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Vol. 19, Issue 10, 4442-4453, October 2008
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*Department of Molecular Biology, University of Geneva, CH-1211 Geneva, Switzerland; and Department of Biology, University of Padova, I-35121 Padova, Italy
Submitted June 4, 2008;
Revised July 10, 2008;
Accepted July 16, 2008
Monitoring Editor: Benjamin Margolis
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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; Shin et al., 2006), and adherens junctions (AJs) play key roles in morphogenesis and tissue sorting (Gumbiner, 2005). TJs comprise membrane proteins, which function as adhesion receptors and as pores/channels for the passage of molecules through the paracellular pathway, and cytoplasmic proteins, which comprise scaffolding and signaling proteins (Schneeberger and Lynch, 2004; Van Itallie and Anderson, 2006; Guillemot et al., 2008). AJs contain transmembrane adhesion receptors of the cadherin and Ig-like cell adhesion molecule families, and cytoplasmic proteins that regulate actin cytoskeleton dynamics and signaling pathways (Perez-Moreno et al., 2003).
Assembly and function of TJs and AJs are dependent on the activities of the Rho family GTPases Cdc42, RhoA, and Rac1 (Nusrat et al., 1995; Jou et al., 1998; Fukata and Kaibuchi, 2001; Braga, 2002), which act as molecular switches that control actin polymerization and dynamics through a variety of effector proteins, including kinases and scaffold proteins (Jaffe and Hall, 2005). In recent years, details have emerged about the roles of Rho family GTPases at epithelial junctions. Rac1 becomes activated at sites of cell–cell adhesion after cadherin-mediated interactions between adjacent cells (Braga et al., 1997; Noren et al., 2001), but it is rapidly down-regulated during expansion of the adhesive contact (Yamada and Nelson, 2007). RhoA activity concentrates at the outer edges of the expanding contact, and it is down-regulated when the monolayer reaches confluence (Coleman et al., 2004; Yamada and Nelson, 2007). Thus, a precise spatial and temporal fine-tuning of the activity of Rho family GTPases is critically important in the establishment and maintenance of junctions. However, little is known about the molecular mechanisms that control RhoA and Rac1 activities during the different phases of junction assembly, and in confluent cells.
Rho family GTPases are regulated by activating factors (guanine nucleotide exchange factors [GEFs]) and inhibiting factors (GTPase activation proteins and guanosine diphosphate-dissociation inhibitors), which in turn bind to adaptor proteins, which may act either to restrict their spatial localization, and/or influence their activity (Mertens et al., 2003; Jaffe and Hall, 2005; Rossman et al., 2005). So far, few junctional proteins have been shown to influence Rho family GTPase activity in epithelial cells (Guillemot et al., 2008). These proteins include Par3, angiomotin, p120catenin, and cingulin (Anastasiadis et al., 2000; Aijaz et al., 2005; Chen and Macara, 2005; Mertens et al., 2005; Sakurai et al., 2006; Wells et al., 2006; Wildenberg et al., 2006). For example, Par3 assembly into junctions is promoted by the Rac1 GEF Tiam1 (Mertens et al., 2005), and in turn Par3 acts to negatively regulate Rac1 activity, by interacting with Tiam1 (Chen and Macara, 2005). At epithelial junctions, the RhoA activator GEF-H1 is inhibited by binding to the TJ protein cingulin (Aijaz et al., 2005). Cingulin is a cytoplasmic TJ protein (Citi et al., 1988), which interacts with zonula occludens (ZO) proteins (ZO-1, ZO-2, and ZO-3) and the actomyosin cytoskeleton (Cordenonsi et al., 1999; D'Atri and Citi, 2001; Guillemot and Citi, 2006b; Guillemot et al., 2008). In cingulin-depleted cells, GEF-H1 sequestration at junctions is decreased (Aijaz et al., 2005), and this results in increased RhoA activity, increased expression of the TJ component claudin-2, and increased cell proliferation (Guillemot and Citi, 2006a).
The recently identified protein JACOP/paracingulin (also identified as cingulin-like1/CGNL1) has a domain organization similar to cingulin, with globular head and tail domains, and a central coiled-coil rod domain, which shows 
40% sequence identity to cingulin (Ohnishi et al., 2004; Guillemot and Citi, 2006b; Guillemot et al., 2008). However, unlike cingulin, which is a TJ-specific protein, paracingulin has been localized at both TJs and AJs (Ohnishi et al., 2004). Because the function of paracingulin is unknown, we decided to explore it using a short hairpin RNA (shRNA)-mediated approach in Madin-Darby canine kidney (MDCK) epithelial cells. Our results show that paracingulin regulates both Rac1 and RhoA activities, through a mechanism involving interaction with and junctional recruitment of Tiam1 and GEF-H1.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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; Paschoud and Citi, 2008). All experiments were carried out at least in triplicate, and values show means and SD. For immunoblots and immunofluorescence data, one representative example is shown.
Antibodies and Plasmids
Polyclonal antibodies to paracingulin were raised in rabbits against bacterially expressed glutathione transferase (GST) fusion proteins encoding either residues 582-940 or residues 493-633 of canine paracingulin, and were used at dilutions of 1:1000 and 1:10,000 for immunofluorescence and immunoblotting, respectively. These antibodies label a single polypeptide of 150 kDa in lysates of MDCK cells, and they do not cross-react with cingulin (Mr 140 kDa), as determined by immunoblotting analysis of cingulin recombinant proteins and immunoblotting and immunofluorescence analysis of cells overexpressing cingulin. Antibodies against 
-catenin, β-catenin, actin, and vesicular stomatitis virus (VSV) were from Sigma-Aldrich (St. Louis, MO). Monoclonal anti-cadherin and anti-Rac1 antibodies were from BD Biosciences (San Jose, CA). Other antibodies were as described previously (Guillemot and Citi, 2006a; Paschoud and Citi, 2008). Monoclonal antibodies against GEF-H1 and a construct encoding VSV-tagged GEF-H1 were a kind gift from K. Matter (University College, London, United Kingdom). Polyclonal antibodies to Tiam1 were a kind gift from J. Collard (Netherlands Cancer Institute, Amsterdam, Holland) and were also obtained from Santa Cruz Biotechnology (Santa Cruz, CA). However, because these antibodies did not immunolocalize endogenous Tiam1 at junctions or label a polypeptide of correct size in MDCK cell lysates, we were obliged to use exogenous hemagglutinin (HA)-tagged Tiam1. Constructs encoding fluorescently- and myc-tagged canine paracingulin and cingulin, and myc-tagged yellow fluorescent protein (YFP) were generated in the mammalian expression vector pcDNA3.1myc-His (Paschoud and Citi, unpublished data) (Paschoud and Citi, 2008). Constructs for the production of His-tagged recombinant proteins in baculovirus-infected Sf29 insect cells were generated in pFastBacHT vectors (Citi et al., 2001). Construct for bacterial expression [in the BL21(DE3) strain] of GST fused to different regions of paracingulin, and the DH/PH domains of Tiam1 were generated by polymerase chain reaction (PCR) amplification and subcloning into the pGEX4T1 vector. Constructs for the expression of the GST-rhotekin/Rho-binding domain and GST-Pak1/p21 binding domain fusion proteins were a kind gift of K. Burridge (University of North Carolina, Chapel Hill, NC). Recombinant proteins were expressed and purified as described previously (Citi et al., 2001). The construct encoding HA-tagged dominant-negative RhoA (RhoAN19) was a kind gift from E. Olson (University of Texas Southwestern Medical Center, Dallas, TX). The construct encoding full-length, HA-tagged Tiam1 was a kind gift from J. Collard (Netherlands Cancer Institute). All new constructs were verified by sequencing.
Cell Fractionation
Cells were grown in 60-mm dishes, washed twice with ice-cold phosphate-buffered saline (PBS), and lysed in 0.25 ml of either radioimmunoprecipitation assay (RIPA) buffer (150 mM NaCl, 40 mM Tris-HCl, pH 7.5, 2 mM EDTA, 10% glycerol, 1% Triton X-100, 0.5 sodium deoxycholate, 0.2% SDS, 5 µg/ml antipain-leupeptin-pepstatin cocktail, and 1 mM phenylmethylsulfonyl fluoride [PMSF]), or cytoskeleton stabilizing buffer (CSK; 10 mM HEPES, pH 6.8, 250 mM sucrose, 150 mM KCl, 1 mM EGTA, 3 mM MgCl2, 0.5% Triton X-100, 1 mM PMSF, and protease inhibitor cocktail). The CSK lysate was centrifuged for 15 min at 13,000 x g, and the pellet was resuspended, washed with CSK buffer, centrifuged again, and taken as "low-speed" cytoskeleton fraction. The soluble fraction was centrifuged for 120 min at 100,000 x g, and the pellet was washed, and taken as the "high-speed" Triton X-100–insoluble cytoskeleton fraction. The supernatant from high-speed centrifugation was taken as "soluble" fraction. Equivalent protein loadings from each fraction were analyzed by SDS-polyacrylamide gel electrophoresis (PAGE) and immunoblotting.
Quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR)
mRNA levels were analyzed by SYBR Green-based real-time PCR as described previously (Guillemot and Citi, 2006b), by using the same primers, except for CGNL1 forward, 5'-CTCAAGGACCTGGAATACGAGC-3' and CGNL1 reverse, 5'-TCCGAGAGCAAATCCGAGTT-3'; and Tiam1 forward, 5'-GGATGCCCAGAACCCGA-3' and Tiam1 reverse, 5'-GGATGGGCTTGATGAGGTAAGA-3'. Student's t tests were performed using the GraphPad Prism 4 software (GraphPad Software, San Diego, CA).
Immunochemical Techniques
Immunoblotting and immunofluorescence were carried out as described previously (Guillemot and Citi, 2006a; Paschoud and Citi, 2008). Cold methanol fixation was used for immunofluorescence, and cells were routinely counterstained with 4.6-diamidino-2-phenylindole to visualize nuclei. For immunoprecipitation, cells in 60-mm dishes were washed twice with ice-cold PBS and lysed in 1 ml of coimmunoprecipitation (CoIP) buffer, pH 7.8 (20 mM Tris-HCl, 150 mM NaCl, 1% NP-40, 1 mM EDTA, 1 mM phenylmethylsulfonyl fluoride, and 10 µg/ml antipain-leupeptin-pepstatin cocktail) for 15 min at 4°C. Lysates were centrifuged at 12,000 x g for 13 min at 4°C, and supernatants were incubated with polyclonal anti-CGNL1 or anti-cingulin antiserum overnight at 4°C. Dynabeads protein G (Invitrogen, Carlsbad, CA) were added for 1 h at 4°C. After washing with CoIP buffer, pH 4.8, proteins were eluted by boiling beads in SDS sample buffer, and immunoprecipitates were analyzed by SDS-PAGE and immunoblotting.
Imaging Techniques
Samples were imaged using an AxiovertS100TV microscope (Carl Zeiss, Jena, Germany), equipped with a Planapochromat 63x objective (1.4 numerical aperture) and excitation/emission filters to detect fluorescein isothiocyanate, tetramethylrhodamine B isothiocyanate, and 4,6-diamidino-2-phenylindole. Images were acquired with a C4742 digital camera (Hamamatsu, Bridgewater, NJ), operated with OpenLab imaging software and exposure times between 100 and 600 ms. Images were processed using Adobe Photoshop (Adobe Systems, Mountain View, CA) to adjust levels and crop images.
GST Pull-Down Assays
GST pull-down assays to isolate active RhoA (using GST-rhotekin/Rho-binding domain) and Rac1 (using GST-Pak1/p21 binding domain) from MDCK cell lysates were carried out using 20 µg of recombinant GST fusion protein and 0.5 to 1 ml of cell lysate per time point, as described previously (Guillemot and Citi, 2006b). For GST pull-down assays with proteins expressed in baculovirus-infected Sf29 insect cells, 5 µg of recombinant GST fusion protein was coupled for 1 h at room temperature to 20 µl of glutathione-Sepharose beads (GE Healthcare, Chalfont St. Giles, United Kingdom), beads were washed with PBS containing 2% bovine serum albumin and 1% NP-40, and incubated for 1 h at 4°C either with 5 µl of purified protein or 5–200 µl of insect cell lysate, followed by washing three times with ice-cold high-stringency wash buffer (0.5 M NaCl, 20 mM Tris-HCl, pH 7.5, 5 mM EDTA, 1% Triton X-100, 5% NP-40, and 0.1% SDS), and once with PBS. Proteins bound to beads were eluted with SDS sample buffer containing freshly added dithiothreitol (0.1 M), and analyzed by SDS-PAGE and immunoblotting. Loadings for immunoblotting were normalized for protein content with total RhoA/Rac1.
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RESULTS |
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1.6-fold increased for claudin-2, and 1.8-fold increased for ZO-3 (Figure 1B). The phenotype was specifically due to paracingulin depletion, because the changes in ZO-3 (protein and mRNA) and claudin-2 (mRNA) expression were rescued by expression of the tetracycline repressor (TetR), which blocks the expression of shRNA (Figure 1, C and D), and restores paracingulin localization at junctions (Figure 1E). The normal levels of claudin-2 protein, despite the increase in the mRNA levels, may be due to regulatory mechanisms that operate at the level of translation efficiency or protein stability.
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-catenin, β-catenin, and E-cadherin (Figure 1F), indicating that paracingulin depletion does not alter the organization of TJs and AJs in confluent monolayers.
Paracingulin Regulates Claudin-2 mRNA Levels and Cell Proliferation in a RhoA-dependent Manner, and Interacts with GEF-H1
To test the hypothesis that the effects of paracingulin depletion on gene expression are correlated with its effects on RhoA activity, as it is the case for cingulin depletion (Guillemot and Citi, 2006a), we measured RhoA activity in lysates of confluent wild-type (WT) and CGNL1(-) cells. RhoA activity was increased in CGNL1(-) cells (Figure 2A), suggesting that one function of paracingulin is to down-regulate RhoA activity at steady state. Importantly, expression of a dominant-negative mutant of RhoA, which decreased RhoA activity (Supplemental Figure S1A) decreased claudin-2 protein levels in both WT and CGNL1(-) cells (Figure 2B) and reversed the increase in claudin-2 mRNA levels observed in CGNL1(-) cells (Figure 2C), whereas it did not affect ZO-3 protein and mRNA levels (Figure 2, B and C). The close correlation between cellular RhoA activity levels and claudin-2 expression indicates that the effects of paracingulin depletion on claudin-2 expression (but not on ZO-3 expression) are dependent on RhoA. Furthermore, a cell proliferation assay showed that paracingulin depletion induced a significant increase in G1/S phase transition in proliferating cells, which was also reversed by inhibition of RhoA activity (Figure 2D).
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Together, these results demonstrate that paracingulin and GEF-H1 physically and functionally interact and that paracingulin helps to recruit and inactivate GEF-H1 at junctions, resulting in a down-regulation of RhoA activity in confluent monolayers.
Paracingulin Depletion Delays TJ Assembly during the Calcium Switch in a Tiam1-dependent and RhoA-independent Manner
To test whether paracingulin regulates TJ assembly, CGNL1(-), control and WT cells were analyzed by measuring the TER in the calcium-switch assay. In this assay, cells are incubated in low calcium for several hours, to induce junction disassembly, and simultaneous formation of junctions is initiated by calcium readdition (Gonzalez-Mariscal et al., 1985). WT cells, and cells expressing a control shRNA, showed the typical pattern in the development of TER, with a rapid increase to a peak of 180 ohm.cm2 8 h after the addition of calcium, followed by a decrease, and stabilization after 24 h (Figure 4A). The molecular mechanism leading to the peak in the TER value after the calcium-switch is unknown, despite that it was first described over 20 years ago (Gonzalez-Mariscal et al., 1985). Strikingly, in CGNL1(-) cells, the characteristic peak of TER at 8 h was abolished, since the TER value only reached 75 ohm.cm2 (Figure 4B). This phenotype was rescued by expression of the TetR (Figure 4B), showing that it was specifically due to paracingulin depletion. Three distinct CGNL1(-) clones showed this phenotype, and expression of the TetR did not affect TER values in WT cells (Supplemental Figure S2).
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TJ assembly and TER development are known to be dependent on the activities of the Rho family GTPases Rac1 and RhoA (Jou et al., 1998; Bruewer et al., 2004). To test whether the effect of paracingulin depletion on TJ assembly correlates with changes in either Rac1 or RhoA activities, we measured these activities both in confluent cells, and during the calcium-switch, by GST pull-down assays. In confluent monolayers, paracingulin depletion had no detectable effect on Rac1 activity (Figure 5A), whereas it increased RhoA activity (Figure 2A). During the calcium-switch, WT cells showed two distinct peaks of Rac1 activation, one peak occurring very early (10–20 min) and one peak at 3–8 h after addition of calcium (Figure 5, B and C; see also Supplemental Figure S3). In CGNL1(-) cells, these peaks of Rac1 activation were either abolished (early peak) or strongly reduced (late peak), as the fraction of active Rac1 remained more uniform throughout the calcium-switch assay (Figure 5, B and C, and Supplemental Figure S3). With regard to RhoA activity, it was high in both WT and CGNL1(-) cells up to the 3-h time point. At 8 h after the calcium-switch, RhoA activity was down-regulated in WT cells, whereas it remained high in CGNL1(-) cells (Figure 5B). In summary, during the calcium-switch paracingulin depletion resulted in down-regulation of Rac1 activity, and no change in RhoA activity.
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; Mertens et al., 2005). We also tested whether down-regulation of RhoA activity affected the phenotype of CGNL1(-) cells, by expressing a dominant-nega-tive mutant of RhoA in CGNL1(-) cells. Notably, CGNL1(-) cells expressing Tiam1 showed increased Rac1 activity (Figure 6A) and in the TER assay they displayed a profile similar to that of WT or control cells, with a peak at 150 ohm·cm2 at 8 h (Figure 6B), indicating that increased Rac1 activity driven by Tiam1 overexpression can rescue the TER phenotype of CGNL1(-) cells, and that the peak in TER is due to Rac1 activation. Conversely, expression of the dominant-negative form of RhoA did not rescue the TER phenotype of CGNL1(-) cells (Figure 6B). In agreement with these observations, occludin localization in CGNL1(-) cells expressing Tiam1 was similar to WT and control cells, whereas in cells expressing the dominant-negative RhoA mutant there was still an impairment of occludin accumulation into junctions (Figure 6C).
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). However, unlike Tiam1 KO cells and CGNL1(-) cells, the Tiam1-depleted clones showed a peak in TER at 24 h after the calcium-switch (Figure 6D and Supplemental Figure S4C). We interpret this as due to the presence of residual Tiam1 protein, which requires longer time to accumulate at junctions in sufficient amounts to induce the peak. Together, these results provide strong evidence that paracingulin depletion delays TJ assembly by interfering with Tiam1-dependent Rac1 signaling at junctions, independently of RhoA.
Paracingulin Interacts with Tiam1, and Recruits Tiam1 to Junctions
We next investigated the molecular mechanism through which paracingulin regulates Rac1 activity. Because overexpression of Tiam1 rescued the functional and structural effects of paracingulin depletion on TJ assembly (Figure 6, B and C), we hypothesized that paracingulin functions by helping to recruit Tiam1 to junctions, and thus promote Rac1 activation during the early phases of junction formation. To test this hypothesis, we first studied the in vivo interaction of paracingulin with Tiam1. Because endogenous Tiam1 could not be immunoprecipitated from MDCK lysates with any of the antibodies we tested, and the yield of endogenous paracingulin was very low, WT MDCK cells were cotransfected with paracingulin and Tiam1 (Supplemental Figure S1B), and paracingulin immunoprecipitates were analyzed by immunoblotting. A similar experiment was carried out with cells expressing cingulin plus either GEF-H1 or Tiam1 (Supplemental Figure S1B), to check whether cingulin, which does not affect TER development (Guillemot and Citi, 2006a), interacts or not with Tiam1. Tiam1 was specifically detected in paracingulin immunoprecipitates (Figure 7A), whereas only GEF-H1, but not Tiam1, was detected in cingulin immunoprecipitates (Figure 7B). Thus, paracingulin but not cingulin can form a complex in vivo with Tiam1. To test whether paracingulin and Tiam1 can interact directly in vitro, we first expressed recombinant full-length His-tagged Tiam1 in insect cells, and we incubated it with GST fusion proteins encoding different regions of paracingulin (Figure 3B). Tiam1 was detected by immunoblotting in GST pull-down experiments by using either construct B (residues 250-420) or construct D (residues 591-882) of paracingulin (Figure 7, C and D). The GST fusion protein D seemed to interact with Tiam1 with higher affinity than the protein B (Figure 7D). Next, we designed and expressed a GST fusion protein with the region comprising the Dbl homology (DH) and pleckstrin homology (PH) domains of Tiam1 (residues 1037-1404). We speculated that this region of Tiam1 may be involved in its interaction with paracingulin, based on our previous observation that interaction of cingulin with GEF-H1 occurs through the DH/PH domains of GEF-H1 (Aijaz et al., 2005). Indeed, the GST fusion protein making up the DH/PH domains of Tiam1 associated specifically with recombinant full-length paracingulin (Figure 7E), showing that the two proteins interact directly in vitro through at least this region of Tiam1.
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Finally, to confirm the molecular mechanism through which paracingulin helps to recruit Tiam1 to junctions, we examined the association of Tiam1 with the membrane–cytoskeleton fraction in CGNL1(-) cells expressing Tiam1, and expressing or not the TetR. Cells were lysed either with RIPA buffer, to determine total Tiam1 levels, or with CSK buffer, followed by fractionation into low-speed and high-speed pellets, and soluble fractions (Figure 7G). Analysis of total cell lysates showed that Tiam1 was expressed at similar levels in CGNL1(-) cells, and in CGNL1(-) cells where paracingulin expression was rescued by the TetR. However, depletion of paracingulin was associated with a significant decrease in the amount of Tiam1 in both the low-speed and the high-speed pellet fractions, and an increase in the soluble fraction (Figure 7G), consistent with the observation that Tiam1 localization at junctions is decreased in CGNL1(-) cells (Figure 7F). Together, these results demonstrate that paracingulin functions by favoring the junctional recruitment of Tiam1, leading to activation of Rac1 at junctions.
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DISCUSSION |
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Regulation of RhoA Signaling by Paracingulin
Paracingulin depletion leads to increased RhoA activity in confluent cells. Because paracingulin levels are higher in confluent than in proliferating cells (Pulimeno and Citi, unpublished observations), and paracingulin interacts with GEF-H1, we conclude that paracingulin contributes to down-regulating RhoA activity as cells become confluent, by recruiting GEF-H1 to junctions and inactivating it. No up-regulation of RhoA during junction assembly was observed in paracingulin-depleted cells, probably due to the fact that maximal activation of RhoA is already occurring under these conditions.
A consequence of paracingulin depletion is the small, but statistically significant increase in cell proliferation and claudin-2 mRNA levels. Because inhibition of RhoA reverses this phenotype, we conclude that it is due, at least in part, to up-regulation of RhoA activity in paracingulin-depleted cells. However Rac1, which is also regulated by paracingulin, is not involved in the regulation of claudin-2 expression and cell proliferation, because the changes in claudin-2 mRNA levels and BrdU incorporation are not rescued by overexpression of Tiam1 (data not shown).
The precise mechanism by which cingulin, paracingulin, and microtubules (Krendel et al., 2002; Aijaz et al., 2005) inhibit GEF-H1 remains unclear. Microtubules bind to the PH domain of GEF-H1, and they are believed to favor its folding into an inhibited state, where access to the substrate is blocked (Birkenfeld et al., 2008). Because cingulin also binds to the PH domain of GEF-H1, it is likely that it also blocks GEF-H1 function by favoring the inhibited state conformation.
Regulation of Rac1 Signaling by Paracingulin
TER development during the calcium-switch assay is the assay of choice to investigate the role of specific proteins in junction assembly. The signaling mechanism leading to the peak in TER observed during junction assembly by the calcium-switch was unknown until now. We show that the peak in TER is due to Rac1 activation, based on the temporal overlap between the second wave of Rac1 activation and the peak in TER, and on the observation that paracingulin depletion, which strongly reduces the waves of Rac1 activation, also strongly reduces the peak in TER. Because this phenotype is rescued by overexpression of Tiam1, and paracingulin interacts with Tiam1, we conclude that Rac1 activation and peak in TER during the calcium-switch depend on the recruitment of Tiam1 to junctions by paracingulin. Interestingly, although Tiam1 recruitment to junctions is impaired in CGNL1(-) cells, overexpression of Tiam1 rescued the TER phenotype of these cells. This apparently contradictory observation can be explained by the following possibilities: 1) in cells overexpressing Tiam1 sufficient amounts of Tiam1 can be recruited to junctions by residual paracingulin, and by other factors (see below); and 2) the overall increase in cellular Rac1 activity is sufficient to drive normal TJ assembly. Conversely, the observation that dominant-negative RhoA did not rescue the TER phenotype of CGNL1(-) cells argues against a role of GEF-H1–paracingulin interaction in mediating the loss of the peak in TER observed in CGNL1(-) cells. Future studies should dissect the role of different Tiam1 domains in rescuing the phenotype of CGNL1(-) cells.
The biological significance of the peak in TER, and how the two transient waves of increased Rac1 activity translate into increased TER, is unclear. The pattern of expression of claudin isoforms is critical in determining the paracellular barrier properties of epithelial tissues (reviewed in Van Itallie and Anderson, 2006). Thus, it is possible that during TJ assembly specific isoforms of claudins that confer high TER are transiently concentrated at junctions, or that the function of the barrier unit is dynamically affected by the local reorganization of the actin cytoskeleton, which interacts with the proteins, such as ZO proteins, that provide a scaffold for claudins.
Our results provide new insights into the mechanism through which Rac1 becomes activated at new sites of cadherin-based cell–cell contact. There is evidence that Rac1 activation is triggered by activation of PI-3 kinase, which results in the formation of phosphoinositides, which could recruit GEFs to the membrane, through interaction with their PH domains (Michiels et al., 1997; Stam et al., 1997; Sander et al., 1998; Pece et al., 1999; Malliri et al., 2004). However, other studies indicate that Rac1 activation occurs independently of phosphatidylinositol 3-kinase activation (Nakagawa et al., 2001; Betson et al., 2002; Ehrlich et al., 2002). Thus, the precise mechanism of Rac1 activation at cadherin-based junctions is controversial. In epithelial cells, Tiam1 is the major GEF activator of Rac1, and it promotes AJ and TJ formation via activation of the Par3 polarity complex (Malliri et al., 2004; Mertens et al., 2005). In turn, Par3 inhibits Tiam1 function and Rac1 activity (Chen and Macara, 2005). Thus, one possibility to explain our results is that paracingulin depletion decreases Rac1 activity indirectly, by increasing the levels of Par3. However, our immunoblot analysis showed that levels of Par3 were unaffected in CGNL1(-) cells, ruling out this possibility. Although Tiam1 has been reported to interact with several proteins, most of these proteins have been identified in fibroblasts/neurons, and not epithelial cell model systems (Mertens et al., 2003; Ten Klooster et al., 2006). Our results provide the first evidence that depletion of a Tiam1-interacting protein results in decreased accumulation of Tiam1 at epithelial junctions, and increased Tiam1 solubility. This, together with the observation that paracingulin depletion inhibits Rac1 activation during junction assembly, identifies paracingulin as a key molecule involved in Rac1 activation during junction assembly, by recruiting Tiam1 to junctions and promoting its association with the junctional acto-myosin cytoskeleton.
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CONCLUSIONS |
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) and interacts with the Rac1 GEF Vav (Noren et al., 2000), it is not clear whether it controls Vav junctional recruitment. In addition, inhibition of RhoA by p120 is thought to occur through direct sequestration of RhoA, rather than interaction with a GEF (Anastasiadis et al., 2000).
The formation of stable E-cadherin-based adhesion sites critically requires Rac1 activity (Braga et al., 1997; Nakagawa et al., 2001), and the formation of stable E-cadherin adhesions is necessary (although not sufficient; Denisenko et al., 1994) for TJ assembly (Gumbiner et al., 1988). Paracingulin is localized both at AJ and TJ in some tissues (Ohnishi et al., 2004), although the precise localizations of both paracingulin and Tiam1 in cultured MDCK cells are not known. We show here that paracingulin depletion delays both AJ and TJ assembly, based on immunofluorescence and TER assays. We propose that paracingulin functions early during the formation of primordial cadherin-based AJ, by helping to recruit Tiam1 and thus activate Rac1 at AJ (Figure 8). Rac1 activation leads to efficient AJ assembly, which in turns allows TJ assembly to occur, through the reorganization of the actin cytoskeleton and the recruitment of membrane and cytoplasmic junctional proteins, including Par3 (Mertens et al., 2005). As junctions mature, Par3 accumulates at junctions and inactivates Tiam1 (Chen and Macara, 2005), in a negative feedback loop (Figure 8). Par3 translocates to cell–cell contract regions later than the formation of the primordial AJ (Suzuki et al., 2002), consistent with our model that Rac1 activation mediated by the paracingulin–Tiam1 interaction at AJ precedes the subsequent inactivation of Tiam1 by Par3 at TJ. Once junctions have matured in confluent cells, Tiam1 activity is negatively regulated by Par3, whereas GEF-H1 and RhoA activity are negatively regulated by cingulin and paracingulin at TJ (Figure 8). It is noteworthy that mature TJ are not as sensitive as AJ to changes in Rac1 activity (Jou et al., 1998; Bruewer et al., 2004). Thus, we propose that the delay of TJ assembly observed in CGNL1(-) cells is a consequence of delayed AJ formation due to decreased Rac1 activation at AJ.
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). Diseases affecting the intestinal, renal, auditory epithelia, as well as invasion of epithelia by pathogens and toxins are associated with changes in the expression of claudins and Rho family GTPase activities (Mankertz and Schulzke, 2007; Birkenfeld et al., 2008). Thus, our results on the role of paracingulin in regulating Rho family GTPases and claudin-2 expression provide new insights and tools to understand the molecular mechanisms underlying the physiology and pathophysiology of epithelial tissues.
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ACKNOWLEDGMENTS |
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Footnotes |
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Address correspondence to: Sandra Citi (sandra.citi{at}unige.ch)
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REFERENCES |
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Anastasiadis, P. Z., Moon, S. Y., Thoreson, M. A., Mariner, D. J., Crawford, H. C., Zheng, Y., and Reynolds, A. B. (2000). Inhibition of RhoA by p120 catenin. Nat. Cell Biol 2, 637–644.[CrossRef][Medline]
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