inflammation is by increasing intestinal barrier permeability through activation of various components of the stress system. Although beneficial to meet the demands necessary during stress, increased intestinal barrier permeability also raises the possibility of the translocation of bacteria and their toxins across the intestinal lumen into the blood circulation. In combination with modern life-style factors, the increase in bacteria/bacterial toxin translocation arising from a more permeable intestinal wall causes a low-grade inflammatory state. We support this hypothesis with numerous studies finding associations with NCDs and markers of endotoxemia, suggesting that this process plays a pivotal and perhaps even a causal role in the development of low-grade inflammation and its related diseases.

Tumor necrosis factor-α induces a biphasic change in claudin-2 expression in tubular epithelial cells: role in barrier functions

The inflammatory cytokine tumor necrosis factor-α (TNF-α) is a pathogenic factor in acute and chronic kidney disease. TNF-α is known to alter expression of epithelial tight junction (TJ) proteins; however, the underlying mechanisms and the impact of this effect on epithelial functions remain poorly defined. Here we describe a novel biphasic effect of TNF-α on TJ protein expression. In LLC-PK1 tubular cells, short-term (1-6 h) TNF-α treatment selectively elevated the expression of the channel-forming TJ protein claudin-2. In contrast, prolonged (>8 h) TNF-α treatment caused a marked downregulation in claudin-2 and an increase in claudin-1, -4, and -7. The early increase and the late decrease in claudin-2 expression involved distinct mechanisms. TNF-α slowed claudin-2 degradation through ERK, causing the early increase. This increase was also mediated by the EGF receptor and RhoA and Rho kinase. In contrast, prolonged TNF-α treatment reduced claudin-2 mRNA levels and promoter activity independent from these signaling pathways. Electric Cell-substrate Impedance Sensing measurements revealed that TNF-α also exerted a biphasic effect on transepithelial resistance (TER) with an initial decrease and a late increase. Thus there was a good temporal correlation between TNF-α-induced claudin-2 protein and TER changes. Indeed, silencing experiments showed that the late TER increase was at least in part caused by reduced claudin-2 expression. Surprisingly, however, claudin-2 silencing did not prevent the early TER drop. Taken together, the TNF-α-induced changes in claudin-2 levels might contribute to TER changes and could also play a role in newly described functions of claudin-2 such as proliferation regulation.

Identification of signalling cascades involved in red blood cell shrinkage and vesiculation

Even though red blood cell (RBC) vesiculation is a well-documented phenomenon, notably in the context of RBC aging and blood transfusion, the exact signalling pathways and kinases involved in this process remain largely unknown. We have established a screening method for RBC vesicle shedding using the Ca²⁺ ionophore ionomycin which is a rapid and efficient method to promote vesiculation. In order to identify novel pathways stimulating vesiculation in RBC, we screened two libraries: the Library of Pharmacologically Active Compounds (LOPAC) and the Selleckchem Kinase Inhibitor Library for their effects on RBC from healthy donors. We investigated compounds triggering vesiculation and compounds inhibiting vesiculation induced by ionomycin. We identified 12 LOPAC compounds, nine kinase inhibitors and one kinase activator which induced RBC shrinkage and vesiculation. Thus, we discovered several novel pathways involved in vesiculation including G protein-coupled receptor (GPCR) signalling, the phosphoinositide 3-kinase (PI3K)–Akt (protein kinase B) pathway, the Jak–STAT (Janus kinase–signal transducer and activator of transcription) pathway and the Raf–MEK (mitogen-activated protein kinase kinase)–ERK (extracellular signal-regulated kinase) pathway. Moreover, we demonstrated a link between casein kinase 2 (CK2) and RBC shrinkage via regulation of the Gardos channel activity. In addition, our data showed that inhibition of several kinases with unknown functions in mature RBC, including Alk (anaplastic lymphoma kinase) kinase and vascular endothelial growth factor receptor 2 (VEGFR-2), induced RBC shrinkage and vesiculation.

After screening two libraries of small bioactive molecules and kinase inhibitors, we identified several signalling pathways to be involved in red blood cell (RBC) shrinkage and vesiculation. These include the Jak (Janus kinase)–STAT (signal transducer and activator of transcription) pathway, phosphoinositide 3-kinase (PI3K)–Akt pathway, the Raf–MEK (mitogen-activated protein kinase kinase)–ERK (extracellular signal-regulated kinase) pathway and GPCR (G protein-coupled receptor) signalling.

Role of NPT2b in health and chronic kidney disease

PURPOSE OF REVIEW

Maintaining phosphate homeostasis is essential and any deviation can lead to several acute and chronic disease states. To maintain normal physiological levels, phosphate needs to be tightly regulated. This is achieved through a complex relationship of organ cross-talk via hormonal regulation of the type II sodium-dependent phosphate co-transporters. This editorial provides evidence of the importance of intestinal NPT2b in health and chronic kidney disease (CKD).

RECENT FINDINGS

The advent of the different Npt2b knockout mice has increased our understanding of how the intestinal phosphate co-transporter contributes to the regulation of systemic phosphate. In addition, these studies have suggested that Npt2b may participate in the phosphate-sensing machinery important for organ cross-talk. Studies using Drosophila have expanded our knowledge of phosphate sensing mechanisms and may provide a foundation for delineating these pathways in humans. Several preclinical studies using different agents to modulate Npt2b, and clinical studies using nicotinamide, have provided evidence that Npt2b is a viable therapeutic target for the management of hyperphosphatemia.

SUMMARY

Over the last couple of years, new experimental approaches have increased our understanding of the important role of Npt2b in maintaining phosphate homeostasis. In addition, several clinical studies have associated the detrimental effects of elevated phosphate with cardiovascular events, and decreased lifespan. Although several key questions about intestinal phosphate transport remain to be answered, it is clear that the intestine is an important player, with current evidence suggesting that it is a prime target for regulating phosphate uptake and improving health outcomes in CKD.

Polymer assisted entrapment of netilmicin in PLGA nanoparticles for sustained antibacterial activity

This study was aimed to develop poly(dl-lactide-co-glycolide) (PLGA) nanoparticle of highly water soluble antibiotic drug, netilmicin sulfate (NS) with improved entrapment efficiency (EE) and antibacterial activity. Dextran sulfate was introduced as helper polymer to form electrostatic complex with NS. Nanoparticles were prepared by double emulsification method and optimized using 2(5-1) fractional factorial design. EE was mainly influenced by dextran sulfate: NS charge ratio and PLGA concentration, whereas particle size (PS) was affected by all factors examined. The optimized NS-loaded-NPs had EE and PS of 93.23 ± 2.7% and 140.83 ± 2.4 nm respectively. NS-loaded-NPs effectively inhibited bacterial growth compared to free NS. Sustained release protected its inactivation and reduced the decline in its killing activity over time even in presence of bronchial cells. A MIC value of 18 μg/mL was observed for NPs on P. aeruginosa. Therefore, NPs with sustained bactericidal efficiency against P. aeruginosa may provide therapeutic benefit in chronic pulmonary infection, like cystic fibrosis.

Paracellular transport through healthy and cystic fibrosis bronchial epithelial cell lines--do we have a proper model?

It has been reported recently that the cystic fibrosis transmembrane conductance regulator (CFTR) besides transcellular chloride transport, also controls the paracellular permeability of bronchial epithelium. The aim of this study was to test whether overexpressing wtCFTR solely regulates paracellular permeability of cell monolayers. To answer this question we used a CFBE41o- cell line transfected with wtCFTR or mutant F508del-CFTR and compered them with parental line and healthy 16HBE14o- cells. Transepithelial electrical resistance (TER) and paracellular fluorescein flux were measured under control and CFTR-stimulating conditions. CFTR stimulation significant decreased TER in 16HBE14o- and also in CFBE41o- cells transfected with wtCFTR. In contrast, TER increased upon stimulation in CFBE41o- cells and CFBE41o- cells transfected with F508del-CFTR. Under non-stimulated conditions, all four cell lines had similar paracellular fluorescein flux. Stimulation increased only the paracellular permeability of the 16HBE14o- cell monolayers. We observed that 16HBE14o- cells were significantly smaller and showed a different structure of cell-cell contacts than CFBE41o- and its overexpressing clones. Consequently, 16HBE14o- cells have about 80% more cell-cell contacts through which electrical current and solutes can leak. Also tight junction protein composition is different in 'healthy' 16HBE14o- cells compared to 'cystic fibrosis' CFBE41o- cells. We found that claudin-3 expression was considerably stronger in 16HBE14o- cells than in the three CFBE41o- cell clones and thus independent of the presence of functional CFTR. Together, CFBE41o- cell line transfection with wtCFTR modifies transcellular conductance, but not the paracellular permeability. We conclude that CFTR overexpression is not sufficient to fully reconstitute transport in CF bronchial epithelium. Hence, it is not recommended to use those cell lines to study CFTR-dependent epithelial transport.

Different transcription regulation routes are exerted by L- and D-amino acid enantiomers of peptide hormones

Conversion of one or more amino acids in eukaryotic peptides to the D-configuration is catalyzed by specific L/D peptide isomerases and it is a poorly investigated post-translational modification. No common modified amino acid and no specific modified position have been recognized and mechanisms underlying changes in the peptide function provided by this conversion were not sufficiently studied. The 72 amino acid crustacean hyperglycemic hormone (CHH) of Astacidea crustaceans exhibits a co-existence of two peptide enantiomers alternately having D- or L-phenylalanine in their third position. It is a pleiotropic hormone regulating several physiological processes in different target tissues and along different time scales. CHH enantiomers differently affect time courses and intensities of examined processes. The short-term effects of the two isomers on gene expression are presented here, examined in the hepatopancreas, gills, hemocytes and muscles of the astacid Pontastacus leptodactylus. Muscles and hemocytes were poorly affected by both isomers. Two CHH modes of action were elucidated in the hepatopancreas and the gills: specific gene induction by D-CHH only, elucidated in both organs and mutual targeted attenuation affected by both enantiomers elucidated in the gills. Consequently a two-receptor system is hypothesized for conveying the effect of the two CHH isomers.

Models and methods to evaluate transport of drug delivery systems across cellular barriers

Sub-micrometer carriers (nanocarriers; NCs) enhance efficacy of drugs by improving solubility, stability, circulation time, targeting, and release. Additionally, traversing cellular barriers in the body is crucial for both oral delivery of therapeutic NCs into the circulation and transport from the blood into tissues, where intervention is needed. NC transport across cellular barriers is achieved by: (i) the paracellular route, via transient disruption of the junctions that interlock adjacent cells, or (ii) the transcellular route, where materials are internalized by endocytosis, transported across the cell body, and secreted at the opposite cell surface (transyctosis). Delivery across cellular barriers can be facilitated by coupling therapeutics or their carriers with targeting agents that bind specifically to cell-surface markers involved in transport. Here, we provide methods to measure the extent and mechanism of NC transport across a model cell barrier, which consists of a monolayer of gastrointestinal (GI) epithelial cells grown on a porous membrane located in a transwell insert. Formation of a permeability barrier is confirmed by measuring transepithelial electrical resistance (TEER), transepithelial transport of a control substance, and immunostaining of tight junctions. As an example, ~200 nm polymer NCs are used, which carry a therapeutic cargo and are coated with an antibody that targets a cell-surface determinant. The antibody or therapeutic cargo is labeled with (125)I for radioisotope tracing and labeled NCs are added to the upper chamber over the cell monolayer for varying periods of time. NCs associated to the cells and/or transported to the underlying chamber can be detected. Measurement of free (125)I allows subtraction of the degraded fraction. The paracellular route is assessed by determining potential changes caused by NC transport to the barrier parameters described above. Transcellular transport is determined by addressing the effect of modulating endocytosis and transcytosis pathways.

Regulation of paracellular permeability: factors and mechanisms

Epithelial permeability is composed of transcellular permeability and paracellular permeability. Paracellular permeability is controlled by tight junctions (TJs). Claudins and occludin are two major transmembrane proteins in TJs, which directly determine the paracellular permeability to different ions or large molecules. Intracellular signaling pathways including Rho/Rho-associated protein kinase, protein kinase Cs, and mitogen-activated protein kinase, modulate the TJ proteins to affect paracellular permeability in response for diverse stimuli. Cytokines, growth factors and hormones in organism can regulate the paracellular permeability via signaling pathway. The transcellular transporters such as Na-K-ATPase, Na(+)-coupled transporters and chloride channels, can interact with paracellular transport and regulate the TJs. In this review, we summarized the factors affecting paracellular permeability and new progressions of the related mechanism in recent studies, and pointed out further research areas.

Central role of the exchange factor GEF-H1 in TNF-α-induced sequential activation of Rac, ADAM17/TACE, and RhoA in tubular epithelial cells

Tumor necrosis factor-α activates the enzyme TACE/ADAM17 through the guanine nucleotide exchange factor GEF-H1, Rac, and p38, leading to activation of the epidermal growth factor. GEF-H1 mediates hierarchical activation of Rac and RhoA through differential phosphorylation.

Transactivation of the epidermal growth factor receptor (EGFR) by tumor necrosis factor-α (TNF-α) is a key step in mediating RhoA activation and cytoskeleton and junction remodeling in the tubular epithelium. In this study we explore the mechanisms underlying TNF-α–induced EGFR activation. We show that TNF-α stimulates the TNF-α convertase enzyme (TACE/a disintegrin and metalloproteinase-17), leading to activation of the EGFR/ERK pathway. TACE activation requires the mitogen-activated protein kinase p38, which is activated through the small GTPase Rac. TNF-α stimulates both Rac and RhoA through the guanine nucleotide exchange factor (GEF)-H1 but by different mechanisms. EGFR- and ERK-dependent phosphorylation at the T678 site of GEF-H1 is a prerequisite for RhoA activation only, whereas both Rac and RhoA activation require GEF-H1 phosphorylation on S885. Of interest, GEF-H1-mediated Rac activation is upstream from the TACE/EGFR/ERK pathway and regulates T678 phosphorylation. We also show that TNF-α enhances epithelial wound healing through TACE, ERK, and GEF-H1. Taken together, our findings can explain the mechanisms leading to hierarchical activation of Rac and RhoA by TNF-α through a single GEF. This mechanism could coordinate GEF functions and fine-tune Rac and RhoA activation in epithelial cells, thereby promoting complex functions such as sheet migration.

Loss of claudins 2 and 15 from mice causes defects in paracellular Na+ flow and nutrient transport in gut and leads to death from malnutrition

BACKGROUND & AIMS

The intestinal symport system moves nutrients across membranes via transporters, and is required for absorption of major nutrients such as glucose, amino acids, and bile acids (which are required for fat absorption). Most of these transporters are regulated by Na(+), but the standard diet does not provide sufficient levels of this ion to the intestinal lumen to support this system. Claudins form paracellular barriers between epithelial cells, and claudin-2 and -15 regulate paracellular ion flow in the intestine. We investigated how cell adherence, tight junction barriers, and claudins regulate the supply of Na(+) to the intestinal lumen in mice.

METHODS

We created Cldn2(-/-)Cldn15(-/-) (double-knockout) mice and analyzed intestinal tissues by reverse-transcription polymerase chain reaction, immunoblot, immunofluorescence, electron microscopy, and H&E analyses. We also measured paracellular Na(+) flow, luminal Na(+) concentration, and absorption of glucose, amino acids, and fats, which were administered orally to the mice.

RESULTS

Paracellular flow of Na(+) from the intestinal submucosa to the lumen, and therefore the concentration of Na(+) in the lumen, was greatly reduced in intestines of Cldn2(-/-)Cldn15(-/-) mice. Absorption of glucose, amino acids, and fats also decreased in the mice, which died by postnatal day 25 from malnutrition.

CONCLUSIONS

The paracellular flow of Na(+) from the intestinal submucosa is regulated by tight junctions that contain claudin-2 and -15. This system is required for the absorption of glucose, amino acids, and fats; disruption of this system in mice leads to infant death as a result of malabsorption.

Atelectrauma disrupts pulmonary epithelial barrier integrity and alters the distribution of tight junction proteins ZO-1 and claudin 4

Mechanical ventilation inevitably exposes the delicate tissues of the airways and alveoli to abnormal mechanical stresses that can induce pulmonary edema and exacerbate conditions such as acute respiratory distress syndrome. The goal of our research is to characterize the cellular trauma caused by the transient abnormal fluid mechanical stresses that arise when air is forced into a liquid-occluded airway (i.e., atelectrauma). Using a fluid-filled, parallel-plate flow chamber to model the "airway reopening" process, our in vitro study examined consequent increases in pulmonary epithelial plasma membrane rupture, paracellular permeability, and disruption of the tight junction (TJ) proteins zonula occludens-1 and claudin-4. Computational analysis predicts the normal and tangential surface stresses that develop between the basolateral epithelial membrane and underlying substrate due to the interfacial stresses acting on the apical cell membrane. These simulations demonstrate that decreasing the velocity of reopening causes a significant increase in basolateral surface stresses, particularly in the region between neighboring cells where TJs concentrate. Likewise, pulmonary epithelial wounding, paracellular permeability, and TJ protein disruption were significantly greater following slower reopening. This study thus demonstrates that maintaining a higher velocity of reopening, which reduces the damaging fluid stresses acting on the airway wall, decreases the mechanical stresses on the basolateral cell surface while protecting cells from plasma membrane rupture and promoting barrier integrity.

Spiral progression in the development of absorption enhancers based on the biology of tight junctions

Epithelium covers the body and, therefore, separates the inner body from the outside environment. Passage across the epithelium is the first step in drug absorption. Tight junctions (TJs) seal the space between adjacent epithelial cells and prevent the free movement of solutes through the paracellular space. Modulation of the epithelial barrier is the most important strategy for enhancing drug absorption. Development of the strategy has accelerated with progress in understanding of the biology of the TJ seal. The first-generation absorption enhancers were screened on the basis of their absorption-enhancing activity in vivo. However, TJs were not well understood initially. The identification of TJ components, including those based on occludin and claudins, has led to the development of new strategies for drug absorption. Accumulation of knowledge of claudins has provided new insights into the paracellular transport of drugs. This review examines the relationship between advances in understanding of TJ biology and paracellular transport of drugs and discusses progress in the development of mucosal absorption enhancers.

p38 MAPK activation and function following osmotic shock of erythrocytes

p38 protein kinase is activated by hyperosmotic shock, participates in the regulation of cell volume sensitive transport and metabolism and is involved in the regulation of various physiological functions including cell proliferation and apoptosis. Similar to apoptosis of nucleated cells, erythrocytes may undergo suicidal death or eryptosis, which is paralleled by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include hyperosmotic shock, which increases cytosolic Ca(2+) activity and ceramide formation. The present study explored whether p38 kinase is expressed in human erythrocytes, is activated by hyperosmotic shock and participates in the regulation of eryptosis. Western blotting was utilized to determine phosphorylation of p38 kinase, forward scatter to estimate cell volume, annexin V binding to depict phosphatidylserine exposure and Fluo3 fluorescence to estimate cytosolic Ca(2+) activity. As a result, erythrocytes express p38 kinase, which is phosphorylated upon osmotic shock (+550 mM sucrose). Osmotic shock decreased forward scatter, increased annexin V binding and increased Fluo3 fluorescence, all effects significantly blunted by the p38 kinase inhibitors SB203580 (2 μM) and p38 Inh III (1 μM). In conclusion, p38 kinase is expressed in erythrocytes and participates in the machinery triggering eryptosis following hyperosmotic shock.

RhoA and Rho kinase mediate cyclosporine A and sirolimus-induced barrier tightening in renal proximal tubular cells

The regulation and maintenance of the paracellular transport in renal tubular epithelia is vital for kidney functions. Combination of the immunosuppressant drugs cyclosporine A (CsA) and sirolimus (SRL) exerts powerful immunosuppression, but also causes nephrotoxicity. We have previously shown that CsA and SRL elevate transepithelial resistance (TER) in kidney tubular cells partly through MEK/ERK1/2. In this work we examined the hypothesis that the RhoA pathway may also be mediating effects of CsA and SRL. We show that CsA and the CsA/SRL combination activated RhoA, induced cofilin phosphorylation and promoted stress fiber generation. The Rho kinase (ROK) inhibitor, Y27632, prevented CsA and CsA/SRL-induced cofilin phosphorylation and actin remodelling, reduced the TER increase and prevented the rise in claudin-7 levels caused by the drugs. Expression of the exchange factor GEF-H1/lfc was elevated in cells treated with CsA and CsA/SRL. GEF-H1 silencing inhibited RhoA activation by ≈50%, and potently reduced cofilin phosphorylation and stress fiber formation induced by CsA and CsA/SRL. However, GEF-H1 downregulation did not prevent the TER change. Thus the Rho/Rho kinase pathway was involved in mediating CsA and CsA/SRL-induced cytoskeleton rearrangement and TER changes via claudin-7 expression. Our data however point to differential regulation of Rho activation involved in central cytoskeleton remodelling, that is GEF-H1-dependent and junctional permeability that does not require GEF-H1.

Paracellular permeability of bronchial epithelium is controlled by CFTR

In normal airway epithelium, the cystic fibrosis transmembrane conductance regulator (CFTR) transports Cl(-) ions to the apical surface of the epithelium paralleled by the flow of water through transcellular and paracellular pathways. The hypothesis was tested whether CFTR not only regulates the transcellular but also the paracellular shunt pathway. Therefore, we performed measurements of transepithelial electrical resistance (TER) and paracellular (14)C-mannitol permeability in wtCFTR (16HBE14o(-)) and delF508-CFTR (CFBE41o(-)) expressing human bronchial epithelial cells. Under resting conditions, CFBE41o(-) cell monolayers exhibit a higher paracellular permeability and lower TER as compared to 16HBE14o(-) monolayers. Stimulation of CFTR by cAMP induces opposite effects in the two cell lines. 16HBE14o(-) monolayers show a sharp decrease of TER, in parallel with a concomitant increase of paracellular permeability. The change in paracellular permeability is mediated by a myosin II dependent mechanism because it can be blocked by the myosin light chain kinase inhibitor ML-7. In contrast, CFBE41o(-) cells respond to cAMP stimulation with a decrease of paracellular permeability, paralleled by slight increase of TER. We conclude that stimulation of wtCFTR increases vectorial transcellular salt transport and, simultaneously, the paracellular permeability allowing water to follow through the paracellular pathway. In contrast, in CF epithelium cAMP stimulation increases neither vectorial salt transport nor paracellular permeability which is likely to contribute to the CF pulmonary phenotype. Taken together, our results link CFTR dysfunction to an improper regulation of the paracellular transport route.

Tight junction proteins and the epidermis

Epithelia are found at the interfaces between body compartments where they act as selective permeability barriers that maintain the unique composition of the compartments on either side. Epithelial barrier function is dependent on tight junctions (TJs), which seal the intercellular or paracellular spaces but may permit selective permeability. In simple epithelia (one cell thick), the function of TJs is relatively well understood. By contrast, our understanding of TJ structure and function in stratified epithelia (e.g. the epidermis) is limited. This article briefly discusses what is known about TJs and their components in simple epithelia and speculates about their roles in the epidermis.

Differential role of Rho GTPases in intestinal epithelial barrier regulation in vitro

Maintenance of intestinal epithelial barrier functions is crucial to prevent systemic contamination by microbes that penetrate from the gut lumen. GTPases of the Rho-family such as RhoA, Rac1, and Cdc42 are known to be critically involved in the regulation of intestinal epithelial barrier functions. However, it is still unclear whether inactivation or activation of these GTPases exerts barrier protection or not. We tested the effects of Rho GTPase activities on intestinal epithelial barrier functions by using the bacterial toxins cytotoxic necrotizing factor 1 (CNF-1), toxin B, C3 transferase (C3 TF), and lethal toxin (LT) in an in vitro model of the intestinal epithelial barrier. Incubation of cell monolayers with CNF-1 for 3 h induced exclusive activation of RhoA whereas Rac1 and Cdc42 activities were unchanged. As revealed by FITC-dextran flux and measurements of transepithelial electrical resistance (TER) intestinal epithelial permeability was significantly increased under these conditions. Inhibition of Rho kinase via Y27632 blocked barrier destabilization of CNF-1 after 3 h. In contrast, after 24 h of incubation with CNF-1 only Rac1 and Cdc42 but not RhoA were activated which resulted in intestinal epithelial barrier stabilization. Toxin B to inactivate RhoA, Rac1, and Cdc42 as well as Rac1 inhibitor LT increased intestinal epithelial permeability. Similar effects were observed after inhibition of RhoA/Rho kinase signaling by C3 TF or Y27632. Taken together, these data demonstrate that both activation and inactivation of RhoA signaling increased paracellular permeability whereas activation of Rac1 and Cdc42 correlated with stabilized barrier functions.

Loss of claudin-15, but not claudin-2, causes Na+ deficiency and glucose malabsorption in mouse small intestine

BACKGROUND & AIMS

In the small intestine, the paracellular transport of Na(+) is thought to be critical for luminal Na(+)-homeostasis and the transcellular absorption of nutrients by Na(+)-driven transporters. Na(+) is supplied to the intestinal lumen from the submucosa and serum through tight junctions, which form a paracellular barrier between the cells of epithelial sheets. However, the molecular basis for this paracellular transport of Na(+) is not well understood. Here, we examined this mechanism by performing loss-of-function studies of claudin-2 and claudin-15, two tight-junctional membrane proteins that are specifically and age-dependently expressed in the villi and/or crypts of small intestinal epithelia.

METHODS

Knockout mice for claudin-2 or claudin-15 were subjected to histologic, cell biologic, electrophysiologic, and physiologic analyses.

RESULTS

Examination of the knockout mice revealed that both claudin-2 and claudin-15 play crucial roles in the transepithelial paracellular channel-like permselectivity for extracellular monovalent cations, particularly Na(+), in infants and adults. Especially in Cldn15(-/-) adults, the luminal Na(+) concentration in the small intestine measured directly in vivo was abnormally low, and glucose absorption was impaired, as assessed by the oral glucose tolerance test and estimation of unabsorbed glucose.

CONCLUSIONS

We propose that the "Na(+)-leaky" claudin-15 is indispensable in vivo for the paracellular Na(+) permeability, luminal Na(+)-homeostasis, and efficient glucose absorption in the small intestine, but claudin-2 is indispensable for only the first of these functions. Claudin-15 knockout leads to Na(+) deficiency and glucose malabsorption in the mouse adult small intestine.

Extracellular signal-regulated kinase and GEF-H1 mediate depolarization-induced Rho activation and paracellular permeability increase

Plasma membrane depolarization activates the Rho/Rho kinase (ROK) pathway and thereby enhances myosin light chain (MLC) phosphorylation, which in turn is thought to be a key regulator of paracellular permeability. However, the upstream mechanisms that couple depolarization to Rho activation and permeability changes are unknown. Here we show that three different depolarizing stimuli (high extracellular K(+) concentration, the lipophilic cation tetraphenylphosphonium, or l-alanine, which is taken up by electrogenic Na(+) cotransport) all provoke robust phosphorylation of ERK in LLC-PK1 and Madin-Darby canine kidney (MDCK) cells. Importantly, inhibition of ERK prevented the depolarization-induced activation of Rho. Searching for the underlying mechanism, we have identified the GTP/GDP exchange factor GEF-H1 as the ERK-regulated critical exchange factor responsible for the depolarization-induced Rho activation. This conclusion is based on our findings that 1) depolarization activated GEF-H1 but not p115RhoGEF, 2) short interfering RNA-mediated GEF-H1 silencing eliminated the activation of the Rho pathway, and 3) ERK inhibition prevented the activation of GEF-H1. Moreover, we found that the Na(+)-K(+) pump inhibitor ouabain also caused ERK, GEF-H1, and Rho activation, partially due to its depolarizing effect. Regarding the functional consequences of this newly identified pathway, we found that depolarization increased paracellular permeability in LLC-PK1 and MDCK cells and that this effect was mitigated by inhibiting myosin using blebbistatin or a dominant negative (phosphorylation incompetent) MLC. Taken together, we propose that the ERK/GEF-H1/Rho/ROK/pMLC pathway could be a central mechanism whereby electrogenic transmembrane transport processes control myosin phosphorylation and regulate paracellular transport in the tubular epithelium.

Purinergic regulation of the epithelial Na+ channel

1. The epithelial Na(+) channel (ENaC) is a major conductive pathway that transports Na(+) across the apical membrane of the distal nephron, the respiratory tract, the distal colon and the ducts of exocrine glands. The ENaC is regulated by hormonal and humoral factors, including extracellular nucleotides that are available from the epithelial cells themselves. 2. Extracellular nucleotides, via the P2Y2 receptors (P2Y2Rs) at the basolateral and apical membrane of the epithelia, trigger signalling systems that inhibit the activity of the ENaC and activate Ca(2+) -dependent Cl(-) secretion. 3. Recent data from our laboratory suggest that stimulation of the P2Y2Rs at the basolateral membrane inhibits ENaC activity by a signalling mechanism that involves G beta gamma subunits freed from a pertussis toxin (PTX)-sensitive G-protein and phospholipase C (PLC) beta 4. A similar signalling mechanism is also partially responsible for inhibition of the ENaC during activation of apical P2Y2Rs. 4. Stimulation of apical P2Y2Rs also activates an additional signalling mechanism that inhibits the ENaC and involves the activated Galpha subunit of a PTX-insensitive G-protein and activation of an unidentified PLC. The effect of this PTX-insensitive system requires the activity of the basolateral Na(+)/K(+)/2Cl(-) cotransporter.

Direct stimulation of the transcellular and paracellular calcium transport in the rat cecum by prolactin

Prolactin (PRL) is reported to stimulate calcium absorption in the rat's small intestine. However, little is known regarding its effects on the cecum, a part of the large intestine with the highest rate of intestinal calcium transport. We demonstrated herein by quantitative real-time polymerase chain reaction and Western blot analysis that the cecum could be a target organ of PRL since cecal epithelial cells strongly expressed PRL receptors. In Ussing chamber experiments, PRL enhanced the transcellular cecal calcium absorption in a biphasic dose-response manner. PRL also increased the paracellular calcium permeability and passive calcium transport in the cecum, which could be explained by the PRL-induced decrease in transepithelial resistance and increase in cation selectivity of the cecal epithelium. PRL actions in the cecum were abolished by inhibitors of phosphoinositide 3-kinase (PI3K), protein kinase C (PKC), and RhoA-associated coiled-coil forming kinase (ROCK), but not inhibitors of gene transcription and protein biosynthesis. In conclusion, PRL directly enhanced the transcellular and paracellular calcium transport in the rat cecum through the nongenomic signaling pathways involving PI3K, PKC, and ROCK.

Enhancement of calcium transport in Caco-2 monolayer through PKCzeta-dependent Cav1.3-mediated transcellular and rectifying paracellular pathways by prolactin

Previous investigations suggested that prolactin (PRL) stimulated the intestinal calcium absorption through phosphoinositide 3-kinase (PI3K), protein kinase C (PKC), and RhoA-associated coiled-coil forming kinase (ROCK) signaling pathways. However, little was known regarding its detailed mechanisms for the stimulation of transcellular and voltage-dependent paracellular calcium transport. By using Ussing chamber technique, we found that the PRL-induced increase in the transcellular calcium flux and decrease in transepithelial resistance of intestinal-like Caco-2 monolayer were not abolished by inhibitors of gene transcription and protein biosynthesis. The PRL-stimulated transcellular calcium transport was completely inhibited by the L-type calcium channel blockers (nifedipine and verapamil) and plasma membrane Ca(2+)-ATPase (PMCA) inhibitor (trifluoperazine) as well as small interfering RNA targeting voltage-dependent L-type calcium channel Ca(v)1.3, but not TRPV6 or calbindin-D(9k). As demonstrated by (45)Ca uptake study, PI3K and PKC, but not ROCK, were essential for the PRL-enhanced apical calcium entry. In addition, PRL was unable to enhance the transcellular calcium transport after PKC(zeta) knockdown or exposure to inhibitors of PKC(zeta), but not of PKC(alpha), PKC(beta), PKC(epsilon), PKC(mu), or protein kinase A. Voltage-clamping experiments further showed that PRL markedly stimulated the voltage-dependent calcium transport and removed the paracellular rectification. Such PRL effects on paracellular transport were completely abolished by inhibitors of PI3K (LY-294002) and ROCK (Y-27632). It could be concluded that the PRL-stimulated transcellular calcium transport in Caco-2 monolayer was mediated by Ca(v)1.3 and PMCA, presumably through PI3K and PKC(zeta) pathways, while the enhanced voltage-dependent calcium transport occurred through PI3K and ROCK pathways.

GEF-H1 mediates tumor necrosis factor-alpha-induced Rho activation and myosin phosphorylation: role in the regulation of tubular paracellular permeability

Tumor necrosis factor-alpha (TNF-alpha), an inflammatory cytokine, has been shown to activate the small GTPase Rho, but the underlying signaling mechanisms remained undefined. This general problem is particularly important in the kidney, because TNF-alpha, a major mediator of kidney injury, is known to increase paracellular permeability in tubular epithelia. Here we aimed to determine the effect of TNF-alpha on the Rho pathway in tubular cells (LLC-PK(1) and Madin-Darby canine kidney), define the upstream signaling, and investigate the role of the Rho pathway in the TNF-alpha-induced alterations of paracellular permeability. We show that TNF-alpha induced a rapid and sustained RhoA activation that led to stress fiber formation and Rho kinase-dependent myosin light chain (MLC) phosphorylation. To identify new regulators connecting the TNF receptor to Rho signaling, we applied an affinity precipitation assay with a Rho mutant (RhoG17A), which captures activated GDP-GTP exchange factors (GEFs). Mass spectrometry analysis of the RhoG17A-precipitated proteins identified GEF-H1 as a TNF-alpha-activated Rho GEF. Consistent with a central role of GEF-H1, its down-regulation by small interfering RNA prevented the activation of the Rho pathway. Moreover GEF-H1 and Rho activation are downstream of ERK signaling as the MEK1/2 inhibitor PD98059 mitigated TNF-alpha-induced activation of these proteins. Importantly TNF-alpha enhanced the ERK pathway-dependent phosphorylation of Thr-678 of GEF-H1 that was key for activation. Finally the TNF-alpha-induced paracellular permeability increase was absent in LLC-PK(1) cells stably expressing a non-phosphorylatable, dominant negative MLC. In summary, we have identified the ERK/GEF-H1/Rho/Rho kinase/phospho-MLC pathway as the mechanism mediating TNF-alpha-induced elevation of tubular epithelial permeability, which in turn might contribute to kidney injury.

The uptake and intracellular fate of PLGA nanoparticles in epithelial cells

Biodegradable polymer nanoparticles (NPs) are a promising approach for intracellular delivery of drugs, proteins, and nucleic acids, but little is known about their intracellular fate, particularly in epithelial cells, which represent a major target. Rhodamine-loaded PLGA (polylactic-co-glycolic acid) NPs were used to explore particle uptake and intracellular fate in three different epithelial cell lines modeling the respiratory airway (HBE), gut (Caco-2), and renal proximal tubule (OK). To track intracellular fate, immunofluorescence techniques and confocal microscopy were used to demonstrate colocalization of NPs with specific organelles: early endosomes, late endosomes, lysosomes, endoplasmic reticulum (ER), and Golgi apparatus. Confocal analysis demonstrated that NPs are capable of entering cells of all three types of epithelium. NPs appear to colocalize with the early endosomes at short times after exposure (approximately 2 h), but are also found in other compartments within the cytoplasm, notably Golgi and, possibly, ER, as time progressed over the period of 4-24 h. The rate and extent of uptake differed among these cell lines: at a fixed particle/cell ratio, cellular uptake was most abundant in OK cells and least abundant in Caco-2 cells. We present a model for the intracellular fate of particles that is consistent with our experimental data.

Prolactin stimulates transepithelial calcium transport and modulates paracellular permselectivity in Caco-2 monolayer: mediation by PKC and ROCK pathways

Prolactin (PRL) was previously demonstrated to rapidly enhance calcium absorption in rat duodenum and the intestine-like Caco-2 monolayer. However, its mechanism was not completely understood. Here, we investigated nongenomic effects of PRL on the transepithelial calcium transport and paracellular permselectivity in the Caco-2 monolayer by Ussing chamber technique. PRL increased the transcellular and paracellular calcium fluxes and paracellular calcium permeability within 60 min after exposure but decreased the transepithelial resistance of the monolayer. The effects of PRL could not be inhibited by RNA polymerase II inhibitor (5,6-dichloro-1-beta-D-ribobenzimidazole), confirming that PRL actions were nongenomic. Exposure to protein kinase C (PKC) or RhoA-associated coiled-coil forming kinase (ROCK) inhibitors (GF-109203X and Y-27632, respectively) abolished the stimulatory effect of PRL on transcellular calcium transport, whereas ROCK inhibitor, but not PKC inhibitor, diminished the PRL effect on paracellular calcium transport. Knockdown of the long isoform of PRL receptor (PRLR-L) also prevented the enhancement of calcium transport by PRL. In addition, PRL markedly increased paracellular sodium permeability and the permeability ratio of sodium to chloride, which are indicators of the paracellular charge-selective property and are known to be associated with the enhanced paracellular calcium transport. The permeability of other cations in the alkali metal series was also increased by PRL, and such increases were abolished by ROCK inhibitor. It could be concluded that PRL stimulated transepithelial calcium transport through PRLR-L and increased paracellular permeability to cations in the Caco-2 monolayer. These nongenomic actions of PRL were mediated by the PKC and ROCK signaling pathways.

Inverse correlation between the extent of N-glycan branching and intercellular adhesion in epithelia. Contribution of the Na,K-ATPase beta1 subunit

The majority of cell adhesion molecules are N-glycosylated, but the role of N-glycans in intercellular adhesion in epithelia remains ill-defined. Reducing N-glycan branching of cellular glycoproteins by swainsonine, the inhibitor of N-glycan processing, tightens and stabilizes cell-cell junctions as detected by a 3-fold decrease in the paracellular permeability and a 2-3-fold increase in the resistance of the adherens junction proteins to extraction by non-ionic detergent. In addition, exposure of cells to swainsonine inhibits motility of MDCK cells. Mutagenic removal of N-glycosylation sites from the Na,K-ATPase beta(1) subunit impairs cell-cell adhesion and decreases the effect of swainsonine on the paracellular permeability of the cell monolayer and also on detergent resistance of adherens junction proteins, indicating that the extent of N-glycan branching of this subunit is important for intercellular adhesion. The N-glycans of the Na,K-ATPase beta(1) subunit and E-cadherin are less complex in tight renal epithelia than in the leakier intestinal epithelium. The complexity of the N-glycans linked to these proteins gradually decreases upon the formation of a tight monolayer from dispersed MDCK cells. This correlates with a cell-cell adhesion-induced increase in expression of GnT-III (stops N-glycan branching) and a decrease in expression of GnTs IVC and V (promote N-glycan branching) as detected by real-time quantitative PCR. Consistent with these results, partial silencing of the gene encoding GnT-III increases branching of N-glycans linked to the Na,K-ATPase beta(1) subunit and other glycoproteins and results in a 2-fold increase in the paracellular permeability of MDCK cell monolayers. These results suggest epithelial cells can regulate tightness of cell junctions via remodeling of N-glycans, including those linked to the Na,K-ATPase beta(1)-subunit.

Interactions of tight junctions with membrane channels and transporters

Tight junctions are unique organelles in epithelial cells. They are localized to the apico-lateral region and essential for the epithelial cell transport functions. The paracellular transport process that occurs via tight junctions is extensively studied and is intricately regulated by various extracellular and intracellular signals. Fine regulation of this transport pathway is crucial for normal epithelial cell functions. Among factors that control tight junction permeability are ions and their transporters. However, this area of research is still in its infancy and much more needs to be learned about how these molecules regulate tight junction structure and functions. In this review we have attempted to compile literature on ion transporters and channels involved in the regulation of tight junctions.

Role of p38 MAP kinase and transforming growth factor-beta signaling in transepithelial migration of invasive bacterial pathogens

Streptococcus pneumoniae and Haemophilus influenzae are human pathogens that often asymptomatically colonize the mucosal surface of the upper respiratory tract, but also occasionally cause invasive disease. The ability of these species to traverse the epithelium of the airway mucosa was modeled in vitro using polarized respiratory epithelial cells in culture. Migration across the epithelial barrier was preceded by loss of transepithelial resistance. Membrane products of S. pneumoniae that included lipoteichoic acid induced disruption of the epithelial barrier in a Toll-like receptor 2-dependent manner. This result correlates with a recent genetic study that associates increased TLR2 signaling with increased rates of invasive pneumococcal disease in humans. Loss of transepithelial resistance by the TLR2 ligand correlated with activation of p38 MAP kinase and transforming growth factor (TGF)-beta signaling. Activation of p38 MAPK and TGF-beta signaling in epithelial cells upon nasal infection with S. pneumoniae was also demonstrated in vivo. Inhibition of either p38 MAPK or TGF-beta signaling was sufficient to inhibit the migration of S. pneumoniae or H. influenzae. Our data shows that diverse bacteria utilize common mechanisms, including MAPK and TGF-beta signaling pathways to disrupt epithelial barriers and promote invasion.