Disruption of circumferential actin filament causes disappearance of occludin from the cell borders of rat hepatocytes in primary culture without distinct changes of tight junction strands

Visualizing the dynamic coupling of claudin strands to the actin cytoskeleton through ZO-1

The organization and integrity of epithelial tight junctions depend on interactions between claudins, ZO scaffolding proteins, and the cytoskeleton. However, although binding between claudins and ZO-1/2/3 and between ZO-1/2/3 and numerous cytoskeletal proteins has been demonstrated in vitro, fluorescence recovery after photobleaching analysis suggests interactions in vivo are likely highly dynamic. Here we use superresolution live-cell imaging in a model fibroblast system to examine relationships between claudins, ZO-1, occludin, and actin. We find that GFP claudins make easily visualized dynamic strand patches between two fibroblasts; strand dynamics is constrained by ZO-1 binding. Claudin association with actin is also dependent on ZO-1, but colocalization demonstrates intermittent rather than continuous association between claudin, ZO-1, and actin. Independent of interaction with ZO-1 or actin, claudin strands break and reanneal; pulse-chase-pulse analysis using SNAP-tagged claudins showed preferential incorporation of newly synthesized claudins into break sites. Although claudin strand behavior in fibroblasts may not fully recapitulate that of epithelial tight junction strands, this is the first direct demonstration of the ability of ZO-1 to stabilize claudin strands. We speculate that intermittent tethering of claudins to actin may allow for accommodation of the paracellular seal to physiological or pathological alterations in cell shape or movement.

INTRACELLULAR LOCALIZATION AND KINETICS OF UPTAKE AND CLEARANCE OF GOLD NANOPARTICLES IN PRIMARY HEPATIC CELLS

Gold nanoparticles (AuNPs) used for therapeutic applications preferentially accumulate in the liver following exposure. However, uptake and clearance by hepatic cells are not well understood. Time-dependent intracellular localization, uptake and clearance of 30 nm AuNPs were monitored in primary rat hepatocytes and liver sinusoidal endothelial cells (LSECs). Confocal Raman microscopy studies demonstrated the differences in the localization of AuNPs in hepatic cells over a 24 h period. The uptake of unmodified AuNPs over 24 h was 17% and 55% for hepatocytes and LSECs. The uptake of poly(ethylene glycol)-coated AuNPs was 3% and 1% over 24 h in hepatocytes and LSECs, respectively. Both cell types expelled approximately 60–70% of intracellular AuNPs within seven days. AuNP accumulation resulted in the disruption of the pericanalicular actin between adjoining hepatocytes. These trends suggest that AuNPs may affect actin organization, which could impair hepatic function long term.

PEGylated silicon nanowire coated silica microparticles for drug delivery across intestinal epithelium

Composite particles made by growing nanoscopic silicon wires from the surface of monodispersed, microsized silica beads were tested in this study for their ability to affect the integrity and permeability of an epithelial cell layer. Polyethylene glycol (PEG) is known to sterically stabilize particles and prevent protein binding; as such, it is a routine way to impart in vivo longevity to drug carriers. The effect of the silica beads, both with and without silicon nanowires and PEG, on the disruption of the tight junctions in Caco-2 cells was evaluated by means of: (a) analysis of the localization of zonula occludens-1 (ZO-1), claudin-1 and f-actin; (b) measurements of trans-epithelial electrical resistance (TEER); (c) real-time quantitative RT-PCR analysis of the expression of PKC-α and PKC-z, which regulate the fluidity of cell membranes, and RhoA and Rac1, which are mainly involved in mechanotransduction processes; and (d) drug permeability experiments with fluorescein-sodium. The results have shown that Si-nanowire-coated silica microparticles added to Caco-2 cells in culture lead to alterations in tight junction permeability and the localization of ZO-1 and f-actin, as well as to decreased width of ZO-1 and claudin-1 at the tight junction and increased expression of PKC transcripts. Si-nanowire-coated silica microparticles increased the permeability of Caco-2 cell monolayers to fluorescein-sodium in proportion to their amount. Effects indicative of loosening the Caco-2 cell monolayers and increasing their permeability were less pronounced for PEGylated particles, owing to their greater supposed inertness in comparison with the non-functionalized beads and nanowires. The analyzed Si-nanowire-coated silica microparticles have thus been shown to affect membrane barrier integrity in vitro, suggesting the possibility of using nanostructured microparticles to enhance drug permeability through the intestinal epithelium in vivo.

K+-ATP-channel-related protein complexes: potential transducers in the regulation of epithelial tight junction permeability

K+-ATP channels are composed of an inwardly rectifying Kir6 subunit and an auxiliary sulfonylurea receptor (SUR) protein. The SUR subunits of Kir6 channels have been recognized as an ATPase, which appears to work as a mechanochemical device like other members of the ABC protein family. Thus, in spite of just gating ions, Kir6/Sur might, in addition, regulate completely different cellular systems. However, so far no model system was available to directly investigate this possibility. Using highly specific antibodies against Kir6.1-SUR2A and an in vitro model system of the rat small intestine, we describe a new function of the Kir6.1-SUR2A complex, namely the regulation of paracellular permeability. The Kir6.1-SUR2A complex localizes to regulated tight junctions in a variety of gastrointestinal, renal and liver tissues of rat, pig and human, whereas it is absent in the urothelium. Changes in paracellular permeability following food intake was investigated by incubating the lumen of morphological well-defined segments of rat small intestine with various amounts of glucose. Variations in the lumenal glucose concentrations and regulators of Kir6.1/SUR2A activity, such as tolbutamide or diazoxide, specifically modulate paracellular permeability. The data presented here shed new light on the physiological and pathophysiological role K+-ATP channels might have for the regulation of tight junctions.

Crosstalk between Rab GTPases and cell junctions

For the past several years, studies from other laboratories, as well as ours, have begun to unravel the mechanism of germ cell movement in the testis by using several in vitro and in vivo models of tight and adherens junction assembly and disassembly, two cellular phenomena that confer cell movement. However, for cell movement to be fully appreciated, the importance of "intracellular" cell movements, such as those involving actin and microtubule filaments, must be better understood. Recent research on Rab GTPases has shown that members of this superfamily function in the trafficking of vesicles containing cargo to distinct subcellular sites such as the plasma membrane while utilizing actin and microtubule filaments as tracks. In this mini-review, we provide an overview of Rab GTPase structure, function, and regulation, while placing added emphasis on the role of Rabs in cell junction dynamics in the testis.

Occludin regulates actin cytoskeleton in endothelial cells

Occludin is a major membrane component of tight junctions of endothelial cells, though the role of this molecule is not fully understood. RLE cells, derived from rat lung endothelial cells, express a negligible level of occludin with clear expression of E-cadherin and ZO-1 at cell junctions. Introduction of occludin by transfection induced clear junctional expression of occludin with few or no changes of expression of E-cadherin and ZO-1. The paracellular barrier function, as determined by transelectrical resistance and flux of non-ionic small molecules, was not detectably upregulated. When cells expressing occludin were cocultured with RLE cells null for occludin, clear junctional expression of occludin was observed irrespective of the expression of occludin on the apposing cells. Cortical actin was developed at the site of these occludin positive cell junctions. Treatment of cells with an actin depolymerizing agent, mycalolide B, abolished junctional expression of occludin together with E-cadherin and circumferential actin. ZO-1 showed relative resistance to this actin depolymerizing treatment and was maintained at the cell junctions, though fragmentation of immunoreactivity was detectable. Collectively, junctional expression of occludin was not associated with paracellular barrier function in this cell line. There was, however, a close correlation of occludin with the actin cytoskeleton, indicating a role of occludin as an important molecule in the regulation of the actin cytoskeleton in endothelial cells.

Show me the pathway! Regulation of paracellular permeability by Na(+)-glucose cotransport

The physiological impact of Na(+)-nutrient cotransport-dependent regulation of intestinal tight junction permeability has been controversial. Nonetheless, increased permeability of small intestinal mucosae and enterocyte tight junctions as a consequence of Na(+)-nutrient cotransport has been documented by a significant number of in vivo and in vitro studies. Some details of the intracellular signaling events that regulate this process have been described recently. The aims of this article are to: (i) review studies of tight junction regulation and paracellular nutrient absorption in mammalian intestine, (ii) identify potential applications of tight junction regulation, and (iii) summarize recent progress in defining molecular mechanisms that lead to altered tight junction permeability.

Uncoupling of gate and fence functions of MDCK cells by the actin-depolymerizing reagent mycalolide B

The tight junction serves as a paracellular gate to seal the paracellular space of apposing cells and as a molecular fence to prevent diffusion of membrane proteins and lipids in epithelial cells. Although involvement of the actin cytoskeleton has been considered to be important in these two functions, it remains to be elucidated whether both functions are regulated in a coupled manner or differentially by actin. Treatment of highly polarized MDCK cells with mycalolide B (MB), a recently developed actin-depolymerizing reagent, induced a decrease of transepithelial resistance in a dose- and time-dependent manner with reversibility when the reagent was washed out. Changes in cytoskeletal actin, such as a reduction of cortical actin, irregularity of stress fibers, and punctated actin aggregates, were observed after MB treatment. However, the fence function, as studied by diffusion of apically labeled sphingomyelin/BSA complex, remained intact in the MB-treated MDCK cells. Localization of junctional molecules and apical marker proteins such as E-cadherin, ZO-1, and 114-kDa protein was shown to be unaffected. Furthermore, freeze-fracture study showed apparent tight junction strands. Collectively, MB treatment abolished the paracellular gate but not the fence function of MDCK cells, suggesting that cytoskeletal actin may play differential roles in the gate and fence functions of the tight junction.

Oncogenic Raf-1 disrupts epithelial tight junctions via downregulation of occludin

Occludin is an integral membrane protein of the epithelial cell tight junction (TJ). Its potential role in coordinating structural and functional events of TJ formation has been suggested recently. Using a rat salivary gland epithelial cell line (Pa-4) as a model system, we have demonstrated that occludin not only is a critical component of functional TJs but also controls the phenotypic changes associated with epithelium oncogenesis. Transfection of an oncogenic Raf-1 into Pa-4 cells resulted in a complete loss of TJ function and the acquisition of a stratified phenotype that lacked cell-cell contact growth control. The expression of occludin and claudin-1 was downregulated, and the distribution patterns of ZO-1 and E-cadherin were altered. Introduction of the human occludin gene into Raf-1-activated Pa-4 cells resulted in reacquisition of a monolayer phenotype and the formation of functionally intact TJs. In addition, the presence of exogenous occludin protein led to a recovery in claudin-1 protein level, relocation of the zonula occludens 1 protein (ZO-1) to the TJ, and redistribution of E-cadherin to the lateral membrane. Furthermore, the expression of occludin inhibited anchorage-independent growth of Raf-1-activated Pa-4 cells in soft agarose. Thus, occludin may act as a pivotal signaling molecule in oncogenic Raf- 1-induced disruption of TJs, and regulates phenotypic changes associated with epithelial cell transformation.

Induction of tight junctions in human connexin 32 (hCx32)-transfected mouse hepatocytes: connexin 32 interacts with occludin

Small gap junction plaques are associated with tight junction strands in some cell types including hepatocytes and it is thought that they may be closely related to tight junctions and the establishment of cell polarity. In order to examine roles of gap junctions in regulating expression and structure of tight junctions, we transfected human Cx32 cDNA into immortalized mouse hepatocytes (CHST8 cells) which lack endogenous Cx32 and Cx26. Immunocytochemistry revealed that endogenous integral tight junction protein occludin was strongly localized and was colocalized with Cx32 at cell borders in transfectants, whereas neither was detected in parental cells. In Northern blots, mRNAs encoding occludin and the other integral tight junction proteins, claudin-1 and -2, were induced in the transfectants compared to parental cells. In Western blots, occludin protein was increased in the transfectants compared to parental cells, and binding of occludin to Cx32 protein was demonstrated by immunoprecipitation. In freeze fracture of the transfectants, tight junction strands were more numerous and complex compared to parental cells, and small gap junction plaques appeared within induced tight junction strands. Nevertheless, no change in barrier function of tight junctions was observed. These results indicate that in hepatocytes, gap junction, and tight junction expression are closely coordinated, and that Cx32 may play a role in regulating occludin expression.

Protein interactions at the tight junction. Actin has multiple binding partners, and ZO-1 forms independent complexes with ZO-2 and ZO-3

Defining how the molecular constituents of the tight junction interact is a prerequisite to understanding tight junction physiology. We utilized in vitro binding assays with purified recombinant proteins and immunoprecipitation analyses to define interactions between ZO-1, ZO-2, ZO-3, occludin, and the actin cytoskeleton. Actin cosedimentation studies showed that ZO-2, ZO-3, and occludin all interact directly with F-actin in vitro, indicating that actin is engaged in multiple interactions at the tight junction. Low speed sedimentation analyses demonstrated that neither ZO-2, ZO-3, nor occludin act as F-actin cross-linking proteins, and further evidence indicates that these proteins do not bind to actin filament ends. The binding interactions of ZO-2, ZO-3, and occludin were corroborated in vivo by immunofluorescence colocalization experiments which showed that all three proteins colocalized with actin aggregates at cell borders in cytochalasin D-treated Madin-Darby canine kidney cells. Exploration of other tight junction protein interactions demonstrated that ZO-2 binds directly to both ZO-1 and occludin. Contrary to previous beliefs, our immunoprecipitation results indicate that ZO-1, ZO-2, and ZO-3 exist in situ primarily as independent ZO-1.ZO-2 and ZO-1.ZO-3 complexes rather than a trimeric ZO-1.ZO-2.ZO-3 grouping. These studies elucidate direct binding interactions among tight junction-associated proteins, giving insight into their organization as a multimolecular structure.