Endothelial cells constituting blood-nerve barrier have highly specialized characteristics as barrier-forming cells

In autoimmune disorders of the peripheral nervous system (PNS) such as Guillain-Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy, breakdown of the blood-nerve barrier (BNB) has been considered as a key step in the disease process. Hence, it is important to know the cellular property of peripheral nerve microvascular endothelial cells (PnMECs) constituting the bulk of BNB. Although many in vitro models of the blood-brain barrier (BBB) have been established, very few in vitro BNB models have been reported so far. We isolated PnMECs from transgenic rats harboring the temperature-sensitive SV40 large T-antigen gene (tsA58 rat) and investigated the properties of these "barrier-forming cells". Isolated PnMECs (TR-BNBs) showed high transendothelial electrical resistance and expressed tight junction components and various types of influx as well as efflux transporters that have been reported to function at BBB. Furthermore, we confirmed the in vivo expression of various BBB-forming endothelial cell markers in the endoneurium of a rat sciatic nerve. These results suggest that PnMECs constituting the bulk of BNB have a highly specialized characteristic resembling the endothelial cells forming BBB.     

Peripheral nerve pericytes modify the blood–nerve barrier function and tight junctional molecules through the secretion of various soluble factors

The objectives of this study were to establish pure blood–nerve barrier (BNB) and blood–brain barrier (BBB)‐derived pericyte cell lines of human origin and to investigate their unique properties as barrier‐forming cells. Brain and peripheral nerve pericyte cell lines were established via transfection with retrovirus vectors incorporating human temperature‐sensitive SV40 T antigen (tsA58) and telomerase. These cell lines expressed several pericyte markers such as α‐smooth muscle actin, NG2, platelet‐derived growth factor receptor β, whereas they did not express endothelial cell markers such as vWF and PECAM. In addition, the inulin clearance was significantly lowered in peripheral nerve microvascular endothelial cells (PnMECs) through the up‐regulation of claudin‐5 by soluble factors released from brain or peripheral nerve pericytes. In particular, bFGF secreted from peripheral nerve pericytes strengthened the barrier function of the BNB by increasing the expression of claudin‐5. Peripheral nerve pericytes may regulate the barrier function of the BNB, because the BNB does not contain cells equivalent to astrocytes which regulate the BBB function. Furthermore, these cell lines expressed several neurotrophic factors such as NGF, BDNF, and GDNF. The secretion of these growth factors from peripheral nerve pericytes might facilitate axonal regeneration in peripheral neuropathy. Investigation of the characteristics of peripheral nerve pericytes may provide novel strategies for modifying BNB functions and promoting peripheral nerve regeneration. J. Cell. Physiol. 226: 255–266, 2010. © 2010 Wiley‐Liss, Inc.     

Pericyte-derived Glial Cell Line-derived Neurotrophic Factor Increase the Expression of Claudin-5 in the Blood–brain Barrier and the Blood-nerve Barrier

The destruction of blood–brain barrier (BBB) and blood-nerve barrier (BNB) has been considered to be a key step in the disease process of a number of neurological disorders including cerebral ischemia, Alzheimer’s disease, multiple sclerosis, and diabetic neuropathy. Although glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) facilitate neuronal or axonal regeneration in the brain or peripheral nerves, their action in the BBB and BNB remains unclear. The purpose of the present study was to elucidate whether these neurotrophic factors secreted from the brain or peripheral nerve pericytes increase the barrier function of the BBB or BNB, using our newly established human brain microvascular endothelial cell (BMEC) line or peripheral nerve microvascular endothelial cell (PnMEC) line. GDNF increased the expression of claudin-5 and the transendothelial electrical resistance (TEER) of BMECs and PnMECs, whereas BDNF did not have this effect. Furthermore, we herein demonstrate that the GDNF secreted from the brain and peripheral nerve pericytes was one of the key molecules responsible for the up-regulation of claudin-5 expression and the TEER value in the BBB and BNB. These results indicate that the regulation of GDNF secreted from pericytes may therefore be a novel therapeutic strategy to modify the BBB or BNB functions and promote brain or peripheral nerve regeneration.     

Peripheral nerve pericytes originating from the blood-nerve barrier expresses tight junctional molecules and transporters as barrier-forming cells

The objective of this study was to establish pure blood-nerve barrier (BNB)-derived peripheral nerve pericyte cell lines and to investigate their unique properties as barrier-forming cells. We isolated peripheral nerve, brain, and lung pericytes from transgenic rats harboring the temperature-sensitive simian virus 40 large T-antigen gene. These cell lines expressed several pericyte markers such as alpha-smooth muscle actin, NG2, osteopontin, and desmin, whereas they did not express endothelial cell markers such as vWF and PECAM. In addition, these cell lines expressed several tight junction molecules such as occludin, claudin-12, ZO-1, and ZO-2. In particular, the expression of occludin was detected in peripheral nerve and brain pericytes, although it was not detected in lung pericytes by a Western blot analysis. An immunocytochemical analysis confirmed that occludin and ZO-1 were localized at the cell-cell boundaries among the pericytes. Brain and peripheral nerve pericytes also showed significantly higher trans-pericyte electrical resistance values and lower inulin clearances than lung pericytes. We considered that occludin localized at the cell-cell boundaries among the pericytes might mechanically stabilize the microvessels of the BNB and the blood-brain barrier. Furthermore, we also showed that these cell lines expressed many barrier-related transporters. ABCG2, p-gp, MRP-1, and Glut-1 were detected by a Western blot analysis and were observed in the cytoplasm and outer membrane by an immunocytochemical analysis. These transporters on pericytes might facilitate the peripheral nerve-to-blood efflux and blood-to-peripheral nerve influx transport of substrates in cooperation with those on endothelial cells in order to maintain peripheral nerve homeostasis.     

The development of an innervated epithelial barrier model using a human corneal cell line and ND7/23 sensory neurons

The corneal epithelium is a highly innervated tissue and hence in vitro models that mimic the effects of chemicals or radiation (e.g. ultra violet) on this important barrier should include consideration of the potential role of innervation. A sensory neural cell line, ND7/23, was incorporated into a 2D and 3D model of a corneal epithelium, using a human corneal cell line, and effects on barrier integrity were neither adverse nor stimulatory. In the 3D model the nerve cell bodies were separated from the corneal epithelium, via a porous polycarbonate insert membrane. The ND7/23 cells were induced to form neurites and cease division when cultured in the keratinocyte medium employed for the corneal cells. In the absence of calcium, the epithelial barrier function was lost, shown by enhanced fluorescein leakage and relocation of ZO-1 and E-cadherin from the cell membrane. At 60 microM calcium, and above, the corneal cells formed tight junctions, with peripheral membrane location of ZO-1 and E-cadherin. The presence of the ND7/23 cells did not compromise or enhance the time taken to form these junctions, when monitored at 24-h intervals over 72 h. Both male- and female-derived human corneal cell lines showed a similar tight junction functional response to different medium calcium concentrations in the presence or absence of the ND7/23 cells. Once differentiated in keratinocyte medium, patch-clamped ND7/23 cells were capable of producing a whole-cell current when exposed to low pH (5.4), indicative of the presence of active pH-gated ion channels.     

Tissue plasminogen activator and neuropathy open the blood-nerve barrier with upregulation of microRNA-155-5p in male rats

The blood-nerve barrier (BNB) consisting of the perineurium and endoneurial vessels is sealed by tight junction proteins. BNB alterations are a crucial factor in the pathogenesis of peripheral neuropathies. However, barrier opening, e.g. by tissue plasminogen activator (tPA), can also facilitate topical application of analgesics. Here, we examined tPA both in the pathophysiology of neuropathy-induced BNB opening or via exogenous application and its effect on the cytoplasmatic tight junction protein anchoring protein, zona occludens-1 (ZO-1), the adherens molecule JAM-C and microRNA(miR)-155-5p. Specifically, we investigated whether tPA alone and barrier opening lead to pain behavioral changes, i.e. hyperalgesia, or whether these effects require further factors.Male Wistar rats underwent chronic constriction injury (CCI) or were treated by a single perisciatic application of recombinant (r)tPA. CCI elicited mechanical allodynia, tPA mRNA upregulation, macrophage invasion, BNB leakage for large molecule tracers, downregulation of ZO-1 and JAM-C mRNA/protein, and a loss of immunoreactivity of both in perineurium and endoneurial cells. Similarly, after perisciatic rtPA injection, ZO-1 and JAM-C mRNA as well as cytosolic/membrane protein and ZO-1 immunoreactivity were downregulated, and the BNB was opened. Neither mechanical hypersensitivity nor macrophage infiltration was observed after rtPA in contrast to CCI. Mechanistically, miR-155-5p, which is known to destabilize barriers and tight junction proteins like claudin-1 and ZO-1, was increased in CCI and to lesser extent after rtPA application. In summary, tPA transiently opens the BNB possibly via miR-155-5p. However, tPA does not provoke allodynia in the absence of a neuropathic stimulus like a ligation or inflammation.     

An Immortalized Human Blood-Nerve Barrier Endothelial Cell Line for In Vitro Permeability Studies

Solute and macromolecular transport studies may elucidate nutritional requirements and drug effects in healthy and diseased peripheral nerves. Endoneurial endothelial cells are specialized microvascular cells that form the restrictive blood-nerve barrier (BNB). Primary human endoneurial endothelial cells (pHEndECs) are difficult to isolate, limiting their widespread availability for biomedical research. We developed a simian virus-40 large T-antigen (SV40-LTA) immortalized human BNB cell line via stable transfection of low passage pHEndECs and observed continuous growth in culture for >45 population doublings. As observed with pHEndECs, the immortalized BNB endothelial cells were Ulex Europaeus agglutinin-1-positive and endocytosed low density lipoprotein, but lost von Willebrand factor expression. Glucose transporter-1, P-glycoprotein (P-gp), γ-glutamyl transpeptidase (γ-GT), large neutral amino acid transporter-1 (LAT-1), creatine transporter (CRT), and monocarboxylate transporter-1 (MCT-1) mRNA expression were retained at all passages with loss of alkaline phosphatase (AP) expression after passages 16–20. Compared with an SV40-LTA immortalized human blood-brain barrier endothelial cell line, there was increased γ-GT protein expression, equivalent expression of organic anion transporting polypeptide-C (OATP-C), organic anion transporter 3 (OAT-3), MCT-1, and LAT-1, and reduced expression of AP, CRT, and P-gp by the BNB cell line at passage 20. Further studies demonstrated lower transendothelial electrical resistance (~181 vs. 191 Ω cm²), equivalent permeability to fluoresceinated sodium (4.84 vs. 4.39 %), and lower permeability to fluoresceinated high molecular weight (70 kDa) dextran (0.39 vs. 0.52 %) by the BNB cell line. This cell line retained essential molecular and biophysical properties suitable for in vitro peripheral nerve permeability studies.     

Hydrocortisone Enhances the Function of the Blood-Nerve Barrier Through the Up-Regulation of Claudin-5

In autoimmune disorders of the peripheral nervous system (PNS), including Guillain–Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy, breakdown of the blood-nerve barrier (BNB) has been considered to be a key step in the disease process. Although glucocorticoids (GCs) have been shown to effectively restore the blood–brain barrier (BBB) in some inflammatory central nervous system diseases such as multiple sclerosis, their action against the BNB has not yet been examined. To elucidate the role of GCs on the BNB, we established a novel human immortalized endothelial cell lines derived from the BNB. The established cell line termed “DH-BNBs” expresses two important tight junction proteins, claudin-5 and occludin. Using DH-BNBs, we analyzed how GCs affect BNB function. We herein report that GCs up-regulate the expression of claudin-5 and increase the barrier properties of the BNB. This is the first report which indicates how GCs affect the blood-nerve barrier.     

Severity and Patterns of Blood-Nerve Barrier Breakdown in Patients with Chronic Inflammatory Demyelinating Polyradiculoneuropathy: Correlations with Clinical Subtypes

Objective

Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is currently classified into clinical subtypes, including typical and atypical forms (multifocal acquired demyelinating sensory and motor neuropathy (MADSAM) and distal acquired demyelinating symmetric neuropathy (DADS)). The aim of this study was to elucidate the patterns and severity of breakdown of the blood-nerve barrier (BNB) in each CIDP subtype.

Methods

We evaluated the effects of sera obtained from patients with typical CIDP, MADSAM and DADS and control subjects on the expression levels of tight junction proteins and transendothelial electrical resistance (TEER) value in human peripheral nerve microvascular endothelial cells (PnMECs).

Results

The sera obtained from the patients with the three clinical phenotypes of CIDP decreased the amount of claudin-5 protein levels and TEER values in the PnMECs. In addition, the sera obtained from typical CIDP patients more prominently reduced claudin-5 protein levels and TEER values in the PnMECs than did that obtained from the MADSAM and DADS patients. Furthermore, the severity of BNB disruption after exposure to the sera was associated with higher Hughes grade, lower MRC score, more pronounced slowing of motor nerve conduction in the median nerve and higher frequency of abnormal temporal dispersion.

Conclusions

Sera derived from typical CIDP patients destroy the BNB more severely than those from MADSAM or DADS patients. The extent of BNB disruption in the setting of CIDP is associated with clinical disability and demyelination in the nerve trunk. These observations may explain the phenotypical differences between CIDP subtypes.     

Human Brain Microvascular Endothelial Cells Derived from the BC1 iPS Cell Line Exhibit a Blood-Brain Barrier Phenotype

The endothelial cells that form capillaries in the brain are highly specialized, with tight junctions that minimize paracellular transport and an array of broad-spectrum efflux pumps that make drug delivery to the brain extremely challenging. One of the major limitations in blood-brain barrier research and the development of drugs to treat central nervous system diseases is the lack of appropriate cell lines. Recent reports indicate that the derivation of human brain microvascular endothelial cells (hBMECs) from human induced pluripotent stem cells (iPSCs) may provide a solution to this problem. Here we demonstrate the derivation of hBMECs extended to two new human iPSC lines: BC1 and GFP-labeled BC1. These hBMECs highly express adherens and tight junction proteins VE-cadherin, ZO-1, occludin, and claudin-5. The addition of retinoic acid upregulates VE-cadherin expression, and results in a significant increase in transendothelial electrical resistance to physiological values. The permeabilities of tacrine, rhodamine 123, and Lucifer yellow are similar to values obtained for MDCK cells. The efflux ratio for rhodamine 123 across hBMECs is in the range 2–4 indicating polarization of efflux transporters. Using the rod assay to assess cell organization in small vessels and capillaries, we show that hBMECs resist elongation with decreasing diameter but show progressive axial alignment. The derivation of hBMECs with a blood-brain barrier phenotype from the BC1 cell line highlights that the protocol is robust. The expression of GFP in hBMECs derived from the BC1-GFP cell line provides an important new resource for BBB research.     

Restoration of tight junction structure and barrier function by down-regulation of the mitogen-activated protein kinase pathway in ras-transformed Madin-Darby canine kidney cells

In the Madin-Darby canine kidney epithelial cell line, the proteins occludin and ZO-1 are structural components of the tight junctions that seal the paracellular spaces between the cells and contribute to the epithelial barrier function. In Ras-transformed Madin-Darby canine kidney cells, occludin, claudin-1, and ZO-1 were absent from cell-cell contacts but were present in the cytoplasm, and the adherens junction protein E-cadherin was weakly expressed. After treatment of the Ras-transformed cells with the mitogen-activated protein kinase kinase (MEK1) inhibitor PD98059, which blocks the activation of mitogen-activated protein kinase (MAPK), occludin, claudin-1, and ZO-1 were recruited to the cell membrane, tight junctions were assembled, and E-cadherin protein expression was induced. Although it is generally believed that E-cadherin-mediated cell-cell adhesion is required for tight junction assembly, the recruitment of occludin to the cell-cell contact area and the restoration of epithelial cell morphology preceded the appearance of E-cadherin at cell-cell contacts. Both electron microscopy and a fourfold increase in the transepithelial electrical resistance indicated the formation of functional tight junctions after MEK1 inhibition. Moreover, inhibition of MAPK activity stabilized occludin and ZO-1 by differentially increasing their half-lives. We also found that during the process of tight junction assembly after MEK1 inhibition, tyrosine phosphorylation of occludin and ZO-1, but not claudin-1, increased significantly. Our study demonstrates that down-regulation of the MAPK signaling pathway causes the restoration of epithelial cell morphology and the assembly of tight junctions in Ras-transformed epithelial cells and that tyrosine phosphorylation of occludin and ZO-1 may play a role in some aspects of tight junction formation.     

Tight junction proteins ZO-1, occludin, and claudins in developing and adult human perineurium.

In peripheral nerves, groups of Schwann cell-axon units are isolated from the adjacent tissues by the perineurium, which creates a diffusion barrier responsible for the maintenance of endoneurial homeostasis. The perineurium is formed by concentric layers of overlapping, polygonal perineurial cells that form tight junctions at their interdigitating cell borders. In this study, employing indirect immunofluorescence and immunoelectron microscopy, we demonstrate that claudin-1 and -3, ZO-1, and occludin, but not claudin-2, -4, and -5, are expressed in the perineurium of adult human peripheral nerve. We also describe the expression of occludin, ZO-1, claudin-1, -3, and -5 in the developing human perineurium, showing that the expressions of claudin-1 and -3, ZO-1, and occludin follow similar spatial developmental expression patterns but follow different timetables in achieving their respective adult distributions. Specifically, claudin-1 is already largely restricted to perineurium-derived structures at 11 fetal weeks, whereas claudin-3 and occludin are weakly expressed in the perineurial structures at this age and acquire a well-defined perineurial distribution only between 22 and 35 fetal weeks. ZO-1 appears to acquire its mature profile even later during the third trimester. The results of the present and previous studies show that the perineurial diffusion barrier matures relatively late during human peripheral nerve development.     

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.     

Lung endothelial cells strengthen,but brain endothelial cells weaken barrier properties of a human alveolar epithelium cell culture model

The blood–air barrier in the lung consists of the alveolar epithelium, the underlying capillary endothelium, their basement membranes and the interstitial space between the cell layers. Little is known about the interactions between the alveolar and the blood compartment. The aim of the present study was to gain first insights into the possible interplay between these two neighbored cell layers. We established an in vitro Transwell model of the alveolar epithelium based on human cell line H441 and investigated the influence of conditioned medium obtained from human lung endothelial cell line HPMEC-ST1.6R on the barrier properties of the H441 layers. As control for tissue specificity H441 layers were exposed to conditioned medium from human brain endothelial cell line hCMEC/D3. Addition of dexamethasone was necessary to obtain stable H441 cell layers. Moreover, dexamethasone increased expression of cell type I markers (caveolin-1, RAGE) and cell type II marker SP-B, whereas decreased the transepithelial electrical resistance (TEER) in a concentration dependent manner. Soluble factors obtained from the lung endothelial cell line increased the barrier significantly proven by TEER values and fluorescein permeability on the functional level and by the differential expression of tight junctional proteins on the molecular level. In contrast to this, soluble factors derived from brain endothelial cells weakened the barrier significantly. In conclusion, soluble factors from lung endothelial cells can strengthen the alveolar epithelium barrier in vitro, which suggests communication between endothelial and epithelial cells regulating the integrity of the blood–air barrier.     

Blood-neural barrier: intercellular communication at glio-vascular interface

The blood-neural barrier (BNB), including blood-brain barrier (BBB) and blood-retinal barrier (BRB), is an endothelial barrier constructed by an extensive network of endothelial cells, astrocytes and neurons to form functional "neurovascular units", which has an important role in maintaining a precisely regulated microenvironment for reliable neuronal activity. Although failure of the BNB may be a precipitating event or a consequence, the breakdown of BNB is closely related with the development and progression of CNS diseases. Therefore, BNB is most essential in the regulation of microenvironment of the CNS. The BNB is a selective diffusion barrier characterized by tight junctions between endothelial cells, lack of fenestrations, and specific BNB transporters. The BNB have been shown to be astrocyte dependent, for it is formed by the CNS capillary endothelial cells, surrounded by astrocytic end-foot processes. Given the anatomical associations with endothelial cells, it could be supposed that astrocytes play a role in the development, maintenance, and breakdown of the BNB. Therefore, astrocytes-endothelial cells interaction influences the BNB in both physiological and pathological conditions. If we better understand mutual interactions between astrocytes and endothelial cells, in the near future, we could provide a critical solution to the BNB problems and create new opportunities for future success of treating CNS diseases. Here, we focused astrocyte-endothelial cell interaction in the formation and function of the BNB.     

Tight Junctions of the Blood–Brain Barrier

1. The blood–brain barrier is essential for the maintainance and regulation of the neural microenvironment. The blood–brain barrier endothelial cells comprise an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. The latter is realized by the tight junctions between the endothelial cells of the brain microvasculature, which are subject of this review. Morphologically, blood–brain barrier-tight junctions are more similar to epithelial tight junctions than to endothelial tight junctions in peripheral blood vessels.2. Although blood–brain barrier-tight junctions share many characteristics with epithelial tight junctions, there are also essential differences. However, in contrast to tight junctions in epithelial systems, structural and functional characteristics of tight junctions in endothelial cells are highly sensitive to ambient factors.3. Many ubiquitous molecular constituents of tight junctions have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin, and 7H6. Signaling pathways involved in tight junction regulation comprise, among others, G-proteins, serine, threonine, and tyrosine kinases, extra- and intracellular calcium levels, cAMP levels, proteases, and TNF. Common to most of these pathways is the modulation of cytoskeletal elements which may define blood–brain barrier characteristics. Additionally, cross-talk between components of the tight junction– and the cadherin–catenin system suggests a close functional interdependence of the two cell–cell contact systems.4. Recent studies were able to elucidate crucial aspects of the molecular basis of tight junction regulation. An integration of new results into previous morphological work is the central intention of this review.     

Connexin 26 expression prevents down-regulation of barrier and fence functions of tight junctions by Na+/K+-ATPase inhibitor ouabain in human airway epithelial cell line Calu-3

Gap junctions are considered to play a crucial role in differentiation of epithelial cells and to be associated with tight junction proteins. In this study, to investigate the role of gap junctions in regulation of the barrier function and fence function on the tight junctions, we introduced the Cx26 gene into human airway epithelial cell line Clau-3 and used a disruption model of tight junctions employing the Na(+)/K(+)-ATPase inhibitor ouabain. In parental Calu-3 cells, gap junction proteins Cx32 and Cx43, but not Cx26, and tight junction proteins occludin, JAM-1, ZO-1, claudin-1, -2, -3, -4, -5, -6, -7, -8, -9, and -14 were detected by RT-PCR. The barrier function and fence function of tight junctions were well maintained, whereas the GJIC was low level. Treatment with ouabain caused disruption of the barrier function and fence function of tight junctions together with down-regulation of occludin, JAM-1, claudin-2, and -4 and up-regulation of ZO-1 and claudin-14. In Cx26 transfectants, Cx26 protein was detected by Western blotting and immunocytochemistry, and many gap junction plaques were observed with well-developed tight junction strands. Expression of claudin-14 was significantly increased in Cx26 transfectants compared to parental cells, and in some cells, Cx26 was co-localized with claudin-14. Interestingly, transfection with Cx26 prevented disruption of both functions of tight junctions by treatment with ouabain without changes in the tight junction proteins. Pretreatment with the GJIC blockers 18beta-glycyrrhetinic acid and oleamide did not affect the changes induced by Cx26 transfection. These results suggest that Cx26 expression, but not the mediated intercellular communication, may regulate tight junction barrier and fence functions in human airway epithelial cell line Calu-3.