Distinct roles of Rab3B and Rab13 in the polarized transport of apical, basolateral, and tight junctional membrane proteins to the plasma membrane

Identification of transcriptome alterations in the prefrontal cortex, hippocampus, amygdala and hippocampus of suicide victims

Suicide is one of the leading causes of death globally for all ages, and as such presents a very serious problem for clinicians worldwide. However, the underlying neurobiological pathology remains to a large extent unknown. In order to address this gap, we have carried out a genome-wide investigation of the gene expression in the amygdala, hippocampus, prefrontal cortex and thalamus in post-mortem brain samples obtained from 20 suicide completers and 7 control subjects. By KEGG enrichment analysis indicated we identified novel clusters of downregulated pathways involved in antigen neutralization and autoimmune thyroid disease (amygdala, thalamus), decreased axonal plasticity in the hippocampus. Two upregulated pathways were involved in neuronal death in the hippocampus and olfactory transduction in the thalamus and the prefrontal cortex. Autoimmune thyroid disease pathway was downregulated only in females. Metabolic pathways involved in Notch signaling amino acid metabolism and unsaturated lipid synthesis were thalamus-specific. Suicide-associated changes in the expression of several genes and pseudogenes that point to various functional mechanisms possibly implicated in the pathology of suicide. Two genes (SNORA13 and RNU4-2) involved in RNA processing were common to all brain regions analyzed. Most of the identified gene expression changes were related to region-specific dysregulated manifestation of genetic and epigenetic mechanisms underlying neurodevelopmental disorders (SNORD114-10, SUSd1), motivation, addiction and motor disorders (CHRNA6), long-term depression (RAB3B), stress response, major depression and schizophrenia (GFAP), signal transduction at the neurovascular unit (NEXN) and inhibitory neurotransmission in spatial learning, neural plasticity (CALB2; CLIC6, ENPP1). Some of the differentially expressed genes were brain specific non-coding RNAs involved in the regulation of translation (SNORA13). One, (PARM1) is a potential oncogene and prognostic biomarker for colorectal cancer with no known function in the brain. Disturbed gene expression involved in antigen neutralization, autoimmunity, neural plasticity, stress response, signal transduction at the neurovascular unit, dysregulated nuclear RNA processing and translation and epigenetic imprinting signatures is associated with suicide and point to regulatory non-coding RNAs as potential targets of new drugs development.

Utility of Claudin-3 in extracellular vesicles from human bile as biomarkers of cholangiocarcinoma

Extracellular vesicles (EVs) are released from all cells. Bile directly contacts bile duct tumor; bile-derived EVs may contain high concentrations of cancer biomarkers. We performed a proteomic analysis of human bile-derived EVs and identified a novel biomarker of cholangiocarcinoma (CCA). EVs were isolated using ultracentrifugation, and chelating agents, ethylenediaminetetraacetic acid and ethylene glycol tetraacetic acid (EDEG) and phosphate buffered saline (PBS) were used as dissolution solutions. Bile was collected from 10 CCA and 10 choledocholithiasis (stones) cases. Proteomic analysis was performed; subsequently, ELISA was performed using the candidate biomarkers in a verification cohort. The vesicles isolated from bile had a typical size and morphology. The expression of exosome markers was observed. RNA was more abundant in the EDEG group. The proportion of microRNA was higher in the EDEG group. EDEG use resulted in the removal of more contaminants. Proteomic analysis identified 166 proteins as CCA-specific. ELISA for Claudin-3 revealed statistically significant difference. The diagnostic accuracy was AUC 0.945 and sensitivity and specificity were 87.5%. We report the first use of EDEG in the isolation of EVs from human bile and the proteomic analysis of human bile-derived EV-proteins in CCA. Claudin-3 in bile-derived EVs is a useful biomarker for CCA.

Role of extra cellular proteins in gastric cancer progression and metastasis: an update

Background

Gastric cancer (GC) is one of the most common cancers in the world with a high ratio of mortality. Regarding the late diagnosis, there is a high ratio of distant metastasis among GC cases. Despite the recent progresses in therapeutic modalities, there is not still an efficient therapeutic method to increase survival rate of metastatic GC cases.

Main body

Apart from the various intracellular signaling pathways which are involved in tumor cell migration and metastasis, the local microenvironment is also a critical regulator of tumor cell migration. Indeed, the intracellular signaling pathways also exert their final metastatic roles through regulation of extra cellular matrix (ECM). Therefore, it is required to assess the role of extra cellular components in biology of GC.

Conclusion

In the present review, we summarize 48 of the significant ECM components including 17 ECM modifying enzymes, seven extracellular angiogenic factors, 13 cell adhesion and cytoskeletal organizers, seven matricellular proteins and growth factors, and four proteoglycans and extra cellular glycoproteins. This review paves the way of determination of a specific extra cellular diagnostic and prognostic panel marker for the GC patients.

Endocytosis of tight junction proteins and the regulation of degradation and recycling

Internalization of tight junction (TJ) proteins from the plasma membrane is a pivotal mechanism regulating TJ plasticity and function in both epithelial and endothelial barrier tissues. Once internalized, the TJ proteins enter complex vesicular machinery, where further trafficking is directly dependent on the initiating stimulus and downstream signaling pathways that regulate the sorting and destiny of TJ proteins, as well as on cell and barrier responses. The destiny of internalized TJ proteins is recycling to the plasma membrane or sorting to late endosomes and degradation. This review highlights recent advances in our knowledge of endocytosis and vesicular trafficking of TJ proteins in both epithelial and endothelial cells. A greater understanding of these processes may allow for the development of methods to modulate barrier permeability for drug delivery or prevent barrier dysfunction in disease states.

Temporal quantitative phosphoproteomics of ADP stimulation reveals novel central nodes in platelet activation and inhibition

Adenosine diphosphate (ADP) enhances platelet activation by virtually any other stimulant to complete aggregation. It binds specifically to the G-protein-coupled membrane receptors P2Y1 and P2Y12, stimulating intracellular signaling cascades, leading to integrin αIIbβ3 activation, a process antagonized by endothelial prostacyclin. P2Y12 inhibitors are among the most successful antiplatelet drugs, however, show remarkable variability in efficacy. We reasoned whether a more detailed molecular understanding of ADP-induced protein phosphorylation could identify (1) critical hubs in platelet signaling toward aggregation and (2) novel molecular targets for antiplatelet treatment strategies. We applied quantitative temporal phosphoproteomics to study ADP-mediated signaling at unprecedented molecular resolution. Furthermore, to mimic the antagonistic efficacy of endothelial-derived prostacyclin, we determined how Iloprost reverses ADP-mediated signaling events. We provide temporal profiles of 4797 phosphopeptides, 608 of which showed significant regulation. Regulated proteins are implicated in well-known activating functions such as degranulation and cytoskeletal reorganization, but also in less well-understood pathways, involving ubiquitin ligases and GTPase exchange factors/GTPase-activating proteins (GEF/GAP). Our data demonstrate that ADP-triggered phosphorylation occurs predominantly within the first 10 seconds, with many short rather than sustained changes. For a set of phosphorylation sites (eg, PDE3A^Ser312, CALDAG-GEFI^Ser587, ENSA^Ser109), we demonstrate an inverse regulation by ADP and Iloprost, suggesting that these are central modulators of platelet homeostasis. This study demonstrates an extensive spectrum of human platelet protein phosphorylation in response to ADP and Iloprost, which inversely overlap and represent major activating and inhibitory pathways.

Regulation of Cancer Cell Behavior by the Small GTPase Rab13

The members of the Rab family of GTPases are master regulators of cellular membrane trafficking. With ∼70 members in humans, Rabs have been implicated in all steps of membrane trafficking ranging from vesicle formation and transport to vesicle docking/tethering and fusion. Vesicle trafficking controls the localization and levels of a myriad of proteins, thus regulating cellular functions including proliferation, metabolism, cell-cell adhesion, and cell migration. It is therefore not surprising that impairment of Rab pathways is associated with diseases including cancer. In this review, we highlight evidence supporting the role of Rab13 as a potent driver of cancer progression.

Storage Temperature Alters the Expression of Differentiation-Related Genes in Cultured Oral Keratinocytes

Purpose

Storage of cultured human oral keratinocytes (HOK) allows for transportation of cultured transplants to eye clinics worldwide. In a previous study, one-week storage of cultured HOK was found to be superior with regard to viability and morphology at 12°C compared to 4°C and 37°C. To understand more of how storage temperature affects cell phenotype, gene expression of HOK before and after storage at 4°C, 12°C, and 37°C was assessed.

Materials and Methods

Cultured HOK were stored in HEPES- and sodium bicarbonate-buffered Minimum Essential Medium at 4°C, 12°C, and 37°C for one week. Total RNA was isolated and the gene expression profile was determined using DNA microarrays and analyzed with Partek Genomics Suite software and Ingenuity Pathway Analysis. Differentially expressed genes (fold change > 1.5 and P < 0.05) were identified by one-way ANOVA. Key genes were validated using qPCR.

Results

Gene expression of cultures stored at 4°C and 12°C clustered close to the unstored control cultures. Cultures stored at 37°C displayed substantial change in gene expression compared to the other groups. In comparison with 12°C, 2,981 genes were differentially expressed at 37°C. In contrast, only 67 genes were differentially expressed between the unstored control and the cells stored at 12°C. The 12°C and 37°C culture groups differed most significantly with regard to the expression of differentiation markers. The Hedgehog signaling pathway was significantly downregulated at 37°C compared to 12°C.

Conclusion

HOK cultures stored at 37°C showed considerably larger changes in gene expression compared to unstored cells than cultured HOK stored at 4°C and 12°C. The changes observed at 37°C consisted of differentiation of the cells towards a squamous epithelium-specific phenotype. Storing cultured ocular surface transplants at 37°C is therefore not recommended. This is particularly interesting as 37°C is the standard incubation temperature used for cell culture.

High-content analysis of Rab protein function at the ER-Golgi interface

The Rab family of small GTPases play fundamental roles in the regulation of trafficking pathways between intracellular membranes in eukaryotic cells. In this short commentary we highlight a recent high-content screening study that investigates the roles of Rab proteins in retrograde trafficking from the Golgi complex to the endoplasmic reticulum, and we discuss how the findings of this work and other literature might influence our thoughts on how the architecture of the Golgi complex is regulated.

Claudin-4 controls the proliferation, apoptosis, migration and in vivo growth of MCF-7 breast cancer cells

Previous studies have shown that the expression of claudin-4 is upregulated in breast cancer. The aim of the present study was to investigate the role and the regulation of claudin-4 in MCF-7 breast cancer cells. For the in vitro experiments, MCF-7 cells were treated with recombinant vectors carrying cDNA for claudin-4 overexpression or short hairpin RNAs (shRNAs) for claudin-4 silencing. Cell proliferation was determined by an MTT assay and cell migration ability was measured by a wound-healing assay. The cell cycle profile and apoptotic rate were analyzed using flow cytometry. The effect of methylation status on claudin-4 expression was determined by PCR and western blotting. For the in vivo tumorigenesis analysis, MCF-7 cells with or without claudin-4 silencing were transplanted into nude mice. In vivo cell growth was evaluated 14 days after transplantation. We found that claudin-4 overexpression increased MCF-7 cell proliferation and migration, and reduced the rate of cell apoptosis. Silencing of claudin-4 induced the opposite effects in MCF-7 cells. In addition, claudin-4 expression was upregulated by demethylation. Moreover, the size of tumor formation was reduced in nude mice transplanted with claudin-4 silenced MCF-7 cells. These observations suggested that claudin-4, which was regulated by methylation status, plays an important role in breast cancer growth and malignancy via the control of cell proliferation, migration and apoptosis.

Claudin-4 expression in gastric cancer cells enhances the invasion and is associated with the increased level of matrix metalloproteinase-2 and -9 expression

Claudin-4 is a member of a large family of transmembrane proteins known as claudins, which are essential for the formation and maintenance of tight junctions. Our previous studies have revealed that claudin-4 proteins are overexpressed in metastatic gastric cancer. To clarify the roles of claudin-4 in gastric cancer metastasis, human gastric adenocarcinoma (AGS) cells constitutively expressing wild-type claudin-4 were generated. Expression of claudin-4 in AGS cells was found to increase cell invasion and migration, as measured by Boyden invasion chamber assays. Moreover, the claudin-4-expressing AGS cells were found to have increased matrix metalloproteinase (MMP)-2 and -9 expression, indicating that claudin-mediated increased invasion may be mediated through the activation of the MMP protein. Overall, the results suggest that claudin-4 overexpression may promote gastric cancer metastasis through the increased invasion of gastric cancer cells.

Rab3Gap1 mediates exocytosis of Claudin-1 and tight junction formation during epidermal barrier acquisition

Epidermal barrier acquisition during late murine gestation is accompanied by an increase in Akt kinase activity and cJun dephosphorlyation. The latter is directed by the Ppp2r2a regulatory subunit of the Pp2a phosphatase. This was accompanied by a change of Claudin-1 localisation to the cell surface and interaction between Occludin and Claudin-1 which are thought to be required for tight junction formation.

The aim of this study was to determine the nature of the barrier defect caused by the loss of AKT/Ppp2r2a function. There was a paracellular barrier defect in rat epidermal keratinocytes expressing a Ppp2r2a siRNA. In Ppp2r2a knockdown cells, Claudin-1 was located to the cytoplasm and its expression was increased. Inhibiting cJun phosphorylation restored barrier function and plasma membrane localisation of Claudin-1. Expression of the Rab3 GTPase activating protein, Rab3Gap1, was restored in Ppp2r2a siRNA cells when cJun phosphorylation was inhibited. During normal mouse epidermal development, Claudin-1 plasma membrane localisation and Rab3Gap1 cell surface expression were co-incident with Akt activation in mouse epidermis, strongly suggesting a role of Rab3Gap1 in epidermal barrier acquisition. Supporting this hypothesis, siRNA knockdown of Rab3Gap1 prevented plasma membrane Claudin-1 expression and the formation of a barrier competent epithelium. Replacing Rab3Gap1 in Ppp2r2a knockdown cells was sufficient to rescue Claudin-1 transport to the cell surface. Therefore these data suggest Rab3Gap1 mediated exocytosis of Claudin-1 is an important component of epidermal barrier acquisition during epidermal development.

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Barrier acquisition correlates with Ppp2r2a and cell surface Claudin-1 expression.

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Ppp2r2a knockdown results in a paracellular barrier defect.

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Ppp2r2a knockdown prevents cell-surface claudin-1 expression in a c-Jun dependent fashion.

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Barrier rescue by inhibition of c-Jun phosphorylation involves exocytosis and Rab3Gap1.

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Rab3Gap1 is induced during barrier acquisition and is necessary for cell surface claudin-1.

ETS2 mediated tumor suppressive function and MET oncogene inhibition in human non-small cell lung cancer

PURPOSE

The ETS2 transcription factor is an evolutionarily conserved gene that is deregulated in cancer. We analyzed the transcriptome of lung adenocarcinomas and normal lung tissue by expression profiling and found that ETS2 was significantly downregulated in adenocarcinomas. In this study, we probed the yet unknown functional role of ETS2 in lung cancer pathogenesis.

EXPERIMENTAL DESIGN

Lung adenocarcinomas (n = 80) and normal lung tissues (n = 30) were profiled using the Affymetrix Human Gene 1.0 ST platform. Immunohistochemical (IHC) analysis was conducted to determine ETS2 protein expression in non-small cell lung cancer (NSCLC) histologic tissue specimens (n = 201). Patient clinical outcome, based on ETS2 IHC expression, was statistically assessed using the log-rank and Kaplan-Meier tests. RNA interference and overexpression strategies were used to assess the effects of ETS2 expression on the transcriptome and on various malignant phenotypes.

RESULTS

ETS2 expression was significantly reduced in lung adenocarcinomas compared with normal lung (P < 0.001). Low ETS2 IHC expression was a significant predictor of shorter time to recurrence in NSCLC (P = 0.009, HR = 1.89) and adenocarcinoma (P = 0.03, HR = 1.86). Moreover, ETS2 was found to significantly inhibit lung cancer cell growth, migration, and invasion (P < 0.05), and microarray and pathways analysis revealed significant (P < 0.001) activation of the HGF pathway following ETS2 knockdown. In addition, ETS2 was found to suppress MET phosphorylation and knockdown of MET expression significantly attenuated (P < 0.05) cell invasion mediated by ETS2-specific siRNA. Furthermore, knockdown of ETS2 augmented HGF-induced MET phosphorylation, cell migration, and invasion.

CONCLUSION(S)

Our findings point to a tumor suppressor role for ETS2 in human NSCLC pathogenesis through inhibition of the MET proto-oncogene.

Novel interaction of Rab13 and Rab8 with endospanins

Rab GTPases regulate vesicular traffic in eukaryotic cells by cycling between the active GTP-bound and inactive GDP-bound states. Their functions are modulated by the diverse selection of effector proteins that bind to specific Rabs in their activated state. We previously described the expression of Rab13 in bone cells. To search for novel Rab13 interaction partners, we screened a newborn rat bone marrow cDNA library for Rab13 effectors with a bacterial two-hybrid system. We found that Rab13 binds to the C-terminus of Endospanin-2, a small transmembrane protein. In addition to Rab13 also Rab8 bound to Endospanin-2, while no binding of Rab7, Rab10, Rab11 or Rab32 was observed. Rab13 and Rab8 also interacted with Endospanin-1, a close homolog of Endospanin-2. Rab13 and Endospanin-2 colocalised in perinuclear vesicular structures in Cos1 cells suggesting direct binding also in vivo. Endospanin-2 is implicated in the regulation of the cell surface growth hormone receptor (GHR), but the inhibition of Rab13 expression did not affect GHR cell surface expression. This suggests that the Rab13–Endospanin-2 interaction may have functions other than GHR regulation. In conclusion, we have identified a novel interaction for Rab13 and Rab8 with Endospanin-2 and Endospanin-1. The role of this interaction in cell physiology, however, remains to be elucidated.

Rab13 is upregulated during osteoclast differentiation and associates with small vesicles revealing polarized distribution in resorbing cells

Osteoclasts are bone-resorbing multinucleated cells that undergo drastic changes in their polarization due to heavy vesicular trafficking during the resorption cycle. These events require the precise orchestration of membrane traffic in order to maintain the unique characteristics of the different membrane domains in osteoclasts. Rab proteins are small GTPases involved in regulation of most, if not all, steps of vesicle trafficking. The investigators studied RAB genes in human osteoclasts and found that at least 26 RABs were expressed in osteoclasts. Out of these, RAB13 gene expression was highly upregulated during differentiation of human peripheral blood monocytic cells into osteoclasts. To study its possible function in osteoclasts, the investigators performed immunolocalization studies for Rab13 and various known markers of osteoclast vesicular trafficking. Rab13 localized to small vesicular structures at the superior parts of the osteoclast between the trans-Golgi network and basolateral membrane domain. Rab13 localization suggests that it is not involved in endocytosis or transcytosis of bone degradation products. In addition, Rab13 did not associate with early endosomes or recycling endosomes labeled with EEA1 or TRITC-conjugated transferrin, respectively. Its involvement in glucose transporter traffic was excluded as well. It is suggested that Rab13 is associated with a putative secretory function in osteoclasts.

A membrane fusion protein αSNAP is a novel regulator of epithelial apical junctions

Tight junctions (TJs) and adherens junctions (AJs) are key determinants of the structure and permeability of epithelial barriers. Although exocytic delivery to the cell surface is crucial for junctional assembly, little is known about the mechanisms controlling TJ and AJ exocytosis. This study was aimed at investigating whether a key mediator of exocytosis, soluble N-ethylmaleimide sensitive factor (NSF) attachment protein alpha (αSNAP), regulates epithelial junctions. αSNAP was enriched at apical junctions in SK-CO15 and T84 colonic epithelial cells and in normal human intestinal mucosa. siRNA-mediated knockdown of αSNAP inhibited AJ/TJ assembly and establishment of the paracellular barrier in SK-CO15 cells, which was accompanied by a significant down-regulation of p120-catenin and E-cadherin expression. A selective depletion of p120 catenin effectively disrupted AJ and TJ structure and compromised the epithelial barrier. However, overexpression of p120 catenin did not rescue the defects of junctional structure and permeability caused by αSNAP knockdown thereby suggesting the involvement of additional mechanisms. Such mechanisms did not depend on NSF functions or induction of cell death, but were associated with disruption of the Golgi complex and down-regulation of a Golgi-associated guanidine nucleotide exchange factor, GBF1. These findings suggest novel roles for αSNAP in promoting the formation of epithelial AJs and TJs by controlling Golgi-dependent expression and trafficking of junctional proteins.

Claudin-4 expression is associated with tumor invasion, MMP-2 and MMP-9 expression in gastric cancer

Claudin-4 is a member of the claudin family, a large family of transmembrane proteins that are essential in the formation and maintenance of tight junctions. Matrix metal-loproteinase (MMP)-2 and -9 degrade type IV collagen of the extracellular matrix and basal membranes. Claudin-4 activates MMP-2, indicating that claudin-mediated increased cancer cell invasion may result from the activation of MMP proteins. In the present study, we used immunohistochemistry to examine the expression levels of claudin-4, MMP-2 and MMP-9 in 189 gastric cancer samples, and analyzed their correlation with tumor invasion, clinicopathologic parameters and clinical outcome. The relationship between claudin-4 expression and MMP-2 and -9 expression was also investigated. The expression of claudin-4 was found to be significantly higher in gastric cancer cases with advanced depth of wall invasion, lymph node metastasis, lymphatic invasion and higher TNM stage. Further analysis revealed claudin-4 expression to be significantly correlated with the expression of MMP-2 and -9. Kaplan-Meier survival analysis indicated that MMP-9 expression was correlated with poor prognosis. These results suggest that claudin-4 expression is associated with tumor invasion and with MMP-2 and -9 expression in gastric cancer. Additionally, MMP-9 expression was demonstrated to serve as a prognostic factor in patients with gastric cancer.

Claudin family of proteins and cancer: an overview

Tight junctions are the apical cell-cell adhesion that regulate paracellular permeability and are critical for epithelial cell polarity. Molecular architecture of tight junction has been studied extensively, which has confirmed that claudin family of proteins is integral component of tight junction. Loss of cell-cell adhesion is central to the cellular transformation and acquisition of metastatic potential; however, the role of claudin family of proteins play in a series of pathophysiological events, including human carcinoma development, is only now beginning to be understood. Several claudin mouse knockout models have been generated and the diversity of phenotypes observed clearly demonstrates their important roles in the maintenance of tissue integrity in various organs and suggest that claudins also participate in cellular contexts other than tight junctions. The mechanisms of claudin regulation and their exact roles in normal physiology and disease are being elucidated, but much work remains to be done. In this review, we have discussed the conceptual framework concerning claudins and their potential implication in cancer. We predict that next several years will likely witness a boom in our understanding of the potential role of claudins in the regulation of tumorigenesis, which may, in turn, provide new approaches for the targeted therapy.

RAB13 participates in ectoplasmic specialization dynamics in the rat testis

During spermatogenesis, leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier (BTB) to gain entry into the adluminal compartment for further development. At the same time, these as well as other germ cell types in the epithelium must retain their close association with Sertoli cells via specialized cell junctions. In this study, we demonstrate that RAB13-a guanosine triphosphatase (GTPase) known to participate in tight junction function in other epithelia-also participates in the dynamics of the ectoplasmic specialization, a testis-specific type of anchoring junction. By immunohistochemistry microscopy, RAB13 localized to the ectoplasmic specialization. Moreover, RAB13 was found to associate with vinculin (VCL) and espin (ESPN), two putative ectoplasmic specialization actin (ACT)-binding proteins, by coimmunoprecipitation and immunofluorescence microscopy experiments. To address the role of RAB13 in ectoplasmic specialization dynamics, an in vivo model was used in which administration of Adjudin induced the disassembly of Sertoli-germ cell anchoring junctions. Following administration of this drug, the RAB13 level decreased steadily when the loss in testicular weight was taken into account. Similarly, the association of RAB13 with VCL decreased but was not completely lost during Adjudin-mediated ectoplasmic specialization restructuring. Taken collectively, these results suggest that RAB13 functions in ectoplasmic specialization dynamics in the testis.

Rab13 regulates membrane trafficking between TGN and recycling endosomes in polarized epithelial cells

To maintain polarity, epithelial cells continuously sort transmembrane proteins to the apical or basolateral membrane domains during biosynthetic delivery or after internalization. During biosynthetic delivery, some cargo proteins move from the trans-Golgi network (TGN) into recycling endosomes (RE) before being delivered to the plasma membrane. However, proteins that regulate this transport step remained elusive. In this study, we show that Rab13 partially colocalizes with TGN38 at the TGN and transferrin receptors in RE. Knockdown of Rab13 with short hairpin RNA in human bronchial epithelial cells or overexpression of dominant-active or dominant-negative alleles of Rab13 in Madin-Darby canine kidney cells disrupts TGN38/46 localization at the TGN. Moreover, overexpression of Rab13 mutant alleles inhibits surface arrival of proteins that move through RE during biosynthetic delivery (vesicular stomatitis virus glycoprotein [VSVG], A-VSVG, and LDLR-CT27). Importantly, proteins using a direct route from the TGN to the plasma membrane are not affected. Thus, Rab13 appears to regulate membrane trafficking between TGN and RE.

Up-regulation of JAM-1 in AR42J cells treated with activin A and betacellulin and the diabetic regenerating islets

Pancreatic AR42J cells demonstrate the pluripotency in precursor cells of the gut endoderm and also provide an excellent model system to study the differentiation of the pancreas. Using the mRNA differential display technique, we identified junctional adhesion molecule-1 (JAM-1), a component of the tight junction, was highly up-regulated during the differentiation of AR42J cells, although junctions were not formed. The expression level of JAM-1 showed an up-regulation in the mRNA level after 3 hours and in the protein level after 24 hours in [activin A + betacellulin]-treated AR42J cells. The expressions of its signaling molecules, PAR-3 and atypical PKC lambda, also increased after the addition of activin A + betacellulin. When JAM-1 was over-expressed in [activin A + betacellulin]-treated AR42J cells, tagged-JAM-1 was observed in cytoplasm as vesicular structures and JAM-1 was colocalized with Rab3B and Rab13, members of the Rab family expressed at tight junctions. In streptozotocin-induced regenerating islets, the expression of JAM-1 was also up-regulated in the mRNA level and the protein level. JAM-1 might therefore play an important role in the differentiation of AR42J cells and the regeneration of pancreatic islets.

Interaction of Rab3B with microtubule-binding protein Gas8 in NIH 3T3 cells

Rab3 subfamily small G proteins (Rab3A, Rab3B, Rab3C, and Rab3D) control the regulated exocytosis in neuronal/secretory cells. Rab3B is also detected and upregulated in non-neuronal/non-secretory cells, whereas its function remains elusive. In the present study, we identified growth-arrest-specific gene 8 (Gas8), an evolutionally conserved microtubule-binding protein that is upregulated in growth-arrested NIH 3T3 cells and involved in the dynein motor regulation in flagellar/ciliary axoneme, as a novel Rab3B-binding protein using a yeast two-hybrid system. Rab3B as well as Gas8 was upregulated in growth-arrested NIH 3T3 cells and enriched in testis and lung with well-developed flagella/cilia. Gas8 was specifically interacted with the GTP-bound form of Rab3B and co-localized with Rab3B at the Golgi in NIH 3T3 cells. Furthermore, Rab3B was relocated upon expression of the Rab3B-binding domain of Gas8. These results suggest that Gas8 links Rab3B to microtubules in NIH 3T3 cells.

Correlation of the tight junction-like distribution of Claudin-1 to the cellular tropism of hepatitis C virus

Claudin-1 (CLDN1), a tight junction (TJ) protein, has recently been identified as an entry co-receptor for hepatitis C virus (HCV). Ectopic expression of CLDN1 rendered several non-hepatic cell lines permissive to HCV infection. However, little is known about the mechanism by which CLDN1 mediates HCV entry. It is believed that an additional entry receptor(s) is required because ectopic expression of CLDN1 in both HeLa and NIH3T3 cells failed to confer susceptibility to viral infection. Here we found that CLDN1 was co-immunoprecipitated with both HCV envelope proteins when expressed in 293T cells. Results from biomolecular fluorescence complementation assay showed that overexpressed CLDN1 also formed complexes with CD81 and low density lipoprotein receptor. Subsequent imaging analysis revealed that CLDN1 was highly enriched at sites of cell-cell contact in permissive cell lines, co-localizing with the TJ marker, ZO-1. However, in both HeLa and NIH3T3 cells the ectopically expressed CLDN1 appeared to reside predominantly in intracellular vesicles. The CLDN1-CD81 complex formed in HeLa cells was also exclusively distributed intracellularly, co-localizing with EEA1, an early endosomal marker. Correspondingly, transepithelial electric resistance, obtained from the naturally susceptible human liver cell line, Huh7, was much higher than that of the HeLa-CLDN1 cell line, suggesting that Huh7 is likely to form functional tight junctions. Finally, the disruption of TJ-enriched CLDN1 by tumor necrosis factor-alpha treatment markedly reduced the susceptibility of Huh7.5.1 cells to HCV infection. Our results suggest that the specific localization pattern of CLDN1 may be crucial in the regulation of HCV cellular tropism.

The interaction of JRAB/MICAL-L2 with Rab8 and Rab13 coordinates the assembly of tight junctions and adherens junctions

The assembly of tight junctions (TJs) and adherens junctions (AJs) is regulated by the transport of integral TJ and AJ proteins to and/or from the plasma membrane (PM) and it is tightly coordinated in epithelial cells. We previously reported that Rab13 and a junctional Rab13-binding protein (JRAB)/molecule interacting with CasL-like 2 (MICAL-L2) mediated the endocytic recycling of an integral TJ protein occludin and the formation of functional TJs. Here, we investigated the role of Rab13 and JRAB/MICAL-L2 in the transport of other integral TJ and AJ proteins claudin-1 and E-cadherin to the PM by using a Ca(2+)-switch model. Although knockdown of Rab13 specifically suppressed claudin-1 and occludin but not E-cadherin transport, knockdown of JRAB/MICAL-L2 and expression of its Rab13-binding domain (JRAB/MICAL-L2-C) inhibited claudin-1, occludin, and E-cadherin transport. We then identified Rab8 as another JRAB/MICAL-L2-C-binding protein. Knockdown of Rab8 inhibited the Rab13-independent transport of E-cadherin to the PM. Rab8 and Rab13 competed with each other for the binding to JRAB/MICAL-L2 and functionally associated with JRAB/MICAL-L2 at the perinuclear recycling/storage compartments and PM, respectively. These results suggest that the interaction of JRAB/MICAL-L2 with Rab8 and Rab13 coordinates the assembly of AJs and TJs.

Involvement of Rab13 and JRAB/MICAL-L2 in epithelial cell scattering

Epithelial cell scattering recapitulates the first steps of carcinoma invasion/metastasis. While the balance between cell-cell adhesive activity and cell motility ultimately determines this process, its molecular mechanisms remain unclear. Adherence junctions and tight junctions (TJs) are primarily responsible for cell-cell adhesive activity and subjected to dynamic remodeling. We previously showed that Rab13 and its effector protein JRAB/MICAL-L2 mediate the endocytic recycling of the integral TJ protein occludin and the assembly of functional TJs. In this study, we examined the role of Rab13 and JRAB/MICAL-L2 in the scattering of Madin-Darby canine kidney (MDCK) cells in response to 12-O-tetradecanoylphorbol-13-acetate (TPA). Knockdown of Rab13 in canine MDCK cells suppressed the TPA-induced scattering, and this phenotype was restored by re-expression of human Rab13. During TPA-induced MDCK cell scattering, Rab13 was transiently activated and returned to its basal level, and both Rab13 and JRAB/MICAL-L2 were colocalized with F-actin at cell-cell contact sites and then accumulated at emerging lamellipodial structures. TPA-induced MDCK cell scattering was also inhibited by knockdown of canine JRAB/MICAL-L2 and rescued by re-expression of mouse JRAB/MICAL-L2. These results indicate that Rab13 and JRAB/MICAL-L2 are involved in epithelial cell scattering.

Tankyrase recruitment to the lateral membrane in polarized epithelial cells: regulation by cell-cell contact and protein poly(ADP-ribosyl)ation

PARsylation [poly(ADP-ribosyl)ation] of proteins is implicated in the regulation of diverse physiological processes. Tankyrase is a molecular scaffold with this catalytic activity and has been proposed as a regulator of vesicular trafficking on the basis, in part, of its Golgi localization in non-polarized cells. Little is known about tankyrase localization in polarized epithelial cells. Using MDCK (Madin-Darby canine kidney) cells as a model, we found that E-cadherin-mediated intercellular adhesion recruits tankyrase from the cytoplasm to the lateral membrane (including the tight junction), where it stably associates with detergent-insoluble structures. This recruitment is mostly completed within 8 h of calcium-induced formation of cell-cell contact. Conversely, when intercellular adhesion is disrupted by calcium deprivation, tankyrase returns from the lateral membrane to the cytoplasm and becomes more soluble in detergents. The PARsylating activity of tankyrase promotes its dissociation from the lateral membrane as well as its ubiquitination and proteasome-mediated degradation, resulting in an apparent protein half-life of approximately 2 h. Inhibition of tankyrase autoPARsylation using H2O2-induced NAD+ depletion or PJ34 [N-(6-oxo-5,6-dihydrophenanthridin-2-yl)-N,N-dimethylacetamide hydrochloride] treatment results in tankyrase stabilization and accumulation at the lateral membrane. By contrast, stabilization through proteasome inhibition results in tankyrase accumulation in the cytoplasm. These data suggest that cell-cell contact promotes tankyrase association with the lateral membrane, whereas PARsylating activity promotes translocation to the cytosol, which is followed by ubiquitination and proteasome-mediated degradation. Since the lateral membrane is a sorting station that ensures domain-specific delivery of basolateral membrane proteins, the regulated tankyrase recruitment to this site is consistent with a role in polarized protein targeting in epithelial cells.

JRAB/MICAL-L2 is a junctional Rab13-binding protein mediating the endocytic recycling of occludin

The dynamic turnover of tight junctions (TJs) is essential for epithelial-mesenchymal transitions and/or mesenchymal-epithelial transitions during epithelial morphogenesis. We previously demonstrated that Rab13 specifically mediates the endocytic recycling of occludin. Here, we identified MICAL-L2 (molecule interacting with CasL-like 2) as a novel Rab13-binding protein. Immunoprecipitation and immunofluorescence microscopy showed that MICAL-L2 specifically bound to the GTP-bound form of Rab13 via its C terminus, which contained a coiled-coil domain, and localized at TJs in epithelial MTD-1A cells. Recycling assay demonstrated that a MICAL-L2 mutant lacking the Rab13-binding domain (MICAL-L2-N) specifically inhibited the endocytic recycling of occludin but not transferrin receptor. Ca2+ switch assay further revealed that MICAL-L2-N as well as Rab13 Q67L inhibited the recruitment of occludin to the plasma membrane, the development of transepithelial electrical resistance, and the formation of a paracellular diffusion barrier. MICAL-L2 was displaced from TJs upon actin depolymerization and was distributed along radiating actin cables and stress fibers in Ca2+-depleted MTD-1A and fibroblastic NIH3T3 cells, respectively. These results suggest that MICAL-L2 mediates the endocytic recycling of occludin and the formation of functional TJs by linking Rab13 to actin cytoskeleton. We rename MICAL-L2 as JRAB (junctional Rab13-binding protein).

Interferon-gamma induces internalization of epithelial tight junction proteins via a macropinocytosis-like process

Increased epithelial permeability is observed in inflammatory states. However, the mechanism by which inflammatory mediators such as IFN-gamma increase epithelial permeability is unknown. We recently observed that IFN-gamma induces disassembly of tight junctions (TJ); in this study we asked whether such TJ disassembly is mediated by endocytosis of junctional proteins. The role of three major internalization pathways in disruption of TJ in IFN-gamma-treated intestinal epithelial cells was analyzed using selective inhibitors and markers of the pathways. No role for the clathrin- and caveolar-mediated endocytosis in the IFN-gamma-induced internalization of TJ proteins was observed. However, inhibitors of macropinocytosis blocked internalization of TJ proteins and junctional proteins colocalized with macropinocytosis markers, dextran and phosphatidylinositol-3,4,5-trisphosphate. Internalized TJ proteins were identified in early and recycling endosomes but not in late endosomes/lysosomes. These results for the first time suggest that IFN-gamma produces a leaky epithelial barrier by inducing macropinoytosis of TJ proteins.

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.

Claudin-3 and claudin-4 expression in ovarian epithelial cells enhances invasion and is associated with increased matrix metalloproteinase-2 activity

Claudin proteins form a large family of integral membrane proteins crucial for tight junction formation and function. Our previous studies have revealed that claudin-3 and claudin-4 proteins are highly overexpressed in ovarian cancer. To clarify the roles of claudins in ovarian tumorigenesis, we have generated human ovarian surface epithelial (HOSE) cells constitutively expressing wild-type claudin-3 and claudin-4. Expression of these claudins in HOSE cells increased cell invasion and motility as measured by Boyden chamber assays and wound-healing experiments. Conversely, small interfering RNA (siRNA)-mediated knockdown of claudin-3 and claudin-4 expression in ovarian cancer cell lines reduced invasion. Claudin expression also increased cell survival in HOSE cells but did not significantly affect cell proliferation. Moreover, the claudin-expressing ovarian epithelial cells were found to have increased matrix metalloproteinase-2 (MMP-2) activity indicating that claudin-mediated increased invasion might be mediated through the activation of MMP proteins. However, siRNA inactivation of claudins in ovarian cancer cell lines did not have a significant effect on the high endogenous MMP-2 activity present in these cells, showing that malignant cells have alternative or additional pathways to fully activate MMP-2. Taken together, our results suggest that claudin overexpression may promote ovarian tumorigenesis and metastasis through increased invasion and survival of tumor cells.

Rab13 Mediates the Continuous Endocytic Recycling of Occludin to the Cell Surface

During epithelial morphogenesis, adherens junctions (AJs) and tight junctions (TJs) undergo dynamic reorganization, whereas epithelial polarity is transiently lost and reestablished. Although ARF6-mediated endocytic recycling of E-cadherin has been characterized and implicated in the rapid remodeling of AJs, the molecular basis for the dynamic rearrangement of TJs remains elusive. Occludin and claudins are integral membrane proteins comprising TJ strands and are thought to be responsible for establishing and maintaining epithelial polarity. Here we investigated the intracellular transport of occludin and claudins to and from the cell surface. Using cell surface biotinylation and immunofluorescence, we found that a pool of occludin was continuously endocytosed and recycled back to the cell surface in both fibroblastic baby hamster kidney cells and epithelial MTD-1A cells. Biochemical endocytosis and recycling assays revealed that a Rab13 dominant active mutant (Rab13 Q67L) inhibited the postendocytic recycling of occludin, but not that of transferrin receptor and polymeric immunoglobulin receptor in MTD-1A cells. Double immunolabelings showed that a fraction of endocytosed occludin was colocalized with Rab13 in MTD-1A cells. These results suggest that Rab13 specifically mediates the continuous endocytic recycling of occludin to the cell surface in both fibroblastic and epithelial cells.

Identification and characterization of Noc2 as a potential Rab3B effector protein in epithelial cells

The Rab3 family small G proteins (Rab3A-D) are involved in the regulated secretory pathway of brain and secretory tissues. Among Rab3-interacting proteins, Rabphilin-3, Rim, and Noc2, all of which contain a conserved Rab3-binding domain (RBD3), are generally recognized Rab3 effector proteins in neurons and secretory cells. Although Rab3B was also detected in epithelial cells, its function remained unknown. We isolated cDNA sequences from human epithelial Caco2-cell mRNA by degenerate RT-PCR based on the conserved amino acid sequence of RBD3. Multiple cDNA clones were identified as encoding Noc2. Northern blot analysis revealed that Noc2 mRNA was expressed not only in secretory tissues but also in epithelial tissues and cell lines. A pull-down assay demonstrated that Noc2 bound to Rab3B in a GTP-dependent manner. When Noc2 was co-expressed with the GTP-bound form of Rab3B, it was recruited from the cytosol to perinuclear membranes. Furthermore, overexpression of Noc2 inhibited the cell-surface transport of basolateral vesicular stomatitis virus glycoprotein. These results suggest that Noc2 functions as a potential Rab3B effector protein in epithelial cells.

Endocytosis of epithelial apical junctional proteins by a clathrin-mediated pathway into a unique storage compartment

The adherens junction (AJ) and tight junction (TJ) are key regulators of epithelial polarity and barrier function. Loss of epithelial phenotype is accompanied by endocytosis of AJs and TJs via unknown mechanisms. Using a model of calcium depletion, we defined the pathway of internalization of AJ and TJ proteins (E-cadherin, p120 and beta-catenins, occludin, JAM-1, claudins 1 and 4, and ZO-1) in T84 epithelial cells. Proteinase protection assay and immunocytochemistry revealed orchestrated internalization of AJs and TJs into a subapical cytoplasmic compartment. Disruption of caveolae/lipid rafts did not prevent endocytosis, nor did caveolin-1 colocalize with internalized junctional proteins. Furthermore, AJ and TJ proteins did not colocalize with the macropinocytosis marker dextran. Inhibitors of clathrin-mediated endocytosis blocked internalization of AJs and TJs, and junctional proteins colocalized with clathrin and alpha-adaptin. AJ and TJ proteins were observed to enter early endosomes followed by movement to organelles that stained with syntaxin-4 but not with markers of late and recycling endosomes, lysosomes, or Golgi. These results indicate that endocytosis of junctional proteins is a clathrin-mediated process leading into a unique storage compartment. Such mechanisms may mediate the disruption of intercellular contacts during normal tissue remodeling and in pathology.