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Vol. 17, Issue 2, 749-759, February 2006
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* Centre de Recherches de Biochimie Macromoléculaire, Centre National de la Recherche Scientifique Formation de Recherche en Evolution 2593, 34293 Montpellier Cedex, France;
Institut de Génétique Humaine, Centre National de la Recherche Scientifique Unité Propre de Recherche 1142, 34396 Montpellier Cedex 5, France
Submitted April 6, 2005;
Revised October 11, 2005;
Accepted November 3, 2005
Monitoring Editor: Ben Margolis
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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1-integrin and ADAM12. Active RhoA induces the entry of M-cadherin into a degradative pathway and thus decreases its stability in correlation with the monoubiquitination of M-cadherin. Moreover, p120 catenin association with M-cadherin is decreased in RhoAV14-expressing cells, which is partially reverted by the inhibition of the RhoA effector Rho-associated kinase ROCK. ROCK inhibition also restores M-cadherin accumulation at the cell-cell contact sites. We propose that the sustained activation of the RhoA pathway inhibits myoblast fusion through the regulation of p120 activity, which controls cadherin internalization and degradation. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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; Taylor, 2000). Each step of this process is highly regulated and although the molecular mechanisms that regulate myogenesis induction are starting to be elucidated, little is known about myoblast fusion. Myoblast fusion is a stepwise process involving the initial recognition and adhesion between fusion-competent myoblasts, subsequent myoblast alignment, and finally membrane breakdown and fusion (Doberstein et al., 1997). This process also occurs when satellite cells fuse with damaged membranes to regenerate muscle fibers. Myoblast fusion has been studied in mammalian myoblastic cell lines, mice, and Drosophila, and some of the molecules involved have been identified. Genetic screens and biochemical analyses have identified many proteins involved in muscle cell fusion located at the cell surface, in the cytoplasm, and in the nucleus (reviewed in Dworak and Sink, 2002; Taylor, 2002). The membrane proteins implicated in myotube formation in vertebrates include 1 integrin, ADAM12 (a member of a disintegrin and metalloproteinase family), NCAM, and M-cadherin (Rosen et al., 1992; Zeschnigk et al., 1995; Fazeli et al., 1996; Huovila et al., 1996; Charlton et al., 2000; Schwander et al., 2003). M-cadherin belongs to the cadherin family of calcium-dependent adhesion molecules. The N-terminal extracellular domain mediates homophilic binding, and the intracellular region binds to - and 
- and p120 catenins. 
-Catenin interacts with - and -catenins and the actin microfilaments. M-cadherin is found predominantly in developing skeletal muscles and is highly expressed during secondary myogenesis. In mature skeletal muscle, M-cadherin is detectable in satellite cells and on the sarcolemma of myofibers underlying satellite cells (Moore and Walsh, 1993; Bornemann and Schmalbruch, 1994; Rose et al., 1994; Cifuentes-Diaz et al., 1995). M-cadherin is also found at neuromuscular junctions, intramuscular nerves, and in two regions of the central nervous system, namely, the spinal cord and the cerebellum (Cifuentes-Diaz et al., 1996; Bahjaoui-Bouhaddi et al., 1997). M-cadherin-deficient mice show no defect in skeletal muscle development, probably because of compensation by other cadherin molecules, in particular N-cadherin (Hollnagel et al., 2002). However, various studies in cultured myoblasts have postulated that M-cadherin may be essential for the fusion of myoblast to myotubes (Donalies et al., 1991; Pouliot et al., 1994; Zeschnigk et al., 1995; Kuch et al., 1997). In addition to cell surface molecules, certain signaling pathways have been implicated in myoblast fusion (Chen and Olson, 2004). In particular, we have shown that RhoA, a member of the Rho GTPase family that is a positive regulator of myogenesis induction, prevents myoblast fusion (Meriane et al., 2000), which was recently confirmed by others (Nishiyama et al., 2004). Cross-talk between Rho GTPases and cadherin molecules has been described in many cases (Charrasse et al., 2002). In the present study, we have investigated the effect of RhoA activation on M-cadherin activity in mouse C2C12 myoblasts. We find that RhoA activity is decreased at the onset of myoblast fusion and that the expression of a constitutive active form of RhoA (RhoAV14) totally inhibits the fusion process. RhoAV14 expression specifically decreases the level of M-cadherin without affecting another molecule of the family, N-cadherin, or 1-integrin and ADAM12, two proteins involved in myoblast fusion. M-cadherin localization is also perturbed in RhoAV14-expressing cells. RhoA induces the ubiquitination and degradation of M-cadherin through a lysosomal-dependent pathway. Finally, we observe that RhoAV14, through the activation of its effector, the Rho-associated kinase ROCK, decreases p120 catenin association with M-cadherin. |
MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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). The complete murine M-cadherin coding sequence (accession no. NM_007662
[GenBank]
) was cloned into the pEGFP-N1 vector (Clontech, Mountain View, CA) using the NheI and SacII sites. The characterization of this construct is shown in Supplemental Data #1. Green fluorescent protein-tagged cystinosin (CTNS-GFP) was kindly provided by Viki Kalatzis (Centre National de la Recherche Scientifique, Montpellier, France).
Establishment of Stable Cell Lines
G418-resistant GP+E-86 clones expressing constitutively activate form of RhoA (RhoAV14) were grown to collect retrovirus-containing cell-free supernatants. Infection of C2C12 myoblasts was performed as described previously (Meriane et al., 2000). Cells were grown continuously in G418.
Cell Culture
C2C12 mouse myoblasts were grown in DMEM/Ham's F-12 (1:1) supplemented with 10% fetal calf serum (FCS) (Hyclone/Perbio Sciences, Brebieres, France). To induce differentiation growth, medium was replaced with differentiation medium consisting of DMEM/Ham's F-12 supplemented with 2% FCS. Stable cell lines derived from C2C12 myoblasts were cultured under the same conditions in medium supplemented with 1 mg/ml G418. HeLa cells (American Type Culture Collection, Manassas, VA) were grown in DMEM supplemented with 10% FCS. The ROCK inhibitor Y-27632 (Sigma-Aldrich, St. Louis, MO) was used at 5 µm for 2-3 h. Chloroquine (Sigma-Aldrich) was used at 100 µM. LysoTracker DND99 (Invitrogen, Cergy Pontoise, France) was used at 50 nM. Fresh cycloheximide (CHX) diluted in phosphate-buffered saline (PBS) was used at 10 µg/ml.
Polyclonal Anti-M-Cadherin Antibody Production
The 750-1765 fragment of M-cadherin (NM_007662
[GenBank]
) corresponding to amino acids 250-590 was cloned in the pGEX5X-2 vector. GST-M-cadherin fragment was produced as described previously (Mary et al., 2002). Three rabbits were injected with 80-100 µg of glutathione S-transferase (GST)-M-cadherin protein. Antisera were tested by immunoblotting and immunocytochemistry. Affinity purification was performed by incubation with the GST-M-cadherin fragment spotted onto nitrocellulose. The characterization of this antibody is shown in Supplemental data #2.
Gel Electrophoresis and Immunoblotting
Cell extracts were prepared as described previously (Charrasse et al., 2002). Protein concentration was determined with a BCA protein assay kit (Pierce Chemical, Rockford, IL). Protein (20 µg) was resolved on polyacrylamide gel (8 and 12%) and transferred onto Immobilon-P. Membranes were incubated with monoclonal antibodies against N-cadherin (1:2000), 1-integrin (1:2500) (both from BD Transduction Laboratories, Lexington, KY), troponin T (1: 1000), myosin (1:2000) (both from Sigma-Aldrich), myogenin (1:500) (BD Biosciences PharMingen, San Diego, CA), and -tubulin (1:100) or with polyclonal (1:1000) or monoclonal (1:200; NanoTools, Munich, Germany) anti-M-cadherin antibody. Membranes were processed as described previously (Charrasse et al., 2002).
Pulse-Chase Experiments
Control and RhoAV14-expressing C2C12 myoblasts where incubated for 30 min in methionine-free media (DMEM minus methionine, 2 mM glutamine, and 2% dialyzed FCS) and pulse labeled for 30 min with 1 mCi/ml [35S]methionine/cysteine (Promix 35S; GE Healthcare, Little Chalfont, Buckinghamshire, United Kingdom). Cells where washed twice in ice-cold PBS and resuspended in complete DMEM/Ham's F-12 medium. At indicated time, cells where harvested and lysed in immunoprecipitation buffer. Extracts were immunoprecipitated using an anti-M-cadherin polyclonal antibody (1/50), separated on an 8% acrylamide gel that was treated with Amplify (GE Healthcare). Radioactive bands were visualized by autoradiography with phosphorscreens and a PhosphorImager (GE Healthcare) and quantified using ImageQuant (GE Healthcare).
Immunoprecipitation
C2C12 cells were cotransfected with ubiquitin-hemagglutinin (HA)-tagged vector (kindly provided by Isabelle Jariel-Encontre, Centre National de la Recherche Scientifique) and with cytomegalovirus--galactosidase vector (1/5) to normalize transfection efficiency. Then, cells were lysed for 10 min in ice-cold extraction protein buffer (Charrasse et al., 2002). Extracts normalized with the amount of -galactosidase were immunoprecipitated using a mouse monoclonal anti-HA antibody from ascites (1/500 dilution), separated on a polyacrylamide gel, and then transferred onto nitrocellulose. Membranes were probed with M-cadherin antibody followed by peroxidase-conjugated anti-rabbit antibody (GE Healthcare). To analyze M-cadherin/p120 catenin and N-cadherin/p120 complexes, monoclonal p120 antibody (BD Transduction Laboratories) was used for immunoprecipitation, and either M-cadherin or N-cadherin detection was performed using monoclonal antibody (1/1000) (BD Transduction Laboratories).
Cell Surface Biotinylation
C2C12 cells grown on 60-mm dishes were incubated with 1 mg/ml sulfosuccinimidyl 2-(biotinamido) ethyl-dithioproprionate (sulfo-NHS-SS-biotin) (Pierce Chemical), followed by washing with sulfo-NHS-SS-biotin blocking reagent (50 mM NH4Cl in PBS containing 1 mM MgCl2 and 0.1 mM CaCl2) to quench free sulfo-NHS-SS-biotin, followed by several further washes in PBS (Le et al., 1999). Cells were then scrapped and lysed as described above before centrifugation to obtain a detergent-soluble supernatant. At this stage, an aliquot of 10 µl was kept (=total fraction), and the remainder was incubated with streptavidin beads to collect biotinylated proteins. Samples were then analyzed by SDS-PAGE and immunoblotting to identify M-cadherin. Different luminescence exposures were collected, and exposures in the linear range were used. Quantification was performed using Aida/2D densitometry software from at least five independent experiments.
siRNA Transfection
Short interfering RNA (siRNA) constructs were made in pSUPER polymerase III expression vector (Charrasse et al., 2004). To suppress endogenous M-cadherin expression, oligonucleotide GATCCCCCAGCAGCTAGGCAGTGTCAttcaagagaTGACACTGCCTA GCTGCTGTTTTTGGAAA was inserted into pSUPER. Bold letters correspond to oligonucleotides 735-753 of the mouse M-cadherin cDNA sequence (NM_007662
[GenBank]
). As a control, R-cadherin siRNA was used (Charrasse et al., 2004). Cells (25,000) were plated in six-well Limbro 1 d before transfection. A first transfection was performed in 20-30% confluent cells with a construct encoding empty pEGFP and either the M-cadherin siRNA or the R-cadherin siRNA (1:5). A second transfection was performed 24 h later when cells were 50-60% confluent. Twenty-four hours after the second transfection, cells from two wells were mixed before addition of differentiation medium. For all transfections, LipofectAMINE (Invitrogen) was used.
Immunocytochemistry
Cells growing onto 35-mm dishes were fixed in 3.7% formaldehyde in PBS followed by a 5-min permeabilization in 0.1% Triton X-100 in PBS and incubated in PBS containing 0.1% bovine serum albumin. Transferrin receptor antibody was from Zymed Laboratories (South San Francisco, CA). Anti-M-cadherin was revealed by either an Alexa Fluor 546-or an Alexa Fluor 488-conjugated goat anti-rabbit antibody (Molecular Probes, Interchim, Montluçon, France). Cells were analyzed as previously described (Charrasse et al., 2002, 2003, 2004).
To label lysosomes, cells were transfected with CTNS-GFP (Kalatzis et al., 2001) or incubated with LysoTracker DND99 (Invitrogen).
Single plane images were captured using a MicroMax 1300 charge-coupled device camera (Princeton Instruments, Trenton, NJ) driven by MetaMorph (Molecular Devices, Sunnyvale, CA) software. Images were deconvolved using the maximum likelihood estimation algorithm (Huygens; Scientific Volume Imaging, Hilversum, The Netherlands). The restored images were saved as Tif files that were mounted using Adobe Photoshop and Adobe Illustrator (Adobe Systems, Mountain View, CA).
|
). For confocal acquisition, a spinning Nipkow disk was used. Time series of captured images were deconvolved, and restored images were saved as Tif files that were compiled into QuickTime movies using MetaMorph.
RhoA GTPase Activity Assay
C2C12 myoblasts either in proliferation or during the course of differentiation were lysed and processed to measure the total and GTP RhoA level as described previously (Charrasse et al., 2002).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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; Wei et al., 1998; Meriane et al., 2000). In addition, the level of RhoA GTP is increased by a pathway that promotes the commitment to myogenesis (Charrasse et al., 2003). We measured RhoA activation at different times after the shift to differentiation medium using pull-down assays (Figure 1, A and B). RhoA GTP levels are substantially reduced after 2 d in differentiation medium (DM), which corresponds to the onset of the fusion process as shown by the morphological analysis of myoblast differentiation (Figure 1C).
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), we observed that RhoAV14 potentiates the differentiation process because myogenin, troponin T, and myosin heavy chain expression was accelerated (our unpublished data). Interestingly, we were unable to detect any myotube formation, as shown microscopically 4 d after DM addition where cells stay aligned and elongated (Figure 2A) and by the measurement of the fusion index (Figure 2B). No myotubes were observed even after 7 d in DM, indicating that the myoblast to myotube transition is effectively blocked and not simply delayed (our unpublished data).
M-Cadherin Is Required for Myoblast Fusion
A previous report has proposed a role for M-cadherin during the fusion process (Zeschnigk et al., 1995). Curiously, mice lacking M-cadherin do not present defects in muscle development, and the authors proposed that this might reflect compensation by N-cadherin (Hollnagel et al., 2002). Because the first study was performed in L6 myoblasts lacking N-cadherin, it was important to know whether M-cadherin is required for myoblast fusion in C2C12 myoblasts, which express both N- and M-cadherins. For this purpose, we used RNA interference technology to lower M-cadherin expression. M-cadherin silencing was analyzed by Western blot during the differentiation process of C2C12 myoblasts (Figure 3A). M-cadherin protein levels were strongly decreased during myogenesis by RNA interference (RNAi). In contrast, N-cadherin and tubulin expression were unaffected, demonstrating the specificity of the RNAi. Interestingly, the expression of the myogenic marker myogenin, troponin T, and myosin heavy chain (MHC) was not affected by M-cadherin gene silencing, indicating that induction of these genes does not require M-cadherin. In contrast, M-cadherin knockdown specifically inhibits myotube formation (Figure 3, B and C). Cells cotransfected with empty GFP and control R-cadherin RNAi were able to fuse, whereas cells cotransfected with empty GPF and M-cadherin RNAi did not fuse (Figure 3B). Our R-cadherin RNAi was previously validated in a rhabdomyosarcoma-derived cell line (Charrasse et al., 2004), and it does not perturb the expression of the myogenic markers described above (our unpublished data). Quantification of the number of transfected cells that have fused in these two conditions is shown in Figure 3C.
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). In contrast, RhoA V14 did not affect N-cadherin, 1-integrin, and tubulin expression levels as well as ADAM-12 (our unpublished data). We then analyzed M-cadherin localization by immunocytochemistry (Figure 5A). In control proliferating C2C12 myoblasts, M-cadherin accumulates at intercellular contacts (Figure 5A, a). After 2 d in differentiation medium, M-cadherin accumulates at myoblast-myotube and myotube-myotube contacts (Figure 5A, b). This agrees with data obtained in rat myoblasts (Eng et al., 1997). In RhoAV14-expressing cells, M-cadherin no longer accumulates at contacts between proliferating cells (Figure 5A, c) or cells in DM (Figure 5A, d). We used a biotinylation assay to further analyze the presence of M-cadherin at the cell surface. We analyzed total and cell surface biotinylated M-cadherin in control and RhoAV14-expressing C2C12 myoblasts cultured in growth or differentiation medium for the indicated period of time (Figure 5B). The histogram shows the level of biotinylated M-cadherin normalized to the amount of total cellular M-cadherin. From these data we conclude that, in RhoAV14-expressing myoblasts, the amount of M-cadherin at the plasma membrane is decreased. In contrast, the amount of N-cadherin at the plasma membrane is not modified by RhoAV14 expression (our unpublished data).
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RhoA Increases M-Cadherin Degradation through the Lysosomal Pathway
The results mentioned above demonstrate that RhoA induces the loss of M-cadherin and its delocalization from the cell surface. We then analyzed whether RhoA affects the turnover of M-cadherin. Control and RhoAV14-expressing myoblasts were treated with CHX, and the amount of M-cadherin was analyzed at different times. We also labeled control and RhoAV14-expressing myoblasts with [35S]methionine/cysteine and performed pulse-chase experiments (Figure 6A). In both cases, we observed a strong decrease of M-cadherin stability in RhoA-expressing cells (Figure 6A). We also analyzed the subcellular localization of M-cadherin in control and RhoAV14-expressing myoblasts. Cells were stained with antibodies directed against the Golgi apparatus, endoplasmic reticulum, and endocytic, recycling, and degradation compartments. RhoA expression increases the colocalization of M-cadherin with the endocytic pathway (our unpublished data). To determine whether the internalized M-cadherin was degraded via a lysosomal pathway, we stained lysosomes by transfection of a plasmid encoding GFP-tagged CTNS (Figure 6B) or using LysoTracker dye (Figures 6C and 7B) (Kalatzis et al., 2001). In control cells, little colocalization between M-cadherin and CTNS was observed (Figure 6B, a-c). In contrast, RhoAV14 expression increased the colocalization of M-cadherin with CTNS (Figure 6B, d-f). Because the visualization of intracellular vesicular compartments is difficult in C2C12 myoblasts, HeLa cells were transfected with M-cadherin/GFP, incubated with LysoTracker dye, and analyzed by time-lapse confocal microscopy. As shown in Figure 6C and the accompanying videos, RhoAV14 increased the colocalization of M-cadherin/GFP with lysosomes. A quantitative analysis is shown in Figure 6D. To strengthen the observation of M-cadherin internalization through the endosome-lysosome pathway, C2C12 myoblasts were treated with chloroquine, an inhibitor of lysosomal activity, and analyzed for M-cadherin localization (Figure 7A). Chloroquine addition increased the vesicular accumulation of M-cadherin (Figure 7A, c), as in RhoAV14-expressing cells (compare Figure 7A, a and b). This treatment induced the appearance of large vesicular structures (as reported previously; Xiao et al., 2003) in which M-cadherin accumulates (Figure 7A, c). These vesicles correspond to lysosomes, as demonstrated by the analysis by time-lapse microscopy of M-cadherin/GFP-expressing C2C12 myoblasts incubated with LysoTracker dye to stain lysosomes. As shown in Figure 7B and the accompanying video, M-cadherin/GFP colocalized with lysosomes. To analyze whether lysosomes were involved in the decrease of M-cadherin levels in RhoAV14-expressing cells, the expression of M-cadherin in chloroquine-treated cells was analyzed by Western blot. In the presence of chloroquine we observe less down-regulation of M-cadherin in RhoAV14-expressing myoblasts (Figure 7A). No effect on M-cadherin expression levels or localization was detected after treatment with proteasomal inhibitors (our unpublished data).
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Finally, we analyzed whether RhoA induces the ubiquitination of M-cadherin. Indeed, polyubiquitination targets proteins for the destruction by the proteasome, whereas monoubiquitination triggers internalization and degradation in lysosomes (Hicke, 2001). The level of ubiquitinated M-cadherin was analyzed by immunoprecipitation of ubiquinated proteins from control (Figure 7C, lanes 3 and 5) and RhoAV14-expressing myoblasts (lanes 4 and 6) transfected with HA-tagged ubiquitin. Expression of RhoAV14 increased the amount of monoubiquitination of M-cadherin. We were unable to detect any N-cadherin ubiquitination either in control or in RhoAV14-expressing myoblasts, showing this is event is specific for M-cadherin (our unpublished data).
RhoA Affects the Association of M-Cadherin with p120 Catenin
To characterize the mechanism by which RhoA is acting, we analyzed the expression level of p120 catenin and its association with M-cadherin. Recent reports suggest that p120 plays a gatekeeper function in determining the fate of cadherin by regulating its delivery to and its stability at the cell surface (Peifer and Yap, 2003). We first observed that p120 levels are maintained throughout myogenesis and are not affected by RhoA expression (Figure 8A). In contrast, the expression of RhoAV14 strongly decreases the association of p120 catenin with M-cadherin (Figure 8B). p120 catenin association with M-cadherin is higher around myoblast fusion, suggesting the formation or the reinforcement of a complex containing these proteins at this time. p120 catenin association with N-cadherin is observed throughout myogenesis and RhoAV14 expression has only a little effect on the association of p120 catenin with N-cadherin (Figure 8B). To visualize this, control and RhoAV14-expressing myoblasts were stained with antibodies directed against p120 catenin and either M-cadherin or N-cadherin. As shown in Figure 8C, the expression of RhoAV14 perturbed M-cadherin accumulation at intercellular contacts (compare Figure 8C, a and c). By contrast, N-cadherin and p120 catenin still accumulated at intercellular contacts (compare Figure 8C, b and d, e and g, and f and h). To elucidate the molecular mechanisms by which RhoA might regulate p120 catenin association with M-cadherin, we studied a RhoA effector, the serine/threonine kinase ROCK/ROK. We tested the effect of Y-27632, a pharmacological ROCK inhibitor, on p120 catenin association with M-cadherin. As shown in Figure 9A, Y-27635 restores partially p120 catenin association with M-cadherin in RhoAV14-expressing cells. Moreover, Y-27632 addition to RhoAV14-expressing cells restored the localization of M-cadherin at the cell-cell contact sites (Figure 9B).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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). Nevertheless, this study used L6 myoblasts that lack N-cadherin, which raises the possibility that, when coexpressed in cells, both molecules might contribute to the control of myoblast fusion. Here, we used mouse C2C12 myoblasts, which express M- and N-cadherin, and showed that M-cadherin inhibition by RNA interference specifically abrogated myoblast fusion. Thus, N-cadherin cannot compensate for M-cadherin in myoblastic cell lines, unlike the situation in vivo (Hollnagel et al., 2002).We have previously shown that N-cadherin function is required for induction of myogenesis (Charrasse et al., 2002); here, we find that M-cadherin controls myoblast fusion without affecting induction of myogenesis. This validates C2C12 myoblasts as a model to study the respective role of N- and M-cadherin in these two important steps of myogenesis. Why the compensatory mechanism observed in vivo does not function in these cell lines remains unclear. Another important question concerns the exact function of M-cadherin during fusion of myoblasts into myotubes. Myoblast fusion results from an ordered sequence of events, including cell migration, alignment, recognition, adhesion, and membrane merging. Anything that hinders one of these steps would likely decrease the fusion rate.
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), and this was recently confirmed in C2C12 myoblasts by others (Nishiyama et al., 2004). This idea is also supported by the decrease of Rho activity we observed at the onset of myoblast fusion. Thus, RhoA GTPase activity seems to be tightly controlled throughout myogenesis to allow its completion. At the very beginning of myogenesis, RhoA is activated through an N-cadherin-mediated pathway (Charrasse et al., 2002), before being down-regulated at the onset of myoblast fusion. The mechanisms responsible for the decrease of RhoA activity at the onset of myoblast fusion remain to be determined. This decrease of RhoA activity is required for the correct function of M-cadherin. Because its inhibition blocks the fusion process, the decrease in M-cadherin might fully explain the lack of fusion we observed in RhoAV14-expressing cells. Attempts to rescue the fusion process by coexpressing exogenous M-cadherin with RhoAV14 have not been successful, because both endogenous and overexpressed M-cadherin were degraded (our unpublished data). Nevertheless, other molecules participating in muscle cell fusion might be affected. This is not the case for 1-integrin and ADAM-12; however, the nuclear accumulation of the transcription factor Forked in human rhabdomyosarcoma is impaired in RhoAV14-expressing cells (Nishiyama et al., 2004).
Furthermore, our data show that sustained RhoA activity leads to the ubiquitination of M-cadherin, resulting in its entry into endocytic vesicles and its degradation by the lysosomal pathway. A similar process has been described for E-cadherin in response to the activation of the tyrosine kinase src (Fujita et al., 2002; Palacios et al., 2005). This was proposed to be an important posttranscriptional mechanism that could be operational during tumor progression and metastases as well as during some processes of normal development. M-cadherin ubiquitination and endocytosis in RhoAV14-expressing cells might result from its failure to associate with p120 catenin (Figure 10). Indeed, endocytosis has emerged as a regulatory mechanism that modulates cadherin cell surface levels in cells (Le et al., 1999; Akhtar and Hotchin, 2001; Palacios et al., 2002), and recent reports have shown that p120 catenin controls VE-cadherin internalization and degradation (Xiao et al., 2003). Thus, p120 is a regulator of cell-cell adhesion through the maintenance of cadherin levels in cells (Ireton et al., 2002; Davis et al., 2003; Peifer and Yap, 2003). Nevertheless, RhoAV14 expression specifically dissociates p120 from M-cadherin without similar effect on N-cadherin. This suggests the existence of specific interplay between the modified form of p120 or different p120 isoforms with N- and M-cadherin. The function of p120 catenin in regulating adhesion is probably controlled by phosphorylation both on tyrosine and serine/threonine residues (Mariner et al., 2001; Xia et al., 2003). Interestingly, we have found that the inhibition of the RhoA effector ROCK partially restores p120 catenin association with M-cadherin and the localization of M-cadherin at cell-cell contact sites. In our hands, ROCK inhibition does not relieve the block of myoblast fusion in C2C12 myoblasts expressing RhoAV14, in contrast to recent data (Nishiyama et al., 2004). Further experiments are necessary to both determine which other pathway(s) downstream of RhoA participate in the process of myoblast fusion and also to identify the targets of ROCK; however, it is fair to speculate that p120 catenin and/or M-cadherin are good candidates.
The identification of a pathway by which RhoA inhibits M-cadherin function will prove useful in understanding the molecular mechanisms involved in muscle diseases where fusion is affected or during the repair process that involves adult satellite-cell fusion.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Footnotes |
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The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org). 
Address correspondence to: Cécile Gauthier-Rouvière (cecile.gauthier{at}crbm.cnrs.fr).
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REFERENCES |
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1000 nuclei were counted.
