Non-canonical Wnt-Frizzled (Fz)/planar cell polarity (PCP) signaling plays a decisive role in cell polarization and coordinated tissue morphogenesis in metazoan development (Gray et al., 2011; Singh and Mlodzik, 2012). Initially, core components of the Wnt-Fz/PCP signaling module were identified in Drosophila to determine coordinated planar polarity of epithelial cells (Gubb and García-Bellido, 1982; Lawrence and Shelton, 1975; Vinson and Adler, 1987; Seifert and Mlodzik, 2007). Subsequently, vertebrate orthologs of key Wnt-Fz/PCP signaling components were shown to be required for both epithelial planar cell polarity and coordinated mesenchymal cell polarization, migration and intercalation (Gray et al., 2011; Singh and Mlodzik, 2012). Notably, signaling molecules and mechanisms determining epithelial cell polarity are largely conserved between vertebrate and invertebrate organisms indicating the general importance of Wnt-Fz/PCP in animal development (Yang and Mlodzik, 2015; Seifert and Mlodzik, 2007).

The key feature of Wnt-Fz/PCP signaling to achieve epithelial cell polarization is the polarized subcellular distribution of Wnt-Fz/PCP components in a tissue-wide and coordinated manner (Strutt, 2001; Feiguin et al., 2001; Tree et al., 2002). This polarized distribution and activation of Wnt-Fz/PCP components is thought to trigger spatially confined changes in the cytoskeletal architecture of epithelial cells leading to their morphological recognizable polarization (Strutt and Warrington, 2008; Ségalen et al., 2010; Strutt et al., 2011; Butler and Wallingford, 2018). In contrast, much less is known about how Wnt-Fz/PCP signaling controls mesenchymal cell polarization and movement (Shindo and Wallingford, 2014).

Previous studies on non-canonical Wnt signaling in mesenchymal cells have demonstrated that Wnt-Fz/PCP signaling components with a known function in epithelial PCP are required for proper cell polarization and intercalation (Gray et al., 2011). Moreover, the Wnt-Fz/PCP core components Prickle (Pk) and Dishevelled (Dsh) have been shown to localize preferentially to the anterior or posterior site of mesenchymal cells, respectively, pointing to the possibility that localized activity of these components triggers mesenchymal cell polarization (Ciruna et al., 2006; Yin et al., 2008). Likewise, studies on the role of non-canonical Wnt signaling in mesenchymal myoblast cells provided evidence for the capacity of graded Wnt11 signals in directing myoblast cell elongation (Gros et al., 2009). These data suggest that non-canonical Wnt signaling instructively polarizes mesenchymal cells similar to its function in epithelial cells (Figure 1A). In contrast, the observation that uniform overexpression of non-canonical Wnt signaling ligands can rescue defective mesenchymal cell polarization in Wnt-Fz/PCP mutant embryos (Heisenberg et al., 2000) points to a permissive function of Wnt-Fz/PCP signaling in this process (Figure 1A). While these overexpression and localization studies suggest both a permissive and instructive role for Wnt-Fz/PCP signaling in mesenchymal cell polarization and intercalation, direct evidence for non-canonical Wnt signaling exerting either of these functions is still lacking.

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Characterization of MZfz7a/b mutants at the embryo and tissue level.

https://doi.org/10.7554/eLife.42093.004

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In zebrafish, Wnt-Fz/PCP signaling is required for directed migration of mesenchymal mesendoderm progenitors during gastrulation. Specifically, migration directionality and coordination of anterior axial mesendoderm, the so-called prechordal plate (ppl) progenitors, is diminished in loss-of-function mutants for the Wnt-Fz/PCP ligand wnt11 (Ulrich et al., 2003; Ulrich et al., 2005). Ppl progenitors are specified at the dorsal germ ring margin at the onset of gastrulation (Warga and Kimmel, 1990; Kimmel et al., 1995; Montero et al., 2005). Upon specification, ppl progenitors internalize by coordinated ingression and then migrate as a coherent cluster of cells in a straight path from the germ ring margin towards the animal pole of the gastrula (Warga and Kimmel, 1990; Montero et al., 2005). Recent work showed that migration of ppl progenitors between the yolk cell and the overlying ectoderm can generate frictional forces, which modulate ectoderm cell movements during gastrulation (Smutny et al., 2017).

We reasoned that instructive and permissive functions of non-canonical Wnt signaling in mesenchymal cell polarization and migration could be distinguished using an approach that allows uniform activation of Wnt-Fz/PCP signaling within the entire cell population. If the ppl was indeed guided by an instructive mechanism, uniform activation should not be able to rescue the ppl cell migration defect in Wnt-Fz/PCP signaling defective embryos. Uniformly activating Wnt-Fz/PCP signaling requires cells to be non-responsive to endogenous Wnt ligands. To achieve ligand-independent uniform activation, we have developed a light-activated version of the Wnt-Fz/PCP receptor Fz7, allowing us to trigger Fz7/PCP signaling in a spatiotemporally controlled manner directly at the receptor level. Using this Opto-Fz7 receptor in fz7 mutant embryos, we show that localized Fz7 signaling has the potential to direct ppl cell migration. Unexpectedly, however, uniform activation of Fz7 signaling was also sufficient to rescue defective ppl cell migration in fz7 mutant embryos. This suggests that endogenous Fz7/PCP signaling has a permissive rather than instructive function in directed ppl cell migration during gastrulation.

For analyzing how Wnt-Fz/PCP signaling functions in directed mesenchymal cell migration, we turned to zebrafish embryos, which have previously been shown to require Wnt-Fz/PCP signaling for directed migration of mesendoderm progenitor cells during gastrulation (Topczewski et al., 2001; Jessen et al., 2002; Ulrich et al., 2003; Dumortier et al., 2012). Specifically, we determined how directed migration of ppl progenitors is affected in mutant embryos for the Wnt11 receptor Fz7 (Djiane et al., 2000; Witzel et al., 2006), which display strongly impaired gastrulation movements (Quesada-Hernández et al., 2010). Our analysis showed that in MZfz7a/b double mutants, beside the previously reported dorsal convergence defects (Quesada-Hernández et al., 2010) (Figure 1—figure supplement 1, Figure 1—source data 1), the ppl was elongated and placed closer to the anterior end of the notochord at the end of gastrulation (bud stage, 10 hrs post-fertilization, hpf) (Figure 1B–D, Figure 1—source data 1). This phenotype points to the possibility that Fz7 is required for directed ppl cell migration in zebrafish.

To investigate this possibility, we determined how the movement velocity of ppl progenitors towards the animal pole of the gastrula and their movement coordination with the overlying ectoderm are affected in MZfz7a/b mutants compared to wild type (wt) embryos. Consistent with previous observations (Smutny et al., 2017), we found that wt ppl progenitors migrated with a velocity of 2–3 μm/min towards the animal pole, and that at late gastrulation (90% epiboly) this movement was highly correlated with the movement of overlying ectoderm progenitors (Figure 1G&H, Video 1, Figure 1—source data 1). In comparison, ppl progenitors in MZfz7a/b mutant embryos slowed down their animal pole directed movement during gastrulation, and eventually even moved backwards towards the vegetal pole from mid-gastrulation on (Figure 1F&H, Video 1, Figure 1—source data 1). Interestingly, movement coordination between ppl and ectoderm progenitors remained unchanged in MZfz7a/b mutant embryos (Figure 1G, Figure 1—source data 1), arguing against a previously suggested function of Fz7 in controlling the interaction between ppl and overlying ectoderm (Winklbauer et al., 2001; Montero et al., 2005).

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To test whether Fz7 is cell-autonomously required for ppl progenitor cell migration, we transplanted ppl cells between wt and MZfz7a/b mutant embryos at the onset of gastrulation (shield stage, 6 hpf). As controls, we performed homotypic transplantations from wt donor into wt host and from mutant donor into mutant host embryos. When wt cells were transplanted into the ppl of MZfz7a/b mutant embryos, these cells typically moved to the leading edge of the ppl and showed lower movement correlation with their neighboring host cells when compared to transplanted mutant cells in mutant embryos (Figure 2A&D, Video 2, Figure 2—source data 1). Conversely, mutant ppl progenitors transplanted into the ppl of wt embryos typically ended up closer to the posterior end of the ppl and showed reduced movement correlation with their wt neighboring cells when compared to transplanted wt cells in wt embryos (Figure 2A&C, Video 2, Figure 2—source data 1). Together, these transplantation experiments suggest that Fz7 functions cell-autonomously in ppl progenitors by determining their ability for undergoing directed migration.

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Characterization of MZfz7a/b mutants cells in homotypic and heterotypic transplantation experiments.

https://doi.org/10.7554/eLife.42093.007

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To identify the cellular effector processes by which signaling through the Fz7 receptor affects directed ppl cell migration, we first took a reductionist approach and cultured isolated ppl progenitor cells in small clusters on Fibronectin-coated substrates and analyzed their protrusive behavior in vitro. We found that after 4 hrs in culture wt progenitors at the margin of these clusters showed pronounced substrate spreading and formed multiple protrusions, resembling lamellipodia (54% of cells on the margin of clusters; Figure 3A&B). Ppl progenitors from MZfz7a/b mutant embryos, in contrast, formed significantly less lamellipodia-like protrusions (27% of marginal cells; Figure 3A&B). This suggests that Fz7 is required for the formation of lamellipodia in ppl progenitors during substrate spreading. To assess the relevance of this in vitro observation for the situation in vivo, we compared protrusion formation of ppl cells in wt and MZfz7a/b mutant embryos between 75% and 90% epiboly stage (8–9 hpf). Consistent with our in vitro observations, ppl cells formed significantly less protrusions resembling lamellipodia or pseudopodia in MZfz7a/b mutants (0.14 ± 0.059 protrusions/cell/minute) compared to wt embryos (0.33 ± 0.075 protrusions/cell/minute), while the number of blebs increased and of protrusions resembling filopodia remained largely unchanged (Figure 3C–F, Video 3). Importantly, the preferential orientation of these different protrusion types in the direction of ppl progenitor cell migration towards the animal pole was indistinguishable between MZfz7a/b and wt embryos (Figure 3G, Figure 3—source data 1). Together, this suggests that Fz7 is required for the type but not orientation of protrusions formed in leading edge ppl cells.

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Protrusion formation in wt and MZfz7a/b mutant embryos.

https://doi.org/10.7554/eLife.42093.010

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To investigate whether spatially restricted activation of Fz7 signaling has the potential to drive ppl cell polarization and direct their migration, we developed a light-activated version of Fz7. The rationale underlying our protein engineering approach was that the functional domains of seven-pass transmembrane (TM) receptors can be delineated in their canonical topology (Airan et al., 2009; Siuda et al., 2015; Li et al., 2015; Gunaydin et al., 2014; Barish et al., 2013; van Wyk et al., 2015; Morri et al., 2018). In particular, the N-terminus, three extracellular loops and seven TM domains are typically associated with ligand binding and receptor activation, whereas the three intracellular loops (ICL; ICL 1–3) and the C-terminus are typically associated with downstream signal transmission. Following this rationale, we substituted the intracellular domains of the light-sensitive receptor rhodopsin with the corresponding domains of fz7 (Figure 4A, Figure 4—figure supplement 1). We identified these domains in rhodopsin and fz7 sequences based on previous domain shuffling experiments of Class A and Class F GPCRs and mutagenesis analysis of frizzled receptors (Liu et al., 2001; Kim et al., 2005; Tauriello et al., 2012). We generated two receptor variants (Figure 4A, Figure 4—figure supplement 1). In the first variant, all intracellular elements of Rhodopsin were substituted by those of Fz7. In the second variant, taking into account that all known Dsh-binding domains of Fz are found in ICL3 and the C-terminus (Wong et al., 2003; Schulte and Bryja, 2007; Tauriello et al., 2012), the ICL1 and ICL2 of rhodopsin were retained to minimize perturbations to the original Rhodopsin fold. Because chimeric receptors of Rhodopsin and members of the more distantly related Class F GPCRs have never been generated, we applied molecular modeling to evaluate receptor compatibility. We found that the inactive and active state conformations of Rhodopsin and Smoothened (Smo), a surrogate receptor for Fz7 and the sole Class F GPCR for which inactive and active state structures are available, can be superimposed revealing a conserved arrangement of the TM helices (Figure 4—figure supplement 2; RMSD 2.8 and 2.6 Å for the inactive and active states, respectively). In addition, analysis of non-covalent contact networks (Venkatakrishnan et al., 2016; Kayikci et al., 2018) revealed similar numbers of inter-helical contacts that stabilize these receptors again in both receptor states (inactive state: 223 and 217 contacts for Rhodopsin and Smo; active state: 197 and 173 contacts for Rhodopsin and Smo). These contacts were distributed in a similar manner with ~70% of contacts occurring between helices TM1 and TM2, TM2 and TM3, TM3 and TM4/5, TM5 and TM6, and TM6 and TM7 in both receptors. Collectively, these results suggest structural similarities toward the design of chimeric proteins that are non-responsive to endogenous ligands and, instead, can be activated by light.

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Opto-Fz7 characterization and its effect on cell migration upon spatially restricted activation.

https://doi.org/10.7554/eLife.42093.018

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To test whether these receptors can be used for spatiotemporally controlled ligand-independent activation of Fz7 signaling within the ppl, we first determined the subcellular localization of the two receptor-variants within progenitor cells. Following ectopic expression by mRNA injection at the one-cell stage, the second receptor variant containing the ICL3 and C-terminal intracellular domains of Fz7 showed clear and uniform localization at the plasma membrane of all progenitor cell types analyzed within the gastrulating embryo at 70% epiboly stage, including ppl, epiblast, nc and paraxial mesendoderm cells (7 hpf; Figure 4B, Figure 4—figure supplement 3A–C, Figure 4—source data 1). The first variant containing all four intracellular domains of Fz7, in contrast, did not clearly localize to the plasma membrane (Figure 4—figure supplement 1B), and we thus decided to solely focus on the second variant (termed Opto-Fz7) for the remainder of our study.

Next, we asked whether Opto-Fz7 is internalized upon light activation, given previous observations that non-canonical Fz receptor activation is followed by receptor internalization (Onishi et al., 2013). We found Opto-Fz7 to re-localize from the plasma membrane to intracellular vesicles upon light exposure (Figure 4C, Figure 4—figure supplement 3A, Figure 4—source data 1), suggesting that Opto-Fz7 signaling is activated by light. Next, we tested whether OptoFz7 specifically triggers non-canonical Wnt signaling, as suggested for Fz7 in zebrafish (Quesada-Hernández et al., 2010). To this end, we analyzed how the subcellular distribution of Dvl and β-catenin change upon Opto-Fz7 activation, previously suggested to be sensitive to non-canonical (Dvl) and canonical Wnt signaling (β-catenin), respectively (Park et al., 2005; Witzel et al., 2006), Yu et al., 2007, Kim et al., 2008a; Gao and Chen, 2010). We found that zebrafish Dvl2-GFP co-localized with Opto-Fz7-mCherry at the plasma membrane, and that Dvl, much like Opto-Fz7, was endocytosed upon light activation (Figure 4—figure supplement 4A&B, Figure 1—source data 1). In contrast, we detected no recognizable changes in nuclear β-catenin localization upon light activation of Opto-Fz7 in embryos at 5.5 hpf, while uniform overexpression of the canonical Wnt ligand Wnt8 elicited nuclear accumulation of β-catenin throughout the embryo (Figure 4—figure supplement 4C–F, Figure 4 – source data; Brunet et al., 2013). Together, these findings indicate that Opto-Fz7 can be used for spatiotemporally controlled activation of non-canonical Fz7 signaling within the ppl.

Next, we asked whether spatially restricted activation of Fz7 signaling could affect ppl progenitor cell migration. To this end, we cultured small clusters of MZfz7a/b mutant ppl progenitor cells expressing Opto-Fz7 on Fibronectin-coated substrates and determined how local activation of Fz7 by light in approximately half of the cluster would affect cluster protrusion formation and movement. We found that local activation of Opto-Fz7 in these clusters triggered movement of the entire cell cluster in the direction of the activated region within the cluster (Figure 4D&E, Video 4, Figure 4—source data 1). This cluster movement was accompanied by increased protrusion formation at the light-exposed side of the explant (Figure 4—figure supplement 5A&B, Figure 4 – source data, Video 4), suggesting that localized stimulation of cell motility by unilateral Opto-Fz7 activation triggers this movement. To test how this in vitro observation relates to the situation in vivo, we expressed Opto-Fz7 in MZfz7a/b mutant embryos and activated Fz7 in one half of the ppl. We then asked whether Fz7 activation of half of the ppl would redirect ppl cell movements. Strikingly, we found that the ppl motion was biased to the direction of the activated side (Figure 4F&G, Figure 4—figure supplement 5E&F, Figure 4 – source data, Video 5), an effect that could be effectively reverted by switching the activation of Fz7 to the initially non-activated side of the ppl (Figure 4G, Figure 4 – source data, Video 5). Furthermore, comparing protrusion formation at the leading edge of the activated versus non-activated side of the ppl showed a higher number of lamellipodia-like protrusions at the activated side (Figure 4—figure supplement 5C&D, Figure 4—source data 1), suggesting that Fz7 redirects ppl migration by triggering protrusion formation locally within the ppl. In contrast, control MZfz7a/b mutant embryos expressing Rhodopsin instead of Opto-Fz7 did not show any redirection of ppl cell migration upon half-sided light stimulation (Figure 4F&G, Figure 4—figure supplement 5E&F, Figure 4—source data 1). Collectively, these findings suggest that locally restricted activation of Fz7 has the capability of redirecting ppl cell migration.

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Our data so far demonstrate that exogenous Fz7 activation can control the migration direction of ppl progenitor cells, pointing to the possibility that localized endogenous Fz7 activation may direct ppl progenitor cell protrusion formation and migration. To address this possibility, we sought to uniformly activate Fz7signaling in ppl progenitors both in vitro and in vivo and determine whether such uniform activation is compatible with ppl cell protrusion formation and directed migration. We reasoned that in case localized endogenous Fz7 activation directs ppl progenitor cell protrusion formation and migration, then uniform activation in MZfz7a/b mutants would be insufficient to restore these processes. First, we asked whether reduced protrusion formation at the margin of small clusters of MZfz7a/b mutant ppl progenitors in culture could be restored by uniform activation of Fz7 signaling. Protrusion formation was fully rescued in cultured clusters of MZfz7a/b mutant ppl progenitors upon uniform light activation of Opto-Fz7 in these cells (Figure 5A&B, Figure 5—source data 1). Moreover, uniform over-activation of Fz7signaling in wt ppl cell clusters by uniform light activation of Opto-Fz7 led to reduced protrusion formation (Figure 5A&B, Figure 5—source data 1), consistent with previous observations that over-activation of Wnt-Fz/PCP signaling mimics the loss-of-function phenotype (Heisenberg et al., 2000; Kilian et al., 2003). This points to the possibility that Fz7 signaling has a mere permissive function in ppl cell protrusion formation. To further test this possibility, we also attempted to rescue the ppl cell movement phenotype in MZfz7a/b mutant embryos by uniform activation of Opto-Fz7. Strikingly, we found that uniform activation of Fz7 signaling in MZfz7a/b mutant embryos expressing Opto-Fz7 fully rescued ppl movement as judged by ppl shape and distance to the anterior tip of the notochord (Figure 5C–E, Figure 5—source data 1). This clearly argues against a critical requirement of localized Fz7 activation for directed ppl cell migration. Likewise, we found that uniform activation of Fz7 in ppl cells could fully rescue their cell-autonomous migration and protrusion formation defects when transplanted into the ppl of a wt embryo (Figure 5F–I, Figure 5—figure supplement 1, Video 6 and 7, Figure 5—source data 1). In contrast, no such rescue activity was detected in control conditions (absence of light, Rhodopsin overexpression together with 9-cis-retinal injection, and 9-cis-retinal injection alone; Figure 5C–E, Figure 5—figure supplement 2, Figure 5—source data 1). Together, these findings suggest that Fz7 signaling has a permissive function in ppl cell protrusion formation and directed migration during gastrulation.

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Rescue of MZfz7a/b embryos by Opto-Fz7 activation.

https://doi.org/10.7554/eLife.42093.026

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The mechanisms by which non-canonical Wnt-Fz/PCP signaling affects mesenchymal cell polarization, directed migration, and intercalation are still debated. There is increasing evidence that non-canonical Wnt-ligands, including Wnt11, are required for directed cell migration, and that gradients of exogenous Wnt11 can direct elongation of myogenic cells. However, whether and how endogenous Wnt11 directs cell migration remains unclear.

To control Wnt-Fz/PCP signaling in a ligand-independent manner, we developed Opto-Fz7, a light-activated variant of the Wnt receptor Fz7, which permits spatio-temporally controlled and ligand-independent Wnt-Fz/PCP signaling activation. The advantage of Opto-Fz7 over, for example, constitutive active versions of this receptor is not only that Opto-Fz7 allows spatiotemporally restricted activation, but also that Opto-Fz7 is entirely insensitive to activation by endogenous Wnt ligands within the embryo. This excludes any potential polarizing influence of endogenous Wnt ligands on Fz7 signaling activity and thus permits truly uniform signaling activation to distinguish between instructive versus permissive functions of Fz7 signaling within the embryo. Our results highlight how this light-activated receptor in the context of the transparent zebrafish model system can be used to determine how signals shape cellular processes during development (Sako et al., 2016; Johnson et al., 2017; Izquierdo et al., 2018).

Our findings that Wnt11 and Fz7 are required for directed ppl cell migration, and that uniform activation of Fz7 signaling in ppl cells is sufficient for their directed migration, suggest that any potentially instructive function of Fz7 signaling in directing ppl cell migration is dispensable. These results are consistent with recent suggestions that polarization of chondrocytes within the developing mouse limb bud is achieved by the combination of permissive Wnt5 and instructive FGF signals (Gao et al., 2018), although direct evidence for the purely permissive nature of Wnt5 signaling is still missing.

While our findings clearly show that Fz7-mediated Wnt-Fz/PCP signaling is required permissively for ppl cell migration, they do not exclude the possibility of any other receptor activating Wnt-Fz/PCP signaling in ppl cells, which as such, might still have an instructive function in directing ppl cell migration. For instance, the receptor-tyrosine kinases Ror2, Ryk and Ptk7 have previously been shown to functionally interact with Wnt11 and/or Fz7 in regulating non-canonical Wnt signaling (Kim et al., 2008b; Gao et al., 2011; Yen et al., 2009; Hayes et al., 2013; Brinkmann et al., 2016), pointing to the possibility that they might also be involved in mediating Wnt11 signaling within the ppl. Also, uniform activation of Fz7 could not rescue the convergence phenotypes of the notochord and the paraxial mesoderm (Figure 5—figure supplement 3, Figure 5—source data 1), suggesting that the mode of Fz7 function might be context-dependent.

While our data exclude a direct function of localized Fz7 signaling in polarizing ppl cell migration, Fz7 signaling in ppl cells is still essential for this process. Our finding that Fz7 signaling is required for the formation of actin-rich lamellipodia-like protrusions in ppl cells, suggests that Fz7 enables directed ppl cell migration by promoting protrusion formation. How Fz7 regulates this process is yet unclear, but previous findings, suggesting that non-canonical Wnt-Fz/PCP signaling can activate small Rho GTPases known to control protrusion formation during migration (Marlow et al., 2002; Eaton et al., 1996), point to Rho GTPases as potential downstream effectors of Fz7 in this process.

A core mechanism by which Wnt-Fz/PCP signaling polarizes epithelial cells is the polarized distribution of different Wnt-Fz/PCP components along the polarization axis (Collu and Mlodzik, 2015). While this is well documented in epithelial cells, it is less clear whether there is any polarized subcellular distribution of Wnt-Fz/PCP components in mesenchymal cells (Roszko et al., 2015; Jussila and Ciruna, 2017). Studies in zebrafish suggest that in neurectoderm progenitor cells, the core Wnt-Fz/PCP component Pk accumulates at the anterior side, while the Wnt-Fz/PCP signaling mediator Dsh localizes to the posterior side of these cells (Ciruna et al., 2006; Yin et al., 2008). In contrast, we found no evidence for a clearly recognizable polarized subcellular distribution of Fz7 or any other Wnt-Fz/PCP components in ppl cells (Figure 4B, Figure 5—figure supplement 4, Figure 5—source data 1), suggesting that endogenous Wnt-Fz/PCP signaling might be uniform rather than polarized in these cells. Yet, local variations in PCP component turnover could contribute to dynamic changes in polarized distribution of PCP components (Strutt et al., 2011; Chien et al., 2015; Shi et al., 2016) that may not have been detected here.

Notably, while our data suggest that Wnt11/Fz7 signaling is required for directed ppl cell migration, ppl cells do not endogenously express Wnt11. However, Wnt11 is expressed in the anterior paraxial mesendoderm close to the ppl (Heisenberg et al., 2000; Kilian et al., 2003), pointing to the possibility that Wnt11 secreted by the anterior paraxial mesendoderm acts on adjacent ppl cells, thereby promoting protrusion formation and directed migration of these cells. Moreover, it is conceivable that Fz7 is not exclusively functioning as a receptor for Wnt11, and that other non-canonical Wnt ligands expressed within the ppl activate Fz7 signaling there.

Non-canonical Wnt-Fz/PCP signaling has long been thought to function as a prime signal polarizing cells in development (Yang and Mlodzik, 2015; Collu and Mlodzik, 2015). While such function has been well established in epithelial cells (Dabdoub et al., 2003; Wu et al., 2013; Chu and Sokol, 2016), the role of non-canonical Wnt-Fz/PCP signaling in mesenchymal cells is less understood. Our findings show that signaling through the Wnt receptor Fz7, although being required for mesenchymal cell polarization and directed migration, has no direct function in polarizing those cells; it rather promotes protrusion formation, allowing the cells to undergo directed migration. This suggests that non-canonical Wnt-Fz/PCP signaling can have very different functions depending on the specific cellular context in which it acts.

All data generated or analysed during this study are included in the manuscript and supporting files. Source data files have been provided for Figure 1—figure supplement 1, Figure 4—figure supplements 1, 3-5 and Figure 5—figure supplements 1-4.

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Author details

1. Daniel Čapek

   Institute of Science and Technology Austria, Klosterneuburg, Austria

   Contribution

   Formal analysis, Investigation, Visualization, Methodology, Writing—original draft, Writing—review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-5199-9940

2. Michael Smutny

   1. Institute of Science and Technology Austria, Klosterneuburg, Austria
   2. Centre for Mechanochemical Cell Biology and Division of Biomedical Sciences, Warwick Medical School, University of Warwick, Coventry, United Kingdom

   Contribution

   Formal analysis, Writing—review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-5920-9090

3. Alexandra-Madelaine Tichy

   1. Australian Regenerative Medicine Institute (ARMI), Faculty of Medicine, Nursing and Health Sciences, Monash University, Clayton, Australia
   2. European Molecular Biology Laboratory Australia (EMBL Australia), Monash University, Clayton, Australia

   Contribution

   Formal analysis

   Competing interests

   No competing interests declared

4. Maurizio Morri

   Institute of Science and Technology Austria, Klosterneuburg, Austria

   Present address

   Chan Zuckerberg Biohub, San Francisco, United States

   Contribution

   Resources

   Competing interests

   No competing interests declared

5. Harald Janovjak

   1. Institute of Science and Technology Austria, Klosterneuburg, Austria
   2. Australian Regenerative Medicine Institute (ARMI), Faculty of Medicine, Nursing and Health Sciences, Monash University, Clayton, Australia
   3. European Molecular Biology Laboratory Australia (EMBL Australia), Monash University, Clayton, Australia

   Contribution

   Conceptualization, Resources, Funding acquisition, Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

6. Carl-Philipp Heisenberg

   Institute of Science and Technology Austria, Klosterneuburg, Austria

   Contribution

   Conceptualization, Funding acquisition, Methodology, Writing—original draft, Project administration, Writing—review and editing

   For correspondence

   heisenberg@ist.ac.at

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-0912-4566

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