Clostridium perfringens enterotoxin fragment removes specific claudins from tight junction strands: Evidence for direct involvement of claudins in tight junction barrier

Distribution of the tight junction proteins ZO-1, occludin, and claudin-4, -8, and -12 in bladder epithelium

In mammals, the bladder stores urine without permitting the passage of urine contents into the bloodstream, a function, in part, of the uroepithelial-associated tight junction complex. The protein constituents that make up this high-resistance barrier in the bladder are currently unknown, although the claudins, a multigene family, are thought to govern paracellular transport in other epithelia. Reverse transcriptase-polymerase chain reaction analysis was used to define that mRNA for claudin-2, -4, -8, -12, and -13 was expressed in mouse bladder tissue. The localization of these claudins, as well as the tight junction-associated proteins zonula occludens-1 (ZO-1) and occludin, within the bladder epithelium was determined by immunofluorescence microscopy. As expected, occludin and ZO-1 were localized to the tight junctions of rat, mouse, and rabbit umbrella cells. Intriguingly, ZO-1 in mouse epithelium, ZO-1 in the dome region of rabbit bladders and occludin in rat and mouse bladders were also expressed in the underlying intermediate and basal cell layers. Claudin-4, -8, and -12 were found in the umbrella cell tight junction; however, additional staining of claudin-4 was observed along the sites of cell-cell contact in the underlying cell layers of rat, mouse, and rabbit tissue. No claudin-2 staining was associated with tight junctions in the uroepithelium. Our results indicate that claudin-4, -8, and -12 are expressed in umbrella cells, where they may impart the high-resistance phenotype associated with this cell type, and that in some instances tight junction proteins are also associated at the sites of cell contact of the underlying cell layers, perhaps playing some role in cell-cell adhesion.

The tight junction: a multifunctional complex

Multicellular organisms are separated from the external environment by a layer of epithelial cells whose integrity is maintained by intercellular junctional complexes composed of tight junctions, adherens junctions, and desmosomes, whereas gap junctions provide for intercellular communication. The aim of this review is to present an updated overview of recent developments in the area of tight junction biology. In a relatively short time, our knowledge of the tight junction has evolved from a relatively simple view of it being a permeability barrier in the paracellular space and a fence in the plane of the plasma membrane to one of it acting as a multicomponent, multifunctional complex that is involved in regulating numerous and diverse cell functions. A group of integral membrane proteins-occludin, claudins, and junction adhesion molecules-interact with an increasingly complex array of tight junction plaque proteins not only to regulate paracellular solute and water flux but also to integrate such diverse processes as gene transcription, tumor suppression, cell proliferation, and cell polarity.

Clostridium perfringens enterotoxin elicits rapid and specific cytolysis of breast carcinoma cells mediated through tight junction proteins claudin 3 and 4

Clostridium perfringens enterotoxin (CPE) induces cytolysis very rapidly through binding to its receptors, the tight junction proteins CLDN 3 and 4. In this study, we investigated CLDN 3 and 4 expression in breast cancer and tested the potential of CPE-mediated therapy. CLDN 3 and 4 proteins were detected in all primary breast carcinomas tested (n = 21) and, compared to normal mammary epithelium, were overexpressed in approximately 62% and 26%, respectively. Treatment of breast cancer cell lines in culture with CPE resulted in rapid and dose-dependent cytolysis exclusively in cells that expressed CLDN 3 and 4. Intratumoral CPE treatment of xenografts of T47D breast cancer cells in immunodeficient mice resulted in a significant reduction in tumor volume (P = 0.007), with accompanying necrosis. Necrotic reactions were also seen in three freshly resected primary breast carcinoma samples treated with CPE for 12 hours, while isolated primary breast carcinoma cells underwent rapid and complete cytolysis within 1 hour. Thus, expression of CLDN 3 and 4 sensitizes primary breast carcinomas to CPE-mediated cytolysis and emphasizes the potential of CPE in breast cancer therapy.

Exploiting immune surveillance mechanisms in mucosal vaccine development

Historically, immune responsiveness was regarded by many as an ability to discriminate self from non-self, but this definition has recently been revised to be a distinction between threatening infectious organisms versus innocuous molecules from autologous tissues. Such distinctions can be made in the context of adjuvant effects from triggering of 'pattern recognition receptors' by pathogen-associated molecules. Mucosal sites such as airway and intestinal passages present a particularly interesting challenge to this system, as distinctions must be effectively made between innocuous non-self molecules associated with food and commensal bacteria versus pathogenic viruses and bacteria. Given the simultaneous presence of all these molecular types at mucosal lymphoid sites, immunological discrimination mechanisms must be especially precise, as immune responses must be directed only at pathogen-associated targets. Ongoing research is identifying genes that may be critical to triggering mucosal immunity; an understanding of their role in discrimination may lead to the development of new vaccines.

A new principle for tight junction modulation based on occludin peptides

The aim of this study was to investigate whether peptides from the extracellular loops of the tight junction protein occludin could be used as a new principle for tight junction modulation. Peptides of 4 to 47 amino acids in length and covering the two extracellular loops of the tight junction protein occludin were synthesized, and their effect on the tight junction permeability in Caco-2 cells was investigated using [14C]mannitol as a para-cellular marker. Lipopeptide derivatives of one of the active occludin peptides (OPs), synthesized by adding a lipoamino acid containing 14 carbon atoms (C14-) to the N terminus of the peptide, were also investigated. Peptides corresponding to the N terminus of the first extracellular loop of occludin increased the permeability of the tight junctions without causing short-term toxicity. However, the peptides had an effect only when added to the basolateral side of the cells, which could be partly explained by degradation by apical peptidases and aggregate formation. By contrast, the lipopeptide C14-OP90-103, which protects the peptide from degradation and aggregation, displayed a rapid apical effect. The l- and d-diastereomers of C14-OP90-103 had distinctly different effects. The d-isomer, which releases intact OP90-103 from the lipoamino acid, displayed a rapid and transient increase in tight junction permeability. The l-isomer, which releases OP90-103 more rapidly, gave a more sustained increase in tight junction permeability. In conclusion, C14-OP90-103 represents a prototype of a new class of tight junction modulators that act on the extracellular domains of tight junction proteins.

Expression patterns of claudins, tight junction adhesion molecules, in the inner ear

Tight junctions (TJs) are indispensable for the establishment of compositionally distinct fluid compartments in the inner ear, but our knowledge of the claudins, TJ adhesion molecules, in the inner ear is still fragmentary. We examined the expression and distribution of claudin-1 to claudin-18 (except for claudin-7, -13 and -17) in the inner ear by immunofluorescence microscopy. In the cochlea, the organ of Corti expressed claudin-1, -2, -3, -9, -10, -12, -14 and -18. In the stria vascularis, claudin-1, -2, -3, -8, -9, -10, -12, -14 and -18 were expressed in the marginal cells, whereas the basal cells were positive only for claudin-11. In Reissner's membrane and the spiral limbus, the expression of claudin-1, -2, -3, -8, -9, -10, -12, -14 and -18 was detected. Furthermore, in the vestibule, claudin-1, -3, -9, -12, -14 and -18 were expressed in the sensory epithelia, whereas in the dark cell area claudin-1, -3, -8, -9, -12, -14 and -18 were detectable. These findings, i.e., very complex expression patterns of claudin species in the inner ear, would reflect the importance and the complexity of the barrier function of TJs in the inner ear.

HIV-1 Tat protein alters tight junction protein expression and distribution in cultured brain endothelial cells

Disruption of the blood-brain barrier (BBB) is widely believed to be the main route of human immunodeficiency virus (HIV) entry into the central nervous system (CNS). Although mechanisms of this process are not fully understood, alterations of tight junction protein expression can contribute, at least in part, to this phenomenon. Tight junctions are critical structural and functional elements of cerebral microvascular endothelial cells and the BBB. The aim of the present study was to examine the effects of HIV-1 Tat protein on expression of tight junction proteins. Primary cultures of brain microvascular endothelial cells (BMEC) were employed in these experiments. A 24-hr exposure of BMEC to Tat(1-72) resulted in a decrease of claudin-1, claudin-5, and zonula occludens (ZO)-2 expression, whereas total levels of occludin and ZO-1 remained unchanged. In addition, a short (3-hr) exposure of BMEC to Tat(1-72) induced cellular redistribution of claudin-5 immunoreactivity. Tat(1-72)-induced alterations of claudin-5 expression also were confirmed in vivo where Tat(1-72) was injected into the right hippocampus of mice. These findings indicate that HIV-1 Tat protein can markedly affect expression and distribution of specific tight junction proteins in brain endothelium. Alterations of only distinct tight junction proteins suggest a finely tuned effect of Tat(1-72) on the BBB. Because tight junction proteins are critical for the barrier function of the BBB, such alterations can lead to disturbances of the BBB integrity and contribute to HIV trafficking into the brain.

The claudin-like megatrachea is essential in septate junctions for the epithelial barrier function in Drosophila

Vertebrate claudin proteins are integral components of tight junctions, which function as paracellular diffusion barriers in epithelia. We identified Megatrachea (Mega), a Drosophila transmembrane protein homologous to claudins, and show that it acts in septate junctions, the corresponding structure of invertebrates. Our analysis revealed that Mega has transepithelial barrier function similar to the claudins. Also, Mega is necessary for normal tracheal cell morphogenesis but not for apicobasal polarity or epithelial integrity. In addition, we present evidence that Mega is essential for localization of the septate junction protein complex Coracle/Neurexin. The results indicate that claudin-like proteins are functionally conserved between vertebrates and Drosophila.

CLMP, a novel member of the CTX family and a new component of epithelial tight junctions

The CTX family is a growing group of type I transmembrane proteins within the immunoglobulin superfamily (IgSF). They localize to junctional complexes between endothelial and epithelial cells and seem to participate in cell-cell adhesion and transmigration of leukocytes. Here, we report the identification of a new member of the CTX family. This protein, which was designated CLMP (coxsackie- and adenovirus receptor-like membrane protein), is composed of 373 amino acids including an extracellular part containing a V- and a C2-type domain, a transmembrane region and a cytoplasmic tail. CLMP mRNA was detected in a variety of both human and mouse tissues and cell lines. The protein migrated with an Mr of around 48 on SDS-PAGE and was predominantly expressed in epithelial cells within different tissues. In cultured epithelial cells, CLMP was detected in areas of cell-cell contacts. When exogenously expressed in polarized MDCK cells, CLMP was restricted to the subapical area of the lateral cell surface, where it co-localized with the tight junction markers ZO-1 and occludin. Also endogenous CLMP showed association with tight junctions, as analyzed in polarized human CACO-2 cells. This suggested a role for CLMP in cell-cell adhesion and indeed, overexpressed CLMP induced aggregation of non-polarized CHO cells. Furthermore, CLMP-expressing MDCK cells showed significantly increased transepithelial resistance, indicating a role for CLMP in junctional barrier function. Thus, we conclude that CLMP is a novel cell-cell adhesion molecule and a new component of epithelial tight junctions. We also suggest, based on phylogenetic studies, that CLMP, CAR, ESAM, and BT-IgSF form a new group of proteins within the CTX family.

Death pathways activated in CaCo-2 cells by Clostridium perfringens enterotoxin

Clostridium perfringens enterotoxin (CPE), a 35-kDa polypeptide, induces cytotoxic effects in the enterocyte-like CaCo-2 cell culture model. To identify the mammalian cell death pathway(s) mediating CPE-induced cell death, CaCo-2 cultures were treated with either 1 or 10 micro g of CPE per ml. Both CPE doses were found to induce morphological damage and DNA cleavage in CaCo-2 cells. The oncosis inhibitor glycine, but not a broad-spectrum caspase inhibitor, was able to transiently block both of those pathological effects in CaCo-2 cells treated with the higher, but not the lower, CPE dose. Conversely, a caspase 3/7 inhibitor (but not glycine or a caspase 1 inhibitor) blocked morphological damage and DNA cleavage in CaCo-2 cells treated with the lower, but not the higher, CPE dose. Collectively, these results indicate that lower CPE doses cause caspase 3/7-dependent apoptosis, while higher CPE doses induce oncosis. Apoptosis caused by the lower CPE dose was shown to proceed via a classical pathway involving mitochondrial membrane depolarization and cytochrome c release. As the CPE concentrations used in this study for demonstrating apoptosis and oncosis have pathophysiologic relevance, these results suggest that both oncosis and apoptosis may occur in the intestines during CPE-associated gastrointestinal disease.

Bryostatin-1 enhances barrier function in T84 epithelia through PKC-dependent regulation of tight junction proteins

Protein kinase C (PKC) is known to regulate epithelial barrier function. However, the effect of specific PKC isozymes, and their mechanism of action, are largely unknown. We determined that the nonphorbol ester PKC agonist bryostatin-1 increased transepithelial electrical resistance (TER), a marker of barrier function, in confluent T84 epithelia. Bryostatin-1, which has been shown to selectively activate PKC-alpha, -epsilon, and -delta (34), was associated with a shift in the subcellular distribution of the tight junction proteins claudin-1 and ZO-2 from a detergent-soluble fraction into a detergent-insoluble fraction. Bryostatin-1 also led to the appearance of a higher-molecular-weight form of occludin previously shown to correspond to protein phosphorylation. These changes were attenuated by the conventional and novel PKC inhibitor Gö-6850 but not the conventional PKC inhibitor Gö-6976 or the PKC-delta inhibitor röttlerin, implicating a novel isozyme, likely PKC-epsilon. The results suggest that enhanced epithelial barrier function induced by bryostatin-1 involves a PKC-epsilon-dependent signaling pathway leading to recruitment of claudin-1 and ZO-2, and phosphorylation of occludin, into the tight junctional complex.

Characteristics of claudin expression in follicle-associated epithelium of Peyer's patches: preferential localization of claudin-4 at the apex of the dome region

SUMMARY

Gut-associated lymphoreticular tissues, such as Peyer's patches and cecal patches, are important inductive sites for mucosal immune responses. As such, gut-associated lymphoreticular tissues may have an epithelial barrier different from that of villous epithelium. In this study, we investigated the immunohistochemical distribution of the claudin family and occludin in the follicle-associated epithelium (FAE) of Peyer's patches and cecal patches of murine intestine. Unique profiles of claudin-2, -3, and -4 and occludin expression were noted in the tight junctions of the FAE: claudin-4 was preferentially expressed in the apex region; claudin-2 was only weakly expressed on the crypt side of the FAE compared with stronger expression on the crypt side of villous epithelial cells; and claudin-3 and occludin were found throughout the dome. These unique expression patterns were present also in cecal patch FAE. We also found that claudin-4 expression in the FAE of Peyer's patches and cecal patches correlated with the presence of TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling)-positive apoptotic cells, and Peyer's patch-deficient mice exhibited expression patterns of claudin and occludin in villous epithelia similar to those in wild-type mice. We conclude that claudin-4 expression was preferentially associated with the dome region of FAE, the mucosal inductive site of the murine intestine. In that location it might correlate with the cell life cycle, help maintain the apex configuration of the dome, or be a factor favoring the uptake of antigens by the FAE.

Molecular anatomy of intercellular junctions in brain endothelial and epithelial barriers: electron microscopist's view

In this review, we have tried to summarize the current knowledge on the distribution of important molecular components of intercellular junctions-both tight junctions (TJs) and adherens junctions (AJs)-at the level of ultrastructure. For this purpose, immunogold procedure was applied to ultrathin sections of brain samples obtained from mice, rats, and humans and embedded in hydrophilic resin Lowicryl K4M. The results of our observations performed with transmission electron microscopy (EM) are discussed and compared with findings of other authors. Although the main structures responsible for the barrier and fence functions of the blood-brain barrier (BBB) and blood-CSF barrier are TJs present between endothelial cells (ECs) of brain capillaries and epithelial cells of the choroid plexus, their functional characteristics (e.g. tightness of the barrier evaluated by electrical resistance) differ significantly. Therefore, our main attention is focused on the presence and distribution of both intrinsic, i.e. integral membrane (transmembrane), molecules such as occludin, claudins, and junctional adhesion molecule (JAM) in TJs, and cadherins in AJs, as well as peripheral molecules of both types of junctions, e.g. zonula occludens (ZO) proteins and catenins. The latter group of molecules connects transmembrane proteins with the cell cytoskeleton. A close spatial association of the TJ proteins with those of AJs indicates that both junctional types are intermingled in the BBB type of endothelium. One of most important purposes of this work is to find out the junction-associated molecules that can serve as sensitive markers of normal or disturbed function of brain barriers. Understanding the structural-functional relations between molecular components of junctional complexes in physiological and experimental conditions of both barriers can provide important information about the etiology of various pathological conditions of the central nervous system and also help to elaborate new therapeutic approaches.

Size-selective loosening of the blood-brain barrier in claudin-5-deficient mice

Tight junctions are well-developed between adjacent endothelial cells of blood vessels in the central nervous system, and play a central role in establishing the blood-brain barrier (BBB). Claudin-5 is a major cell adhesion molecule of tight junctions in brain endothelial cells. To examine its possible involvement in the BBB, claudin-5-deficient mice were generated. In the brains of these mice, the development and morphology of blood vessels were not altered, showing no bleeding or edema. However, tracer experiments and magnetic resonance imaging revealed that in these mice, the BBB against small molecules (<800 D), but not larger molecules, was selectively affected. This unexpected finding (i.e., the size-selective loosening of the BBB) not only provides new insight into the basic molecular physiology of BBB but also opens a new way to deliver potential drugs across the BBB into the central nervous system.

Holey barrier: claudins and the regulation of brain endothelial permeability

Endothelial tight junctions (TJs)* are an important functional part of the blood-brain barrier (BBB). In this issue, Nitta et al. (2003) demonstrate that claudin-5, a transmembrane protein of TJs, is a critical determinant of BBB permeability in mice. Unexpectedly, knockout of claudin-5 did not result in a general breakdown of TJs but in a selective increase in paracellular permeability of small molecules. This suggests that the BBB can be manipulated to allow selective diffusion of small molecules and makes claudin-5 a possible target for the development of drugs for this purpose.

Tight junctions in the skin

Tight junctions (=zonulae occludentes, TJs) function as an effective barrier in simple epithelia. Recent developments in the molecular biology of TJs revealed that TJs also exist in the stratum granulosum and contribute to barrier function in epidermis. Furthermore, several TJ-related junctions were identified in epidermis. In this review article, the history of the investigation into TJs in epidermis and the perspectives of investigation into TJs in dermatology are described.

Synergistic effects of neurons and astrocytes on the differentiation of brain capillary endothelial cells in culture

Brain capillary endothelial cells form a functional barrier between blood and brain, based on the existence of tight junctions that limit paracellular permeability. Occludin is one of the major transmembrane proteins of tight junctions and its peripheral localization gives indication of tight junction formation. We previously reported that RBE4.B cells (brain capillary endothelial cells), cultured on collagen IV, synthesize occludin and correctly localize it at the cell periphery only when cocultured with neurons. In the present study, we describe a three-cell type-culture system that allowed us to analyze the combined effects of neurons and astrocytes on differentiation of brain capillary endothelial cells in culture. In particular, we found that, in the presence of astrocytes, the neuron-induced synthesis and localization of occludin is precocious as compared to cells cocultured with neurons only.

Inhibitors of glycosphingolipid biosynthesis reduce transepithelial electrical resistance in MDCK I and FRT cells

Madin-Darby canine kidney (MDCK) I and Fisher rat thyroid (FRT) cells exhibit transepithelial electrical resistance (TER) values in excess of 5,000 Omega. cm(2). When these cells were incubated in the presence of various inhibitors of sphingolipid biosynthesis, a >5-fold reduction of TER was observed without changes in the gate function for uncharged solutes or the fence function for apically applied fluorescent lipids. The localization of ZO-1 and occludin was not altered between control and inhibitor-treated cells, indicating that the tight junction was still intact. Furthermore, the complexity of tight junction strands, analyzed by freeze-fracture microscopy, was not reduced. Once the inhibitor was removed and the cells were allowed to synthesize sphingolipids, a gradual recovery of the TER was observed. Interestingly, these inhibitors did not attenuate the TER of MDCK II cells, a cell line that typically exhibits values below 800 omega x cm(2.) These results suggest that glycosphingolipids play a role in regulating the electrical properties of epithelial cells.

Melatonin induced cyclic modulation of vectorial water transport in kidney-derived MDCK cells

BACKGROUND

Melatonin, newly synthesized by the pineal gland, is rapidly released to general circulation reaching a nanomolar concentration. Cyclic production of melatonin synchronizes body rhythms with the photoperiod. Moreover, changes in urine production and osmolarity have been observed in the kidney during the night. However, the precise mechanisms by which plasma-circulating melatonin modifies renal physiology are not clearly understood.

METHODS

Madin-Darby canine kidney (MDCK) cell monolayers transport water vectorially from the apical to the basolateral side forming blisters or domes. Transport in epithelial cells is regulated by tight junction sealing, ion pumps and channels, and cytoskeleton organization, among other processes. MDCK cells were used to study vectorial water transport to determine the role of microfilament organization and protein kinase C (PKC) in dome formation in culture conditions that mimic the cyclic pattern of melatonin circulation in plasma.

RESULTS

Melatonin cyclically increased dome formation by 50% and caused enlargement and thickening of stress fibers in cells surrounding the domes. Optimal increase in dome formation was observed at nanomolar concentrations of melatonin after 6 hours, concomitantly with a 28% decrease in the transepithelial electrical resistance, which remained low for up to 12 hours, without apparent change in fluorescein isothiocyanate (FITC)-dextran flux. A blockage in dome formation elicited by melatonin was observed in monolayers preincubated with the Na+-K+-ATPase or PKC inhibitors.

CONCLUSION

The results obtained indicate that melatonin cyclically modifies the transepithelial permeability in kidney-derived cells through PKC activation and microfilament reorganization, and supports the hypothesis that melatonin may synchronize daily body rhythms through cyclic cytoskeletal rearrangements.

Intestinal epithelial responses to enteric pathogens: effects on the tight junction barrier, ion transport, and inflammation

The effects of pathogenic organisms on host intestinal epithelial cells are vast. Innumerable signalling pathways are triggered leading ultimately to drastic changes in physiological functions. Here, the ways in which enteric bacterial pathogens utilise and impact on the three major physiological functions of the intestinal epithelium are discussed: alterations in the structure and function of the tight junction barrier, induction of fluid and electrolyte secretion, and activation of the inflammatory cascade. This field of investigation, which was virtually non-existent a decade ago, has now exploded, thus rapidly expanding our understanding of bacterial pathogenesis. Through increased delineation of the ways in which microbes alter host physiology, we simultaneous gain insight into the normal regulatory mechanisms of the intestinal epithelium.

Clostridium perfringens epsilon toxin rapidly decreases membrane barrier permeability of polarized MDCK cells

Epsilon toxin is produced by Clostridium perfringens types B and D which are responsible for fatal intestinal diseases in animals. The main biological activity of epsilon toxin is the production of oedema in various organs. We have previously found that epsilon toxin forms a large membrane complex in MDCK cells which is not internalized into cell, and induces cell volume enlargement and loss of cell viability (Petit, L., Gibert, M., Gillet, D., Laurent-Winter, C., Boquet, P., Popoff, M. R. (1997) J Bacteriol 179, 6480-6487). Here, we show that epsilon toxin is very potent to decrease the trans-epithelial electrical resistance of polarized MDCK cells grown on filters without altering the organization of the junctional complexes. The dose-dependent decrease in trans-epithelial electrical resistance, more marked when the toxin was applied to the apical side than to the basal side of MDCK cells, was associated with a moderate increase of the paracellular permeability to low-molecular-weight compounds but not to macromolecules. Epsilon toxin probably acts by forming large membrane pores which permit the flux of ions and other molecules such as the entry of propidium iodide and finally to the loss of cell viability.

Tight junction proteins

A fundamental function of epithelia and endothelia is to separate different compartments within the organism and to regulate the exchange of substances between them. The tight junction (TJ) constitutes the barrier both to the passage of ions and molecules through the paracellular pathway and to the movement of proteins and lipids between the apical and the basolateral domains of the plasma membrane. In recent years more than 40 different proteins have been discovered to be located at the TJs of epithelia, endothelia and myelinated cells. This unprecedented expansion of information has changed our view of TJs from merely a paracellular barrier to a complex structure involved in signaling cascades that control cell growth and differentiation. Both cortical and transmembrane proteins integrate TJs. Among the former are scaffolding proteins containing PDZ domains, tumor suppressors, transcription factors and proteins involved in vesicle transport. To date two components of the TJ filaments have been identified: occludin and claudin. The latter is a protein family with more than 20 members. Both occludin and claudins are integral proteins capable of interacting adhesively with complementary molecules on adjacent cells and of co-polymerizing laterally. These advancements in the knowledge of the molecular structure of TJ support previous physiological models that exhibited TJ as dynamic structures that present distinct permeability and morphological characteristics in different tissues and in response to changing natural, pathological or experimental conditions.

Claudin-2 expression induces cation-selective channels in tight junctions of epithelial cells

Tight junctions seal the paracellular pathway of epithelia but, in leaky tissues, also exhibit specific permeability. In order to characterize the contribution of claudin-2 to barrier and permeability properties of the tight junction in detail, we studied two strains of Madin-Darby canine kidney cells (MDCK-C7 and MDCK-C11) with different tight junctional permeabilities. Monolayers of C7 cells exhibited a high transepithelial resistance (>1 kOhms cm(2)), compared with C11 cells (<100 Ohms cm(2)). Genuine expression of claudin-1 and claudin-2, but not of occludin or claudin-3, was reciprocal to transepithelial resistance. However, confocal microscopy revealed a marked subjunctional localization of claudin-1 in C11 cells, indicating that claudin-1 is not functionally related to the low tight junctional resistance of C11 cells. Strain MDCK-C7, which endogenously does not express junctional claudin-2, was transfected with claudin-2 cDNA. In transfected cells, but not in vector controls, the protein was detected in colocalization with junctional occludin by means of immunohistochemical analyses. Overexpression of claudin-2 in the originally tight epithelium with claudin-2 cDNA resulted in a 5.6-fold higher paracellular conductivity and relative ion permeabilities of Na(+) identical with 1, K(+)=1.02, NMDG(+)=0.79, choline(+)=0.71, Cl(-)=0.12, Br(-)=0.10 (vector control, 1:1.04:0.95:0.94:0.85:0.83). By contrast, fluxes of (radioactively labeled) mannitol and lactulose and (fluorescence labeled) 4 kDa dextran were not changed. Hence, with regular Ringer's, Na(+) conductivity was 0.2 mS cm(-2) in vector controls and 1.7 mS cm(-2) in claudin-2-transfected cells, while Cl(-) conductivity was 0.2 mS cm(-2) in both cells. Thus, presence of junctional claudin-2 causes the formation of cation-selective channels sufficient to transform a 'tight' tight junction into a leaky one.

Intestinal infection with Giardia spp. reduces epithelial barrier function in a myosin light chain kinase-dependent fashion

BACKGROUND & AIMS

Giardiasis causes malabsorptive diarrhea, and symptoms can be present in the absence of any significant morphologic injury to the intestinal mucosa. The effects of giardiasis on epithelial permeability in vivo remain unknown, and the role of T cells and myosin light chain kinase (MLCK) in altered intestinal barrier function is unclear. This study was conducted to determine whether Giardia spp. alters intestinal permeability in vivo, to assess whether these abnormalities are dependent on T cells, and to assess the role of MLCK in altered epithelial barrier function.

METHODS

Immunocompetent and isogenic athymic mice were inoculated with axenic Giardia muris trophozoites or sterile vehicle (control), then assessed for trophozoite colonization and gastrointestinal permeability. Mechanistic studies using nontransformed human duodenal epithelial monolayers (SCBN) determined the effects of Giardia on myosin light chain (MLC) phosphorylation, transepithelial fluorescein isothiocyanate-dextran fluxes, cytoskeletal F-actin, tight junctional zonula occludens-1 (ZO-1), and MLCK.

RESULTS

Giardia infection caused a significant increase in small intestinal, but not gastric or colonic, permeability that correlated with trophozoite colonization in both immunocompetent and athymic mice. In vitro, Giardia increased permeability and phosphorylation of MLC and reorganized F-actin and ZO-1. These alterations were abolished with an MLCK inhibitor.

CONCLUSIONS

Disruption of small intestinal barrier function is T cell independent, disappears on parasite clearance, and correlates with reorganization of cytoskeletal F-actin and tight junctional ZO-1 in an MLCK-dependent fashion.

Claudin-based barrier in simple and stratified cellular sheets

For homeostasis in multicellular organisms, isolation and compartmentalisation of the internal environment are essential, and are established by various cellular sheets. For these cellular sheets to function as barriers, the intercellular route must be sealed. Recent advances reveal that claudins - major cell adhesion molecules in tight junctions - are directly involved in this intercellular sealing, not only in simple but also in stratified cellular sheets in vertebrates.

Cry1A toxins of Bacillus thuringiensis bind specifically to a region adjacent to the membrane-proximal extracellular domain of BT-R(1) in Manduca sexta: involvement of a cadherin in the entomopathogenicity of Bacillus thuringiensis

Many subspecies of the soil bacterium Bacillus thuringiensis produce various parasporal crystal proteins, also known as Cry toxins, that exhibit insecticidal activity upon binding to specific receptors in the midgut of susceptible insects. One such receptor, BT-R(1) (210 kDa), is a cadherin located in the midgut epithelium of the tobacco hornworm, Manduca sexta. It has a high binding affinity (K(d) approximately 1nM) for the Cry1A toxins of B. thuringiensis. Truncation analysis of BT-R(1) revealed that the only fragment capable of binding the Cry1A toxins of B. thuringiensis was a contiguous 169-amino acid sequence adjacent to the membrane-proximal extracellular domain. The purified toxin-binding fragment acted as an antagonist to Cry1Ab toxin by blocking the binding of toxin to the tobacco hornworm midgut and inhibiting insecticidal action. Exogenous Cry1Ab toxin bound to intact COS-7 cells expressing BT-R(1) cDNA, subsequently killing the cells. Recruitment of BT-R(1) by B. thuringiensis indicates that the bacterium interacts with a specific cell adhesion molecule during its pathogenesis. Apparently, Cry toxins, like other bacterial toxins, attack epithelial barriers by targeting cell adhesion molecules within susceptible insect hosts.

Occludin TM4-: an isoform of the tight junction protein present in primates lacking the fourth transmembrane domain

The tight junction protein occludin possesses four transmembrane domains,two extracellular loops, and cytoplasmic N- and C-termini. Reverse transcription-PCR analysis of human tissues, embryos and cells using primers spanning the fourth transmembrane domain (TM4) and adjacent C-terminal region revealed two products. The larger and predominant product corresponded in sequence to canonical occludin (TM4+), while the smaller product exhibited a 162 bp deletion encoding the entire TM4 and immediate C-terminal flanking region (TM4-). Examination of the genomic occludin sequence identified that the 162 bp sequence deleted in TM4-coincided precisely with occludin exon 4, strongly suggesting that TM4- is an alternative splice isoform generated by skipping of exon 4. Indeed, the reading frame of downstream exons is not affected by exclusion of exon 4. The presence of both TM4+ and TM4- occludin isoforms was also identified in monkey epithelial cells but TM4-was undetected in murine and canine tissue and cells, indicating a late evolutionary origin for this alternative splicing event. Conceptual translation of TM4- isoform predicts extracellular localisation of the C-terminus. Immunocytochemical processing of living human Caco-2 cells using a C-terminal occludin antibody revealed weak, discontinuous staining restricted to the periphery of subconfluent islands of cells, or islands generated by wounding confluent layers. In occludin immunoblots, a weak band at ∼58 kDa, smaller than the predominant band at 65 kDa and corresponding to the predicted mass of TM4- isoform, is evident and upregulated in subconfluent cells. These data suggest that the TM4- isoform may be translated at low levels in specific conditions and may contribute to regulation of occludin function.

Tight junctions and compositionally related junctional structures in mammalian stratified epithelia and cell cultures derived therefrom

The occurrence of extended tight junction (TJ) structures, including zonulae occludentes (ZO), and the spatial arrangement of TJ proteins in stratified mammalian epithelia has long been controversially discussed. Therefore, we have systematically examined the localization of TJ proteins in diverse stratified epithelial tissues (e.g., epidermis, heel pad, snout, gingiva, tongue, esophagus, exocervix, vagina, urothelium, cornea) of various species (human, bovine, rodents) as well as in human cell culture lines derived from stratified epithelia, by electron microscopy as well as by immunocytochemistry at both the light and the electron microscopic level, using antibodies to TJ proteins such as occludin, claudins 1 and 4, protein ZO-1, cingulin and symplekin. We have found an unexpected diversity of TJ-related structures of which only those showing colocalization with the most restricted transmembrane TJ marker protein, occludin, are presented here. While in epidermis and urothelium occludin is restricted to the uppermost living cell layer, TJ-related junctions are abundant in the upper third or even in the majority of the suprabasal cell layers in other stratified epithelia. Interfollicular epidermis contains, in the stratum granulosum, extended, probably continuous ZO-like structures which can also be traced at least through the Henle cell layer of hair follicles. Similar apical ZO-like structures have been seen in the upper living cell layers of all other stratified epithelia and cell cultures examined, but in most of them we have noticed, in addition, junctional regions showing relatively broad, ribbon-like membrane contacts which in cross-section often appear pentalaminar, with an electron-dense middle lamella ("lamellated TJs", coniunctiones laminosae). In suprabasal layers of several stratified epithelia we have further observed TJ protein-containing junctions of variable sizes which are characterized by a 10-30-nm dense lamina interposed between the two membranes ("sandwich junctions"; iuncturae structae). Moreover, we have often observed variously sized regions in which the intermembrane distance is rather regularly bridged by short rod-like elements ("cross-bridged cell walls"; parietes transtillati), often in close vicinity of TJ-related structures or desmosomes. The significance of these structures and their possible biological importance are discussed.

Tight junctions of the blood-brain barrier: development, composition and regulation

1. The blood-brain barrier is essential for the maintenance and regulation of the neural microenvironment. The main characteristic features of blood-brain barrier endothelial cells are an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. 2. Endothelial blood-brain barrier tight junctions differ from epithelial tight junctions, not only by distinct morphological and molecular properties, but also by the fact that endothelial tight junctions are more sensitive to microenvironmental than epithelial factors. 3. Many ubiquitous molecular tight junction components have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin and 7H6. Signaling pathways involved in tight junction regulation include G-proteins, serine-, threonine- and tyrosine-kinases, extra and intracellular calcium levels, cAMP levels, proteases and cytokines. Common to most of these pathways is the modulation of cytoskeletal elements and the connection of tight junction transmembrane molecules to the cytoskeleton. Additionally, crosstalk between components of the tight junction- and the cadherin-catenin system of the adherens junction suggests a close functional interdependence of the two cell-cell contact systems. 4. Important new molecular aspects of tight junction regulation were recently elucidated. This review provides an integration of these new results.

Shigella flexneri regulates tight junction-associated proteins in human intestinal epithelial cells

Shigella spp. are a group of Gram-negative enteric bacilli that cause acute dysentery in humans. We demonstrate that Shigella flexneri has evolved the ability to regulate functional components of tight junctions after interaction at the apical and basolateral pole of model intestinal epithelia. In the regulation of tight junctional protein assemblies, S. flexneri can engage serotype-specific mechanisms, which targets not only expression, but also cellular distribution and membrane association of components of tight junctions. Distinct mechanisms resulting in the regulation of tight junction-associated proteins are initiated after either apical or basolateral interactions. S. flexneri serotype 2a has the ability to remove claudin-1 from Triton X-insoluble protein fractions upon apical exposure to T-84 cell monolayers. S. flexneri serotype 2a and 5, but not the non-invasive Escherichia coli strain F-18, share the ability to regulate expression of ZO-1, ZO-2, E-cadherin and to dephosphorylate occludin. The disruption of tight junctions is dependent on direct interaction of living Shigella with intestinal epithelial cells and is supported by heat-stable secreted bacterial products. Intestinal epithelial cells have the ability to compensate in part for S. flexneri induced regulation of tight junction-associated proteins.

Claudin-based tight junctions are crucial for the mammalian epidermal barrier: a lesson from claudin-1-deficient mice

The tight junction (TJ) and its adhesion molecules, claudins, are responsible for the barrier function of simple epithelia, but TJs have not been thought to play an important role in the barrier function of mammalian stratified epithelia, including the epidermis. Here we generated claudin-1-deficient mice and found that the animals died within 1 d of birth with wrinkled skin. Dehydration assay and transepidermal water loss measurements revealed that in these mice the epidermal barrier was severely affected, although the layered organization of keratinocytes appeared to be normal. These unexpected findings prompted us to reexamine TJs in the epidermis of wild-type mice. Close inspection by immunofluorescence microscopy with an antioccludin monoclonal antibody, a TJ-specific marker, identified continuous TJs in the stratum granulosum, where claudin-1 and -4 were concentrated. The occurrence of TJs was also confirmed by ultrathin section EM. In claudin-1-deficient mice, claudin-1 appeared to have simply been removed from these TJs, leaving occludin-positive (and also claudin-4-positive) TJs. Interestingly, in the wild-type epidermis these occludin-positive TJs efficiently prevented the diffusion of subcutaneously injected tracer (approximately 600 D) toward the skin surface, whereas in the claudin-1-deficient epidermis the tracer appeared to pass through these TJs. These findings provide the first evidence that continuous claudin-based TJs occur in the epidermis and that these TJs are crucial for the barrier function of the mammalian skin.

Maintenance of the epithelial barrier in a bronchial epithelial cell line is dependent on functional E-cadherin local to the tight junctions

Tight junctions (TJ) are essential components of polarized epithelia, and E-cadherin is important for their formation and maintenance. The bronchial epithelial cell line, 16HBE14o-expresses E- and P-cadherin, but not N-cadherin. E- and P-cadherin levels changed during culture, the former increasing after confluence, and the latter were markedly reduced. All detectable E-cadherin was bound to beta- and gamma-catenins. We investigated involvement of E-cadherin with epithelial integrity using an E-cadherin specific, function-blocking antibody, SHE78-7. Surprisingly, apical SHE78-7 exposure caused a prompt fall in transepithelial resistance (TER), while TER remained unchanged for 8 hrs after basal exposure then dropped. SHE78-7 exposure increased epithelial permeability to mannitol, inulin, and 9.5 kDa and 77 kDa dextrans and caused fragmentation and loss of the tight junction protein, ZO-1, from the cell borders in some areas. Ultrastructural studies showed that all junctional intercellular contact was lost in the center of SHE78-7 induced lesions. Near the lesion periphery, epithelial structure was maintained, but TJs were dysfunctional as shown by ruthenium red penetration. Analysis of epithelial penetration by SHE78-7 revealed discrete, local defects in the apical barrier at the top of some cell hills that permitted rapid access of the antibody to E-cadherin near the apical surface. In contrast, after basal exposure, antibody initially engaged with E-cadherin nearer the basal surface and only accessed apical E-cadherin later. Taken together with the TER measurements, these data suggest compartmentalization of E-cadherin function within 16HBE14o-cells, with only the apical E-cadherin adjacent to the tight junctions contributing to the function of the latter.

claudin-18, a novel downstream target gene for the T/EBP/NKX2.1 homeodomain transcription factor, encodes lung- and stomach-specific isoforms through alternative splicing

T/EBP/NKX2.1, a member of the NKX family of homeodomain-containing transcription factors, regulates the expression of a number of genes in lung and thyroid. Here we describe the isolation and characterization of a novel target gene, termed claudin-18, that is down-regulated in the lungs of T/ebp/Nkx2.1-null mouse embryos. The gene product exhibits an amino acid sequence similar to those of the claudin multigene family of proteins that constitute tight junction strands in epithelial cells. The gene was localized by fluorescence in situ hybridization to mouse chromosome 9 at region 9E3-F1 and to human chromosome 3 at region 3q21-23. The claudin-18 gene has two promoters, each with its own unique exon 1 that is spliced to common exons 2 through 5. Alternative usage of these promoters leads to production of lung and stomach-specific transcripts. The downstream lung-specific promoter contains two T/EBP/NKX2.1 binding sites responsible for trans activation of the gene by T/EBP/NKX2.1 in lung cells. Only claudin-18 was down-regulated in T/ebp/Nkx2.1-null embryo lungs among 11 claudin transcripts examined. Furthermore, the claudin-18 transcript has an alternative 12-bp insertion derived from the 5' end of intron 4, which produces a C-terminally truncated isoform in lung and stomach. Immunohistochemistry demonstrated complete membrane localization of claudin-18 with small focal dots in the lung and stomach epithelial cells. Immunogold electron microscopy analysis revealed that claudin-18 is concentrated at the cell-cell borders of epithelial cells. These unique features suggest a potentially important role for claudin-18 in the structure and function of tight junctions in lung and stomach.

The complex interactions between Clostridium perfringens enterotoxin and epithelial tight junctions

Clostridium perfringens enterotoxin (CPE) is responsible for the diarrheal symptoms of C. perfringens type A food poisoning and antibiotic-associated diarrhea. The CPE protein consists of a single 35 kDa polypeptide with a C-terminal receptor-binding region and an N-terminal toxicity domain. Under appropriate conditions, CPE can interact with structural components of the epithelial tight junctions, including certain claudins and occludin. Those interactions can affect tight junction structure and function, thereby altering paracellular permeability and (possibly) contributing to CPE-induced diarrhea. However, the tight junction effects of CPE require cellular damage as a prerequisite. CPE induces cellular damage via its cytotoxic activity, which results from plasma membrane permeability alterations caused by formation of a approximately 155 kDa CPE-containing complex that may correspond to a pore. Thus, CPE appears to be a bifunctional toxin that first induces plasma membrane permeability alterations; using the resultant cell damage, CPE then gains access to tight junction proteins and affects tight junction structure and function.

Clostridium perfringens type A enterotoxin forms mepacrine-sensitive pores in pure phospholipid bilayers in the absence of putative receptor proteins

Clostridium perfringens enterotoxin (CPE) is an important cause of food poisoning with no significant homology to other enterotoxins and its mechanism of action remains uncertain. Although CPE has recently been shown to complex with tight junction proteins, we have previously demonstrated that CPE increases ionic permeability in single Caco-2 cells using the whole-cell patch-clamp technique, thereby excluding any paracellular permeability. In this paper we demonstrate that CPE forms pores in synthetic phospholipid membranes in the absence of receptor proteins. The properties of the pores are consistent with CPE-induced permeability changes in Caco-2 cells suggesting that CPE has innate pore-forming ability.

Talin, a host cell protein, interacts directly with the translocated intimin receptor, Tir, of enteropathogenic Escherichia coli, and is essential for pedestal formation

Enteropathogenic Escherichia coli (EPEC) is able to inject its own receptor, a transmembrane protein called translocated intimin receptor, Tir, into the host epithelial cell. The bacterium then uses an outer membrane protein, intimin, to bind to Tir and remains firmly attached to the host cell surface for the duration of the infection. The bacterium is also able to trigger the rearrangement of several host cell proteins, culminating with the formation of an actin-rich, pedestal-like structure beneath the EPEC adherence site. Although several cytoskeletal proteins are rearranged following EPEC infection, the exact role played by these proteins during pedestal formation remains unknown. We report here that talin, an integrin-binding protein, is recruited by EPEC and associates directly with Tir. By surface plasmon resonance (SPR), the predicted value for the dissociation constant (KD) for Tir-talin binding was 1.86 x 10(-7) M. We also demonstrate that microinjection of anti-talin antibodies into HeLa cells resulted in the complete inability to focus actin filaments beneath the attached bacterium. These findings demonstrate that talin is essential for EPEC-induced pedestal formation in infected cells.

Mucosal barrier and immune mediators

The intestinal mucosa functions is an immunologic organ that plays a major role in the development of oral tolerance and host-defense mechanisms. Antigens must cross the intestinal epithelium in a controlled manner to interact with dendritic antigen-presenting cells, because bacteria or their products are a primary risk factor for the development of intestinal inflammation. Therefore, the regulation of the intestinal epithelial cell barrier is central to the development of intestinal immunity and inflammation, but the involved mechanisms are largely unknown. Intestinal barrier function relies on the formation of tight junctions at the apical contact areas of intestinal epithelial cells. Tight junctions have a highly dynamic structure whose permeability, assembly, or disassembly can be regulated by a variety of cellular and metabolic mediators, including cytokines, which have major functions in the immune system. Immune modulators control tight junction dependent intestinal barrier function during development, wound healing, and pathologic processes such as cancer, infection, and chronic inflammation.

Influence of enterotoxin on mucosal intranet: selective inhibition of extrathymic T cell development in intestinal intraepithelial lymphocytes by oral exposure to heat-labile toxin

We tested the possibility that heat-labile enterotoxin of Escherichia coli (LT) affects the development of extrathymic T cells in the intraepithelial lymphocyte (IEL) compartment. After oral administration of LT, the number of extrathymic CD8alphaalpha+ IEL was selectively and significantly diminished when compared with the corresponding cells in phosphate-buffered saline-fed control mice. To clarify the mechanism behind this selective reduction of CD8alphaalpha+ IEL, we analyzed the expression of essential cytokines and their corresponding receptors for the mucosal intranet formed by intestinal epithelial cells (IEC) and IEL. The expression levels of stem cell factor, interleukin (IL)-7, and IL-15 in IEC, and their corresponding receptors, i. e. c-kit, IL-7 receptor, and IL-15 receptor, in CD8alphaalpha+ IEL were reduced following oral feeding with LT. These findings suggest that LT negatively regulates development of CD8alphaalpha+ IEL via the disruption of mucosal intranet-associated cytokine and cytokine receptors, which are required for the development and/or expansion of extrathymically developed T cells. Further, LT-induced destruction of the mucosal intranet resulted in the impairment of IEC generation via an increase of apoptosis.

Interleukin-2 receptor beta subunit-dependent and -independent regulation of intestinal epithelial tight junctions

Interleukin (IL)-15 is able to regulate tight junction formation in intestinal epithelial cells. However, the mechanisms that regulate the intestinal barrier function in response to IL-15 and the involved subunits of the IL-15 ligand-receptor system are unknown. We determined the IL-2Rbeta subunit and IL-15-dependent regulation of tight junction-associated proteins in the human intestinal epithelial cell line T-84. The IL-2Rbeta subunit was expressed and induced signal transduction in caveolin enriched rafts in intestinal epithelial cells. IL-15-mediated tightening of intestinal epithelial monolayers correlated with the enhanced recruitment of tight junction proteins into Triton X-100-insoluble protein fractions. IL-15-mediated up-regulation of ZO-1 and ZO-2 expression was independent of the IL-2Rbeta subunit, whereas the phosphorylation of occludin and enhanced membrane association of claudin-1 and claudin-2 by IL-15 required the presence of the IL-2Rbeta subunit. Recruitment of claudins and hyperphosphorylated occludin into tight junctions resulted in a more marked induction of tight junction formation in intestinal epithelial cells than the up-regulation of ZO-1 and ZO-2 by itself. The regulation of the intestinal epithelial barrier function by IL-15 involves IL-2Rbeta-dependent and -independent signaling pathways leading to the recruitment of claudins, hyperphosphorylated occludin, ZO-1, and ZO-2 into the tight junctional protein complex.

Comparative biochemical and immunocytochemical studies reveal differences in the effects of Clostridium perfringens enterotoxin on polarized CaCo-2 cells versus Vero cells

Since most in vitro studies exploring the action of Clostridium perfringens enterotoxin (CPE) utilize either Vero or CaCo-2 cells, the current study directly compared the CPE responsiveness of those two cell lines. When CPE-treated in suspension, both CaCo-2 and Vero cells formed SDS-resistant, CPE-containing complexes of approximately 135, approximately 155, and approximately 200 kDa. However, confluent Transwell cultures of either cell line CPE-treated for 20 min formed only the approximately 155-kDa complex. Since those Transwell cultures also exhibited significant (86)Rb release, approximately 155-kDa complex formation is sufficient for CPE-induced cytotoxicity. Several differences in CPE responsiveness between the two cell lines were also detected. (i) CaCo-2 cells were more sensitive when CPE-treated on their basal surface, whereas Vero cells were more sensitive when CPE-treated on their apical surface; those sensitivity differences correlated with CPE binding the apical versus basolateral surfaces of these two cell lines. (ii) CPE-treated Vero cells released (86)Rb into both Transwell chambers, whereas CaCo-2 cells released (86)Rb only into the CPE-containing Transwell chamber. (iii) Vero cells express the tight junction (TJ) protein occludin but (unlike CaCo-2 cells) cannot form TJs. The ability of TJs to affect CPE responsiveness is supported by the similar effects of CPE on Transwell cultures of CaCo-2 cells and Madin-Darby canine kidney cells, another polarized cell forming TJs. Confluent CaCo-2 Transwell cultures CPE-treated for >1 h formed the approximately 200-kDa CPE complex (which also contains occludin), exhibited morphologic damage, and had occludin removed from their TJs. Collectively, these results identify CPE as a bifunctional toxin that, in confluent polarized cells, first exerts a cytotoxic effect mediated by the approximately 155-kDa complex. Resultant damage then provides CPE access to TJs, leading to approximately 200-kDa complex formation, internalization of some TJ proteins, and TJ damage that may increase paracellular permeability and thereby contribute to the diarrhea of CPE-induced gastrointestinal disease.

Claudin-4: a new target for pancreatic cancer treatment using Clostridium perfringens enterotoxin

BACKGROUND & AIMS

Recently, several members of the claudin family have been identified as integral constituents of tight junctions. Using expression profiling, we previously found claudin-4 to be overexpressed in pancreatic cancer. Because claudin-4 has been described as a receptor for the cytotoxic Clostridium perfringens enterotoxin (CPE), we investigated the effect of CPE on pancreatic cancer cells.

METHODS

Expression of claudin-4 was analyzed by Northern blots. In vitro toxicity of CPE was determined by trypan blue exclusion and the (86)Rb-release assay. The in vivo effect of CPE was studied in claudin-4-expressing nude mouse xenografts of the Panc-1 cell line.

RESULTS

Expression analyses showed that claudin-4 was overexpressed in most pancreatic cancer tissues and cell lines and several other gastrointestinal tumors. CPE led to an acute dose-dependent cytotoxic effect, restricted to claudin-4-expressing cells and dependent on claudin-4 expression levels. Furthermore, transforming growth factor beta was identified as a negative modulator of both claudin-4 expression and susceptibility to CPE. In vivo, intratumoral injections of CPE in Panc-1 xenografts led to large areas of tumor cell necrosis and significant reduction of tumor growth.

CONCLUSIONS

Our findings suggest that targeting claudin-4-expressing tumors with CPE represents a promising new treatment modality for pancreatic cancer and other solid tumors.

Claudin-1, claudin-2 and claudin-11 are present in tight junctions of choroid plexus epithelium of the mouse

The choroid plexus epithelium forms the blood-cerebrospinal fluid (CSF) barrier and is responsible for the secretion of the CSF from the blood. The morphological correlate of the blood-CSF barrier are the tight junctions of choroid plexus epithelium. By freeze-fracture electron microscopy it has been demonstrated that choroid plexus epithelial tight junctions form parallel strands resembling those of Sertoli cells building the blood-testis barrier and those of the myelin sheaths of oligodendrocytes. As the oligodendrocyte specific protein/claudin-11 has been shown to be the central mediator of parallel-array tight junctions in Sertoli cells and myelin sheaths in mice, we asked whether claudin-11 is present in the tight junctions of choroid plexus epithelial cells of the mouse. Here, we present the first direct evidence that claudin-11 besides claudin-1 and -2, occludin and the zonula occludens protein ZO-1 is present in choroid plexus epithelial tight junctions. During inflammation in the central nervous system such as experimental autoimmune encephalomyelitis, the molecular composition of choroid plexus epithelial tight junctions does not change considerably. Their unique molecular composition, with claudin-11 accompanied by claudin-1 and claudin-2 points to a unique regulatory mechanism of the blood-CSF-barrier function.

Regulated expression of claudin-4 decreases paracellular conductance through a selective decrease in sodium permeability

Tight junctions regulate paracellular conductance and ionic selectivity. These properties vary among epithelia but the molecular basis of this variation remains unknown. To test whether members of the claudin family of tight junction proteins influence paracellular ionic selectivity, we expressed human claudin-4 in cultured MDCK cells using an inducible promoter. Overexpression increased the complexity of tight junction strands visible by freeze-fracture microscopy without affecting the levels of claudin-1, -2, or -3, occludin, or ZO-1. A decrease in conductance correlated directly with the kinetics of claudin-4 induction. Dilution potentials revealed that the decrease in paracellular conductance resulted from a selective decrease in Na(+) permeability without a significant effect on Cl(-) permeability. Flux for an uncharged solute, mannitol, and the rank order of permeabilities for the alkali metal cations were unchanged. A paracellular site for these effects was supported by the lack of apical/basal directionality of the dilution potentials, the linearity of current-voltage relationships, and the lack of influence of inhibitors of major transcellular transporters. These results provide, to our knowledge, the first direct demonstration of the ability of a claudin to influence paracellular ion selectivity and support a role for the claudins in creating selective channels through the tight-junction barrier.

Molecular structure of tight junctions and their role in epithelial transport

Tight junctions create a paracellular barrier with physiological properties that differ among epithelia. Among these differences are electrical resistance and discrimination for solute size and charge. Emerging evidence suggests that a large family of transmembrane proteins called the claudins create these variable properties.

Conversion of zonulae occludentes from tight to leaky strand type by introducing claudin-2 into Madin-Darby canine kidney I cells

There are two strains of MDCK cells, MDCK I and II. MDCK I cells show much higher transepithelial electric resistance (TER) than MDCK II cells, although they bear similar numbers of tight junction (TJ) strands. We examined the expression pattern of claudins, the major components of TJ strands, in these cells: claudin-1 and -4 were expressed both in MDCK I and II cells, whereas the expression of claudin-2 was restricted to MDCK II cells. The dog claudin-2 cDNA was then introduced into MDCK I cells to mimic the claudin expression pattern of MDCK II cells. Interestingly, the TER values of MDCK I clones stably expressing claudin-2 (dCL2-MDCK I) fell to the levels of MDCK II cells (>20-fold decrease). In contrast, when dog claudin-3 was introduced into MDCK I cells, no change was detected in their TER. Similar results were obtained in mouse epithelial cells, Eph4. Morphometric analyses identified no significant differences in the density of TJs or in the number of TJ strands between dCL2-MDCK I and control MDCK I cells. These findings indicated that the addition of claudin-2 markedly decreased the tightness of individual claudin-1/4-based TJ strands, leading to the speculation that the combination and mixing ratios of claudin species determine the barrier properties of individual TJ strands.

Multifunctional strands in tight junctions

Tight junctions are one mode of cell-cell adhesion in epithelial and endothelial cellular sheets. They act as a primary barrier to the diffusion of solutes through the intercellular space, create a boundary between the apical and the basolateral plasma membrane domains, and recruit various cytoskeletal as well as signalling molecules at their cytoplasmic surface. New insights into the molecular architecture of tight junctions allow us to now discuss the structure and functions of this unique cell-cell adhesion apparatus in molecular terms.