Abstract
Cell migration entails networks and bundles of actin filaments termed lamellipodia and microspikes or filopodia, respectively, as well as focal adhesions, all of which recruit Ena/VASP family members hitherto thought to antagonize efficient cell motility. However, we find these proteins to act as positive regulators of migration in different cell lines. CRISPR/Cas9-mediated loss of Ena/VASP proteins reduced lamellipodial actin assembly and perturbed lamellipodial architecture, as evidenced by changed network geometry as well as reduction of filament length and number that was accompanied by abnormal Arp2/3 complex and heterodimeric capping protein accumulation. Loss of Ena/VASP function also abolished the formation of microspikes normally embedded in lamellipodia, but not of filopodia capable of emanating without lamellipodia. Ena/VASP-deficiency also impaired integrin-mediated adhesion accompanied by reduced traction forces exerted through these structures. Our data thus uncover novel, cellular Ena/VASP functions of these actin polymerases that are fully consistent with their promotion of cell migration.
Introduction
Adhesion and migration are invariably driven by continuous and dynamic actin cytoskeleton remodeling (Blanchoin et al., 2014). The major protrusive structure of migrating cells on flat and rigid substrata is the sheet-like lamellipodium (Letort et al., 2015; Rottner et al., 2017). The extension of dense and branched actin filaments in the lamellipodium drives its growth and pushes the membrane forward (Koestler et al., 2008; Mullins et al., 1998; Pollard and Borisy, 2003; Vinzenz et al., 2012). Branching by Actin-related protein (Arp) 2/3 complex is activated by the WAVE regulatory complex (WRC) downstream of Rac subfamily GTPase signaling (Eden et al., 2002; Ismail et al., 2009; Molinie and Gautreau, 2018). Consistently, knockdown or knockout of essential Arp2/3 complex (Suraneni et al., 2012; Wu et al., 2012) or WRC subunits (Innocenti et al., 2004; Schaks et al., 2018; Steffen et al., 2004) or Rac GTPases (Schaks et al., 2018; Steffen et al., 2013) all abrogated lamellipodium formation. The lateral flow of actin filaments and bundles embedded in the lamellipodium, called microspikes, is driven by actin assembly at the membrane (Oldenbourg et al., 2000). When microspikes transform and protrude beyond the lamellipodium edge, they are called filopodia (Small, 1988; Svitkina et al., 2003). Yet, in spite of both being built of parallel actin filament bundles and sharing constituents, filopodia and microspikes display important differences. As opposed to microspikes as integral parts of lamellipodial networks, filopodia can kink and bend (Nemethova et al., 2008), arising independently and even in excess in the absence of lamellipodia (Gupton et al., 2005; Koestler et al., 2013; Steffen et al., 2006; Wu et al., 2012). Consistently, filopodia can form around the entire cell periphery and at the dorsal surface (Block et al., 2008; Bohil et al., 2006; Pellegrin and Mellor, 2005). Moreover, rates of actin polymerization in microspikes and lamellipodia are indistinguishable from each other (Lai et al., 2008; Oldenbourg et al., 2000), whereas those in filopodia appear to be regulated independently.
Filament elongation is driven by formins and Ena/VASP proteins. Formins are dimeric multi domain proteins that remain tightly associated with growing filament barbed ends while accelerating their growth (Kovar et al., 2006). Several formins such as mDia2 (Block et al., 2008; Yang et al., 2007) and the formin-like family members 2 (FMNL2) and 3 (Block et al., 2012; Harris et al., 2010; Kage et al., 2017; Young et al., 2018) localize at lamellipodia and filopodia tips, and have been implicated in driving these protrusions and cell migration.
Ena/VASP family proteins localize to sites of active actin assembly including focal adhesions (FA) (Gertler et al., 1996; Reinhard et al., 1995), stress fibers (Gateva et al., 2014; Rottner et al., 2001), and lamellipodial or filopodial tips (Rottner et al., 1999; Svitkina et al., 2003). Vertebrates harbor Vasodilator-stimulated phosphoprotein (VASP), mammalian Enabled (Mena), and Ena-VASP-like (Evl). All family members are tetramers encompassing domains allowing for interactions with FPPPP-containing proteins, actin monomers, profilin-actin complexes and actin filaments (Bear and Gertler, 2009). Ena/VASP tetramers are poorly processive, but processivity and resistance against heterodimeric capping protein that stops barbed end growth, increases markedly upon oligomerization or clustering (Breitsprecher et al., 2011a, 2008; Brühmann et al., 2017; Hansen and Mullins, 2015).
Genetic removal of all three Ena/VASP proteins was previously shown to abrogate filopodia formation in neuronal cells (Kwiatkowski et al., 2007). The functions of Ena/VASP proteins in cell motility have remained more controversial. Although VASP accumulation at lamellipodia tips positively correlates with protrusion rates (Rottner et al., 1999), genetic inactivation of VASP and Mena or mitochondrial Ena/VASP sequestration in fibroblasts was reported to increase cell migration (Bear et al., 2002, 2000). This phenotype was explained by lamellipodia protruding more persistently after interference with Ena/VASP function, and containing shorter and more branched filaments. Excess of Ena/VASP, in contrast, generated lamellipodia with longer, less branched filaments, prone to lifting rearwards during membrane ruffling, therefore driving migration less efficiently (Bear et al., 2002). Despite this negative role in fibroblast motility, the proposed positive regulatory function of Ena/VASP on lamellipodial filaments was consistent with their clear relevance in the motility of Listeria harboring the Ena/VASP ligand ActA (Geese et al., 2002; Loisel et al., 1999; Skoble et al., 2001). Whether the reported negative correlation of Ena/VASP activity with migration can be generalized, has remained unanswered.
Finally, although Ena/VASP proteins are prominent FA components, their roles in cell substrate adhesion and FA formation have also remained controversial. Both inhibitory (Galler et al., 2006) and stimulatory roles (Gupton et al., 2012; Kang et al., 2010; Puleo et al., 2019; Young and Higgs, 2018) on adhesion were reported, whereas one study did not detect any effect in fibroblasts lacking Mena and VASP (Bear et al., 2000). To resolve these discrepancies, we characterized cells disrupted for individual, two or all three Ena/VASP family members in distinct types of parental cells, and explored the impact of their individual and collective removal on formation of different protrusion types, migration efficiency and adhesion.
Results
Loss of Ena/VASP impairs cell migration in B16-F1 cells
To evaluate the physiological function of Ena/VASP proteins in a rapidly migrating cell type, we sequentially inactivated the three Ena/VASP paralogues Evl, VASP and Mena using CRISPR/Cas9 technology in B16-F1 mouse melanoma cells. Respective protein loss in independent clonal cell lines was confirmed by immunoblotting and verified by sequencing of genomic target sites (Figure 1A, Supplement 1). Migration rates of B16-F1 wild-type and two independent lines of each genotype were then analyzed on laminin by phase-contrast, time-lapse microscopy. Interestingly, consecutive Ena/VASP member removal caused an increasing phenotype in migration rate, ranging from a comparably modest reduction of 1.25 ± 0.37 µm/min and 1.19 ± 0.47 µm/min for single Evl KO mutants (E-KO) as compared to control (1.44 ± 0.4 µm/min), to 0.62 ± 0.25 µm/min and 0.57 ± 0.27 µm/min in Evl and VASP double-KOs (EV-KO), and 0.47 ± 0.19 µm/min and 0.43 ± 0.22 µm/min in triple-KOs (EVM-KO) (Figure 1B, Figure 1-video 1). This was accompanied by a gradual increase in directionality culminating in EVM-KO cells that were 46% more directional as compared to wild-type B16-F1 (Figure 1C). Quantitative analyses of the turning angles revealed an incrementally increased frequency at 0° from 10.2% in wild-type to 12.3% in single KO, 16.1% in double KO and 16.9% in triple KO cells, in support of the higher directionality in mutant cells that coincided with their reduced motility (Figure 1D). Since independently generated Ena/VASP mutant cell lines behaved highly similar in distinct assays, data derived from one triple-KO clone (#23.7.66) are shown below. To ensure that observed phenotypes were Ena/VASP-specific, the same parameters were analyzed in reconstituted EVM-KO mutant cells ectopically expressing EGFP-tagged VASP, Evl or Mena. Re-expression of VASP, which was found to contribute most efficiently to motility, largely rescued cell speed, directionality and frequency of turning angles (Figure 1E-F, Figure 1-figure supplement 1A). The rescue experiments with EGFP-tagged Evl or Mena yielded similar results, but these proteins were slightly less effective in restoring cell speed (Figure 1E-F). Finally, we calculated the mean square displacement (MSD) in wild-type and the entire collection of mutant cells to assess their effective directional movement. Despite their higher directionality, presumably owing to their considerably slower motility, all Ena/VASP mutant cells displayed incrementally decreasing and lower MSD values as compared to control (Figure 1G). Notably, in EVM-KO cells rescued with EGFP-tagged VASP, Evl or Mena, the MSD values were again markedly increased (Figure 1-figure supplement 1B).
Ena/VASP-deficiency consistently diminishes cell migration in fibroblasts
To corroborate our findings in strongly adherent cell types, we first examined MVD7 mouse embryonic fibroblasts. This cell line was described previously to derive from Mena/VASP-deficient mice and selected initially to lack detectable expression of Evl (Bear et al., 2000), albeit later stated to contain trace amounts (Auerbuch et al., 2003). We did confirm the absence of Mena and VASP in these cells by immunoblotting (Figure 2A). Surprisingly, however, our newly-generated antibodies raised against and affinity-purified with Evl (see Materials and Methods) now identified the protein at low, but clearly detectable levels in this cell line. Thus, to obtain fibroblasts devoid of all three Ena/VASP proteins, we employed CRISPR/Cas9 to eliminate Evl in MVD7 cells, with Evl disruption confirmed by immunoblotting and sequencing as described for B16-F1 cells above (Figure 2A, Supplement 1). Since distinct triple-KO cell lines behaved again in a highly comparable fashion, only data from clone #31 referred to as MVE-KO are shown. We first analyzed migration on fibronectin by phase-contrast time-lapse imaging of MVD7 and MVE-KO as well as MVE-KO cells rescued with EGFP-tagged Evl. In MVE-KO, cell speed (0.41 ± 0.17 µm/min) was significantly reduced as compared to MVD7 control (0.56 ± 0.19 µm/min), whereas migration of MVE-KO cells expressing EGFP-Evl (0.68 ± 0.28 µm/min) was markedly increased (Figure 2B). Remarkably, in spite of their higher cell speed, MVD7 exhibited even lower MSD values than MVE-KO cells (Figure 2C), apparently caused by the lower directionality of the former, and as evidenced by their trajectories in radar plots (Figure 2-figure supplement 1A-B). As expected, triple-KO cells reconstituted with Evl displayed strongly increased MSD values (Figure 2C) and decreased directionality (Figure 2-figure supplement 1A-B).
Additionally, we analyzed directional cell migration of these cell lines in wound closure scratch assays (Figure 2D). Average wound closure rates were reduced by 24% in MVE-KO cells (18121 ± 2658 µm2h-1) as compared to MVD7 cells (23906 ± 2042 µm2h-1) (Figure 2E). Interestingly, reconstitution of MVE-KO cells with stable Evl expression significantly increased wound closure to rates virtually identical to MVD7 controls (Figure 2F). Thus, despite comparably low expression of Evl in MVD7 fibroblasts, its elimination caused clearly detectable impairment of cell migration.
To assess the contribution of the two other Ena/VASP family members in fibroblast motility, we also disrupted the genes encoding Mena and VASP individually or in combination in mouse NIH 3T3 fibroblasts by CRISPR/Cas9 technology, and again confirmed elimination of respective proteins by immunoblotting and sequencing (Figure 2G, Supplement 1). Migration rates of NIH 3T3 wild-type and two independent lines of each genotype were then again analyzed on fibronectin by phase-contrast time-lapse imaging. Comparable to the findings in B16-F1 cells, consecutive removal of Mena and VASP in NIH 3T3 cells also caused a step-wise increase in migration phenotype, ranging from a rather modest average reduction of about 10.6% for single Mena KO mutants (M-KO) to 31.6% in Mena and VASP double-KO (MV-KO) mutants as compared to control (Figure 2H). Consistently, these Ena/VASP mutant cells also exhibited incrementally decreasing and lower MSD values as compared to NIH 3T3 wild-type controls (Figure 2I). All these findings combined, strongly suggest, quite strikingly, that Ena/VASP proteins execute conserved and prominent, positive regulatory functions in cell migration, in a fashion irrespective of cell type (melanoma cells and fibroblast alike) and thus type of mesenchymal migration and/or signaling condition, as induced by extracellular matrices (laminin versus fibronectin).
To relate the specific contributions of individual Ena/VASP family members to cell migration, we finally determined the cellular concentrations of expressed orthologues in B16-F1, NIH 3T3 and MVD7 cells. For this, we titrated defined amounts of recombinant proteins with total cell lysates from a given number of cells in immunoblots, and calculated resulting absolute levels of each protein in the different cell types (Figure 2J). Interestingly, in spite of considerably higher expression of Mena in B16-F1 and NIH 3T3 cells as compared to VASP, and in particular relative to Evl, loss of Mena affected migration much less strongly than, for instance, removal of VASP. This suggests a certain differentiation of Ena/VASP family member function despite clearly overlapping functions.
Loss of Ena/VASP reduces lamellipodium width and abrogates microspike formation
Since migration is mainly driven by actin assembly in the lamellipodium, we then analyzed actin filament (F-actin) distribution in B16-F1 and EVM-KO cells after phalloidin staining. As opposed to B16-F1 control cells, which displayed prominent lamellipodia with numerous microspikes, EVM-KO cells developed strongly compromised lamellipodia largely lacking microspikes (Figure 3A, Figure 3-video 1). Quantification of F-actin contents in lamellipodia revealed about 45% reduction in the triple mutant, and the average width of lamellipodia was diminished by about 65% to 0.8 ± 0.2 µm when compared to wild-type with 2.3 ± 0.7 µm. In mutant cells reconstituted with EGFP-VASP, F-actin intensity increased even 40% above wild-type levels, and lamellipodium width was readily restored to 2.7 ± 0.8 µm (Figure 3B-C). Similar, but less pronounced effects were found after rescue with EGFP-tagged Evl or Mena as compared to VASP (Figure 3-figure supplement 1A-C). Given the evident lack of microspikes, we stained for the actin-crosslinking protein fascin, a well-established constituent of microspikes in B16-F1 cells (Vignjevic et al., 2006). Again, the triple mutant virtually lacked fascin-containing bundles, as opposed to wild-type and reconstituted mutants expressing VASP, despite unchanged expression of fascin in the mutant (Figure 3D-G). This was corroborated by imaging of wild-type and EVM-KO cells expressing EGFP-fascin (Figure 3-video 2). Finally, we examined the gene dose-dependent requirement for Ena/VASP family members in microspike formation and found that virtually complete loss of microspikes was observed only upon disruption of all three family members (Figure 3H).
Potent filopodia inducers fail to rescue microspike formation in Ena/VASP-deficient cells
We then asked whether microspikes can be reformed in the triple-mutant upon transient expression of potent factors known to induce peripheral protrusions, and in particular related protrusive structures such as filopodia (Block et al., 2008; Bohil et al., 2006; Kage et al., 2017; Pellegrin and Mellor, 2005). We also used all Ena/VASP members as controls. Evl and Mena both rescued canonical lamellipodia with microspikes in EVM-KO comparable to VASP, with Mena forming microspikes above average length (Figure 4A, Figure 4-figure supplement 1A-B). Strikingly, none of the agents established previously to be capable of inducing filopodia formation, i.e. constitutively active variants of the small Rho GTPase Rif (L77), the formins mDia2 (mDia2ΔDAD), FMNL3 (E275) and to lesser extent FMNL2 (E272), or unconventional myosin X were able to restore microspike-containing lamellipodia, indicating that microspike formation requires more specific activities that are critically dependent on Ena/VASP function (Figure 4A).
To explore this potential molecular difference between microspikes and filopodia more directly, we induced filopodia in the absence of lamellipodia in B16-F1 and EVM-KO cells. Since it is commonly assumed that lamellipodia emerge by extension of Arp2/3 complex-generated, dendritic filament networks downstream of WRC and Rac signaling (Pollard, 2007; Schaks et al., 2018; Steffen et al., 2013; Wu et al., 2012), we inhibited Arp2/3 complex with high concentrations of CK666 in combination with cell seeding on low concentrations of laminin, which aided inhibitor-mediated suppression of lamellipodia formation in these conditions (data not shown). Analyses of phalloidin-stained cells revealed that albeit reduced in formation frequency per cell, filopodia formation was still possible in EVM-KO cells (66% compared to B16-F1 control on low laminin). This confirmed that Ena/VASP family can indeed contribute to filopodia formation, as previously observed in D. discoideum (Han et al., 2002). At the same time, these data also illustrated that filopodia can indeed form to significant extent without lamellipodia, e.g. upon inhibition of Arp2/3 complex and thus in the absence of microspikes (Figure 4B-C), in spite of the reduced frequencies of filopodia formation observed here. Conversely, however, induction of filopodia formation by expression of constitutive active mDia2 (Block et al., 2008; Yang et al., 2007) was virtually identical in the EVM-KO mutant as compared to B16-F1 cells (Figure 4D-E), supporting the view of multiple pathways leading to filopodium formation (Young et al., 2015).
Loss of Ena/VASP changes distribution of Arp2/3 complex and capping protein and affects lamellipodial dynamics
To explore if or how loss of Ena/VASP function affected Arp2/3 complex accumulation, we immunolabelled the cells for the Arp2/3 complex and found that it accumulated in a peripheral band much narrower than commonly observed in wild-type lamellipodia (reduced by 64%), but with roughly 75% higher intensity at the tips of mutant lamellipodia (Figure 5A-C). The mutant phenotype was again Ena/VASP-specific, since it was perfectly rescued by transient expression of EGFP-tagged VASP and Evl, and at least partly by Mena. More specifically, VASP was slightly more effective than Evl, and Mena was capable of mediating a statistically significant rescue only concerning width of observed Arp2/3 complex signal (Figure 5A-C, Figure 5-figure supplement 1A-C). We also monitored the distributions of heterodimeric capping protein (CP), the F-actin binding protein cortactin and the WRC-subunit WAVE2. CP and cortactin distributions were highly reminiscent of changes in Arp2/3, again much narrower (reduced by 60% and 67%, respectively), and with approximately 75% and 60% increased intensities in EVM-KO as compared to controls (Figure 5D-F, Figure 5-figure supplement 1D-I). Interestingly, we found no noticeable difference in distribution and intensity of WAVE2 in wild-type versus EVM-KO (Figure 5-figure supplement 1J-L). This together with reversion to or close to wild-type levels of Arp2/3 complex, CP and cortactin by EGFP-tagged Ena/VASP family members illustrated their impact on the lamellipodial Arp2/3 complex machinery downstream, and thus independent of its major lamellipodial activator, WRC.
Next, we asked whether or to which extent lamellipodia protrusion was affected. To this end, we recorded wild-type, EVM-KO and VASP-reconstituted cells randomly migrating on laminin, and determined respective protrusion rates by kymograph analyses (Figure 5G-H, Figure 5-video 1). Quantification revealed lamellipodia protrusion to be reduced by 35% in EVM-KO cells as compared to control or EVM-KO rescued with EGFP-VASP (Figure 5I). Notably, despite reduced protrusion, lamellipodia persistence was at best slightly reduced in EVM-KO cells, but not in a statistically significant fashion (Figure 5J), confirming that protrusion effectivity can be readily uncoupled from persistence (Block et al., 2012; Kage et al., 2017).
Ena/VASP proteins are potent actin polymerases (Breitsprecher et al., 2011a, 2008; Hansen and Mullins, 2010), implying that these factors could affect dynamics of actin assembly in lamellipodia. To test this experimentally, we assessed lamellipodial actin network assembly rates in wild-type and EVM-KO cells (Figure 5K, Figure 5-video 2). Lamellipodial actin network translocation was also reduced by about 22% in EVM-KO as compared to B16-F1 controls, suggesting that the effects on protrusion described above can be explained, at least in part, by reduced actin network polymerization (Figure 5L).
Loss of Ena/VASP deteriorates lamellipodium architecture
To gain quantitative insights into the ultrastructural details of lamellipodial architecture at the single filament level, we employed three-dimensional (3D) electron microscopy of negatively stained B16-F1 wild-type and EVM-KO samples, allowing computer-assisted tracing of single filaments in 3D space (Mueller et al., 2017; Winkler et al., 2012). Consistent with actin assembly in lamellipodia tips (Lai et al., 2008), filaments are oriented with their barbed ends facing the membrane (Narita et al., 2012). In analogy to recent work (Mueller et al., 2017), barbed ends were scored in digitalized tomograms as the ends of filaments proximal to the leading edge and pointed ends as those distal to the leading edge. Barbed ends either represent actively growing or capped filaments, whereas pointed ends designate Arp2/3 complex-generated branch sites, severed or debranched filaments. In contrast to lamellipodia of wild-type cells, EVM-deficient lamellipodia contained much sparser and less organized filament networks (Figure 6A, Figure 6-video 1), which is consistent with the F-actin stainings in Figure 3A-B. Consistently, assessment of actin filaments in the front region (within a ∼1µm broad zone behind the edge) revealed considerably shorter filaments in the mutant (Figure 6B). Moreover, towards the interior of the lamellipodium, filament density in the EVM-mutant decreased noticeably faster as compared to control, while at the edge, barbed ends occurred in excess in the mutant (Figure 6C). Consistent with reduced filament lengths (Figure 6B) and increased Arp2/3 intensities at the edge of EVM-KO cells (Figure A-B), assessed pointed end densities were significantly above control values in particular within the first half micron of the edge (Figure 6C). Finally, although both control and mutant lamellipodia exhibited a wide array of filament angles abutting the protruding front, with a clear maximum around 90° relative to the edge (Figure 6D), the distribution of filament angles in EVM-deficient lamellipodia was distinct, with the 90° peak being clearly less prominent (17% lower than in control). Conversely, filament fractions with diagonal angles were about doubled compared to controls. These data suggest that reduced filament mass and disordered filament network geometry in the mutant are causative for reduced protrusion and network assembly.
Loss of Evl impairs cell spreading and FA formation
To assess Ena/VASP functions in adhesion, we first analyzed spreading of MVD7, derived MVE-KO and reconstituted fibroblasts expressing EGFP-Evl on fibronectin by time-lapse imaging (Figure 7A, Figure 7-video 1). Notably, MVE-KO cells displayed a substantial spreading defect that was fully rescued by Evl re-expression (Figure 7B-C). Consistent with the removal of Evl in MVD7 fibroblasts, B16-F1-derived EVM-KO cells also spread considerably slower than wild-type or reconstituted mutant cells expressing EGFP-tagged VASP or Evl (Figure 7-figure supplement 1A-B). In these experiments, Evl rescued more effectively compared to VASP (Figure 7-figure supplement 1C). Then, we examined FA morphologies in MVE-KO fibroblasts expressing different Ena/VASP family members fused to EGFP. Untransfected MVE-KO cells formed noticeably smaller FAs on average than the same cells expressing distinct Ena/VASP family members, but again, the rescue with Evl appeared most effective (Figure 7D). To explore potential effects of Ena/VASP proteins on FA formation more systematically, MVD7, MVE-KO and reconstituted cell lines migrating on fibronectin were labelled for the FA-marker protein vinculin and assessed for various features (Figure 7E). Images captured at identical settings were processed by a web-based FA analysis tool (Berginski and Gomez, 2013), allowing a global and unbiased assessment of multiple parameters. Consistent with previous work, untransfected MVD7 double-mutant cells still contained prominent FAs (Bear et al., 2000). Notably however, vinculin intensity was clearly reduced in the triple mutant by almost 30% as compared to the parental MVD7 cell line, and again fully rescued by Evl expression, while rescue of MVE-KO cells with VASP or Mena was less effective (Figure 7F). Consistently, FA size in MVE-KO cells was reduced by 18% as compared to MVD7 cells (Figure 7G), despite the low abundance of Evl in this cell line (Figure 2J). Furthermore, expression of VASP or Mena in MVE-KO cells increased FA size only by 11% or 16%, respectively, while expression of Evl increased FA size by more than 24%. Accordingly, Evl expression was most efficient in rescuing the average number of FAs per cell as compared to VASP and Mena (Figure 7H). Together, despite clearly overlapping functions of Ena/VASP proteins in various actin-dependent processes, Evl appeared more critical in the regulation of focal adhesions than VASP or Mena.
Loss of Evl diminishes generation of traction forces
Since Evl proved most effective in rescuing parameters addressing FA properties, we finally explored the functional consequence of Evl function in traction force development. To compare traction forces exerted onto a polyacrylamide substratum independently of cell size and geometry, MVD7, MVE-KO cells, and reconstituted cells expressing EGFP-Evl were grown on top of crossbow-shaped micropatterns coated with fibronectin (Vignaud et al., 2014) (Figure 8A). Interestingly, the contractile energy was significantly reduced in MVE-KO cells as compared to MVD7 (Figure 8B-C), and rescued upon stable expression of EGFP-tagged Evl (Figure 8D-E). Moreover, the contractile energies of Evl-rescued MVE-KO and parental MVD7 cells were highly similar to each other. Comparable results were obtained with unconfined cells, i.e. homogenous fibronectin coating on polyacrylamide substrata, and thus in the absence of size- and geometry-specific constrains (Figure 8-figure supplement 1A-B).
Discussion
In contrast to earlier studies (Bear et al., 2002, 2000), we show that Ena/VASP actin polymerases positively regulate cell migration, since their consecutive loss in B16-F1 cells results in cumulative, reconstitutable motility phenotypes. Thus, despite increased directionality, the reduced net cell translocation rates observed in EVM-KOs as compared to wild-type occur because of reduced lamellipodial actin assembly and protrusion rates. Diminished migration was also observed in MVD7 fibroblasts after genetic removal of Evl as well as in NIH 3T3 fibroblast after consecutive disruption of Mena and VASP, supporting the view that Ena/VASP operate in a mechanistically conserved fashion, reconciling previous controversies (Auerbuch et al., 2003; Geese et al., 2002; Loisel et al., 1999; Rottner et al., 1999). But how can these discrepancies be explained? Firstly, MVD7 cells proved to be an excellent tool in the field for over two decades now, but were largely considered to be Ena/VASP-free, which we here show not to be the case. Secondly, the concept of Ena/VASP-proteins antagonizing efficient cell motility in fibroblasts was largely developed further by inactivating these proteins in Rat2 cells through mitochondrial sequestration versus activating them through CAAX-mediating targeting to the plasma membrane (Bear et al., 2002). It was clearly shown in previous studies that mitochondrial sequestration of actin-binding proteins can co-recruit multiple additional interactors of such factors (Fradelizi et al., 2001), so the specificity of this approach for Ena/VASP proteins might be worth reconsidering. Furthermore, it is certainly worth noting that targeting through the general, isoprenylation-prone motif derived from small GTPases, the so called CAAX-box, is a very efficient means of targeting proteins to plasma membranes, but those membranes cannot be equalized to physiological accumulation of Ena/VASP proteins at the membrane, such as focal adhesions as well as tips of lamellipodia and filopodia (Rottner et al, 1999; Svitkina et al., 2003). In this respect, we should emphasize not to question the experimental outcome of previously published data. We should rather emphasize instead that previously employed approaches cannot be directly compared to the loss of function approaches developed here.
In line with markedly diminished migration, EVM-KO cells display aberrant, narrow lamellipodia with reduced F-actin densities, shorter filaments and perturbed network geometry. Nevertheless, we should stress the fact that for many years, it had remained unclear in the community whether or not Ena/VASP proteins were essential for the formation of lamellipodial actin networks, which we are clearly able to clarify here not to be the case. But what are the precise consequences of loss of Ena/VASP activity in those lamellipodia still formed in their absence? Interestingly, lamellipodial accumulation of Arp2/3 complex, CP and cortactin is markedly enhanced upon Ena/VASP removal. While CP and cortactin localization frequently coincides with Arp2/3 complex, the latter has indeed already been observed to be negatively correlated with Ena/VASP function (Samarin et al., 2003). Thus, it is tempting to speculate that loss of processive filament elongation upon Ena/VASP removal can also increase capping of filaments and Arp2/3 complex-mediated, lamellipodial branching activity (Samarin et al., 2003). This would also be consistent with shorter and more highly branched filaments observed by platinum replica EM upon mitochondrial Ena/VASP sequestration (Bear et al., 2002). Future cryo-EM tomography studies will be required to assess in 3D to what extent increased Arp2/3 complex intensities found in EVM-KOs indeed correlate with enhanced branching.
Nevertheless, EVM-KO cells are still capable of impaired lamellipodia formation and migration, raising the question of alternative actin polymerases partly taking over. Previous studies have implicated formins in lamellipodia protrusion in B16-F1, including mDia2 (Yang et al., 2007) and FMNL 2 and -3 (Kage et al., 2017). Indeed, FMNL2/3 removal caused defects partly reminiscent of what’s seen here, at least concerning impact on lamellipodial protrusion, actin filament densities or microspike formation (see below), although phenotypes seen upon Ena/VASP deficiency were much more severe. Thus, the extent of potential redundancy between all these factors will only be revealed by systematic, combinatorial loss of function studies and side-by-side, phenotypic comparison (Young et al., 2018). Derived conclusions will also have to consider the exciting, recently discovered ‘distributive polymerase’ activity ascribed to WAVE (Bieling et al., 2018).
As yet, authors of even highly relevant literature have decided not to clearly distinguish between filopodia and microspikes to facilitate analyses of finger-like protrusions (Svitkina et al., 2003; Vignjevic et al., 2006). However, the virtual absence of microspikes, but not of filopodia, in EVM-KO cells combined with the inability of established filopodia inducers such as mDia2, FMNL3, Myosin X and Rif to rescue microspikes clearly establishes, perhaps for the first time fundamental differences between these molecular entities. The mechanisms underlying filopodia assembly are still under debate: In the convergent elongation model, filopodia form by coalescence of lamellipodial filaments (Svitkina et al., 2003), whereas the de novo nucleation model proposes filopodia to arise independently of lamellipodial actin networks, as effected by formin-mediated filament nucleation and elongation (Faix et al., 2009; Mellor, 2010). The findings provided here strongly suggest that in contrast to filopodia, microspikes form exclusively by convergent elongation, as previously suggested (Svitkina et al., 2003), whereas filopodia may emerge by multiple, distinct mechanisms (Young et al., 2015), resolving a long-standing controversy in the field.
Ena/VASP proteins are well established to accumulate in FAs of adherent cells, and known to reside in the force transduction layer in-between talin and ends of stress fiber filaments (Kanchanawong et al., 2010). However, the presence of multiple orthologues and analyses of only partially-depleted cells hitherto prevented careful functional assessment of their precise contributions to integrin-dependent adhesion. Here we show that lack of Ena/VASP proteins consistently impairs integrin-dependent spreading and changes in FA patterns. Moreover, reduction of adhesion size and adhesion component intensity observed in fibroblasts well correlates with diminished traction forces upon Ena/VASP protein disruption. This clearly illustrates the positive regulatory function of Ena/VASP family members in adhesion formation and exertion of traction forces onto the substratum. The particularly important role of Evl in integrin-dependent adhesion is consistent with very recent work showing its specific requirement for FA maturation, cell-matrix adhesion and mechanosensing (Puleo et al., 2019).
Finally, we have not only sought to interpret all our phenotypes in the context of various combinations of gene deletions employing up to three distinct cell types, but also assessed their expression levels in different cell types relative to each other, which certainly adds an additional layer of complexity. Two major points worth being more systematically followed in the future are worth mentioning here. First, Mena is by far the most abundant protein present in both melanoma and fibroblast cell lines, but in particular considering this fact, phenotypes generated by its depletion are comparably modest. In spite of its abundance, we are tempted to speculate therefore that it simply does not sufficiently find its relevant, specific interactors that might promote its impact in other structures such as cell-cell contacts (Scott et al., 2006) or tissues like the nervous system (Kwiatkowski et al., 2007). Finally, given their comparably low abundance, VASP, and in particular Evl, are displaying surprisingly significant relevance in lamellipodia protrusion and adhesion formation, respectively, the molecular nature of which also remains to uncovered in future projects.
Materials and methods
Constructs
cDNAs encoding murine VASP (isoform 1), Mena (isoform 2), and Evl (isoform 2) were amplified by PCR from a NIH 3T3 cDNA library and ligated into suitable sites of pEGFP-C1 (Clontech, Palo Alto, CA). VASP cDNA was additionally inserted into the BglII and SalI sites of pmCherry (Addgene ID: 632524). For generation of stably transfected cell lines, the 1.3 kb Puro cassette was amplified by PCR using pPur (Clontech) as template and inserted into the Asp718 and BamHI sites of pEGFP-C1 to yield pEGFP-C1 Puro. Subsequently, the cDNA fragment encoding Evl was inserted into the BspEI and XhoI sites of pEGFP-C1 Puro. For generation of constitutively active murine mDia2 (Drf3) lacking the C-terminal regulatory DAD domain (1-1.036), the corresponding cDNA fragment was amplified from a NIH 3T3 cDNA library and inserted into the BspEI and Asp718 sites of pEGFP-C1. Plasmids pEGFP-C1 Lifeact-EGFP (Riedl et al., 2008), pEGFP-Rif-L77 (Aspenström et al., 2004), pEGFP-MyoX (Berg and Cheney, 2002), EGFP-FMNL2-E272 and EGFP-FMNL3-E275 (Kage et al., 2017) and pEGFP-fascin (Adams and Schwartz, 2000) have been described. pEGFP-βActin was from Clontech. For generation of recombinant antigens, respective sequences encoding for murine Evl (1-414), VASP (1-375), Mena (1-241), and WAVE2-WCA (419-497) were amplified from plasmid DNA and inserted into BamHI/SalI sites of pGEX-6P-1 (GE Healthcare, Munich, Germany). Plasmids pGEX-6P-2-cortactin (Lai et al., 2009) and pGEX-6P-1-fascin (Breitsprecher et al., 2011b) have been described. Fidelity of generated plasmids was confirmed by sequencing.
Cell culture and transfection
B16-F1 mouse melanoma cells (CRL-6323) and mouse embryonic NIH 3T3 fibroblast (CRL-1658) were purchased from ATCC. The mouse embryonic fibroblasts cell line MVD7, which expresses a temperature-sensitive version of large T antigen, was derived from Mena/VASP-deficient mice (Bear et al., 2000). For this study, MVD7 cells were additionally immortalized with a SV40 wild-type variant of large T antigen-transducing retrovirus by standard procedures, to allow cultivation at 37°C. Expression of Evl in original and derived MVD7 cells, immortalized with SV40 wild-type large T antigen, was not affected as assessed by immunoblotting (data not shown). B16-F1, NIH 3T3 and immortalized MVD7 cells were cultured at 37°C and 5% CO2 in high glucose DMEM culture medium (Lonza, Cologne, Germany) supplemented with 10% FBS (Biowest), 2 mM UltraGlutamine (Lonza) and 1% Penicillin-Streptomycin (Biowest). B16-F1 cells were transfected with 1 µg plasmid DNA and MVD7 and NIH 3T3 cells with 3 µg plasmid DNA, respectively using JetPRIME transfection reagent (PolyPlus, Illkirch, France) in 35 mm diameter wells (Sarstedt, Nümbrecht, Germany) according to the manufacturer’s protocol. Absence of mycoplasma in cell lines was routinely checked by the VenorGeM Mycoplasma Detection Kit (Sigma, St. Louis, MO).
Genome editing by CRISPR/Cas9
DNA target sequences were pasted into CRISPR/Cas9 design tool (http://crispr.mit.edu/) to generate sgRNAs of 20 nucleotides with high efficiency scores. In case of Evl, the targeting sequence 5’-GATCGGTACCCACTTCTTAC-3’ was used to cover all possible splice variants. Accordingly, genome editing of VASP was performed with 5’-GTAGATCTGGACGCGGCTGA-3’, and for disruption of Mena expression, the sgRNA 5’-AAGGGAGCACGTGGAGCGGC-3’ was used. Respective sequences were ligated into expression plasmid pSpCas9(BB)-2A-Puro(PX459)V2.0 (Addgene plasmid ID: 62988) using BbsI (Ran et al., 2013). Validation of CRISPR construct sequences was performed using a 5’-GGACTATCATATGCTTACCG-3’ sequencing primer. 24 h after transfection with the CRISPR constructs, cells were selected in culture medium containing puromycin (B16-F1: 2 µg/mL, MVD7: 4 µg/mL and NIH 3T3: 5 µg/mL) for 4 days, and then cultivated for 24 h in the absence of puromycin. For isolation of clonal knockout cell lines, single cells were seeded by visual inspection into 96 well microtiter plates and expanded in pre-conditioned culture medium containing 20% BriClone supplement (NICB, Dublin City University, Ireland). Clones were analyzed by the TIDE sequence trace decomposition web tool (https://tide.deskgen.com/, (Brinkman et al., 2014)) and immunoblotting using specific antibodies, and subsequently verified by sequencing (Supplementary file 1). For the latter, respective DNA fragments of about 500 bp encompassing the target sites were first amplified from genomic DNA using the Evl primer combination 5’-AAGCCATGAGTCTCCCAAGC-3’ and 5’-GTCTCACGCTTTGGCTCTCA-3’, the VASP primer combination 5’-GTGTGGCCTGCCTATCTGTT-3’ and 5’-CAGAGGGACAGAGGGACAGA-3’, and the Mena primer pair 5’-TCAGGCAACTGCAAGAACAG-3’ and 5’-CATCTCGGCTGTAGGAGGTG-3’. Subsequently, amplified fragments were inserted into pJet1.2 vector (Thermo Fisher Scientific, Carlsbad, CA) and used for transformation of E. coli host DH5α (Thermo Fisher Scientific). At least 30 sequences were analyzed for each knockout clone.
Generation of recombinant proteins and antibodies
For expression of recombinant proteins, the E. coli strain Rossetta 2 (Novagen) was used. Expression of GST-tagged fusion proteins was induced using 1 mM isopropyl-ß-D-thiogalactoside (IPTG) (Carl Roth, Karlsruhe, Germany) at 21°C for 14 h. Purification of respective proteins from bacterial extracts was performed by affinity chromatography using Protino glutathione-conjugated agarose 4B (Macherey-Nagel, Düren, Germany). Except for GST-WAVE2-WCA, the GST-tag was removed by proteolytic cleavage with PreScission Protease (GE Healthcare), followed by a final polishing step of the proteins by size-exclusion chromatography on an Äkta Purifier System using either HiLoad 26/600 Superdex 200 or HiLoad 26/75 Superdex columns (GE Healthcare). Purified proteins were dialyzed against immunization buffer (150 mM NaCl, 1 mM dithiothreitol (DTT), and 20 mM Tris/HCl pH 7.4) and stored in aliquots at −20°C. Polyclonal antibodies against Evl, VASP, Mena, cortactin and WAVE2 were raised by immunizing New Zealand white rabbits with respective recombinant proteins following standard procedures. Evl, Mena and cortactin polyclonal antibodies were subsequently purified by affinity chromatography using antigens conjugated to sepharose. GST antibodies in the anti GST-WAVE2-WCA serum were absorbed by GST-affinity chromatography. Monoclonal antibodies against fascin were produced upon injecting recombinant fascin mixed with CpG-DNA as adjuvant (Magic Mouse, Creative, NY, USA) into 7 week-old, female mice (Charles River, USA) using standard hybridoma technology and antibody screening procedures in the PROCOMPAS graduate programme-funded monoclonal antibody facility (Braunschweig Integrated Centre of Systems Biology - BRICS, Technische Universität Braunschweig). Hybridoma clone 5E2 was selected and kindly provided by Sabine Buchmeier and Prof. Dr. em. Brigitte Jockusch (TU Braunschweig).
Antibodies used
Immunoblotting was performed according to standard protocols using rabbit polyclonal antibodies directed against Evl (1:1000 dilution), VASP (1:1000 dilution), Mena (1:1000 dilution), WAVE2 (1:1000 dilution), GFP (1:2000) (Faix et al., 2001) or mouse monoclonal antibody against glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (1:1000; #CB1001-500UG, Merck (Darmstadt, Germany)) and anti-fascin antibody 5E2 (undiluted hybridoma supernatant). Primary antibodies in immunoblots were visualized using phosphatase-coupled anti-mouse (1:1,000 dilution; #115-055-62, Dianova (Hamburg, Germany)) or anti-rabbit antibodies (1:1,000 dilution; #115-055-144, Dianova). For immunofluorescence, the following primary antibodies were used: rabbit anti-cortactin antibodies (1:1000 dilution), mouse monoclonal anti-fascin antibody 5E2 (undiluted hybridoma supernatant), mouse monoclonal anti-ArpC5A (p16-ArcA) from hybridoma culture clone 323H3 (Olazabal et al., 2002), mouse monoclonal anti-capping protein α1/α2 subunits (hybridoma supernatant; 1:4 dilution; Developmental Hybridoma Bank, University of Iowa, Iowa City, IA) and mouse monoclonal anti-vinculin antibody (1:1,000 dilution, #V9131, clone hVIN-1, Sigma). Primary antibodies were visualized in immunohistochemistry with polyclonal Alexa-555-conjugated goat-anti-rabbit (1:1,000 dilution; #A21429, Invitrogen (Carlsbad, CA)) or goat-anti-mouse (1:1,000 dilution, #A32727, Invitrogen) and Alexa-488-conjugated goat-anti-rabbit (1:1,000 dilution; #A-11034, Invitrogen) or goat-anti-mouse antibodies (1:1,000 dilution; #A-11029, Invitrogen). To enhance EGFP signals, Alexa488-conjugated nanobodies from Chromotek (Chromotek, (Planegg-Martinsried, Germany)) (1:200 dilution; #gba488) were used. Atto550-phalloidin (1:250 dilution, #AD 550-82, Atto-Tec (Siegen, Germany)) was used for visualization of F-actin.
Immunoblotting
For preparation of total cell lysates, cells were cultured to confluency and trypsinized. Cell pellets were washed twice with cold PBS and lysed with cold RIPA buffer (150 mM NaCl, 1.0% Triton X-100, 0.5% sodium deoxycholate, 0.1% sodium dodecyl sulfate (SDS), 50 mM Tris, pH 8.0) supplemented 5 mM benzamidine (Carl Roth), 0.1 mM AEBSF (AppliChem, Darmstadt, Germany) and Benzonase (1:1000, Merck) for 1 h at 4°C on a wheel rotator. Cell lysates were subsequently homogenized by passing the lysate 10 times through a syringe cannula (Braun). Protein contents of total cell lysates were determined by Pierce BCA assay (Thermo Fisher Scientific) using a Synergy 4 fluorescence microplate reader (Biotek, Bad Friedrichshall, Germany) according to manufacturer’s protocol. 50 µg (B16-F1) or 100 µg (MEF) of total proteins per lane were subjected to SDS-PAGE, and transferred by semi-dry blotting onto nitrocellulose membranes (Hypermol, Hannover, Germany). Blotting membranes were then blocked with NCP buffer (10 mM Tris/HCL, 150 mM NaCl, 0.05% Tween-20, 0.02% NaN3, pH 8.0,) containing 4% bovine serum albumin (BSA) for 1 h and incubated with primary antibodies overnight in the same buffer. After extensive washing of membranes and incubation with secondary, phosphatase-conjugated antibodies for at least 2 hours, blots were developed with 20 mg/mL of 5-brom-4-chlor-3-indolylphosphat-p-toluidin (BCIP) in NaHCO3, pH 10.0.
Quantification of cellular Ena/VASP protein concentrations
Total cellular concentrations of Ena/VASP family members in used cell lines were quantified from immunoblots using orthologue-specific antibodies by titrating total cell lysates corresponding to a defined number of B16-F1, NIH 3T3 and MVD7 cells with dilution series containing defined amounts of recombinant proteins loaded on the same gels. Band intensities were quantified by ImageJ software. The cell volume for each cell type was calculated from phase-contrast images of freshly trypsinized, non-adherent cells (>50 for each cell line). The volume of the nucleus was subtracted from the cell volume to obtain the volume of the cytoplasm.
Live cell imaging
Time-lapse imaging of cells was performed using an Olympus XI-81 inverted microscope (Olympus, Hamburg, Germany) driven by Metamorph software (Molecular Devices, San Jose, CA) and equipped with objectives specified below and a CoolSnap EZ camera (Photometrics, Photometrics, Tucson, AZ). Cells were seeded onto 35 mm glass bottom dishes (Ibidi, Planegg-Martinsried, Germany) coated with either 25 µg/mL laminin (Sigma) in case of B16-F1 cells and derived clones, or with 10 µg/mL fibronectin (Roche, Penzberg, Germany) in case of MVD7, NIH 3T3 and their derivatives, and maintained in imaging medium composed of F-12 Ham Nutrient Mixture with 25 mM HEPES (Sigma), the latter to compensate for the lack of CO2, and supplemented with 10% FBS (Biowest, Nuaillé, France), 1% Penicillin-Streptomycin (Biowest), 2 mM stable L-glutamine (Biowest), and 2.7 g/L D-glucose (Carl Roth) in an Ibidi Heating System at 37 °C. For random motility assays, B16-F1cells were seeded at low density onto the dishes and allowed to adhere for 3 h. Subsequently, the medium was exchanged with imaging medium, the chamber mounted into a heating system, and cells recorded by time-lapse phase-contrast imaging at 60 sec intervals for 3 h using an Uplan FL N 4x/0.13NA objective (Olympus). NIH 3T3, MVD7 and derived cells were allowed to settle for 6 h after seeding and imaged at 10 min intervals for 10 h using an UPlan FL N 10x/0.30NA objective (Olympus). Single cell tracking was performed in ImageJ by MTrackJ. Analyses of cell speed and cell trajectories, turning angles, and mean square displacements were performed in Excel (Microsoft, Redmond, WA) using a customized macro (Litschko et al., 2018). Cells that contacted each other or divided were excluded from analyses. Directionality index ratio was determined by dividing the shortest distance between starting and end points (d) by the actual trajectories (D). For wound healing assays, MVD7 cells were seeded onto uncoated dishes and expanded to confluency. Subsequently, the monolayer was scratched with a 200 µL pipette tip, the cells washed three times with warm imaging medium, and recorded by phase-contrast time-lapse imaging at 10 min intervals for 20 h using an UPlan FL N 10x/0.30NA objective (Olympus). Wound closure rates were determined in ImageJ by measuring decrease of scratch area over time. Lamellipodial protrusions were recorded at 5 sec intervals for 10 min using an UPlan FL 40x/0.75NA objective (Olympus). Protrusion rates of advancing lamellipodia assessed over a time period of 2.5 min were quantified by first generating kymographs from time-lapse movies of the cell periphery using ImageJ, followed by slope determination from these kymographs. Lamellipodial persistence of randomly migrating B16-F1 and derived cells on laminin was determined by phase-contrast time-lapse microscopy using an UPlan FL N 10x/0.30NA objective and a frame rate of 1 frame per minute. Lamellipodial persistence was defined as time in min from initiation till collapse of the lamellipodium. Spreading of B16-F1 and derived clones was monitored at 30 sec intervals by time-lapse phase-contrast imaging for 30 min using an UPlan FL N 10x/0.30NA objective immediately after seeding, and spreading of MVD7 and derived lines were imaged at 30 sec intervals for 1 h using an Olympus LUCPlan FL N 20x/0.45NA objective. Quantification of cell spreading was executed with ImageJ from time-lapse movies by measuring increase of cell area over time. For epifluorescence imaging of fluorescently-labelled cells, B16-F1, EVM-KO and reconstituted EVM-KO cells expressing mCherry-VASP were transfected with either pEGFP-Lifeact or pEGFP-fascin. 24 h post transfection, cells were seeded into imaging medium, and after 3 h, migrating cells recorded by time-lapse imaging at 10 sec intervals for 10 min using an Olympus 40x/0.75NA Uplan FL objective. In motility and spreading assays, reconstituted cells expressing EGFP-tagged VASP, Evl or Mena were identified by epifluorescence imaging.
Immunofluorescence
If not indicated otherwise for immunofluorescence labelling, cells were fixed in pre-warmed, 4% PFA in PBS, pH 7.3 for 20 min, subsequently washed three times with PBS supplemented with 100mM glycine, permeabilized with 0.1% Triton X-100 in PBS for 3 min and blocked in PBG (PBS, 0.045% cold fish gelatin (Sigma), and 0.5% BSA). For immunolabeling with fascin, cells were fixed in −20°C cold methanol. For immunolabeling of capping protein, the cells were fixed with 3% glyoxal at pH 5.0 as described (Richter et al., 2018) and then treated as above. To reduce cytoplasmic background in vinculin stainings, cells were fixed for 1 min with 2% PFA in PBS containing 0.3% Triton X-100 and then postfixed with 4% PFA in PBS for 20 min as described (Kage et al., 2017). Primary antibodies were incubated overnight, followed by extensive washing of the specimens with PBG and incubation with respective secondary antibodies for at least 2 h. GFP signals were enhanced with Alexa488-conjugated nanobodies. F-actin was visualized with Atto550 phalloidin. Imaging of fixed cells was performed with an Olympus XI-81 inverted microscope equipped with an UPlan FI 100x/1.30NA oil immersion objective or an LSM510 Meta confocal microscope (Carl Zeiss, Jena, Germany) equipped with a Plan-Neofluar 63x/1.3NA oil immersion objective using 488 nm and 543 nm laser lines.
Fluorescence intensities of phalloidin or lamellipodial proteins were measured from still images captured at identical settings using ImageJ software after background subtraction. Mean pixel intensities in lamellipodial regions of interest are shown as whiskers-box plots including all data points.
Induction of filopodia in absence of lamellipodia
B16-F1 wild-type and EVM-KO cells were seeded onto low laminin (1µg/mL) for 1 h, a concentration lower than the threshold for inducing prominent lamellipodia in this cell type and conditions (data not shown). Then, the cells were incubated for 2 h with the Arp2/3 complex inhibitor CK666 (200 μM) to completely abolish lamellipodia and trigger filopodia formation. Subsequently, cells were fixed and stained with phalloidin for the actin cytoskeleton followed by confocal imaging.
Determination of actin assembly rates in lamellipodia
Quantification of lamellipodial actin polymerization rates were performed in B16-F1 and EMV-KO cells transiently expressing EGFP-ß-actin, by photobleaching protruding lamellipodial regions and subsequently measuring the distance of fluorescence recovery from the lamellipodium tip over time, essentially as described (Dimchev and Rottner, 2018). In brief, photobleaching of EGFP-ß-actin was accomplished on an epi-FRAP system, equipped with a 405-nm diode laser (settings: 65 mW laser power, 10-pixel laser beam diameter, 1ms bleach dwell time/pixel), which was manually triggered in selected regions during image acquisition (exposure time of 500 ms). Time-lapse imaging was performed with an interval of 1.5 sec between frames. Images were acquired with a 100x/1.4NA Plan apochromatic oil immersion objective on an inverted Axio Observer (Carl Zeiss, Jena, Germany) using a DG4 light source (Sutter Instrument, Novato, CA) for epifluorescence illumination and a CoolSnap-HQ2 camera (Photometrics, Tucson, AZ), driven by VisiView software (Visitron Systems, Puchheim, Germany).
Electron tomography
B16-F1 and EMV-KO cells were grown on Formvar-coated copper-palladium grids coated with 25 µg/mL laminin (Sigma) in laminin coating buffer (150 mM NaCl, 50 mM Tris, pH 7.5). The cells were simultaneously extracted and fixed for 1 min with 0.5% Triton X-100 (Fluka, Buchs, Switzerland) and 0.25% glutaraldehyde (Agar Scientific, Stansted, United Kingdom) in cytoskeleton buffer (10 mM MES buffer, 150 mM NaCl, 5 mM EGTA, 5 mM glucose and 5 mM MgCl2, pH 6.1), followed by a second fixation step for 15 minutes in cytoskeleton buffer (pH 7.0) containing 2% glutaraldehyde. The coverslips were incubated for additional 4-12 h in cytoskeleton buffer containing 2% glutaraldehyde at 4°C, and were then stained for electron microscopy. Negative staining was performed in mixtures of 4-6% sodium silicotungstate (Agar Scientific) at pH 7.0, containing 10 nm gold colloid saturated with BSA, and diluted 1:10 from a gold stock (Urban et al., 2010). Double tilt series of negatively stained cytoskeletons were acquired on a FEI Tecnai G20 transmission electron microscope (Tecnai, Hillsboro, OR) operated at 200 kV driven by SerialEM 3.x (Mastronarde, 2005) equipped with an Eagle 4k HS CCD camera (Gatan, Pleasanton, CA). Double axis tilt series were acquired with typical tilt angles from −65° to +65° and 1° increments following the Saxton scheme at a primary on-screen magnification of 25,000x. Actin filaments were tracked automatically using a Matlab-based tracking algorithm (Mueller et al., 2017; Winkler et al., 2012).
Focal adhesion analysis
Focal adhesions in MVD7 and derived cells were analyzed from confocal images captured at identical settings. Images of cells labelled for vinculin were first processed in ImageJ by background subtraction using a rolling ball radius of 40 pixels, exclusion of nuclear regions due to high background staining, and by contouring of cell perimeters. Preprocessed images were then analyzed by the Focal Adhesion Analysis Server (Berginski and Gomez, 2013). The parameters were set to imaging frequency 0 min, detection threshold 3.5 and minimal adhesion size of 10 pixels. Obtained values for FA intensity and size as well as number of FAs/cell were subsequently analyzed in Excel.
Traction force analysis
Patterned polyacrylamide hydrogels were fabricated according to the Mask method (Vignaud et al., 2014). A quartz photomask was first cleaned through oxygen plasma (AST product, 300W) for 3.5 min at 200 W. Areas containing crossbow patterns were then incubated with 0.1 mg/mL Poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) (JenKem Technology, Plano, TX) in 10mM HEPES, pH 7.4 for 30 min. After a rapid de-wetting step, PLL-PEG was burned using deep-UV exposure for 6 min. Patterns on the mask were then incubated with a mix of 10 µg/mL fibronectin (Sigma) and 10 µg/mL fibrinogen Alexa Fluor 647 conjugate (Invitrogen) in 100 mM sodium bicarbonate buffer pH, 8.4 for 30 min. A mix of 8% acrylamide (Sigma) and 0.264% bis-acrylamide solution (Sigma) was degassed for 30 min, mixed with 0.2 µm diameter PLL-PEG-coated fluorescent beads (Fluorosphere #F8810, Life Technologies (Carlsbad, CA)) and sonicated before addition of APS and TEMED. 25 µL of this solution was added onto the micropatterned photomask, covered with a silanized coverslip, and allowed to polymerize for 25 min before being gently detached in sodium bicarbonate buffer. Micropatterns were stored overnight in sodium bicarbonate buffer at 4°C before plating cells. Non-patterned hydrogels were prepared using the same polyacrylamide polymerization procedure followed by a step of oxygen plasma treatment for 30 sec at 30 W of the coverslips and subsequent incubation with fibronectin for 30 min.
For identification of untransfected MVE-KO cells on patterns, the cells were stained with 1 µM CellTracker Green (Invitrogen) for 5 min followed by removal of unbound dye with PBS. In case of reconstituted MVE-KO cells, only cells expressing EGFP-Evl that exhibited appropriate localization of the fusion protein at focal adhesions on the patterns were selected for performing traction force experiments. Images were acquired on an EclipseTi-E Nikon confocal spinning disk microscope equipped with a CSUX1-A1 Yokogawa confocal head. Data were analyzed with a set of macros in Fiji (Martiel et al., 2015). Displacement fields were obtained from bead images prior and after removal of cells by trypsin treatment. Bead images were first paired and aligned to correct for experimental drift. Displacement field was calculated by particle imaging velocimetry (PIV) on the base of normalized cross-correlation following an iterative scheme. Erroneous vectors where discarded owing to their low correlation value and replaced by the median value of neighboring vectors. Traction-force field was subsequently reconstructed by Fourier Transform Traction Cytometry, with a regularization parameter set to 3.2×10-10.
Statistical analyses
Quantitative experiments were performed at least in triplicates to avoid any potential bias of environmental influences or unintentional error. Impact on lamellipodial or adhesion phenotypes derived from analyses on fixed samples and living cells were systematically obtained from sample sizes of dozens or hundreds of cells, respectively. Raw data were processed in Excel (Microsoft). Statistical analyses were performed using SigmaPlot 11.0 software (Systat Software, Erkrath, Germany) or GraphPad Prism 5 (GraphPad, San Diego, CA). All data sets were tested for normality by the Shapiro-Wilk test. Statistical differences between not normally distributed datasets of two groups were determined by the non-parametric Mann-Whitney U rank sum test. For comparison of more than 2 groups, statistical significance of normally-distributed data was examined by one-way ANOVA and Tukey Multiple Comparison test. In case of not normally-distributed data, the non-parametric Kruskal-Wallis test and Dunn’s Multiple Comparison test were used. Statistical differences were defined as *p≤0.05, **p≤0.01, ***p≤0.001 as well as n.s., not significant, and are displayed and mentioned in figures and figure legends, respectively. Graphs were created with Origin 2018G (OriginLab, Northampton, Ma), and final figures prepared with Photoshop (Adobe, San Jose, CA) and CorelDraw Graphics Suite 6X (CorelDraw, Ottawa, On).
Competing interests
The authors declare that no competing interests exist.
The following video and figure supplement are available for figure 1:
Figure 1-video 1. Loss of Ena/VASP proteins in B16-F1 cells impairs cell migration, related to Figure 1. Random migration of B16-F1, and derived Evl-KO (E-KO), Evl/VASP-KO (EV-KO) and Evl/VASP/Mena-KO (EVM-KO) mutants on laminin. Cells were recorded by phase-contrast time-lapse imaging using a 4x objective and tracked by MTrackJ to illustrate representative cell trajectories. Note more directional movement of EVM-KO cells. Time is indicated in h:min. Bar, 300 µm.
The following figure supplement is available for figure 2:
The following video and figure supplement are available for figure 3:
Figure 3-video 1. Elimination of all three Ena/VASP proteins impairs lamellipodium formation and abolishes microspikes, related to Figure 3. Displayed cells were transfected with EGFP-LifeAct to monitor actin dynamics during migration on laminin by time-lapse imaging using a 40x objective. EVM-KO cells were additionally transfected with mCherry-VASP (not shown). Reconstitution of EVM-KO cells with mCherry-VASP restored lamellipodium and microspike formation. Time is indicated in min:s. Bar, 15 µm.
Figure 3-video 2. Elimination of all three Ena/VASP proteins abrogates microspikes, related to Figure 3. The cells shown were transfected with EGFP-fascin to monitor microspikes in cells migrating on laminin by time-lapse imaging using a 40x objective. EVM-KO cells were additionally transfected with mCherry-VASP (not shown). Reconstitution of EVM-KO cells with mCherry-VASP fully restored microspike formation (right panel). Time is indicated in min:s. Bar, 15 µm.
The following figure supplement is available for figure 4:
The following video and figure supplement are available for figure 5:
Figure 5-video 1. Impaired protrusion of EVM-KO cells migrating on laminin, related to Figure 5. Reconstitution of the triple mutant cells with EGFP-VASP fully restored lamellipodial protrusion. Cells were recorded by time-lapse phase-contrast imaging using a 40x objective. Time is indicated in min:s. Bar, 15 µm.
Figure 5-video 2. Loss of Ena/VASP in B16-F1 cells reduces actin network polymerization, related to Figure 5. Representative time-lapse movies showing FRAP experiments on lamellipodia of B16-F1 wild-type or EVM-KO cells, as indicated, expressing EGFP-β-actin on laminin. Time-lapse movies show periods before and after bleaching of lamellipodial regions with bleaching time points set to 0 sec in each case. Bar, 3 µm.
The following video is available for figure 6:
Figure 6-video 1. Loss of Ena/VASP deteriorates lamellipodium architecture in B16-F1 cells, related to Figure 6. Animated visualization of the three-dimensional organization of lamellipodia of B16-F1 wild-type and EVM-KO cells resolved by electron tomography. A 3D model of the actin filament network corresponds to Figure 6A and was obtained by automatic tracking of filaments through the tomogram slices.
The following video and figure supplement are available for figure 7:
Figure 7-video 1. Loss of Evl in MVD7 cells diminishes spreading on fibronectin, related to Figure 7. MVD7, MVE-KO and reconstituted cells expressing EGFP-Evl were seeded onto fibronectin and recorded by time-lapse phase-contrast imaging using a 20x objective. Reconstituted cells are highlighted by asterisks. Note enhanced spreading in MVE-KO cells expressing EGFP-Evl. Time, min:s. Scale bar, 25 µm.
The following figure supplement is available for figure 8:
Acknowledgements
This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG), grants FA330/11-1 (to J.F.), RO2414/5-1 PROCOMPAS graduate program GRK2223/1 (to K.R.) as well as by ERC Advanced grants AAA 741773 (to L.B.) and CoG 724373 (to M.S.), and the Austrian Science Foundation (FWF) (to M.S.). We also thank Prof. Dr. em. Brigitte Jockusch and Sabine Buchmeier (PROCOMPAS monoclonal antibody facility, TU Braunschweig) for antibody generation, and the ANR-17-EURE-003 for sponsoring the live microscopy facilities of DBSCI department of IRIG (MuLife).