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Vol. 17, Issue 3, 1194-1203, March 2006
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* Membrane Traffic in Neuronal and Epithelial Morphogenesis, Institut National de la Santé et de la Recherche Médicale Avenir Team, 75005 Paris, France;
Institut Jacque Monod, Unité Mixte de Recherche 7592 Centre National de la Recherche Scientifique, Universities Paris 6 and 7, 75005 Paris, France;
Institut National de la Santé et de la Recherche Médicale U536, Institut du Fer-à-Moulin, 75005 Paris, France; and
|| "Rho GTPases, Adhesion and Skeletal Muscle," Centre de Recherches de Biochimie Macromoléculaire-Centre National de la Recherche Scientifique Formation de Recherche en Evolution 2593, 34293 Montpellier, France
Submitted July 18, 2005;
Revised December 5, 2005;
Accepted December 20, 2005
Monitoring Editor: Jennifer Lippincott-Schwartz
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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; da Silva and Dotti, 2002), no link has yet been found between the cytoskeleton and the membrane trafficking machinery that delivers membrane vesicles for growth.
Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) constitute the core machinery for membrane fusion and are involved in all of the fusion events along the biosynthetic and endocytic pathways (for review, see Rothman, 2002). The membrane fusion mechanism involved in neurite outgrowth differs from the "classical" synaptic core machinery inasmuch as it is unaffected by treatment with tetanus neurotoxin (TeNT), which cleaves the classical synaptic vesicle SNARE synaptobrevin 2 (Syb2) and so blocks neurotransmitter release (Osen-Sand et al., 1996; Grosse et al., 1999). Accordingly, the brains of Syb2 null mice develop normally up to birth (Schoch et al., 2001). In contrast, a specific membrane fusion machinery may be involved in neuronal development because a mutant of the exocyst component Sec5 shows impaired neurite outgrowth but normal release of neurotransmitter in Drosophila melanogaster (Murthy et al., 2003). We have shown that a TeNT-insensitive vesicle-associated membrane protein (TI-VAMP; also called VAMP-7), which is also a member of a subfamily of vesicular SNAREs (Filippini et al., 2001; Rossi et al., 2004), plays a major role in neurite outgrowth (Martinez-Arca et al., 2000, 2001; Alberts et al., 2003). However, the precise mechanism by which TI-VAMP-mediated fusion leads to growth and how it is regulated are not known.
In this article, we set out to test the possible coordination of TI-VAMP-mediated trafficking with actin in neuronal growth cones as a way to integrate membrane traffic and cytoskeleton dynamics in neurite elongation and polarization.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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). The polyclonal anti-green fluorescent protein (GFP) antibody we used was described previously (Martinez-Arca et al., 2001). Mouse mAb against Syb2 (clone 69.1) was a generous gift from R. Jahn (Max Planck Institute, Goettingen, Germany). Mouse monoclonal antibodies against EEA1 and CD63 were from BD Biosiences (Franklin Lakes, NJ) and from Ancell (Bayport, MN), respectively. Polyclonal anti-red fluorescent protein (RFP) antibody (Ab3216) was from Chemicon International (Temecula, CA). Affinity-purified Cy3- and Cy5-coupled goat anti-rabbit and anti-mouse immunoglobulins were from Jackson ImmunoResearch Laboratories (West Grove, PA). Alexa 488-coupled goat anti-rabbit and anti-mouse immunoglobulins and phalloidin coupled to Alexa 564 or 488 were from Molecular Probes (Carlsbad, CA). Fluorescent cholera toxin was a generous gift from L. Johannes (Institut Curie, Paris, France).
Plasmids
TI-VAMP-ecliptic pHLuorin (TIVAMP-epHL) was described previously (Martinez-Arca et al., 2000). The mutations (F64L and S65T) required to obtain the super ecliptic variant (Sankaranarayanan et al., 2000) were obtained by site-directed mutagenesis in TIVAMP-epHL. Chimeras between red fluorescent protein (mRFP) and Rho GTPases were obtained by inserting an Nhe1/BspE1 fragment from the mRFP1 (a kind gift from R. Tsien, University of California, San Diego, La Jolla, CA) into pEGFP-RhoAL63, pEGFP-Rac1V12 or pEGFP-Cdc42V12 (Gauthier-Rouviere et al., 1998).
Cell Culture and Transfection
Cos7 cells were maintained in DMEM supplemented with 10% fetal calf serum. Transfection of Cos7 cells was performed using LipofectAMINE, 2000 according to the manufacturer's instructions. Hippocampal and cortical neurons were prepared from E18 rat brain and cultured as described previously (Chang and De Camilli, 2001). Transfection of cortical neurons was performed using the Amaxa Nucleofector (Amaxa, Köln, Germany) according to the manufacturer's recommendations for transfection of primary neurons.
Qualitative and Quantitative Immunocytochemistry
Cells in culture were fixed with 4% paraformaldehyde/4% sucrose and processed for immunofluorescence microscopy as described previously (Coco et al., 1999). Confocal laser-scanning microscopy was performed using a SP2 confocal microscope (Leica Microsystems, Mannheim, Germany). Images were assembled using Adobe Photoshop (Adobe Systems, San Jose, CA).
For comparing TI-VAMP- or Syb2-expression with F-actin content in growth cones, cells were double-labeled with monoclonal antibodies to TI-VAMP or Syb2 and Alexa564-coupled phalloidin. Images were acquired by confocal microscopy based on the actin staining and therefore blind for TI-VAMP- or Syb2-associated immunoreactivity. The fluorescence intensity of single confocal planes within growth cones was analyzed using the region measurement function of the MetaMorph software (Universal Imaging, Evry France) and a regression analysis was performed to statistically evaluate differences. For evaluation of TIV-, Syb2-, and actin fluorescence intensity in growth cones expressing RFP or Cdc42-RFP, images were acquired from triple-labeled cells (phalloidin A488, RFP, and anti-mouse Cy5) based on RFP signal, i.e., blind for the associated TI-VAMP or actin signal, and analysis was performed as described above. Differences were evaluated statistically with the Mann–Whitney nonparametric test.
Bead–Cell Adhesion Assay
Purification of L1Fc, production of L1-coated beads, and incubation of hippocampal neurons with L1-coated beads was performed as described in Alberts et al. (2003).
Imaging and Quantification of the Exocytosis of TIV-pHL
TIV-pHL-expressing cells were placed in DMEM without riboflavine in a Ludin Chamber type 1 (LIS, Reinach, Switzerland), and the microscope temperature was controlled using a Cube & Box control system (LIS, Reinach, Switzerland). Movies were acquired on a Leica DM IRBE inverted microscope, under mercury lamp illumination with filters (GFP, excitation 475 ± 20 nm; RFP, excitation 525 ± 45 nm) piloted by a Lambda 10-2 filter wheel (AutoMate Scientific, San Francisco, CA), and acquired with a Cascade amplified camera (Photometrics/Roper Scientific, Evry, France). Images were acquired every 2 s with exposure times between 400 and 600 ms using MetaMorph and treated using the haze removal filtering. Quantification of the number of events was obtained by manual inspection of the movies frame by frame. Fifteen TIV-pHL-expressing cells were analyzed for each experimental condition.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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). This localization of TI-VAMP to the F-actin-rich region of growth cones was specific because, in agreement with our previous findings (Coco et al., 1999), Syb2 was largely excluded from the growth cone periphery (Figure 1B). The immunofluorescent staining of TI-VAMP in actin-rich growth cones outlined the periphery of the growth cone, suggesting that some of the protein was associated with the plasma membrane. To confirm this conclusion, we labeled the plasma membrane of living neurons with a fluorescently labeled B subunit of cholera toxin (CtxB), which binds to GM1 (Guirland et al., 2004), and costained for TI-VAMP after fixation and permeabilization. As shown in Figure 1C, TI-VAMP (green) and CtxB (red) colocalized in the periphery of the growth cone (indicated by arrows) in regions that are yellow in the merge, confirming the plasma membrane location of TI-VAMP and suggesting that TI-VAMP-mediated vesicle fusion with the plasma membrane might occur at the leading edge of growth cones.
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; Song and Poo, 2001), we hypothesized that variations in TI-VAMP staining intensity might be related to the assembly state of the actin cytoskeleton. As shown in Figure 2A, intense labeling of F-actin in growth cones coincided with intense labeling of TI-VAMP (top, marked with +), whereas growth cones that stained little for F-actin also had little labeling for TI-VAMP (top, marked with –). By contrast, the F-actin content did not correlate with Syb2 labeling intensity in growth cones (Figure 2A, bottom). To validate these observations, we quantified the F-actin labeling intensity and compared it with the TI-VAMP or Syb2 labeling intensity in a number of growth cones (Figure 2B). Whereas the F-actin intensity correlated closely with TI-VAMP labeling (R2 = 0.818; Figure 2B), we observed no correlation between F-actin and Syb2 labeling intensity in growth cones (R2 = 0.123; Figure 2B). Thus, TI-VAMP expression in growth cones is positively correlated with the regulated assembly of F-actin.
We went on to test whether F-actin formation might play a role in the accumulation of TI-VAMP in growth cones by treating neurons with low concentrations of F-actin-disrupting drugs. As shown in Figure 2C, treatment of hippocampal neurons with cytochalasin B for short times led to a redistribution of F-actin into distinct, highly fluorescent foci (Figure 2C, middle right) and to a change in growth cone morphology. Interestingly, TI-VAMP also redistributed to these F-actin-rich foci (Figure 2C, middle left, arrows). Treatment with the cytochalasin B solvent dimethyl sulfoxide (DMSO) alone had no effect on F-actin distribution, growth cone morphology, or on the appearance of TI-VAMP in actin-rich structures (Figure 2C, top). Treatment of neurons with latrunculin A disrupted the actin cytoskeleton more dramatically. Under these conditions, TI-VAMP staining became diffuse, and no accumulation of the protein was observed (Figure 2C, bottom). In contrast, Syb2 did not redistribute to F-actin-rich foci upon treatment with cytochalasin B (Figure 2D, middle) nor did Syb2 staining become diffuse after incubation with latrunculin A (Figure 2D, bottom) compared with its distribution in the control condition (DMSO treatment; Figure 2D, top). Therefore, TI-VAMP accumulation in growth cones depends upon the integrity of the actin cytoskeleton, whereas the localization of Syb2 in growth cones is not obviously affected by actin-sequestering drugs.
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). Furthermore, extensive remodeling of the actin cytoskeleton has been described when growth cones establish contact with a target cell (Lin and Forscher, 1993). Therefore, we made use of the previously described bead assay to mimic acutely forming L1-dependent contacts in hippocampal neurons (Alberts et al., 2003). The behavior of the TI-VAMP compartment was analyzed with reference to the actin cytoskeleton after incubation of neurons with L1-coated beads. Figure 3A, top, shows that localized ligation of L1 on growth cones leads to a strong accumulation of both TI-VAMP and F-actin to sites of bead–cell contact. We validated these observations by quantifying the fluorescence intensity of TI-VAMP, and, as a control, synaptobrevin 2 associated with actin-rich structures induced by L1-coated beads and compared with actin-rich structures in growth cones not incubated with beads. As shown in Figure 3B, acutely forming L1-dependent contacts induce a local accumulation of both F-actin and TI-VAMP at contact sites compared with control growth cones. In contrast, synaptobrevin 2 seems to be excluded from actin-rich structures forming around L1 beads as the intensity of synaptobrevin 2 staining decreases. Importantly, both the formation of F-actin-rich structures and the accumulation of TI-VAMP by L1-beads were abolished in neurons treated with cytochalasin B (Figure 3A) or latrunculin A (our unpublished observations). Thus, L1-dependent contact formation in growth cones leads to the formation of F-actin-rich structures, which are necessary for the recruitment of the TI-VAMP compartment to sites of bead–growth cone contact.
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; Li et al., 2000; Dickson, 2002; Schwamborn and Puschel, 2004; Van Aelst and Cline, 2004). Furthermore, Rac and Cdc42 were already shown to stimulate exocytosis in mast cells (HongGeller and Cerione, 2000) and Cdc42 regulates exocytosis in adrenal chromaffin cells (Gasman et al., 2004). To investigate whether these GTPases contribute to actin-dependent recruitment of TI-VAMP into growth cones, we expressed dominant-positive (i.e., constitutively active) mutants of RhoA, Rac1, and Cdc42 in cortical neurons. In agreement with previous findings in chick neurons (Brown et al., 2000), we often observed enlarged growth cones and axonal shafts when we expressed dominant-positive Cdc42 mutants in cortical neurons (note the difference in magnification in Figure 4, A and B). A constitutively active Cdc42 mutant, Cdc42V12, expressed as a fusion protein with RFP, localized to the plasma membrane where it induced formation of F-actin-rich filopodia all along the axonal shaft (Figure 4B). In these neurons, TI-VAMP localized to the F-actin-rich filopodia outside the growth cone, accompanied by a large decrease in the intensity of staining for F-actin and TI-VAMP at the leading edge of growing axons (Figure 4B). In control cells, expressing RFP alone (Figure 4A), actin and TI-VAMP remained heavily enriched in the growth cones, as in untransfected neurons (Figures 1 and 2). Despite morphological effects on the growth cones, neither the constitutively active form of Rac (RacV12) or RhoA (RhoL63), expressed as RFP fusion proteins, induced F-actin-rich filopodia outside the growth cone (Figure 4, C and D), and both TI-VAMP and F-actin accumulated normally in the growth cones of cortical neurons expressing either of these mutant proteins.
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50% compared with RFP alone, whereas dominant-negative Cdc42N17-RFP interfered with the accumulation of these proteins to a lesser, although significant extent. The intensity of staining for Syb2 in actin-rich domains of growth cones was low under all conditions tested, and no statistically significant differences were detected compared with cells expressing RFP alone. Therefore, expression of dominant positive or dominant negative Cdc42 in developing neurons strongly affects TI-VAMP localization and induces a loss of polarized expression of both TI-VAMP and actin in growth cones. In contrast, localization of Syb2 positive organelles seems to be excluded from actin-rich structures in the developing neuron under all conditions tested.
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We next established an assay to directly quantify TI-VAMP-dependent exocytosis and evaluate the involvement of the actin cytoskeleton and Rho GTPases in this process. To this end, we generated a pH-sensitive GFP-tagged (Miesenbock et al., 1998; Sankaranarayanan et al., 2000) form of TI-VAMP (TIV-pHL). First, we localized TIV-pHL in Cos7 fibroblastic cells by confocal microscopy and found that its subcellular distribution was identical to the endogenous protein with a high degree of colocalization with CD63 as already published (Martinez-Arca et al., 2003a, b) and a low degree of colocalization with early endosomal antigen 1 (EEA1), an early endosomal marker, as already shown (Coco et al., 1999) (Figure 7A). We were able to show that, as in the case of synapto-pHL, the signal emitted by plasma membrane resident TIV-pHL as measured by epifluorescence drops reversibly when living cells are incubated at pH 5 and that neutralization of intraorganelle pH with NH4Cl results in a strong increase of signal because of the signal from intravesicular TIV-pHL (Figure 7B) (Sankaranarayanan et al., 2000; Sankaranarayanan and Ryan, 2000). These results indicate that TIV-pHL fluorescence is quenched intracellularly because of the acidic pH of endosomal organelles and could therefore be used to selectively visualize surface expression of TIV-pHL. When expressed in Cos7 cells, we observed transient puffs of light by time-lapse live cell microscopy that seem similar to secretory vesicle fusion events visualized using synaptopHL (Figure 7C and Movie 1) (Miesenbock et al., 1998; Sankaranarayanan and Ryan, 2000). Additionally, "worm-like" events could be observed that are likely to represent the fusion of tubules with the plasma membrane (Figure 7D and Movie 1). As indicated by the arrows in Figure 7D, tubular carriers of considerable size (distance between the two arrows in micrograph 7B "4:06" corresponds to 
18 µM) could be observed.
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Because dominant-positive Cdc42V12-RFP stimulated the formation of filopodia containing F-actin and TI-VAMP in neurons, we tested the effect of expressing the same construct on the rate of exocytosis of TI-VAMP and compared it with the effect of expressing constitutively active RhoA and Rac1. Unfortunately, expression levels of TIV-pHL were insufficient for detection in neurons and therefore analysis was performed in Cos7 cells. Immunofluorescence analysis showed no obvious differences in localization or expression levels of TIVpHL when RFP or different GTPases were coexpressed (our unpublished data). Cdc42V12-RFP had a very strong stimulatory effect on the exocytosis of TIV-pHL compared with cells expressing RFP alone (see Movie 2 and the corresponding micrographs in Figure 8A). As shown in Table 2, Cdc42V12-RFP increased by >10 the rate of TIV-pHL exocytosis, measured as the number of exocytic events per minute (one representative cell for each condition is presented in Figure 8A). RacV12-RFP had a moderate stimulatory effect (Table 2), whereas RhoL63-RFP was weakly inhibitory (our unpublished data). Importantly, the stimulation by Cdc42V12-RFP depended upon the presence of F-actin because treatment with cytochalasin B strongly inhibited Cdc42V12-RFP-stimulated exocytosis (see Movie 3 and corresponding micrographs in Figure 8B). Thus, enhanced Cdc42 activity stimulates the exocytosis of TI-VAMP in an actin-dependent manner. Taking these data together with those presented in Figure 5, we suggest that the activity of Cdc42, and to a lesser extent, of Rac1 control a pathway linking actin remodeling to the fusion of TI-VAMP-containing vesicles with the plasma membrane.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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We found that TI-VAMP but not Syb2 concentrates in neuronal growth cones in an actin-dependent manner, suggesting differential regulation of the compartmentalization of these two vesicular populations in immature neurons. This view is supported by our finding that acute local stimulation of the cell adhesion molecule L1 induces a site-directed accumulation of actin and TI-VAMP, but not synaptobrevin 2 (Figure 3; Alberts et al., 2003). It was recently shown that the movements of GFP-Syb2 in growth cones are affected by nocodazole but not cytochalasin B (Sabo and McAllister, 2003), thus pointing to the fact that the dynamics of Syb2 depend more on microtubules than on actin. We also tested the effect of nocodazole and taxol and did not find any significant difference in the polarized concentration of TI-VAMP in growth cones (our unpublished observations). The present article and the study by Sabo et al., (2003) therefore suggest that actin and tubulin play different functions in the compartmentalization of TI-VAMP and Syb2 in the growth cone. A likely explanation of our findings would be that F-actin targets and/or restrains the TI-VAMP, but not the Syb2, vesicles into the growth cone. Syb2 containing organelles may only be transported to the leading edge when a synapse is being generated or stabilized, as suggested by the redistribution of Syb2 to the leading edge of growth cones activated by presynaptic AMPA receptors (Schenk et al., 2003).
We do not know whether the TI-VAMP vesicles are mobile within the actin-rich growth cones or whether they are sequestered, thus constituting a reserve pool similar to synaptic vesicles in the mature synapse, which are tethered to an actin scaffold by synapsins and are mobilized by a phosphorylation-dependent mechanism (Sudhof, 2004). Our data indicate that TI-VAMP vesicles are directly linked to actin and rapidly mobilized to sites of growth upon signal-induced actin remodeling. TI-VAMP redistributes to actin foci after treatment with cytochalasin B or expression of Cdc42 V12 and is recruited to actin-rich structures forming around L1-coated beads. Moreover, actin might play a direct role in the motility and exocytosis of TI-VAMP-containing vesicles toward the plasma membrane because we find TI-VAMP associated with the plasma membrane in actin-rich growth cones (Figure 1) and detect an actin-dependent exocytosis in Cos 7 cells expressing Cdc42 V12 (Figure 8). A role for actin in the movement of endosomes during stimulus-induced transit to the membrane has been shown recently in fibroblasts (Sandilands et al., 2004). Our current working model would predict that actin dynamics control the localization and possibly exocytosis of the TI-VAMP compartment at the leading edge of growth cones and that this process provides an amplification of directed growth by polarized exocytosis.
The variability in actin and TI-VAMP expression in different growth cones suggests that signaling mechanisms exist that lead to the correlated accumulation of both markers. Indeed, Cdc42 activity might represent such a signaling mechanism, because both dominant-positive as well as dominant-negative Cdc42, but not Rho or Rac, interfere with the polarized expression of both actin and TI-VAMP. In comparison with Cdc42V12-RFP, the less dramatic effect of Cdc42N17-RFP, particularly on actin expression in growth cones, may indicate that Cdc42-independent signaling pathways are involved in the control of TI-VAMP and/or actin accumulation in growth cones. Alternatively, expression levels of the constructs might not have been sufficient to completely abrogate Cdc42-dependent signaling.
At this point, it is unclear whether growth cones rich in actin and TI-VAMP represent fast-growing axons. Indeed, we used concentrations of actin-depolymerizing drugs that were previously shown to have no effect on axonal outgrowth in hippocampal neurons (Rajnicek and McCaig, 1997; Bradke and Dotti, 1999; Dent and Kalil, 2001) but that were shown to abrogate attraction or repulsion of growth cones by guidance cues (Zheng et al., 1996; Rajnicek and McCaig, 1997). Thus, the accumulation of the TI-VAMP-positive membrane compartment might not be necessary for efficient neurite outgrowth. Rather, it can be speculated that the actin-dependent accumulation of the TI-VAMP compartment in the growth cone periphery may represent a mechanism to integrate actin- and membrane-dynamics to support directed growth. In this light, our finding that Cdc42 and the cell adhesion molecule L1, both of which play important roles in axonal pathfinding (Cohen et al., 1998; Yuan et al., 2003) can coordinate the localization of both actin and TI-VAMP in neuronal growth cones is particularly intriguing.
We did not find any Cdc42 effectors in the several yeast two-hybrid screens that we have carried out in fly embryo, human placenta (Martinez-Arca et al., 2003b; Formstecher et al., 2005), and human fetal brain (our unpublished observations) libraries using the Longin domain or the full cytoplasmic domain of TI-VAMP as baits. Furthermore, the recombinant cytosolic domain of TI-VAMP did not affect directly actin dynamics in vitro (Carlier and Thierry, unpublished observations). Therefore, the details of the molecular mechanism linking Cdc42 activation, TI-VAMP, and actin are likely to involve a complex network that will require further investigations.
In conclusion, our results are consistent with a model in which the restricted activity of Cdc42 within the growth cone mediates the polarized accumulation and exocytosis of TI-VAMP in growth cones through its regulatory action on the actin cytoskeleton. The importance of spatially restricted Cdc42 activation for directed cellular movement was demonstrated before as both dominant-positive and dominant-negative Cdc42 mutants inhibit cellular polarization and directed migration of macrophages and astrocytes (Allen et al., 1998; Etienne-Manneville and Hall, 2001). We do not know the molecular cascade leading to the concentration of TI-VAMP in the growth cone and the stimulation of its exocytosis by Cdc42 in a physiological context. Yet local activation of the cell adhesion molecule L1 in neuronal growth cones induces an actin-dependent recruitment of TI-VAMP-, but not Syb2-containing vesicles, to sites of contact (Figure 3; Alberts et al., 2003). Thus, it seems likely that signaling events induced by adhesion molecules like L1 activate Cdc42 leading to the actin dependent accumulation and exocytosis of TI-VAMP. The mechanism proposed here is similar to that described for phagocytosis in macrophages, another cellular process that also requires localized TI-VAMP trafficking (Braun et al., 2004) upon receptor and Cdc42 activation (Hoppe and Swanson, 2004) and actin remodeling (May and Machesky, 2001). Thus, it is tempting to speculate that TI-VAMP-mediated exocytosis and actin dynamics are tightly coupled to direct the formation of cell protrusions not only in neurons and macrophages but also in other cell types, as a more general mechanism for the formation of directed cellular extensions.
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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). 
Present address: Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06520.
Address correspondence to: Thierry Galli (thierry{at}tgalli.net).
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