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Vol. 18, Issue 7, 2579-2591, July 2007

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Ena/VASP Proteins Have an Anti-Capping Independent Function in Filopodia Formation

Derek A. Applewhite^*, Melanie Barzik, Shin-ichiro Kojima^*, Tatyana M. Svitkina, Frank B. Gertler, and Gary G. Borisy^*,

*Department of Cell and Molecular Biology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611; Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Biology, University of Pennsylvania, Philadelphia, PA 19104; and Marine Biological Laboratory, Woods Hole, MA 02453

Submitted November 6, 2006; Revised March 28, 2007; Accepted April 24, 2007
Monitoring Editor: David Drubin

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Filopodia have been implicated in a number of diverse cellular processes including growth-cone path finding, wound healing, and metastasis. The Ena/VASP family of proteins has emerged as key to filopodia formation but the exact mechanism for how they function has yet to be fully elucidated. Using cell spreading as a model system in combination with small interfering RNA depletion of Capping Protein, we determined that Ena/VASP proteins have a role beyond anticapping activity in filopodia formation. Analysis of mutant Ena/VASP proteins demonstrated that the entire EVH2 domain was the minimal domain required for filopodia formation. Fluorescent recovery after photobleaching data indicate that Ena/VASP proteins rapidly exchange at the leading edge of lamellipodia, whereas virtually no exchange occurred at filopodial tips. Mutation of the G-actin–binding motif (GAB) partially compromised stabilization of Ena/VASP at filopodia tips. These observations led us to propose a model where the EVH2 domain of Ena/VASP induces and maintains clustering of the barbed ends of actin filaments, which putatively corresponds to a transition from lamellipodial to filopodial localization. Furthermore, the EVH1 domain, together with the GAB motif in the EVH2 domain, helps to maintain Ena/VASP at the growing barbed ends.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Actin-based protrusions known as filopodia, first described by Porter, Claude, and Fullam as early as 1945 (Porter et al., 1945; Albrecht-Buehler, 1976), are composed of tightly bundled parallel filaments 5–50 µm long and 0.1–0.5 µm thick (Small, 1988; Mitchison and Cramer, 1996). Filopodia have been implicated in a number of cellular processes including neuronal growth cone pathfinding, embryonic development, wound healing, and metastasis (Jacinto et al., 2000, 2001; Vasioukhin and Fuchs, 2001; Dent and Gertler, 2003; Hashimoto et al., 2005). Despite the importance of filopodia to so many diverse cellular functions, the exact mechanism governing their initiation and formation has yet to be fully explained. A number of actin-binding proteins have been implicated in the formation of filopodia, most recently myosin X and the formin family of proteins, which catalyze the formation of long unbranched actin filaments (Evangelista et al., 2003; Zigmond, 2004; Bohil et al., 2006; Kovar, 2006). However, in several cell types, filopodia and the sheet-like Arp2/3-based lamellipodia, rapidly interchange during protrusion, suggesting commonalities between the two structures (Svitkina et al., 2003). The type of protrusion that dominates in the subcellular compartment critically depends on whether filaments are allowed to elongate persistently or are capped shortly after nucleation. Accordingly, when the heterodimeric Capping Protein (CP), the major barbed end terminator in lamellipodia, is silenced by small interfering RNA (siRNA), the balance between the two structures, is shifted to favor filopodial formation (Mejillano et al., 2004). A similar phenomenon has been observed in vitro where a decrease in CP levels led to a transition from a dendritic-like network reminiscent of lamellipodia to a bundled filopodial-like actin organization (Vignjevic et al., 2003; Haviv et al., 2006).

Although it appears that CP levels determine which actin-based protrusive organelle dominates at the cell periphery, it does not function in isolation. The Ena/VASP family of proteins, which includes three mammalian homologues, Mena (Mammalian Enabled), VASP (Vasodilator-Stimulated Phosphoprotein), and EVL (Ena-VASP-Like), also influence the underlying actin architecture (Waldmann et al., 1987; Reinhard et al., 1992; Gertler et al., 1995, 1996; Bear et al., 2002). By cellular localization alone, Ena/VASP proteins appear to be vital to this lamellipodial-filopodial transition because they localize to both the leading edge of protruding lamellipodia as well as filopodial tips (Reinhard et al., 1992; Gertler et al., 1996; Lanier et al., 1999; Rottner et al., 1999). Recent work in neuronal cells demonstrates the crucial role Ena/VASP proteins have in filopodia formation. Targeting of Ena/VASP proteins to the inner leaflet of the cell membrane led to an increase in the number and length of filopodia in primary hippocampal neurons. Conversely, sequestration of Ena/VASP proteins to mitochondria led to decreased filopodia formation and inhibition of filopodia induction upon treatment with the neuronal guidance factors Netrin-1 or forskolin (Lebrand et al., 2004). Similarly, in Dictyostelium, genetic ablation of dVASP, the sole Ena/VASP member present in the organism, decreased filopodia formation and caused chemotactic defects (Han et al., 2002). Finally, experimental evidence demonstrates that the lamellipodial-to-filopodial transition that occurs upon depletion of CP requires Ena/VASP proteins. Depletion of CP in cells lacking VASP and Mena and containing only trace amounts of EVL caused the cells to lose their ability to robustly form filopodia. The reintroduction of a single Ena/VASP member at physiological levels was needed to recapitulate the hyper-filopodial phenotype that occurs upon CP depletion (Mejillano et al., 2004).

Whereas experimental evidence supports a positive role for Ena/VASP proteins in filopodia formation, a clear mechanism of how they are involved in the process has yet to be elucidated. Biochemically, Ena/VASP proteins have been shown to antagonize filament termination, thereby increasing the length of actin filaments; however, the anti-capping function of Ena/VASP is still a point of controversy in the field and some investigators have reported otherwise (Bearer et al., 2000, 2002; Samarin et al., 2003; Barzik et al., 2005; Schirenbeck et al., 2006). Ena/VASP can bind to both barbed ends and sides of actin filaments through G-actin binding (GAB) and F-actin binding (FAB) motifs, respectively, that are harbored within the C-terminal EVH2 domain. Additionally, through a coiled-coil (COCO) domain at the extreme C-terminus, Ena/VASP forms tetramers (Huttelmaier et al., 1999; Walders-Harbeck et al., 2002; Zimmermann et al., 2002; Krause et al., 2003; Kuhnel et al., 2004; Barzik et al., 2005). Ena/VASP proteins are targeted to sites of dynamic actin turnover including focal adhesions, lamellipodial and filopodial tips, and ActA of Listeria monocytogenes, via their N-terminal EVH1 domain, which binds the consensus sequence D/E FPPPPXD/E (Chakraborty et al., 1995; Niebuhr et al., 1997; Carl et al., 1999; Fedorov et al., 1999; Laurent et al., 1999). Through a central poly-proline domain, Ena/VASP binds to profilin, and to SH3 and WW domain–containing proteins (Gertler et al., 1996; Ermekova et al., 1997; Ahern-Djamali et al., 1999; Krause et al., 2003). Thus, Ena/VASP proteins possess the requirements needed for filopodia formation through the antagonism of CP and oligomerization of actin filaments and serve as adapter proteins for the assembly of multiprotein complexes for putative interactions with a number of actin-binding proteins.

Given the multidomain organization of the proteins it is likely that Ena/VASP proteins are involved in several steps in the filopodia initiation and formation process. If Ena/VASP proteins functioned only to antagonize barbed end capping as suggested initially, then under the conditions where CP is depleted, the necessity for Ena/VASP proteins in filopodia formation would be eliminated. Continuous elongation of actin filaments is a necessary, but not sufficient condition for filopodia formation. If actin filaments remain uncapped and continue to elongate, it is likely that the force generated by the growing filaments would be insufficient to effectively push against the membrane because of the intrinsic flexibility of long actin filaments (Mogilner and Edelstein-Keshet, 2002; Pollard and Borisy, 2003; Mogilner and Rubinstein, 2005). Therefore, Ena/VASP proteins may also function to aggregate or cross-link actin barbed ends to provide stability and create force. The fact that Ena/VASP proteins have been shown to bundle actin filaments supports this hypothesis, and the C-terminal tetramerization domain of Ena/VASP provides an additional tool for bringing barbed ends into close proximity (Bachmann et al., 1999; Huttelmaier et al., 1999; Laurent et al., 1999; Bearer et al., 2000; Kuhnel et al., 2004; Barzik et al., 2005). This initial aggregation of actin filaments could potentially play an additional role in recruiting actin bundling proteins such as fascin that bundle parallel actin filaments (Svitkina et al., 2003; Vignjevic et al., 2003, 2006). Finally, the simultaneous binding of G-actin and profilin Ena/VASP proteins could sequester actin monomers primed for polymerization to the tips of filopodia where the barbed ends are located (Harbeck et al., 2000; Chereau and Dominguez, 2006).

To test the specific mechanisms of Ena/VASP proteins in filopodia formation and to determine which domain or domains are necessary to this function, we analyzed various mutants of Ena/VASP in a mouse fibroblastic MV^D7 cell line. The MV^D7 line was generated from a VASP/Mena double-knockout mouse expressing only trace amounts of EVL. Therefore reintroduced wild-type or mutant Mena/VASP could be analyzed in the virtual absence of the three mammalian Ena/VASP family members (Bear et al., 2000; Geese et al., 2002; Loureiro et al., 2002). Though lamellipodial protrusion and Listeria movement were well characterized in this system, filopodia formation has not been documented. One of the difficulties in analyzing filopodia formation in the MV^D7 cell lines is that the cells do not robustly make filopodia once fully spread and are less protrusive in comparison to other cell lines. In this work, we overcame this obstacle by observing the cells during the spreading phase. Furthermore, we adopted the method of RNA interference of CP to decrease the barbed end termination of actin filaments within the lamellipodia. This approach allowed us to answer the question of whether Ena/VASP proteins act only to antagonize the capping activity by CP or play additional roles in filopodia formation. Through detailed analysis, we concluded that the EVH2 domain is the minimal domain needed to enhance filopodia formation. Furthermore, any defect to the GAB, FAB, or COCO domains found within the EVH2 domain compromised the ability of filopodia induction even under conditions where CP was depleted. These results indicate a novel role of Ena/VASP in filopodia formation beyond bundling, as suggested for dVASP (Schirenbeck et al., 2005). An additional question is how to reconcile the differences in Ena/VASP function based on dual localization in both lamellipodia and filopodia. Through fluorescent recovery after photobleaching (FRAP) analysis we discovered a dramatic change of molecular dynamics of Ena/VASP associated with a transition from lamellipodial leading edge localization to filopodial tips. Finally, results from FRAP analysis also indicate the importance of the G-actin–binding domain to the stabilization of Ena/VASP at actin barbed ends.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Expression Plasmids
The pDsRed-Super small hairpin RNA (shRNA) vector was constructed by replacing GFP cDNA from pG-Super (Kojima et al., 2004) with DsRed2 cDNA from pDsRed2-N1 (Clontech, Palo Alto, CA) using SacII-NotI sites. The EVH1-only (amino acid residues 1–113) and EVH2-only (residues 225–340) fragments were PCR-amplified from human VASP gene and subcloned into pEGFP-N1 vector (Clontech) using Bam H1-XhoI sites. pmCherry-Actin was constructed from transferring human -actin cDNA from pEGFP-Actin (Clontech) to pmCherry-C1 (Vignjevic et al., 2006). pEGFP-VASP and pmCherry-fascin are described elsewhere (Vignjevic et al., 2006).

Cell Culture and Microscopy
MV^D7 VASP derivative cell lines including VASP EVH2, VASPFAB, VASPCOCO, VASP EVH2-FAB, VASP EVH2-COCO, VASP GAB mutant (R232E,K233E), VASP(S153A), VASP (S235A,S274A), and VASP (S153A, S235A,S273A) were generated as previously described (Loureiro et al., 2002) and MV^D7 cells including VASP and Mena derivative cell lines (Mena EVH2, Mena [S236A,S376A], Mena [S236D,S376D], MenaPRO, MenaFAB, Mena COCO; Loureiro et al., 2002) and were cultured as described (Bear et al., 2000). B16F1 mouse melanoma cells were provided by C. Ballestrem (Weizmenn Institute of Science) and were maintained as previously described (Ballestrem et al., 1998). COS7 green monkey kidney cells were obtained from ATCC (Manassas, VA) and maintained in DMEM supplemented with 10% fetal bovine serum (FBS). Transfection was performed using TransIT-LT1 (Mirrus, Boston, MA) for MV^D7 lines and COS7, or FuGENE6 (Roche, Indianapolis, IN) for B16F1 according to manufacturer's recommendations.

Light microscopy was performed using an inverted microscope (Eclipse TE2000 or Diaphot 300, Nikon, Melville, NY) equipped with a plan 100x 1.3 NA objective and a back-illuminated cooled CCD camera (model CH250 and CH350, Roper Scientific, Tucson, AZ) driven by Metamorph imaging software (Universal Imaging, West Chester, PA). Live cell imaging of MV^D7 cell lines was carried out in L-15 media containing recombinant mouse -interferon (Invitrogen, Carlsbad, CA; 10⁶ units/ml) and 15% FBS. For live cell imaging of B16F1, cells were transferred to L-15 supplemented with 10% FBS. During live cell imaging, the microscope stage was maintained at 37°C for B16F1 or at 32°C for MV^D7 derivative cell lines.

Immunofluorescence
Immunostaining for CP was performed by treating cells for 5 min with 1% Triton X-100 in PEM buffer (100 mM Pipes, pH 6.9, 1 mM MgCl₂, 1 mM EGTA) containing 2% PEG MW 40,000 (Serva, Paramus, NJ) and 2 µM phalloidin, followed by fixation by 0.2% glutaraldehyde in 100 mM coxyladilic acid and quenching with NaBH₄. Anti-CP antibodies were obtained from D. Schafer (University of Virginia). Immunostaining for Arp2/3 complex and lamellipodin was performed by fixing cells with 4% paraformaldehyde, followed by permeating with 1% Triton X-100. The rabbit polyclonal Arp 3 antibody was purchased from Upstate Biotechnology (Lake Placid, NY) and the rabbit polyclonal anti-lamellipodin was previously prepared in Krause et al. (2004). Immunostaining for antiphosphorylated tyrosine (4G10, Upstate Biotechnology) was performed by simultaneously extracting with 0.5% Triton X-100 and fixing with 0.25% glutaraldehyde in PEM buffer, followed by fixation in 0.2% glutaraldehyde in 100 mM coxyladilic acid and quenching with NaBH₄. Immunostaining for fascin was performed by methanol fixation at –20°C. The mouse monoclonal antibody (clone 55K-2) was purchased DakoCytomation (Fort Collins, CO). TRITC-conjugated anti-rabbit IgG and anti-mouse IgG were purchased from Jackson ImmunoResearch (West Grove, PA). F-actin was stained with 0.033 µM of AlexaFlour350-Texas Red-X-, or AlexaFlour647-phalloidin (Molecular Probes, Eugene, OR).

Cell-spreading Assay
The cell-spreading assay was performed by trypsinizing (0.05% trypsin and 0.53 mM EDTA) MV^D7 cells and replating them on laminin (20 µg/ml)-coated coverslips. Cells were allowed to attach and spread for 10–15 min and then were imaged for 20–40 min or until cells were fully spread. Transfected cells (containing the shRNA vector targeted against CP) were identified by DsRed2-soluble marker by epifluorescence, whereas imaging and phenotyping was done by phase-contrast microscopy.

FRAP Analysis
FRAP experiments were performed on a Zeiss LSM 510 confocal microscope (Carl Zeiss, Oberkochen, Germany) with a 110x/1.3 NA planapochromat oil objective and an argon laser (1 mW). B16F1 mouse melanoma cell or MV^D7 fibroblast were maintained at 37 or 32°C, respectively. Enhanced green fluorescent protein (EGFP)-tagged full-length VASP or the EVH1 domain from VASP were transiently expressed B16F1 cells, whereas EGFP-EVH2 (VASP) was analyzed in MV^D7 cells after transient transfection. For analysis of mutant Ena/VASP proteins, MV^D7 derivatives stably expressing EGFP-VASP mutants were used. Cells were bleached by defining a rectangular region (8–15 µm²) perpendicular to the direction of protrusion or at the filopodia tips (2–8 µm²) for 1 s using the 488-nm laser line at 100% laser power (25 mW). Subsequent GFP fluorescent images were acquired at 0.5–3-s intervals with the excitation line of the 4880-nm laser line and phase images by a halogen light source. The Co-FRAP experiments were performed sequentially in the multitrack mode using both a 488-nm line from a 25-mW Argon laser and the 543-nm line from a 5-mW helium-neon laser at 100% power for bleaching of EGFP-VASP and mCherry-actin, respectively. B16F1 cells transfected with pEGFP-VASP and pmCherry-Actin plasmids were used. A 4–10-µm² rectangular region was bleached the distal portion of a protruding filopodium. Subsequent EGFP, mCherry, and phase images were acquired at 0.5–3-s intervals with 488- and 543-nm laser lines and halogen light, respectively.

The obtained images were analyzed by Metamorph software (Universal Imaging). EGFP-tagged protein fluorescent intensities were measured in the bleach zone in each frame. The loss of fluorescent intensity due to photofading was determined as follows: fluorescent intensity in a nonbleached region was measured over time and was fitted to an exponential decay function, exp(–k_fadet), where k_fade is the decay constant and t is time. The intensity of the bleach zone was corrected for photofading by multiplying a correction factor, exp(k_fadet) at each time point. The corrected fluorescent recovery intensities were then curved fitted using the expression I(t) = I_f + (I_PB – I_f) exp(–k_rect), where I(t) = intensity at final time, I_PB = intensity at photobleaching event, and k_rec = fluorescence recovery rate constant. Half-time of recovery time was calculated as ln2/k_rec.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
MV^D7 Cells Have Three Distinct Spreading Modes
To study protrusion we used cell spreading as a model system. Spreading cells are particularly protrusive and have many characteristics general to migrating cells (Dubin-Thaler et al., 2004). MV^D7 mouse fibroblasts, parental or derivative, have three different spreading modes: Smooth-Edged, Filopodial, or Ruffling (Figure 1, A–C). Cells with the Smooth-Edged phenotype are characterized by a "fried-egg"-like appearance in phase microscopy, expanding evenly at the perimeter (Figure 1A). Wide-field fluorescence microscopy reveals an outer perimeter characteristic of protruding lamellipodia with increased fluorescence intensity after phalloidin staining. The Filopodial phenotype (Figure 1B) is characterized by filopodia, dynamic finger-like protrusions, which elongate as the cell spreads a hallmark of genuine filopodia. In addition, these filopodia contain actin along the shaft (Figure 1B). The Ruffling phenotype (Figure 1C) was defined by phase-dense wave-like structures, usually undergoing cycles of protrusion and retraction throughout the time of spreading. The phase-dense regions were enriched in actin as revealed by phalloidin staining. Time-lapsed movies demonstrate that different phenotypes were likely a result of a dynamic, protrusive process rather than retraction (Supplementary Movies 1–3). These three phenotypes represent the intrinsic heterogeneity of the MV^D7 and derivative cell lines during spreading and were used to characterize the protrusive behavior of cells depending on the presence of individual Ena/VASP proteins.

View larger version (99K): [in this window] [in a new window]   Figure 1. Three characteristic spreading phenotypes. Phase-contrast (phase) and phalloidin-stained (actin) images. (A) Smooth-Edged phenotype. Characterized by a flat, evenly spreading leading edge (phase) enriched in actin as visualized by phalloidin staining. Scale bar, 10 µm. (B) Filopodial phenotype. Characterized by long actin-containing protrusions. (C) Ruffling phenotype. Characterized by phase-dense undulations with corresponding actin-rich regions.

View larger version (78K): [in this window] [in a new window]   Figure 2. Lamellipodial and filopodial markers in spreading cells. Low magnification images show actin distribution. Scale bar, 10 µm. High magnification images of boxed regions are presented to the right of the low magnification images. Scale bar, 2 µm. (A) Higher magnification of actin distribution (cyan), Arp 2/3 complex localized by immunostaining for Arp 3 (red), and a merge of the actin and Arp2/3 distributions (yellow) in a cell in the Smooth-Edged spreading phenotype. Arp 2/3 present at the spreading edge of the lamellipodia. (B) Actin distribution at high magnification (cyan), CP localized by immunostaining for the CP subunit (red) and a merged image of the actin and CP distributions (yellow) in a cell in the Smooth-Edged spreading phenotype. CP is present at the spreading edge of the lamellipodia. (C) Actin distribution at high magnification (cyan), fascin localized by expression of mCherry-fascin (red), and a merged image of actin and fascin (yellow) in a cell in the Filopodial spreading phenotype. Fascin colocalized with F-actin bundles present along the length of filopodia. (D) Actin distribution along the length of a filopodium (cyan), VASP localized by expression of EGFP-VASP in MVD7-VASP stable cell line (green), and the combination of actin and VASP distributions in the merged image (yellow) of a cell in the Filopodial phenotype. VASP is accumulated at tips of filopodia.

Contribution of Ena/VASP Protein Domains during Filopodia Formation
The above results ensured the feasibility of the spreading assay for analysis of filopodia formation. We sought to determine how the various domains of Ena/VASP proteins contribute to filopodia formation by using the cell-spreading assay. We analyzed the frequency of the Smooth-Edged, Filopodial, and Ruffling phenotypes occurring in the MV^D7 derivative cell lines by phase-contrast live-cell imaging (Figure 3A). It was during this highly dynamic, protrusive period that the cells were scored and binned into one of the phenotypic categories. It is important to note that the initial observations were made during a finite period of cell spreading and therefore that we cannot exclude the possibility that the cells may have switched spreading modes once this period of data acquisition ended. The tested mutants (Figure 3A) included deletion of the EVH1, Proline-rich (PRO), and/or EVH2 domain(s). In addition, we dissected the EVH2 domain in more detail using mutations of the GAB motif (R232E, R233E) and deletion of the FAB or COCO motif. The GAB motif of mammalian Ena/VASP proteins share close homology with that of the -thymosin family of proteins, and it is thought that the G-actin–binding site is needed to mediate the formation of a complex between Ena/VASP, profilin-actin, and actin filament barbed ends (Harbeck et al., 2000; Walders-Harbeck et al., 2002; Barzik et al., 2005). It is likely that this complex formation would support positioning and maintenance of Ena/VASP proteins at filopodial tips. Point mutations at key positive residues have been shown to ablate the ability of these proteins to bind G-actin (Huttelmaier et al., 1999; Harbeck et al., 2000; Walders-Harbeck et al., 2002). We infected MV^D7 cells with retroviruses for EGFP-tagged VASP or Mena mutants and used FACS to select cells with a certain range of expression level of EGFP-fusion proteins. Therefore differences the in expression level among a series of mutants was virtually negligible.

View larger version (29K): [in this window] [in a new window]   Figure 3. Cell-spreading assay, under normal conditions. In the cell-spreading assay, phenotypes were assessed by phase-contrast microscopy in live cells during the spreading phase (see Materials and Methods for detail). (A) The schematic diagram of the mutated/deleted Ena/VASP proteins represents the cell lines used in the assay. These proteins were stably expressed as EGFP-tagged form in MVD7 fibroblasts using retroviral vectors. The bar graph depicts percentage of cells displaying Smooth-Edged, Filopodial, and Ruffling phenotypes for each cell line. (B) The column bar graph represents the average percentages of the Smooth-Edged, Filopodial, and Ruffling phenotypes for groups containing or lacking wild-type EVH2 given in A. The average percentages of the three phenotypes in the EVH2-containing group were used for 2-test for each line. The asterisks in A indicate statistical significance (p < 0.05).

The EVH2 Domain of Ena/VASP Proteins Is Required to Promote Filopodia Formation in the Absence of Capping Protein
Although biochemical data indicates that Ena/VASP proteins function as an antagonist for filament termination, presumably this is not their only role in filopodia formation. To test the functions of Ena/VASP proteins beyond anticapping activity in filopodia formation, we depleted CP, which is the major barbed end terminator, using a plasmid-based shRNA mammalian expression vector and performed the cell-spreading assay. The hairpin RNA expression plasmid contains a DsRED2 expression cassette that allowed us to identify knockdown by red fluorescence. CP depletion led to a robust increase in the Filopodial spreading phenotype in the wild-type EVH2 group, increasing from an average frequency of 24% in non–CP-depleted cells to over 60% in CP-depleted cells (Figure 4, A and B). A similar robust change occurred in the mutant EVH2 group, however, instead of an increase in the Filopodial phenotype, CP depletion increased the frequency of Ruffling cells from 21 to over 60%. Both groups underwent a concomitant decrease in the Smooth-Edged phenotype after CP depletion (Figure 4, A and B).

View larger version (29K): [in this window] [in a new window]   Figure 4. Cell-spreading assay, CP-depleted. MVD7 and derivative cell lines were transfected with CP-depleting plasmid (pDsRed-SUPER-CP-T1), which expressed DsRed2 marker and shRNA against CP simultaneously. Four days after transfection, the phenotypes were assessed as in Figure 3. Transfectants were identified by DsRed-soluble marker. (A) The schematic diagram of the mutated/deleted Ena/VASP protein and the bar graph of the percentages of the Smooth-Edged, Filopodial, and Ruffling phenotypes. (B) The average percentages of the three phenotypic categories for the wild-type and mutant groups of MVD7 derivative cell lines given in A. The asterisks indicate statistically significant difference (p < 0.01, 2-test) from the average of the wild-type group.

Phosphorylation at Key Serine Residues in Mena Promotes Filopodia Formation
Ena/VASP function is regulated by phosphorylation of serine/threonine residues by cAMP/cGMP-dependent PKA and PKG, respectively (Waldmann et al., 1987; Gertler et al., 1996; Lambrechts et al., 2000). In neurite shafts and neurons, the treatment with Netrin-1, a neuronal chemotactic factor, coincided with increased Mena phosphorylation at serine 236 adjacent to the PRO domain, resulting in enhanced filopodia formation (Lebrand et al., 2004). To test whether regulation of Ena/VASP by phosphorylation could also control filopodia formation, we used MV^D7 cell lines stably expressing phospho-mimetic and nonphosphorylatable mutants of Ena/VASP (Table 1). We first tested the two sites within Mena: the conserved Serine 236 common to all vertebrate Ena/VASP family members and Serine 376 adjacent to the GAB site. Substituting both phosphorylation sites with alanine to mimic the unphosphorylated state (Mena-AA) did not result in efficient induction of filopodia upon CP depletion. Instead the knockdown of CP caused over 70% of transfected cells to show the Ruffling phenotype (Table 1). Conversely, substituting these residues with aspartic acid to mimic the phosphorylated state (Mena-DD) led to filopodia formation in 57% (n = 61) of cells after CP depletion (Table 1), a level equivalent to that of wild-type protein. Accordingly, the distribution of phenotypes in Mena-AA mutant cells, but not Mena-DD cells, was significantly different from cells with wild-type EVH2 domains (Figure 4 and Table 1).

The EVH2 Domain of VASP But Not Full-Length VASP Bundles Actin In Vivo To Initiate Robust Filopodia Formation
Our experiments demonstrate that the EVH2 domain is essential to filopodia formation in MV^D7 derivative cell lines. To determine whether the EVH2 domain is also sufficient to induce filopodia in other cell lines, we transiently expressed EGFP-EVH2 (VASP) in COS7 cell lines. COS7 cells, when plated on fibronectin, rarely form filopodia under normal cell culture conditions (Bohil et al., 2006; Loitto et al., 2007). This system was, therefore, ideal for testing the ability of EGFP-EVH2 to induce filopodia (Figure 5A). On expression of EGFP-EVH2, over 80% (n = 201) of transfected COS7 cells formed filopodia-like protrusions at the cell periphery, which was visible by both phase-contrast microscopy and fluorescent microscopy when stained for phalloidin (Figure 5B). These protrusions were also visible on the dorsal surface (Figure 5B, inset). Although COS7 cells are a fibroblastic cell line that usually do not form microvilli, we confirmed that these protrusions were genuine filopodia by immunostaining for fascin, a predominately filopodial bundler (Adams and Schwartz, 2000; Bartles, 2000; Loomis et al., 2003; Tilney et al., 2004; Vignjevic et al., 2006). Both the EVH2 domain of VASP and fascin localize to these dorsal filopodia (Figure 5C). Approximately 20% of COS7 cells expressing the EGFP-control vector showed this hyper-filopodial phenotype. These results demonstrate that the potential of the Ena/VASP EVH2 domain to induce filopodia is not only limited to MV^D7 cells.

View larger version (59K): [in this window] [in a new window]   Figure 5. EVH2 domain is sufficient to induce filopodia in other cell lines. (A–D) Induction of filopodia by the EVH2 domain of VASP in COS7 cells. (A and B) F-actin organization of nontransfected (A) and EGFP-EVH2-expressing (B) COS7 cells. Phase-contrast and phalloidin-staining images are shown. Expression of the EVH2 domain induced numerous filopodia. (C) Immunostaining of fascin (middle panel; red in right panel) in COS7 cells overexpressing EGFP-EVH2 (left panel; green in right panel). The inset in right panel shows higher magnification of the boxed region. Fascin and EGFP-EVH2 are colocalized in dorsal filopodia. (D) Quantification of hyper-filopodial phenotype in COS7 cells. Bar graph indicates the percentage of the hyper-filopodial phenotype in cells expressing EGFP-control and EGFP-EVH2 (**p < 0.02, Student's t test, n = 3). (E–I) Overexpression of the EVH2 domain of VASP induces hyper-filopodial formation in B16F1 mouse melanoma cells. (E) F-actin staining (left panel; red in right panel) and EGFP-EVH2 (middle panel; green in right panel). EGFP-EVH2 localizes along the length of filopodia as well as stress fibers. Boxed region shows, at higher magnification, colocalization of EVH2 domain and actin. Scale bar, 10 µm. (F) Actin by phalloidin staining of a nontransfected control B16F1 displaying typical filopodial amounts and distributions. (G and H) Localization of the filopodial tip markers in EVH2-induced filopodia. B16F1 cell expressing EGFP-EVH2 (green), immunostained with anti-phospho-tyrosine (G, red) or anti-lamellipodin (H, red). F-actin (blue) was visualized by phalloidin staining. Scale bar, 10 µm. Boxed region is higher magnification of EVH2 induced filopodia. Scale bar, 2 µm. Both molecular markers were concentrated at the filopodia tips. (I) Box-and-whisker plot comparing the number of filopodia per B16F1 cell transiently expressing full-length VASP, EVH1 domain of VASP, EVH2 domain of VASP, and EGFP-control. The small square indicates the mean, the line in the middle of the box the median. The top of the box indicates the third quartile, the bottom of the both the first quartile. The upper and lower boundary of each whisker represents 90th and 10th percentile of the data respectively (*p < 0.02, one-way ANOVA).

VASP Exchanges Rapidly at the Leading Edge of Lamellipodia But Very Slowly at Filopodial Tips
The Ena/VASP family of proteins localizes to both the leading edge of lamellipodia and filopodial tips where they function to regulate actin dynamics. Productive lamellipodial protrusion requires a defined length of actin filaments. To prevent overelongation of actin filaments in lamellipodia, the association between VASP and barbed ends must be transient in vivo. In contrast, if the role of Ena/VASP proteins is to cross-link actin barbed ends and prevent premature filament termination, as predict, then the association between VASP and barbed ends in filopodial tips would be relatively stable.

To test these concepts, we used FRAP to measure the recovery dynamics of full-length VASP, EVH1, or EVH2 domain of VASP, at filopodial tips and the leading edge of protruding lamellipodia. We used B16F1 cells that transiently expressed EGFP-tagged VASP or the EVH1 domain of VASP because these cells readily from both lamellipodia and filopodia and are easily transfectable, making them amenable to imaging. This was important because our FRAP system utilizes a laser scanning confocal microscope, and exogenous expression of EGFP-VASP in B16 was brighter than EGFP-tagged Ena/VASP proteins stably integrated in MV^D7 cells because their expression levels were carefully selected by FAC sorting (Geese et al., 2002; Loureiro et al., 2002). In addition, we used MV^D7 cells expressing EGFP-EVH2 to prevent tetramerization with endogenous Ena/VASP proteins. The results of the FRAP analysis of full-length EGFP-VASP in B16F1 cells yielded different half-times of recovery in lamellipodia and filopodia. In filopodial tips we observed virtually no recovery of fluorescence (n = 15; Figure 6A). We also bleached both protruding as well as stable filopodia, which did not reveal significant fluorescent recovery. This same trend was found in cells expressing EGFP-Mena (unpublished data). An initial concern regarding the lack of fluorescence recovery of VASP at filopodial tips was that the narrow structure of filopodia might represent a barrier to diffusion. Because the EVH1 domain of VASP readily localized to filopodial tips, we extended our FRAP analysis to this domain (Figure 6B). The recovery of the EVH1 domain was extremely rapid, t_1/2 = 3.3 ± 4.7 s (n = 15). This result demonstrates that the lack of recovery we observed with full-length VASP at filopodial tips was not a result of limited diffusion within the structure.

View larger version (55K): [in this window] [in a new window]   Figure 6. Fluorescent recovery after photobleaching (FRAP) of VASP, EVH1, and EVH2 Domain of VASP. (A–D) Graphs are representative of FRAP data of the full-length VASP (A and C), the EVH1 domain (B), and the EVH2 domain (D). FRAP analysis are shown for filopodia (A, B, and D) and lamellipodia (C). In lower magnification images (top left) show the prebleach images. The regions indicated by gray rectangles are shown in the time series of fluorescence recovery (bottom). The white arrowheads indicate either filopodial tips (A and B) or shaft (D). Gray boxes denote the bleached regions subject to quantification. The fluorescence recovery for each sample was presented as a graph (top right). The gray triangles represent theoretical values of decrease in fluorescence intensity due to laser illumination during image acquisition. The gray circles represent fluorescent intensity normalized to the pre-bleached images. The black squares represent the relative intensity after correction for photofading during image acquisition. Time is given in seconds. (E) Box-and-whisker plot of the distribution of the half recovery times of FRAP analysis in the leading edges of lamellipodia (VASP), leading edge and filopodial shafts (EVH2), and filopodial tips (EVH1; *p < 0.001, Student's t test).

To further investigate this concept we coexpressed mCherry-actin and EGFP-VASP in B16F1 cells and sequentially photobleached both tagged molecules at the distal end of a protruding filopodium (Figure 7A). B16F1 cells show a considerably higher transfection efficiency than MV^D7 derivative cells, which made using them much more convenient for this study. Again, we observed a lack of fluorescent recovery by EGFP-VASP at the filopodial tip despite continued filopodial protrusion; however, we did observe fluorescent recovery of mCherry-actin. The mCherry-actin first appeared at the distal portion of the filopodia and underwent retrograde flow within the filopodium as it the filopodium continued to protrude. This result demonstrates that at the resolution of the light microscope, Ena/VASP allows for insertional polymerization of actin while maintaining filopodial tip localization. In summary, Ena/VASP proteins transition from a lamellipodial to filopodial localization, and in doing so, undergoes a change in kinetics, that is, once filopodia formation is initiated, Ena/VASP proteins transition to a more "static" association with actin filaments. In addition Ena/VASP facilitates insertional polymerization of actin at filopodial tips. Collectively, these results illustrate how Ena/VASP functions as a barbed end cross-linker that facilitates the transition from a lamellipodial to a filopodial mode of protrusion.

View larger version (55K): [in this window] [in a new window]   Figure 7. Co-FRAP of EGFP-VASP and mCherry-Actin. Sequential photobleaching of EGFP-VASP (cyan) and mCherry-Actin (red) of a protruding filopodium. Lower magnification image shows prebleach fluorescence image, with a cyan box representing the region of higher magnification. In higher magnification, time-lapsed fluorescence images are shown with the bleached region indicated by gray box. Two phase-contrast images at the beginning (top) and end (bottom) of time sequence, with black arrows denoting the tip of bleached filopodium. Note that the filopodium continued protrusion during the course of time- lapsed imaging.

View larger version (155K): [in this window] [in a new window]   Figure 8. FRAP of filopodial tips in MVD7-derivative stable cell lines. Time-lapsed fluorescence and phase-contrast images of photobleached filopodial tips in MVD7 derivative cell lines. (A) MVD7-VASP, (B) MVD7-GAB mutant (VASP), and (C) MVD7-FAB(VASP). The first image of the fluorescence sequence shows the prebleached filopodium with gray boxes indicating the regions of quantification in Supplementary Figure 2. Phase-contrast images are shown at the prebleach and the final time points. Black arrows, which denote filopodial tips, demonstrate filopodia protrusion during image acquisition.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

View larger version (22K): [in this window] [in a new window]   Figure 9. Model of Ena/VASP-initiated filopodia formation. (A) Ena/VASP molecules can be divided into two modules: membrane-interacting module and actin-interacting module. (B, 1–3) Initiation of filopodia by Ena/VASP proteins. (B, 1) Ena/VASP is recruited to membrane ligands via a transient association with the EVH1 domain. (B, 2) Additional interactions with the G-actin– and F-actin–binding domain help to stabilize molecule at filopodial tips. (B, 3) The complex formed between the G-actin–binding domain of Ena/VASP, the actin barbed end and G-actin is also somewhat transient in nature to allow for insertional polymerization. Collectively the interaction between the EVH1 domain and the membrane-bound ligand, the G-actin–binding domain, and the actin barbed end, and tetramerization of the coiled-coil domain allows for the entire molecule to be "static" and filopodial tips, but flexible enough to for continued insertional actin polymerization.

Schirenbeck et al. (2006) hypothesized that the bundling activity of the FAB motif in dVASP was required for filopodia formation. Our data suggest that in addition to the bundling function of the FAB site, the GAB motif is required. Moreover, the FAB motif of Ena/VASP has been shown to be important for localization of Ena/VASP to actin bundles, anticapping, and the incorporation of G-actin into filaments (Bachmann et al., 1999; Bear et al., 2002; Loureiro et al., 2002; Barzik et al., 2005). Analysis of a mutant VASP protein harboring a deletion of the F-acting–binding motif does not distinguish between these different functions. Additionally, there are major differences between dVASP and the mammalian counterparts, including the lack of serine/threonine phosphorylation sites (Schirenbeck et al., 2006).

Structural and biochemical work from Chereau and Dominguez (2006) indicates that the GAB motif has a higher affinity for profilin-actin then for G-actin, which led the authors to propose a model whereby the PRO domain adjacent to the GAB motif cooperates in the "priming" of profilin-actin to the barbed end of filaments, which leads to a processive stepwise mechanism to explain the association of Ena/VASP with the barbed ends during filament elongation. Barzik et al. (2005) also hypothesize that Ena/VASP maintains their barbed end localization while simultaneously preventing filament termination, promoting filament elongation. Results from our FRAP analysis of Ena/VASP at filopodial tips lends support to these models. We did not observe fluorescent recovery of VASP at filopodial tips, indicating a lack of exchange of molecules even as filopodia continue to grow. When we bleached filopodial tips in the cell line harboring mutations in the GAB motif, we observed limited fluorescent recovery, suggesting that the GAB motif plays an integral role in the maintenance of filopodial tip localization and further supports models proposed by Chereau and Dominguez, Barzik, and others. In addition, we directly observed the fluorescent recovery of mCherry-actin at the distal tip of filopodia despite the lack of recovery of EGFP-VASP, indicating that at the level of the light microscope, Ena/VASP allows for insertional polymerization of actin. A similar processive stepwise model has also been proposed to explain how the formin family of molecules maintains their association with actin filaments but allows for continued actin polymerization (Zigmond et al., 2003).

Co et al. (2007) demonstrated that the WH2 domain of N-WASP, which shares homology to the GAB motifs in Ena/VASP, is essential to barbed end capture. Although the concept of membrane anchoring is not new (Laurent et al., 1999; Samarin et al., 2003), this is first time it has been narrowed to this domain. Similar mutations that inhibited barbed end capture by the WH2 domain were made to the GAB motif and tested here. It is likely that the deficiencies we found in filopodia formation and stabilization of the GAB mutant at filopodial tips is a result of the similar inhibition of barbed end capture and implicates a possible role of barbed end capture by the GAB motif of Ena/VASP.

Both formins and Ena/VASP are part of the filopodial tip complex (Reinhard et al., 1992; Lanier et al., 1999; Rottner et al., 1999; Pellegrin and Mellor, 2005). It has been hypothesized that formins and VASP cooperate during filopodia formation (Schirenbeck et al., 2005). Unlike the Dictyostelium dDia2 and dVASP, the mammalian counterparts induce filopodia independently of each other (Barzik and Gertler, unpublished data). This can probably be explained by the structural differences between dVASP and the mammalian Ena/VASP proteins.

Phosphorylation of Mena in neurons has a positive effect on filopodia formation (Lebrand et al., 2004). Results from our filopodia formation assay conclude that phosphorylation positively regulates filopodia formation by Mena, however, we found that phosphorylation of VASP had a more intermediate phenotype. Phosphorylation of Ena/VASP had differential effects in anticapping (Loureiro et al., 2002; Barzik et al., 2005) and bundling (Harbeck et al., 2000; Barzik et al., 2005), depending on which residue is phosphorylated. Howe et al. (2002) demonstrated that VASP undergoes cycles of global phosphorylation and dephosphorylation as cells adhere and spread. It has yet to be determined whether these cyclic phosphorylation events affect the total cellular pool or only bound VASP. It is not clear from this study if Mena cycles through similar phosphorylation-dephosphorylation states. VASP is distinguished from Mena by an additional PKA/PKG site, Threonine 274, which might be required for fine-tuning of VASP during special activities. Our data from the CP-depletion assay suggests that filopodia formation could be such an activity during which VASP and Mena might be regulated differently by phosphorylation.

	ACKNOWLEDGMENTS

 
We thank Drs. Edwin Taylor, Sarah Rice, and Omar Quintero for critical reading of the manuscript and Dr. Teng-Leong Chew and Yvonne Aratyn for assistance with the photobleaching experiments. This work was supported in part by National Institutes of Health Grants GM7542201 to D.A.A., GM58801 to F.B.G., and GM62431 to G.G.B. and by Cell Migration Consortium Grants GM64346 to D.A.A and G.G.B.

	Footnotes

 
This was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E06-11-0990) on May 2, 2007.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Gary G. Borisy (gborisy{at}mbl.edu)

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Adams, J. C., and Schwartz, M. A. (2000). Stimulation of fascin spikes by thrombospondin-1 is mediated by the GTPases Rac and Cdc42. J. Cell Biol 150, 807–822.[Abstract/Free Full Text]

Ahern-Djamali, S. M., Bachmann, C., Hua, P., Reddy, S. K., Kastenmeier, A. S., Walter, U., and Hoffmann, F. M. (1999). Identification of profilin and src homology 3 domains as binding partners for Drosophila enabled. Proc. Natl. Acad. Sci. USA 96, 4977–4982.[Abstract/Free Full Text]

Albrecht-Buehler, G. (1976). Filopodia of spreading 3T3 cells. Do they have a substrate-exploring function? J. Cell Biol 69, 275–286.[Abstract/Free Full Text]

Bachmann, C., Fischer, L., Walter, U., and Reinhard, M. (1999). The EVH2 domain of the vasodilator-stimulated phosphoprotein mediates tetramerization, F-actin binding, and actin bundle formation. J. Biol. Chem 274, 23549–23557.[Abstract/Free Full Text]

Ballestrem, C., Wehrle-Haller, B., and Imhof, B. A. (1998). Actin dynamics in living mammalian cells. J. Cell. Sci 111, (Pt 12), 1649–1658.[Abstract]

Bartles, J. R. (2000). Parallel actin bundles and their multiple actin-bundling proteins. Curr. Opin. Cell Biol 12, 72–78.[CrossRef][Medline]

Barzik, M., Kotova, T. I., Higgs, H. N., Hazelwood, L., Hanein, D., Gertler, F. B., and Schafer, D. A. (2005). Ena/VASP proteins enhance actin polymerization in the presence of barbed end capping proteins. J. Biol. Chem 280, 28653–28662.[Abstract/Free Full Text]

Bear, J. E., Loureiro, J. J., Libova, I., Fassler, R., Wehland, J., and Gertler, F. B. (2000). Negative regulation of fibroblast motility by Ena/VASP proteins. Cell 101, 717–728.[CrossRef][Medline]

Bear, J. E. et al. (2002). Antagonism between Ena/VASP proteins and actin filament capping regulates fibroblast motility. Cell 109, 509–521.[CrossRef]