Expression and polarized targeting of a rab3 isoform in epithelial cells

JEAP, a novel component of tight junctions in exocrine cells

Tight junctions (TJs) consist of transmembrane proteins and many peripheral membrane proteins. To further characterize the molecular organization of TJs, we attempted here to screen for novel TJ proteins by the fluorescence localization-based expression cloning method. We identified a novel peripheral membrane protein at TJs and named it junction-enriched and -associated protein (JEAP). JEAP consists of 882 amino acids with a calculated molecular weight of 98,444. JEAP contained a polyglutamic acid repeat at the N-terminal region, a coiled-coil domain at the middle region, and a consensus motif for binding to PDZ domains at the C-terminal region. Exogenously expressed JEAP co-localized with ZO-1 and occludin at TJs in polarized Madin-Darby canine kidney cells, but not with claudin-1, JAM, or ZO-1 in L cells. Endogenous JEAP localized at TJs of exocrine cells including pancreas, submandibular gland, lacrimal gland, parotid gland, and sublingual gland, but not at TJs of epithelial cells of small intestine or endothelial cells of blood vessels. The present results indicate that JEAP is a novel component of TJs, which is specifically expressed in exocrine cells.

Direct interaction between Rab3b and the polymeric immunoglobulin receptor controls ligand-stimulated transcytosis in epithelial cells

We have examined the role of rab3b in epithelial cells. In MDCK cells, rab3b localizes to vesicular structures containing the polymeric immunoglobulin receptor (pIgR) and located subjacent to the apical surface. We found that GTP-bound rab3b directly interacts with the cytoplasmic domain of pIgR. Binding of dIgA to pIgR causes a dissociation of the interaction with rab3b, a process that requires dIgA-mediated signaling, Arg657 in the cytoplasmic domain of pIgR, and possibly GTP hydrolysis by rab3b. Binding of dIgA to pIgR at the basolateral surface stimulates subsequent transcytosis to the apical surface. Overexpression of GTP-locked rab3b inhibits dIgA-stimulated transcytosis. Together, our data demonstrate that a rab protein can bind directly to a specific cargo protein and thereby control its trafficking.

Cell Junction Dynamics in the Testis: Sertoli-Germ Cell Interactions and Male Contraceptive Development

Spermatogenesis is an intriguing but complicated biological process. However, many studies since the 1960s have focused either on the hormonal events of the hypothalamus-pituitary-testicular axis or morphological events that take place in the seminiferous epithelium. Recent advances in biochemistry, cell biology, and molecular biology have shifted attention to understanding some of the key events that regulate spermatogenesis, such as germ cell apoptosis, cell cycle regulation, Sertoli-germ cell communication, and junction dynamics. In this review, we discuss the physiology and biology of junction dynamics in the testis, in particular how these events affect interactions of Sertoli and germ cells in the seminiferous epithelium behind the blood-testis barrier. We also discuss how these events regulate the opening and closing of the blood-testis barrier to permit the timely passage of preleptotene and leptotene spermatocytes across the blood-testis barrier. This is physiologically important since developing germ cells must translocate across the blood-testis barrier as well as traverse the seminiferous epithelium during their development. We also discuss several available in vitro and in vivo models that can be used to study Sertoli-germ cell anchoring junctions and Sertoli-Sertoli tight junctions. An in-depth survey in this subject has also identified several potential targets to be tackled to perturb spermatogenesis, which will likely lead to the development of novel male contraceptives.

Pilt, a novel peripheral membrane protein at tight junctions in epithelial cells

Tight junctions (TJs) serve as a barrier that prevents solutes and water from passing through the paracellular pathway, and as a fence between the apical and basolateral plasma membranes in epithelial cells. TJs consist of transmembrane proteins (claudin, occludin, and JAM) and many peripheral membrane proteins, including actin filament (F-actin)-binding scaffold proteins (ZO-1, -2, and -3), non-F-actin-binding scaffold proteins (MAGI-1), and cell polarity molecules (ASIP/PAR-3 and PAR-6). We identified here a novel peripheral membrane protein at TJs from a human cDNA library and named it Pilt (for protein incorporated later into TJs), because it was incorporated into TJs later after the claudin-based junctional strands were formed. Pilt consists of 547 amino acids with a calculated M(r) of 60,704. Pilt has a proline-rich domain. In cadherin-deficient L cells stably expressing claudin or JAM, Pilt was not recruited to claudin-based or JAM-based cell-cell contact sites, suggesting that Pilt does not directly interact with claudin or JAM. The present results indicate that Pilt is a novel component of TJs.

Synapsin I is expressed in epithelial cells: localization to a unique trans-Golgi compartment

Synapsin I is abundant in neural tissues. Its phosphorylation is thought to regulate synaptic vesicle exocytosis in the pre-synaptic terminal by mediating vesicle tethering to the cytoskeleton. Using anti-synapsin antibodies, we detected an 85 kDa protein in liver cells and identified it as synapsin I. Like brain synapsin I, non-neuronal synapsin I is phosphorylated in vitro by protein kinase A and yields identical 32P-peptide maps after limited proteolysis. We also detected synapsin I mRNA in liver by northern blot analysis. These results indicate that the expression of synapsin I is more widespread than previously thought. Immunofluorescence analysis of several non-neuronal cell lines localizes synapsin I to a vesicular compartment adjacent to trans-elements of the Golgi complex, which is also labeled with antibodies against myosin II; no sub-plasma membrane synapsin I is evident. We conclude that synapsin I is present in epithelial cells and is associated with a trans-Golgi network-derived compartment; this localization suggests that it plays a role in modulating post-TGN trafficking pathways.

Subcellular distribution and function of Rab3A-D in pancreatic acinar AR42J cells

Members of the Rab3 subfamily have been linked to the regulation of exocytosis in secretory cells. We have recently shown by Northern blot analysis that pancreatic acinar-like AR42J cells express all four Rab3 isoforms (Rab3A-D). In the present study, we examined the subcellular distribution of endogenously expressed Rab3 proteins and their relation to the amylase-containing secretory compartment in dexamethasone-differentiated AR42J cells. Rab3A and Rab3C were enriched in the cytosol, Rab3B and Rab3D in the membrane fraction. Accordingly, confocal immunocytochemistry revealed that Rab3B and Rab3D were located in a compartment close to the plasma membrane, whereas anti-Rab3A and Rab3C mainly stained the cytosol. Sucrose density gradient centrifugation showed overlapping, but distinct localization of each Rab3 isoform. The order of banding from lighter to more dense fractions was Rab3C < Rab3A < Rab3B < Rab3D. All Rab3 proteins at least partially colocalized with amylase immunoreactivity. Transient overexpression of Rab3 proteins showed that Rab3A inhibited cholecystokinin (CCK)-induced amylase secretion, whereas overexpression of other Rab3 isoforms had no significant effect. In conclusion, our data indicate that the different Rab3 proteins show distinct subcellular distribution, suggesting different impact on exocrine secretory response in dexamethasone-differentiated AR42J cells.

Rho kinase regulates tight junction function and is necessary for tight junction assembly in polarized intestinal epithelia

BACKGROUND & AIMS

Tight junctions are crucial determinants of barrier function in polarized intestinal epithelia and are regulated by Rho guanosine triphosphatase. Rho kinase (ROCK) is a downstream effector of Rho.

METHODS

A specific inhibitor of ROCK, Y-27632, was used to examine the role of ROCK in the regulation of tight junctions in model intestinal (T84) cells by electrophysiologic, biochemical, morphologic, and molecular biologic approaches.

RESULTS

ROCK inhibition induced reorganization of apical F-actin structures and enhanced paracellular permeability but did not alter the distribution or detergent solubility of tight junction proteins. Confocal microscopy showed colocalization of a subpool of ROCK with the tight junction protein zonula occludens 1. Inhibition of ROCK function by a dominant negative mutant of ROCK also produced reorganization of apical F-actin structures without disruption of tight junctions. ROCK inhibition in calcium switch assays showed that ROCK is necessary for the assembly of tight and adherens junctions. Upon calcium repletion, occludin, zonula occludens 1, and E-cadherin failed to redistribute to the intercellular junctions; assembly of the apical F-actin cytoskeleton was prevented; and barrier function failed to recover.

CONCLUSIONS

We suggest that ROCK regulates intact tight junctions via its effects on the F-actin cytoskeleton. ROCK is also critical for assembly of the apical junctional proteins and the F-actin cytoskeleton organization during junctional formation.

OSP/claudin-11 forms a complex with a novel member of the tetraspanin super family and beta1 integrin and regulates proliferation and migration of oligodendrocytes

Oligodendrocyte-specific protein (OSP)/claudin-11 is a major component of central nervous system myelin and forms tight junctions (TJs) within myelin sheaths. TJs are essential for forming a paracellular barrier and have been implicated in the regulation of growth and differentiation via signal transduction pathways. We have identified an OSP/claudin-11-associated protein (OAP)1, using a yeast two-hybrid screen. OAP-1 is a novel member of the tetraspanin superfamily, and it is widely expressed in several cell types, including oligodendrocytes. OAP-1, OSP/claudin-11, and beta1 integrin form a complex as indicated by coimmunoprecipitation and confocal immunocytochemistry. Overexpression of OSP/claudin-11 or OAP-1 induced proliferation in an oligodendrocyte cell line. Anti-OAP-1, anti-OSP/claudin-11, and anti-beta1 integrin antibodies inhibited migration of primary oligodendrocytes, and migration was impaired in OSP/claudin-11-deficient primary oligodendrocytes. These data suggest a role for OSP/claudin-11, OAP-1, and beta1 integrin complex in regulating proliferation and migration of oligodendrocytes, a process essential for normal myelination and repair.

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.

Regulation of tight junction permeability and occludin phosphorylation by Rhoa-p160ROCK-dependent and -independent mechanisms

In epithelial and endothelial cells, tight junctions regulate the paracellular permeability of ions and proteins. Disruption of tight junctions by inflammation is often associated with tissue edema, but regulatory mechanisms are not fully understood. Using ECV304 cells as a model system, lysophosphatidic acid and histamine were found to increase the paracellular permeability of the tracer horseradish peroxidase. Cytoskeletal changes induced by these agents included stimulation of stress fiber formation and myosin light chain phosphorylation. Additionally, occludin, a tight junction protein, was a target for signaling events triggered by lysophosphatidic acid and histamine, events that resulted in its phosphorylation. A dominant-negative mutant of RhoA, RhoA T19N, or a specific inhibitor of Rho-activated kinases, Y-27632, prevented stress fiber formation, myosin light chain phosphorylation, occludin phosphorylation, and the increase in tracer flux in response to lysophosphatidic acid. In contrast, although RhoA T19N and Y-27632 blocked the cytoskeletal events induced by histamine, they had no effect on the stimulation of occludin phosphorylation or increased tracer flux, indicating that occludin phosphorylation may regulate tight junction permeability independently of cytoskeletal events. Thus, occludin is a target for receptor-initiated signaling events regulating its phosphorylation, and this phosphorylation may be a key regulator of tight junction permeability.

Occludin and claudin-1 concentrate in the midbody of immortalized mouse hepatocytes during cell division

It has been believed that epithelial cells maintain tight junctions at all times, including during cell division, to provide a continuous epithelial seal. However, changes in localization of integral tight junction proteins during cell division have not been examined. In this study, using SV40-immortalized mouse hepatocytes transfected with human Cx32 cDNA, in which tight junction strands and the endogenous tight junction proteins occludin, claudin-1, ZO-1, and ZO-2 were induced, we examined changes in localization of the tight junction proteins at all stages of cell division. All tight junction proteins were present between mitotic cells and neighboring cells throughout cell division. In late telophase, the integral tight junction proteins occludin and claudin-1, but not the cytoplasmic proteins ZO-1 and ZO-2, were concentrated in the midbody between the daughter cells and were observed at cell borders between the daugher and neighboring cells. These results indicate that the integral tight junction proteins are regulated in a different manner from the cytoplasmic proteins ZO-1 and ZO-2 during cytokinesis.

Clostridium difficile toxins disrupt epithelial barrier function by altering membrane microdomain localization of tight junction proteins

The anaerobic bacterium Clostridium difficile is the etiologic agent of pseudomembranous colitis. C. difficile toxins TcdA and TcdB are UDP-glucosyltransferases that monoglucosylate and thereby inactivate the Rho family of GTPases (W. P. Ciesla, Jr., and D. A. Bobak, J. Biol. Chem. 273:16021-16026, 1998). We utilized purified reference toxins of C. difficile, TcdA-10463 (TcdA) and TcdB-10463 (TcdB), and a model intestinal epithelial cell line to characterize their influence on tight-junction (TJ) organization and hence to analyze the mechanisms by which they contribute to the enhanced paracellular permeability and disease pathophysiology of pseudomembranous colitis. The increase in paracellular permeability induced by TcdA and TcdB was associated with disorganization of apical and basal F-actin. F-actin restructuring was paralleled by dissociation of occludin, ZO-1, and ZO-2 from the lateral TJ membrane without influencing the subjacent adherens junction protein, E-cadherin. In addition, we observed decreased association of actin with the TJ cytoplasmic plaque protein ZO-1. Differential detergent extraction and fractionation in sucrose density gradients revealed TcdB-induced redistribution of occludin and ZO-1 from detergent-insoluble fractions constituting "raft-like" membrane microdomains, suggesting an important role of Rho proteins in maintaining the association of TJ proteins with such microdomains. These toxin-mediated effects on actin and TJ structure provide a mechanism for early events in the pathophysiology of pseudomembranous colitis.

Cx32 but not Cx26 is associated with tight junctions in primary cultures of rat hepatocytes

On freeze-fracture replicas, gap junctions are frequently colocalized with tight junctions. In this study, to elucidate the relationship between gap- and tight-junction proteins, we investigated the localization of gap-junction proteins Cx32 and Cx26 and tight-junction proteins occludin, claudin-1, ZO-1, and ZO-2 in primary cultured rat hepatocytes, using confocal laser microscopy. In hepatocytes cultured in 2% DMSO and 10(-7) M glucagon medium, Cx32- but not Cx26-immunoreactive lines were observed on the most subapical plasma membrane at cell borders, while on the basolateral membrane both Cx32- and Cx26-positive spots were colocalized. Occludin-, claudin-1-, ZO-1-, and ZO-2-immunoreactive lines were also linearly observed on the most subapical plasma membrane and were colocalized with only Cx32-immunoreactive lines. In freeze-fracture analysis, many small gap-junction plaques were observed within a well-developed tight-junction strand network. The fence function of tight junctions in the cells, as examined by diffusion of labeled sphingomyelin, was well maintained. We also carried out Western blotting for Cx32 following immunoprecipitation with anti-occludin, anti-claudin-1, or anti-ZO-1 antibodies. Cx32 was detectable in all immunoprecipitates. These results suggest that Cx32 gap junctions, but not those with Cx26, are closely coordinated with the expression and function of tight junctions in hepatocytes and that Cx32 gap-junction formation may affect cell polarity through modification of tight-junction expression.

Immunohistochemical localization and mRNA detection of Rab3D and/or Rab3B in rat von Ebner's glands, parotid gland, pancreas, and liver

In examining the secretory mechanism of exocrine glands, we focused on the small GTP-binding proteins, Rab3D and Rab3B, which function in the final steps of exocytosis in non-neuronal tissues. These proteins were observed in von Ebner's glands by 32P-GTP overlay. mRNA isolated from von Ebner's glands, the pancreas, parotid glands, and liver was subjected to reverse transcription PCR and probed with primers and nested primers for Rab3D and Rab3B. Rab3D was found in all three exocrine glands and the liver, while Rab3B was found in the liver. Utilizing immunofluorescence histochemistry, Rab3D was localized to hepatocytes of the liver and to secretory granules of the exocrine glands, and Rab3B to liver and pancreatic islets. Isoproterenol evoked decreases in alpha-amylase- and Rab3D-labelled parotid secretory granules, and pilocarpine stimulated decreases in secretory granules labelled for lingual lipase and Rab3D from von Ebner's glands, and amylase and Rab3D from pancreas. Neither secretagogue affected Rab3B in pancreatic islets. These observed parallel decreases in response to beta-adrenergic (parotid) or cholinergic (von Ebner's and pancreas) secretagogues indicate that the function of Rab3D in exocytosis in these exocrine organs is similar and that the type of secretagogue does not determine the function.

Role of vinculin in the maintenance of cell-cell contacts in kidney epithelial MDBK cells

Microinjection of fluorophore-tagged cytoskeletal proteins has been a useful tool in studies of formation of focal adhesions (FA). We used this method to study the maintenance of adherens junctions (AJ) and tight junctions (TJ) of epithelial Madin-Darby bovine kidney cells. We chose alpha-actinin and vinculin as markers, because they are present both at adherens junctions and focal adhesions and their binding partners have been well characterized. Isolated FITC-labelled chicken alpha-actinin and vinculin were injected into confluent cells where they were rapidly incorporated both in FAs and AJs. The FAs remained unchanged, whereas cell-cell contacts began to fade within an hour after injection and the cells were joined to polykaryons having 5 to 13 nuclei. Short fragments of cell membranes containing injected proteins, actin, beta-catenin, cadherin, claudin, occludin and ZO-1 were visible inside the polykaryons indicating that both AJs and TJs were disintegrated as a single complex. Microinjected FITC-labelled vinculin head domain was also incorporated to both AJs and FAs, but instead of fusions it rapidly induced the detachment of the cells from the substratum probably due to high affinity of vinculin head to talin. Vinculin tail domain had no apparent effect on the cell morphology. Since small GTPases are involved in the building up of AJs, we injected active and inactive forms of cdc42 and rac proteins together with vinculin to see their effect. Active forms reduced the formation of polykaryons presumably by strengthening AJs, whereas inactive forms had no apparent effect. We suggest that excess alpha-actinin and vinculin uncouple the cell-cell adhesion junctions from the intracellular cytoskeleton which leads to fragmentation of junctional complexes and subsequent cell fusion. The results show that cell-cell adhesion sites are more dynamic and more sensitive than FAs to an imbalance in the amount of free alpha-actinin and intact vinculin.

Alternative splicing of the human Rab6A gene generates two close but functionally different isoforms

Analysis of the human Rab6A gene structure reveals the presence of a duplicated exon, and incorporation of either of the two exons by alternative splicing is shown to generate two Rab6 isoforms named Rab6A and Rab6A', which differ in only three amino acid residues located in regions flanking the PM3 GTP-binding domain of the proteins. These isoforms are ubiquitously expressed at similar levels, exhibit the same GTP-binding properties, and are localized to the Golgi apparatus. Overexpression of the GTP-bound mutants of Rab6A (Rab6A Q72L) or Rab6A' (Rab6A' Q72L) inhibits secretion in HeLa cells, but overexpression of Rab6A' Q72L does not induce the redistribution of Golgi proteins into the endoplasmic reticulum. This suggests that Rab6A' is not able to stimulate Golgi-to-endoplasmic reticulum retrograde transport, as described previously for Rab6A. In addition, Rab6A' interacts with two Rab6A partners, GAPCenA and "clone 1," but not with the kinesin-like protein Rabkinesin-6, a Golgi-associated Rab6A effector. Interestingly, we found that the functional differences between Rab6A and Rab6A' are contingent on one amino acid (T or A at position 87). Therefore, limited amino acid substitutions within a Rab protein introduced by alternative splicing could represent a mechanism to generate functionally different isoforms that interact with distinct sets of effectors.

Differentiation of the epithelial apical junctional complex during mouse preimplantation development: a role for rab13 in the early maturation of the tight junction

We have investigated the mechanisms by which the epithelial apicolateral junctional complex (AJC) is generated during trophectoderm differentiation in the mouse blastocyst using molecular, structural and functional analyses. The mature AJC comprises an apical tight junction (TJ), responsible for intercellular sealing and blastocoel formation, and subjacent zonula adherens E-cadherin/catenin adhesion complex which also extends along lateral membrane contact sites. Dual labelling confocal microscopy revealed that the AJC derived from a single 'intermediate' complex formed following embryo compaction at the 8-cell stage in which the TJ-associated peripheral membrane protein, ZO-1alpha- isoform, was co-localized with both alpha- and beta-catenin. However, following assembly of the TJ transmembrane protein, occludin, from the early 32-cell stage when blastocoel formation begins, ZO-1alpha- and other TJ proteins (ZO-1alpha+ isoform, occludin, cingulin) co-localized in an apical TJ which was separate from a subjacent E-cadherin/catenin zonula adherens complex. Thin-section electron microscopy confirmed that a single zonula adherens-like junctional complex present at the AJC site following compaction matured into a dual TJ and zonula adherens complex at the blastocyst stage. Embryo incubation in the tracer FITC-dextran 4 kDa showed that a functional TJ seal was established coincident with blastocoel formation. We also found that rab13, a small GTPase previously localized to the TJ, is expressed at all stages of preimplantation development and relocates from the cytoplasm to the site of AJC biogenesis from compaction onwards with rab13 and ZO-1alpha- co-localizing precisely. Our data indicate that the segregation of the two elements of the AJC occurs late in trophectoderm differentiation and likely has functional importance in blastocyst formation. Moreover, we propose a role for rab13 in the specification of the AJC site and the formation and segregation of the TJ.

Protein kinase C activation leads to dephosphorylation of occludin and tight junction permeability increase in LLC-PK1 epithelial cell sheets

Activation of protein kinase C by exposure of LLC-PK1 renal epithelial cells to 10(−7) M TPA, a tumor promoting phorbol ester, results in a rapid and sustained increase in paracellular permeability as evidenced by a decrease in transepithelial electrical resistance. Occludin, the first identified transmembrane protein to be localized to the tight junction of both epithelial and endothelial cells is thought play an important role in tight junction barriers. Although transepithelial electrical resistance fell to less than 20% of initial values within 1 hour of TPA exposure, transmission electron microscopy showed no change in the gross morphology of the tight junction of cells treated with 10(−7) M TPA for up to 2 hours. Immunofluorescence microscopy revealed a more rapid change in the membrane distribution of ZO-1 compared to occludin in the TPA-treated cells. Immunoblot analysis indicated that occludin levels in total cell lysates as well as cytosolic, membrane (Triton-X soluble) and cytoskeletal (Triton-X insoluble) fractions remained unchanged for at least 2 hours in cells treated with 10(−7) M TPA compared to their corresponding control cells. As the phosphorylation state of occludin is thought to be important in both tight junction assembly and regulation, the effect of phorbol ester treatment on the phosphorylation of occludin was investigated. Surprisingly, activation of protein kinase C with 10(−7) M TPA resulted in a time-dependent decrease in threonine phosphorylation of occludin which correlated closely with the rapid decrease in transepithelial electrical resistance. This dephosphorylation of occludin, occurring after activation of a serine/threonine kinase by TPA, suggested that protein kinase C was not acting directly on this tight junction target protein. If occludin dephosphorylation is involved in increasing tight junction permeability, then protein kinase C is apparently further upstream in the signaling pathway regulating epithelial barrier function, with a downstream serine/threonine phosphatase acting upon occludin.

The mechanisms of aquaporin control in the renal collecting duct

The antidiuretic hormone arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Central to its antidiuretic action in mammals is the exocytotic insertion of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the apical membrane of principal cells, an event initiated by an increase in cAMP and activation of protein kinase A. Water is then reabsorbed from the hypotonic urine of the collecting duct. The water channels aquaporin-3 (AQP3) and aquaporin-4 (AQP4), which are constitutively present in the basolateral membrane, allow the exit of water from the cell into the hypertonic interstitium. Withdrawal of the hormone leads to endocytotic retrieval of AQP2 from the cell membrane. The hormone-induced rapid redistribution between the interior of the cell and the cell membrane establishes the basis for the short term regulation of water permeability. In addition water channels (AQP2 and 3) of principal cells are regulated at the level of expression (long term regulation). This review summarizes the current knowledge on the molecular mechanisms underlying the short and long term regulation of water channels in principal cells. In the first part special emphasis is placed on the proteins involved in short term regulation of AQP2 (SNARE proteins, Rab proteins, cytoskeletal proteins, G proteins, protein kinase A anchoring proteins and endocytotic proteins). In the second part, physiological and pathophysiological stimuli determining the long term regulation are discussed.

Rab3D, a small GTP-binding protein implicated in regulated secretion, is associated with the transcytotic pathway in rat hepatocytes

Rab3 isotypes are expressed in regulated secretory cells. Here, we report that rab3D is also expressed in rat hepatocytes, classic models for constitutive secretion. Using reverse transcriptase polymerase chain reaction (RT-PCR) with primers specific for rat rab3D, we amplified a 151 base pair rab3D fragment from total RNA extracted from primary cultures of rat hepatocytes. Immunoblot analysis using polyclonal antibodies to peptides representing the N- and C-terminal hypervariable regions of murine rab3D recognized a protein of approximately 25 kd in hepatocyte lysates, hepatic subcellular fractions, and tissue extracts. The distribution of rab3D was primarily cytosolic; however, only membrane-associated rab3D significantly bound guanosine triphosphate (GTP) in overlay assays. Several lines of investigation indicate that rab3D is associated with the transcytotic pathway. First, rab3D was enriched in a crude vesicle carrier fraction (CVCF), which includes transcytotic carriers. Vesicular compartments immunoisolated from the CVCF on magnetic beads coated with anti-rab3D antibody were enriched in the transcytosed form of the polymeric IgA receptor (pIgA-R), but lacked not only the pIgA-R precursor form associated with the secretory pathway, but also a Golgi marker protein. Second, indirect immunofluorescence on frozen liver sections and in polarized cultured hepatocytes localized rab3D-positive sites at or near the apical plasma membrane and to the pericanalicular cytoplasm. Finally, cholestasis induced by bile duct ligation (BDL), a manipulation known to slow transcytosis, caused rab3D to accumulate in the pericanalicular cytoplasm of cholestatic hepatocytes. Our results indicate that rab3D plays a role in the regulation of apically directed transcytosis in rat hepatocytes.

The small GTPase Rab6B, a novel Rab6 subfamily member, is cell-type specifically expressed and localised to the Golgi apparatus

Members of the Rab subfamily of small GTPases play an important role in the regulation of intracellular transport routes. Rab6A has been shown to be a regulator of membrane traffic from the Golgi apparatus towards the endoplasmic reticulum (ER). Here, we report on the identification of a Rab6 isoform, termed Rab6B. The corresponding full-length cDNA was isolated from a Caco-2 cell library. The deduced amino acid sequence showed 91% identity with the Rab6A protein and revealed that sequence divergence is dispersed over a large region of the COOH-terminal domain. Rab6B is encoded by an independent gene which is located on chromosome 3 region q21-q23. In contrast to Rab6A whose expression is ubiquitous, northern blot analysis, immunohistochemistry, and immunofluorescence demonstrated that Rab6B is expressed in a tissue and cell-type specific manner. Rab6B is predominantly expressed in brain and the neuroblastoma cell line SK-N-SH. In brain, Rab6B was found to be specifically expressed in microglia, pericytes and Purkinje cells. Endogenous Rab6B localises to the Golgi apparatus and to ERGIC-53-positive vesicles. Comparable studies between Rab6A and Rab6B revealed distinct biochemical and cellular properties. Rab6B displayed lower GTP-binding activities and in overexpression studies, the protein is distributed over Golgi and ER membranes, whereas Rab6A is more restricted to the Golgi apparatus. Since the GTP-bound form of Rab6B (Rab6B Q72L) does interact with all known Rab6A effectors, including Rabkinesin-6, the results suggest a cell-type specific role for Rab6B in retrograde membrane traffic at the level of the Golgi complex.

Molecular physiology and pathophysiology of tight junctions I. Tight junction structure and function: lessons from mutant animals and proteins

Tight junctions form the major paracellular barrier in epithelial tissues. Barrier-sealing properties are quite variable among cell types in terms of electrical resistance, solute and water flux, and charge selectivity. A molecular explanation for this variability appears closer following identification of the transmembrane proteins occludin and members of the claudin multigene family. For example, the human phenotype of mutations in claudin-16 suggests that it creates a channel that allows magnesium to diffuse through renal tight junctions. Similarly, a mouse knockout of claudin-11 reveals its role in formation of tight junctions in myelin and between Sertoli cells in testis. The study of other claudins is expected to elucidate their contributions to creating junction structure and physiology in all epithelial tissues.

Genomic organization, chromosomal localization, and expression of the murine RAB3D gene

Rab proteins, members of the Ras superfamily of small GTPases, play regulatory roles in intercompartmental vesicular transport. Each step of traffic seems to require the participation of at least one distinct Rab, with the Rab3 subfamily involved in stimulated exocytosis. We report our studies on the murine rab3D gene, one of the four mammalian Rab3 isoforms. We located this gene on chromosome 13, region A(2-3). The rab3D gene consists of 5 exons spanning 10.6 kb, and the structural gene is contained in exons 2 through 5 with one canonical GTP-binding motif in each exon. Organization of the rab3D gene is identical to that of rab3A but different from other rab genes. Alternative poly-A(+) signals in the 3' untranslated region account for the identities of multiple transcripts detected by Northern blot analysis. Rab3D is expressed in all tissues studied, predominantly in heart, lung, and liver, and binding sites for multiple transcription factors are found in the TATA-less promoter region.

Interaction of junctional adhesion molecule with the tight junction components ZO-1, cingulin, and occludin

Junctional adhesion molecule (JAM) is an integral membrane protein that has been reported to colocalize with the tight junction molecules occludin, ZO-1, and cingulin. However, evidence for the association of JAM with these molecules is missing. Transfection of Chinese hamster ovary cells with JAM (either alone or in combination with occludin) resulted in enhanced junctional localization of both endogenous ZO-1 and cotransfected occludin. Additionally, JAM was coprecipitated with ZO-1 in the detergent-insoluble fraction of Caco-2 epithelial cells. A putative PDZ-binding motif at the cytoplasmic carboxyl terminus of JAM was required for mediating the interaction of JAM with ZO-1, as assessed by in vitro binding and coprecipitation experiments. JAM was also coprecipitated with cingulin, another cytoplasmic component of tight junctions, and this association required the amino-terminal globular head of cingulin. Taken together, these data indicate that JAM is a component of the multiprotein complex of tight junctions, which may facilitate junction assembly.

Rab3B regulates ZO-1 targeting and actin organization in PC12 neuroendocrine cells

Rab3B is a monomeric GTPase that modulates norepinephrine secretion when expressed in PC12 neuroendocrine cells. In the present study we determined whether rab3B also regulates the organization of intercellular junctions, since this GTPase localizes to regions of cell contact in multiple cell types. The stable expression of rab3B, but not the closely related rab3A, led to two morphological phenotypes in PC12 cells: (i) reorganization of F-actin into long filopodia and (ii) redistribution of the junction-associated protein ZO-1. ZO-1 localization was not appreciably affected by the expression of a GTP binding mutant of rab3B (N135I) that stimulates norepinephrine secretion by PC12 cells. The apparent diversity of these rab3B phenotypes implies that this GTPase is capable of influencing cell signaling pathways that in turn modulate the cytoskeleton and junction organization. In support of this hypothesis we observed that rab3B expression also altered the profile of proteins that interact with the signaling molecule, phosphatidylinositol 3-kinase (PI3-kinase). The effect of rab3B on protein interactions with PI3-kinase was reversed by inhibitors of this kinase. Furthermore, PI3-kinase inhibitors virtually abolished ZO-1 localization at the surfaces of cells that express rab3B, but not rab3A, whereas these inhibitors had no effect on rab3B-dependent norepinephrine secretion. Our results indicate that rab3B can influence junctional protein targeting and secretion by distinct mechanisms.

Tight junctions are membrane microdomains

Tight junctions (TJ) of polarized epithelial cells regulate barrier function at mucosal surfaces. Structural proteins of TJs include hyperphosphorylated occludin (HO) and the peripheral membrane protein, ZO-1. Since TJs are dynamically regulated, and lipid-modified signal transduction proteins localize to TJs, we considered the possibility that the TJ itself is composed of microdomains with unique structure. Differential detergent extraction and isopycnic sucrose density gradients were utilized to isolate TJ-enriched membranes from a polarized intestinal epithelial cell line, T84. Here we report that major pools of hyperphosphorylated occludin (HO) and ZO-1 are found in raft-like membrane microdomains with characteristics of the previously described detergent-insoluble glycolipid rafts (DIGs). Properties of such gradient fractions included Triton X-100 (TX-100) insolubility, light scattering at 600 nm, buoyant density of approximately 1.08 g/cm(3) and increased cholesterol content compared to high density fractions. Similar results were obtained using natural epithelium. Unlike the TJ proteins HO and ZO-1, other basolateral transmembrane proteins including E-cadherin, c-met and β 1 integrin were not increased in DIG-like fractions. Immunoprecipitation studies revealed coprecipitation of a pool of occludin with caveolin-1, a scaffolding protein abundant in DIGs. Coprecipitation results were supported by immunofluorescence and immunogold labeling studies demonstrating caveolin-1 localization in the apical membrane and focal colocalization with occludin in TJs. TJ disassembly by calcium chelation resulted in displacement of TJ proteins from the ‘raft-like’ compartment. Our findings suggest that raft-like compartments play an important role in the spatial organization of TJs and probably in regulation of paracellular permeability in epithelial cells.

Rotavirus-induced structural and functional alterations in tight junctions of polarized intestinal Caco-2 cell monolayers

We provide here new insights into rotavirus (RRV) pathogenicity by showing that RRV infection promotes structural and functional injuries localized at the tight junctions (TJ) in the cell-cell junctional complex of cultured polarized human intestinal Caco-2 cells forming monolayers. RRV infection resulted in a progressive increase in the paracellular permeability to [(3)H]mannitol as a function of the time postinfection. We observed a disorganization of the TJ-associated protein occludin as a function of the time postinfection, whereas distribution of the zonula adherens associated E-cadherin was not affected. These structural and functional RRV-induced TJ injuries were not accompanied by alteration in cell and monolayer integrity, as assessed by the lack of change in transepithelial membrane resistance and lactate dehydrogenase release. Finally, using the stabilizer of actin filaments Jasplakinolide, we demonstrated that the RRV-induced structural and functional alterations in TJ are independent of the RRV-induced apical F-actin rearrangements.

Multiple aspects of Rab protein action in the secretory pathway: focus on Rab3 and Rab6

Rab proteins form the largest branch of the Ras superfamily of GTPases. They are localized to the cytoplasmic face of organelles and vesicles involved in the biosynthetic/secretory and endocytic pathways in eukaryotic cells. It is now well established that Rab proteins play an essential role in the processes that underlie the targeting and fusion of transport vesicles with their appropriate acceptor membranes. They perform this task through interactions with a wide variety of effector molecules. In this review, we illustrate recent advances in the field of Rab GTPases, taking as examples two proteins involved in the biosynthetic pathway, Rab3 and Rab6.

Post-translational control of occludin membrane assembly in mouse trophectoderm: a mechanism to regulate timing of tight junction biogenesis and blastocyst formation

The mouse blastocyst forms during the 32-cell stage with the emergence of the blastocoelic cavity. This developmental transition is dependent upon the differentiation and transport function of the trophectoderm epithelium which forms the wall of the blastocyst and exhibits functional intercellular tight junctions (TJs) to maintain epithelial integrity during blastocoele expansion. To investigate mechanisms regulating the timing of blastocyst formation, we have examined the dynamics of expression of occludin, an integral membrane protein of the TJ. Confocal microscopy of intact embryos and synchronised cell clusters revealed that occludin first assembles at the apicolateral membrane contact site between nascent trophectoderm cells usually during the early 32-cell stage, just prior to the time of blastocoele cavitation. This is a late event in the assembly of TJ-associated proteins within trophectoderm which, from our previous data, spans from 8- to 32-cell stages. Occludin membrane assembly is dependent upon prior E-cadherin-mediated cell-cell adhesion and is sensitive to brefeldin A, an inhibitor of Golgi-to-membrane transport. Occludin is delivered to the TJ site in association with the TJ plaque protein, ZO-1(α)+, which we have shown previously is newly transcribed and translated during late cleavage. Immediately after assembly and before cavitation, occludin localised at the TJ site switches from a Triton X-100-soluble to -insoluble form indicative of actin cytoskeletal and/or membrane anchorage. Occludin mRNA and protein are detectable throughout cleavage by RT-PCR and immunoblotting, respectively, indicating that timing of membrane assembly is not controlled by expression alone. Rather, we have identified changes in the pattern of different occludin forms expressed during cleavage which, using phosphatase treatment of embryo lysates, include post-translational modifications. We propose that the phosphorylation of one form of occludin (band 2, 65–67 kDa) during late cleavage, which leads to its exclusive conversion from a Triton X-100-soluble to -insoluble pool, may regulate occludin association with ZO-1(α)+ and membrane assembly, and thereby act to control completion of TJ biogenesis and the timing of blastocyst formation.

Actin coating of secretory granules during regulated exocytosis correlates with the release of rab3D

The present study describes a novel phenomenon in pancreatic acinar cells undergoing regulated exocytosis. When acinar cell preparations were challenged with the secretagogue carbamylcholine, a subpopulation of zymogen granules became coated with filamentous actin. These zymogen granules were always in proximity of the acinar cell apical membrane (the site of exocytosis) but did not appear to have fused yet. They were distinct from regular zymogen granules not only because of their association with filamentous actin, but also because the majority of them lacked the zymogen granule marker rab3D, a small GTPase implicated in regulated exocytosis. The apparent loss of rab3D, presumed to result from the release of rab3D from the granule membranes, could be prevented by agents that modulate the actomyosin system as well as by GTP[gammaS]. These data suggest that zymogen granules engaging in exocytosis become coated with actin before fusion and that this actin coating is tightly coupled to the release of rab3D. We propose that rab3D is involved in the regulation of actin polymerization around secretory granules and that actin coating might facilitate the movement of granules across the subapical actin network and toward their fusion site.

Direct binding of three tight junction-associated MAGUKs, ZO-1, ZO-2, and ZO-3, with the COOH termini of claudins

ZO-1, ZO-2, and ZO-3, which contain three PDZ domains (PDZ1 to -3), are concentrated at tight junctions (TJs) in epithelial cells. TJ strands are mainly composed of two distinct types of four-transmembrane proteins, occludin, and claudins, between which occludin was reported to directly bind to ZO-1/ZO-2/ZO-3. However, in occludin-deficient intestinal epithelial cells, ZO-1/ZO-2/ZO-3 were still recruited to TJs. We then examined the possible interactions between ZO-1/ZO-2/ZO-3 and claudins. ZO-1, ZO-2, and ZO-3 bound to the COOH-terminal YV sequence of claudin-1 to -8 through their PDZ1 domains in vitro. Then, claudin-1 or -2 was transfected into L fibroblasts, which express ZO-1 but not ZO-2 or ZO-3. Claudin-1 and -2 were concentrated at cell-cell borders in an elaborate network pattern, to which endogenous ZO-1 was recruited. When ZO-2 or ZO-3 were further transfected, both were recruited to the claudin-based networks together with endogenous ZO-1. Detailed analyses showed that ZO-2 and ZO-3 are recruited to the claudin-based networks through PDZ2 (ZO-2 or ZO-3)/PDZ2 (endogenous ZO-1) and PDZ1 (ZO-2 or ZO-3)/COOH-terminal YV (claudins) interactions. In good agreement, PDZ1 and PDZ2 domains of ZO-1/ZO-2/ZO-3 were also recruited to claudin-based TJs, when introduced into cultured epithelial cells. The possible molecular architecture of TJ plaque structures is discussed.

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.

Immunolocalization of tight junction proteins, occludin and ZO-1, and glucose transporter GLUT1 in the cells of the blood-nerve barrier

Facilitated-diffusion glucose transporter GLUT1 is abundant in the blood-nerve barrier. To observe the relationship between glucose transfer across the barrier and the molecular architecture of the barrier, we examined the localization of GLUT1 and tight junction proteins, occludin and zonula occludens-1 (ZO-1), by immunofluorescence microscopy and immunogold electron microscopy in the rat sciatic nerve. GLUT1 was enriched at the whole aspects of the plasma membranes of the cells of the barrier: perineurial cells, and endothelial cells of the blood vessels in the endoneurium. These GLUT1-positive cells were also positive for occludin and ZO-1, both of which were localized at tight junctions. ZO-1 additionally was present in the GLUT1-negative cells not serving as the blood-nerve barrier. These observations suggest that occludin in the tight junctions and GLUT1 at the plasma membranes in the cells of the barrier may constitute a mechanism for the selective transfer of glucose across the barrier while preventing the non-specific flow of blood constituents.

Requirement for Ras and phosphatidylinositol 3-kinase signaling uncouples the glucocorticoid-induced junctional organization and transepithelial electrical resistance in mammary tumor cells

In Con8 rat mammary epithelial tumor cells, the synthetic glucocorticoid dexamethasone stimulates the remodeling of the apical junction (tight and adherens junctions) and the transepithelial electrical resistance (TER), which reflects tight junction sealing. Indirect immunofluorescence revealed that dexamethasone induced the recruitment of endogenous Ras and the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase to regions of cell-cell contact, concurrently with the stimulation of TER. Expression of dominant-negative RasN17 abolished the dexamethasone stimulation in TER, whereas, dexamethasone induced the reorganization of tight junction and adherens junction proteins, ZO-1 and beta-catenin, as well as F-actin, to precise regions of cell-cell contact in a Ras-independent manner. Confocal microscopy revealed that RasN17 and the p85 regulatory subunit of PI 3-kinase co-localized with ZO-1 and F-actin at the tight junction and adherens junction, respectively. Treatment with either of the PI 3-kinase inhibitors, wortmannin or LY294002, or the MEK inhibitor PD 098059, which prevents MAPK signaling, attenuated the dexamethasone stimulation of TER without affecting apical junction remodeling. Similar to dominant-negative RasN17, disruption of both Ras effector pathways using a combination of inhibitors abolished the glucocorticoid stimulation of TER. Thus, the glucocorticoiddependent remodeling of the apical junction and tight junction sealing can be uncoupled by their dependence on Ras and/or PI 3-kinase-dependent pathways, implicating a new role for Ras and PI 3-kinase cell signaling events in the steroid control of cell-cell interactions.

Expression and regulation of RAB3 proteins in osteoclasts and their precursors

The ruffled membrane, the resorptive organelle of the osteoclast, is generated by fusion of intracytoplasmic acidifying vesicles with the plasma membrane, an event analogous to regulated exocytosis. While the ruffled membrane is essential to the bone resorptive process, the mechanisms governing its generation are unknown. However, regulated exocytosis is mediated, in part, by isoforms of the Rab3 subset of Rab GTPases. Because of similarities between exocytosis and ruffled membrane formation, we asked if Rab3 proteins are expressed by osteoclasts or their precursors, and if so, are these molecules regulated by agents known to prompt the osteoclast phenotype? We find murine osteoclast precursors, in the form of bone marrow macrophages (BMMs), express at least two Rab3 isoforms, namely A and B/C, which are individually enhanced by a variety of hematopoietic cytokines. Consistent with the osteoclastogenic properties of a number of these cytokines, differentiation of BMMs into osteoclasts, in vitro, is associated with increased expression of both isoforms, particularly Rab3B/C. Finally, Rab3B/C localizes with the avian osteoclast H+ATPase (vacuolar proton pump) and pp60c-src, both intracellularly and within acidifying vesicles derived largely from the ruffled membrane. Thus, expression of specific rab3 proteins, an event which may control formation of the osteoclast ruffled membrane, is modulated by cytokines during osteoclastogenesis.

Structural and signalling molecules come together at tight junctions

Tight junctions (TJs) have been suggested to act both as barriers and fences, but lack of information on the constituents of TJ strands has hampered the direct assessment of these functions. Over the past year, our understanding of the molecular architecture of TJ strands has increased markedly and we are ready to experimentally examine how TJs are involved in their dual functions.

High affinity Rab3 binding is dispensable for Rabphilin-dependent potentiation of stimulated secretion

Rabphilin is a protein that associates with the GTP-bound form of Rab3, a small GTPase that controls a late step in Ca(2+)-triggered exocytosis. Rabphilin is found only in neuroendocrine cells where it co-localises with Rab3A on the secretory vesicle membrane. The Rab3 binding domain (residues 45 to 170), located in the N-terminal part of Rabphilin, includes a cysteine-rich region with two zinc finger motifs that are required for efficient interaction with the small GTPase. To determine whether binding to Rab3A is necessary for the subcellular localisation of Rabphilin, we synthesised point mutants within the Rab3-binding domain. We found that two unique mutations (V61A and L83A) within an amphipathic alpha-helix of this region abolish detectable binding to endogenous Rab3, but only partially impair the targetting of the protein to secretory vesicles in PC12 and pancreatic HIT-T15 cells. Furthermore, both mutants transfected in the HIT-T15 beta cell line stimulate Ca(2+)-regulated exocytosis to the same extent as wild-type Rabphilin. Surprisingly, another Rabphilin mutant, R60A, which possesses a wild-type affinity for Rab3, and targets efficiently to membranes, does not potentiate regulated secretion. High affinity binding to Rab3 is therefore dispensable for the targetting of Rabphilin to secretory vesicles and for the potentiation of Ca(2+)-regulated secretion. The effects of Rabphilin on secretion may be mediated through interaction with another, unknown, factor that recognizes the Rab3 binding domain.

Modulation of glioma cell migration and invasion using Cl(-) and K(+) ion channel blockers

Human malignant gliomas are highly invasive tumors. Mechanisms that allow glioma cells to disseminate, migrating through the narrow extracellular brain spaces are poorly understood. We recently demonstrated expression of large voltage-dependent chloride (Cl(-)) currents, selectively expressed by human glioma cells in vitro and in situ (Ullrich et al., 1998). Currents are sensitive to several Cl(-) channel blockers, including chlorotoxin (Ctx), (Ullrich and Sontheimer; 1996; Ullrich et al; 1996), tetraethylammonium chloride (TEA), and tamoxifen (Ransom and Sontheimer, 1998). Using Transwell migration assays, we show that blockade of glioma Cl(-) channels specifically inhibits tumor cell migration in a dose-dependent manner. Ctx (5 microM), tamoxifen (10 microM), and TEA (1 mM) also prevented invasion of human glioma cells into fetal rat brain aggregates, used as an in vitro model to assess tumor invasiveness. Anion replacement studies suggest that permeation of chloride ions through glioma chloride channel is obligatory for cell migration. Osmotically induced cell swelling and subsequent regulatory volume decrease (RVD) in cultured glioma cells were reversibly prevented by 1 mM TEA, 10 microM tamoxifen, and irreversibly blocked by 5 microM Ctx added to the hypotonic media. Cl(-) fluxes associated with adaptive shape changes elicited by cell swelling and RVD in glioma cells were inhibited by 5 microM Ctx, 10 microM tamoxifen, and 1 mM TEA, as determined using the Cl(-)-sensitive fluorescent dye 6-methoxy-N-ethylquinolinium iodide. Collectively, these data suggest that chloride channels in glioma cells may enable tumor invasiveness, presumably by facilitating cell shape and cell volume changes that are more conducive to migration and invasion.

Modulation of glioma cell migration and invasion using Cl(-) and K(+) ion channel blockers

Human malignant gliomas are highly invasive tumors. Mechanisms that allow glioma cells to disseminate, migrating through the narrow extracellular brain spaces are poorly understood. We recently demonstrated expression of large voltage-dependent chloride (Cl(-)) currents, selectively expressed by human glioma cells in vitro and in situ (Ullrich et al., 1998). Currents are sensitive to several Cl(-) channel blockers, including chlorotoxin (Ctx), (Ullrich and Sontheimer; 1996; Ullrich et al; 1996), tetraethylammonium chloride (TEA), and tamoxifen (Ransom and Sontheimer, 1998). Using Transwell migration assays, we show that blockade of glioma Cl(-) channels specifically inhibits tumor cell migration in a dose-dependent manner. Ctx (5 microM), tamoxifen (10 microM), and TEA (1 mM) also prevented invasion of human glioma cells into fetal rat brain aggregates, used as an in vitro model to assess tumor invasiveness. Anion replacement studies suggest that permeation of chloride ions through glioma chloride channel is obligatory for cell migration. Osmotically induced cell swelling and subsequent regulatory volume decrease (RVD) in cultured glioma cells were reversibly prevented by 1 mM TEA, 10 microM tamoxifen, and irreversibly blocked by 5 microM Ctx added to the hypotonic media. Cl(-) fluxes associated with adaptive shape changes elicited by cell swelling and RVD in glioma cells were inhibited by 5 microM Ctx, 10 microM tamoxifen, and 1 mM TEA, as determined using the Cl(-)-sensitive fluorescent dye 6-methoxy-N-ethylquinolinium iodide. Collectively, these data suggest that chloride channels in glioma cells may enable tumor invasiveness, presumably by facilitating cell shape and cell volume changes that are more conducive to migration and invasion.

Expression and localization of Rab3D in rat parotid gland

Rab3 proteins (isoforms A, B, C and D) are low molecular weight GTP-binding proteins proposed to be involved in regulated exocytosis. In the present study, Rab3 protein expression and localization was examined in rat parotid gland by reverse transcription (rt) PCR, Western blotting and immunocytochemistry. An approximately 200 bp PCR product was obtained from parotid RNA by rtPCR and this fragment was cloned and sequenced. Nucleotide and deduced amino acid sequences obtained from five clones were identical to rab3D. Membrane and cytosolic fractions prepared from parotid acini were immunoblotted with antisera specific for each of the four Rab3 isoforms. A 28 kDa protein was detected with Rab3D-specific antisera in both fractions with staining being more intense in the membrane fraction. No other Rab3 isoforms were detected by immunoblotting, a result consistent with those obtained by rtPCR. Rab3D was enriched in zymogen granule membranes and Triton X-114 extraction revealed that this isoform is predominantly lipid-modified in parotid. Localization of Rab3D was done on frozen sections of parotid gland by immunofluorescence microscopy. Staining was observed primarily in the acinar cells and was adjacent to the acinar lumen. Incubation of dispersed acini with isoproterenol and substance P stimulated amylase secretion 4- and 2-fold above basal, respectively. Isoproterenol, but not substance P, induced redistribution of Rab3D from the cytosol to the membrane fraction in dispersed parotid acini. Consistent with these findings, isoproterenol injections into fasted rats also resulted in increased membrane-associated Rab3D in the parotid acini. These results indicate that Rab3D is: (1) the major Rab3 isoform expressed in rat parotid gland; (2) localized to zymogen granule membranes; and (3) involved with regulated enzyme secretion in acinar cells.

Occludin and claudins in tight-junction strands: leading or supporting players?

Tight junctions have attracted much interest from cell biologists, especially electron microscopists, since on freeze-fracture electron microscopy they appear as a well-developed network of continuous, anastomosing intramembranous strands (tight-junction strands). These strands might be directly involved in the 'barrier' as well as 'fence' functions in epithelial and endothelial cell sheets, but until recently little was known of their constituents. This review discusses current understanding of the molecular architecture of tight-junction strands, focusing on the recent discovery of two distinct types of tight-junction-specific integral membrane proteins, occludin and claudins.

Comparison of the effects on secretion in chromaffin and PC12 cells of Rab3 family members and mutants. Evidence that inhibitory effects are independent of direct interaction with Rabphilin3

The Rab class of low molecular weight GTPases has been implicated in the regulation of vesicular trafficking between membrane compartments in eukaryotic cells. The Rab3 family consisting of four highly homologous isoforms is associated with secretory granules and synaptic vesicles. Many different types of experiments indicate that Rab3a is a negative regulator of exocytosis and that its GTP-bound form interacts with Rabphilin3, a possible effector. Overexpression of Rabphilin3 in chromaffin cells enhances secretion. We have investigated the expression, localization, and effects on secretion of the various members of the Rab3 family in bovine chromaffin and PC12 cells. We found that Rab3a, Rab3b, Rab3c, and Rab3d are expressed to varying degrees in PC12 cells and in a fraction enriched in chromaffin granule membranes from the adrenal medulla. Immunocytochemistry revealed that all members of the family when overexpressed in PC12 cells localize to secretory granules. Binding constants for the interaction of the GTP-bound forms of Rab3a, Rab3b, Rab3c, and Rab3d with Rabphilin3 were comparable (Kd = 10-20 nM). Overexpression of each of the four members of the Rab3 family inhibited secretion. Mutations in Rab3a were identified that strongly impaired the ability of the GTP-bound form to interact with Rabphilin3. The mutated proteins inhibited secretion similarly to wild type Rab3a. Although Rab3a and Rabphilin3 are located on the same secretory granule or secretory vesicle and interact both in vitro and in situ, it is concluded that the inhibition of secretion by overexpression of Rab3a is unrelated to its ability to interact with Rabphilin3.

Role of tyrosine phosphorylation in the reassembly of occludin and other tight junction proteins

After the simulation of anoxia by ATP depletion of MDCK cell monolayers with metabolic inhibitors, the tight junction (TJ) is known to become structurally perturbed, leading to loss of the permeability barrier. Peripheral TJ proteins such as zonula occludens 1 (ZO-1), ZO-2, and cingulin become extremely insoluble and associate into large macromolecular complexes (T. Tsukamoto and S. K. Nigam. J. Biol. Chem. 272: 16133-16139, 1997). For up to 3 h, this process is reversible by ATP repletion. We now show that the reassembly process depends on tyrosine phosphorylation. Recovery of transepithelial electrical resistance in ATP-replete monolayers was markedly inhibited by the tyrosine kinase inhibitor, genistein. Indirect immunofluorescence revealed a decrease in staining of occludin, a membrane component of the TJ, in the region of the TJ after ATP depletion, which reversed after ATP repletion; this reversal process was inhibited by genistein. Examination of the Triton X-100 solubilities of occludin and several nonmembrane TJ proteins revealed a shift of occludin and nonmembrane TJ proteins into an insoluble pool following ATP depletion. These changes reversed after ATP repletion, and the movement of insoluble occludin, ZO-1, and ZO-2 back into the soluble pool was again via a genistein-sensitive mechanism. Rate-zonal centrifugation analyses of detergent-soluble TJ proteins showed a reversible increase in higher density fractions following ATP depletion-repletion, although this change was not affected by genistein. In 32P-labeled cells, dephosphorylation of all studied TJ proteins was observed during ATP depletion, followed by rephosphorylation during ATP repletion; rephosphorylation of occludin was inhibited by genistein. Furthermore, during the ATP repletion phase, tyrosine phosphorylation of Triton X-100-insoluble occludin, which is localized at the junction, as well as ZO-2, p130/ZO-3 (though not ZO-1), and other proteins was evident; this tyrosine phosphorylation was completely inhibited by genistein. This indicates that tyrosine kinase activity is necessary for TJ reassembly during ATP repletion and suggests an important role for the tyrosine phosphorylation of occludin, ZO-2, p130/ZO-3, and possibly other proteins in the processes involved in TJ (re)formation.

Subcellular distribution and function of Rab3A, B, C, and D isoforms in insulin-secreting cells

Insulin-secreting cells express four GTPases of the Rab3 family. After separation of extracts of INS-1 cells on a sucrose density gradient, the bulk of the A, B, and C isoforms was recovered in the fractions enriched in insulin-containing secretory granules. Rab3D was also mainly associated with secretory granules, but a fraction of this isoform was localized on lighter organelles. Analyses by confocal microscopy of immunostained HIT-T15 cells transfected with epitope-tagged constructs confirmed the distribution of the Rab3 isoforms. Transfection of HIT-T15 cells with GTPase-deficient mutants of the Rab3 isoforms decreased nutrient-induced insulin release to different degrees (D>B>A>>C), while overexpression of Rab3 wild types had minor or no effects. Expression of the same Rab3 mutants in PC12 cells provoked an inhibition of K+-stimulated secretion of dense core vesicles, indicating that, in beta-cells and neuroendocrine cells, the four Rab3 isoforms play a similar role in exocytosis. A Rab3A/C chimera in which the carboxyterminal domain of A was replaced with the corresponding region of C inhibited insulin secretion as Rab3A. In contrast, a Rab3C/A chimera containing the amino-terminal domain of C was less potent and reduced exocytosis as Rab3C. This suggests that the degree of inhibition obtained after transfection of the Rab3 isoforms is determined by differences in the variable amino-terminal region.

Co-localization of Rab3B and oxytocin to electron dense granules of the sheep corpus luteum during the estrous cycle

Oxytocin and its carrier protein, neurophysin, are both associated with luteal secretory granules which migrate from the paranuclear region to the cell membrane where exocytosis takes place. Rab3 proteins are thought to be associated with membrane vesicles or granules undergoing exocytotic fusion with the plasma membrane. The objective of this study was to determine whether Rab3B is co-localized with oxytocin within the same secretory granules of large luteal cells obtained from corpora lutea of 16 Merino cross ewes at day 3, 7, 12 or 15 of the estrous cycle using immunocytochemistry. The mean granule density (granules/microm3) was not significantly different (P > 0.05) between the days examined. Electron microscopic immunocytochemistry showed that oxytocin and Rab3B were co-localized to the secretory granules on all days evaluated. Rab3B immunostaining was primarily located within secretory granules scattered throughout the cytoplasm. The mean intensity of labelling (number of gold particles) for oxytocin per microm2 cytoplasmic luteal tissue was significantly decreased on day 15 compared to those observed on days 3, 7 and 12 of estrous cycle. No significant changes were observed in the mean intensity of the Rab3B label at the different times of the cycle. The present study provides evidence that a member of the subfamily of Rab proteins, Rab3B, is present and co-localized with oxytocin in the same secretory granules of the ovine corpus luteum. These results implicate Rab3B protein directly or indirectly in the hormone secretory pathway of ovarian tissue.

The tight junction: morphology to molecules

The tight junction forms a regulated barrier in the paracellular pathway between epithelial and endothelial cells. This intercellular junction also demarcates the compositionally distinct apical and basolateral membranes. While the existence of a paracellular barrier in epithelia was hypothesized by physiologists over a century ago, the molecular characterization of the tight junction is a relatively new and rapidly expanding area of research. It is now recognized that the tight junction is comprised of at least nine peripheral and one integral membrane proteins. This complex includes members of a protein family related to tumor suppression and signal transduction, a rab protein, and a Ras target protein. The characteristics of, interactions between, and potential physiological roles of these proteins at the tight junction are discussed.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.