Dickkopf-1 regulates gastrulation movements by coordinated modulation of Wnt/βcatenin and Wnt/PCP activities, through interaction with the Dally-like homolog Knypek

Abstract

Dickkopf-1 (Dkk1) is a secreted protein that negatively modulates the Wnt/βcatenin pathway. Lack of Dkk1 function affects head formation in frog and mice, supporting the idea that Dkk1 acts as a “head inducer” during gastrulation. We show here that lack of Dkk1 function accelerates internalization and rostral progression of the mesendoderm and that gain of function slows down both internalization and convergence extension, indicating a novel role for Dkk1 in modulating these movements. The motility phenotype found in the morphants is not observed in embryos in which the Wnt/βcatenin pathway is overactivated, and that dominant-negative Wnt proteins are not able to rescue the gastrulation movement defect induced by absence of Dkk1. These data strongly suggest that Dkk1 is acting in a βcatenin independent fashion when modulating gastrulation movements. We demonstrate that the glypican 4/6 homolog Knypek (Kny) binds to Dkk1 and that they are able to functionally interact in vivo. Moreover, Dkk1 regulation of gastrulation movements is kny dependent. Kny is a component of the Wnt/planar cell polarity (PCP) pathway. We found that indeed Dkk1 is able to activate this pathway in both Xenopus and zebrafish. Furthermore, concomitant alteration of the βcatenin and PCP activities is able to mimic the morphant accelerated cell motility phenotype. Our data therefore indicate that Dkk1 regulates gastrulation movement through interaction with LRP5/6 and Kny and coordinated modulations of Wnt/βcatenin and Wnt/PCP pathways.

The anterior brain is first defined as a specific portion of rostral neural ectoderm expressing a set of early neural markers such as hesx1/anf1 and otx2. This initial territory is progressively refined into specific presumptive midbrain and forebrain areas (Wilson and Houart 2004). The anterior–posterior (AP) patterning of the CNS is thought to arise, in early gastrula, through the action of posteriorizing signals, coming from the newly formed mesoderm, on an “anterior” neural ectoderm (Stern 2001). The anterior neural plate therefore needs to be protected from “posteriorizing” influence to maintain rostral neural identities such as forebrain and midbrain. This is achieved by both moving away from, and expressing antagonists to, these signals (Wilson and Houart 2004).

Among the most studied posteriorizing signals are the secreted Wnt molecules (Erter et al. 2001; Kiecker and Niehrs 2001a; Lekven et al. 2001). Wnts are secreted by the mesendoderm at the marginal zone, as gastrulation proceeds and gastrula embryos with increased Wnt activity fail to develop rostral neural identity and show a posterior transformation of the anterior neural plate (Kim et al. 2000; Kiecker and Niehrs 2001b; Lekven et al. 2001). A variety of molecules, acting as secreted antagonists of the Wnt pathway, have been identified in the Spemann Organizer. Most of them are related to the extracellular domain of the Wnt receptor Frizzled and act by direct binding to Wnt proteins (Leyns et al. 1997; Wang et al. 1997; Hsieh et al. 1999; Kawano and Kypta 2003).

Distinct from these, the Dickkopf family of secreted molecules (Dkk) (Glinka et al. 1998; Kawano and Kypta 2003) influences the reception of Wnt signals by binding to the Frizzled coreceptors LRP5/6 trans-membrane proteins (Mao et al. 2001) and Kremen (Mao et al. 2002). Among them, Dkk1 is shown to have a strict inhibitory effect on Frizzled receptors (Kazanskaya et al. 2000). It is first expressed in the forming mesendoderm of the late zebrafish, Xenopus, and mouse blastula and then localized specifically in the nascent Spemann Organizer of the early gastrula (Kazanskaya et al. 2000; Shinya et al. 2000). Moreover, both in zebrafish and mouse, dkk1 is expressed in early extra-embryonic tissue (yolk syncytial layer [YSL] and anterior visceral endoderm [AVE]).

In frog and fish, Dkk1 overexpression is able to anteriorize neural tissue (Kazanskaya et al. 2000; Shinya et al. 2000). It can also induce a secondary head if coexpressed with BMP antagonists in ventral blastomeres of Xenopus early blastula embryos. These data led to the conclusion that Dkk1 acts as a “head inducer” through inhibition of the Wnt/βcatenin posteriorizing activity in early gastrula embryos (Niehrs et al. 2001). Requirement for Dkk1 function in head formation has been further supported by the characterization of mouse embryos lacking Dkk1 gene function. Indeed, the genetic knock-out of the mouse dkk1 leads to the formation of headless embryos due to lack of maintenance of anterior neural identity during gastrulation (Mukhopadhyay et al. 2001). Interestingly, failure in establishing the forebrain territories in these embryos is not due to requirement of Dkk1 in the AVE, as mosaic embryos with a dkk1^−/− AVE but containing a functional dkk1 gene in all other tissues develop normally (Mukhopadhyay et al. 2001). Thus, Dkk1 has been undoubtedly shown to be required for establishment of the forebrain in vertebrates, and its biochemical properties have been reasonably well understood (van Tilbeurgh et al. 1999). However, the mechanism by which Dkk1 acts on the neural ectoderm to support forebrain formation is yet to be unraveled.

Late in gastrulation, local inhibition of Wnt signaling is also required to maintain telencephalon and eye identities inside the vertebrate anterior neural plate (Heisenberg et al. 2001; Houart et al. 2002; Kim et al. 2002; Lagutin et al. 2003). One of Dkk1’s possible functions may therefore be to ensure the expression of specific secreted Frizzled-Related Proteins (sFRPs) such as tlc inside the anterior neural border (ANB). We found that such is the case, as lack of Dkk1 dramatically reduces the expression of tlc (L. Caneparo, R. Ahrendt, J. Peres, M. Kapsimali, and C. Houart, in prep.).

In this study, we address the role of Dkk1 in the mesendoderm during gastrulation. Dkk1 is thought to act through vertical signaling from the anterior mesendoderm (Kiecker and Niehrs 2001b; Niehrs et al. 2001). We show that in zebrafish embryos with little or no Dkk1 activity, very little change in the molecular identity of the internalized axial tissue is observed, suggesting that Dkk1 protein may also act on planar signaling events prior to internalization. More importantly, our data show that gain and loss of Dkk1 function both lead to defects in gastrulation movements. Indeed, lack of Dkk1 function accelerates internalization and rostral progression of the mesendoderm, while gain of function slows down both internalization and convergence extension. These findings point to a role for Dkk1 in modulating these movements. Surprisingly, we show that up-regulation of the βcatenin pathway is not inducing the gastrulation movement defects observed in the Dkk1 morphants, strongly suggesting that Dkk1 is able to interact with another signaling pathway. We therefore tested the possible interaction of Dkk1 with known regulators of gastrulation movements. We demonstrate that Dkk1 is able to bind in vivo to the glypican4/6 member Knypek (Kny, a Dally-like homolog). We find that Kny and Dkk1 are able to potentiate each other’s activity when coexpressed in early embryos. Complementarily, propagation of the secreted Dkk1 is greatly reduced in kny mutants. These data demonstrate that propagation and range of action of Dkk1 are likely to be dependent on interaction with the heparan sulfate proteoglycan (HSPG) Kny. Potentiation of Kny function by Dkk1 and Kny requirement for acceleration of gastrulation movements in the Dkk1 morphants together strongly suggest that Dkk1 cooperates with Kny in regulating gastrulation movements. We therefore tested whether Dkk1 was able to modulate the Wnt/planar cell polarity (PCP) pathway. Our results show that Dkk1 overexpression mimics up-regulation of the Wnt/PCP pathway both in Xenopus and zebrafish and dramatically increases JNK phosphorylation, strongly suggesting that Dkk1 is positively modulating the Wnt/PCP pathway. However, decrease of Wnt/PCP activity in the Dkk1 morphants is not the only cause of the gastrulation movement phenotype observed, as loss of PCP function in vertebrates is not associated with acceleration of movements. In fact, we find that increase of Wnt/βcatenin activity accompanied by mild down-regulation of the Wnt/PCP pathway is a condition for which acceleration of internalization is often observed. We therefore propose that Dkk1 modulates gastrulation movements by coordinated modulation of the Wnt//βcatenin and PCP pathways, through interaction with both Kny and the LRP/Kremen complex. All together, our results led us to propose a model by which Dkk1, via endocytosis of LRP5/6, may transform the biochemical properties of the Frizzled receptors and/or an interacting cytoplasmic component from Wnt/βcatenin to a Wnt/PCP conformation, thereby up-regulating the latter pathway while inhibiting the “canonical” one.

Results

Dkk1 modulates gastrulation movements

To study the possible role of Dkk1 in induction of the anterior neural border signaling center (Houart et al. 1998), we used specific antisense morpholino oligonucleotides (Nasevicius and Ekker 2000) that nearly completely inhibit Dkk1 translation in injected zebrafish embryos (Fig. 1G–J). The morphant embryos show a severe reduction of forebrain territories (Fig. 1A–F), as predicted by previous studies of Dkk1 loss of function in Xenopus and mouse (Glinka et al. 1998; Mukhopadhyay et al. 2001). It has been strongly suggested that vertical signals emanating from the anterior axial mesendoderm/prechordal plate are required for the establishment of the anterior neural territory (Kazanskaya et al. 2000; Kiecker and Niehrs 2001b) and that Dkk1 is one of the key players in this signaling function. However, in the absence of dkk1 in both fish (L. Caneparo, R. Ahrendt, J. Peres, M. Kapsimali, and C. Houart, in prep.) and mouse (Mukhopadhyay et al. 2001), otx2 is properly induced in the anterior neural territory, suggesting that some initial patterning events occur in the absence of Dkk1.

Figure 1.

dkk1MO efficiently represses dkk1 translation, which induces severe reduction of the forebrain. (A–F) Loss of Dkk1 function represses formation of eye and telencephalon. Lateral views, anterior to the left, of heads of wild-type (A,C,E) and dkk1MO-injected (B,D,F) embryos. (G–J′) Rescue of dkk1GFP overexpression by Dkk1MO. (G–J) Lateral view of live 80% epiboly (under UV illumination, G′–J′) and prim5 (G,J) embryos after one-cell-stage injection of 50 ng/μL dkk1GFP alone (G,G′) or together with 0.4 mg/mL (H,H′), 1 mg/mL (I,I′), and 1.5 mg/mL (J,J′) Dkk1MO. With the exception of C and D, the embryos shown in this figure are alive. Bar, 100 μm.

In Dkk1 antibody injected Xenopus embryos hhex and Blimp1, two anterior endoderm markers and gsc and shh in prechordal plate are down-regulated (Kazanskaya et al. 2000). In the mouse dkk1/noggin double mutants, Hesx1 expression is down-regulated in the presumptive prechordal plate (del Barco et al. 2003). These results indicate that the most rostral population of axial mesendoderm is impaired in the absence of dkk1. In order to assess whether the zebrafish Dkk1 morphant (dkk1MO) embryos also present some axial mesendodermal defects, we analyzed the expression pattern of a set of axial markers (Fig. 2A–H; data not shown). No change in expression has been found (between 41 and 70 injected embryos tested for seven axial markers at 60%–65%, 80%–90% epiboly and bud stages) except for a significant reduction of the anterior prechordal plate marker gsc (n = 17/58) (Fig. 2G,H), confirming the defect observed in Xenopus and mouse.

Figure 2.

Dkk1 regulates gastrulation movements. Lateral (A,B,E,F,I–Y), dorsal (C,D), and animal pole (G,H) views of control (A,C,E,G,I,K,N,Q–S), Dkk1MO-injected (B,D,F,H,J,L,O,T–V), and dkk1RNA-injected (M,P,W–Y) gastrula embryos. A–J and Q–Y show progression of the mesendoderm by markers in fixed embryos (A–J) and by observation of the GFP-expressing axial mesendoderm in live embryos (Q–Y). The black arrowhead indicates the position of the rostral-most mesendoderm. The red frame highlights the stages at which the difference in mesendoderm progression is obvious in the live Dkk1 morphants. (K–P) Lateral views of, in blue, neural plate (K–M) and epidermal (N–P) territories in wild-type (K,N), dkk1MO (L,O), and dkk1RNA (M,P) embryos. Red arrowhead shows the caudally shifted anterior edge of the neural territory.

In the course of this analysis, we noticed a striking difference in position of the rostral-most limit of the axial mesendoderm (Fig. 2A–F). This observation has been made looking at the expression of a set of axial markers and suggested acceleration in axial mesendoderm progression. In order to address this possibility, we followed the movement of the axial mesendoderm in live transgenic embryos containing the insertion of a gene coding for the green fluorescent protein (GFP) under the control of the gsc promoter (n = 48/55) (Fig. 2Q–Y). In wild-type embryos, the rostral tip of the internalized mesendoderm is progressively moving toward the animal pole during gastrulation and reaches the pole by 90% epiboly. In dkk1MO embryos, the rostral axial mesendoderm reaches the animal pole by 75% epiboly (Fig. 2U,V), 45–60 min earlier than in wild-type embryos raised at 28°C. We concurrently observed that dkk1MO embryos show a slightly narrower anterior neural plate by the end of gastrulation, although no defect in BMP signaling has been detected rostrally (n = 31/31, Fig. 2N,O), and no anterior neural plate size difference is visible earlier in gastrulation (n = 35/35) (Fig. 2K,L). Finally, the rostral limit of expression of snail1a (data not shown) and sprouty4 (Fig. 2I,J) in nonaxial mesendoderm is also shifted anteriorly in morphant embryos, due to change in cell identity and/or cell motility. Both patterning and motility phenotype observed in the morphants can be corrected by coinjection of both dkk1MO and dkk1 full-length transcripts lacking the MO target sequence (Supplementary Fig. S1). These observations suggest that although epiboly movements are not visibly perturbed in dkk1MO morphants, some gastrulation movements are accelerated in these embryos.

In order to test this further, we assessed directionality and speed of mesendodermal movements in Dkk1 morphant embryos. We first assessed movements of lateral mesoderm by transplanting in the germ ring, on either side of the shield (at 35° from it), rhodamin-labeled wild-type and fluorescein-tagged dkk1MO cells in a dkk1MO shield stage host embryo. We followed their progression by confocal time-lapse analysis (see Materials and Methods; n = 56, Fig. 3A–C). In most cases, dkk1MO cells move faster toward the animal pole than the wild-type clones (n = 48/56). A difference in cell behavior is visible from the onset of gastrulation as dkk1MO cells move under the epiblast more frequently than wild-type cells (Fig. 3B, graph in I). This acceleration does not seem to be dependent on the clone size (seen for wild-type and dkk1MO clones of either five, 15, or 25 transplanted cells). Expectedly, wild-type and dkk1MO clones move in very similar ways if transplanted in wild-type hosts (n = 25/25) (Fig. 3D), as secreted Dkk1 from host cells almost certainly compensates for the absence of protein in the dkk1MO grafted cells. Similarly, if wild-type and dkk1MO clones are grafted adjacent inside the shield of dkk1MO host embryos (Fig. 3E,F), most cells are moving together (defective cells receiving Dkk1 from secreted wild-type neighbors), with some dkk1MO cells running in front and wild-type cells trailing behind (n = 9/16 and 7/16 without a significant difference in position between the two cell types).

Figure 3.

More frequent mesendoderm internalization and faster rostral progression in the Dkk1 morphants and slowed-down internalization and CE defects in Dkk1 gain of function. (A–H) Dorsal (B–F), animal pole (A), and lateral (G,H) views of dkk1MO (A–C,E,F) or wild-type (D,G,H) embryos in which wild-type (red in live embryos and brown in fixed specimen) and dkk1MO (green in live embryos and dark blue in fixed specimen) cells have been transplanted inside the germ ring at onset of gastrulation. A–C show the same embryo at progressively later stages of gastrulation. Note that H shows transplanted dkk1RNA-expressing cells in dark blue. Dashed line indicates the midline. (I) Timing of internalization of 15 wild-type (blue) and dkk1MO (red) cells transplanted in the margin of early gastrula hosts. Measure of transplants in the lateral margin are represented by full dots and transplants in the axial margin (shield) are represented by empty dots. X represents time and Y represents the number of cells. (J) Measure of the AP length of the transplanted cell population at bud stage in microns (Y axis). Wild-type clones are shown in blue, dkk1MO are shown in red, and dkk1RNA are shown in green. (K) Measure of the distance of the leading transplanted cell from the embryo margin at 95% epiboly in microns (Y axis). Wild-type clones are shown in blue, dkk1MO are shown in red, and dkk1RNA are shown in green. (L) Expression of dkk1 in the marginal mesendoderm. Animal pole view of a shield stage wild-type embryo. (S) Shield (Spemann Organizer). Note the absence of transcript in the ventral margin and the graded level of expression from dorsal (S, shield) to lateral. (M,N) Cell shape and cohesion in the lateral (M) and dorsal (N) mesendoderm (arrows) as it internalizes in a live shield stage embryo. Inset in M shows a close-up of the internalizing lateral mesendoderm cell, emphasizing the mesenchymal shape of the lateral mesendoderm cells.

To quantify further the changes observed in the morphant gastrulae, we followed the progression of 15 cells transplanted into the germ ring of a morphant host and took three sets of measurements: the number of internalized cells/minute (Fig. 3I), the AP length of the clone at 90% epiboly (Fig. 3J), and the distance of the rostral-most cell from the posterior margin at 95%–100% epiboly (Fig. 3K). In our experimental conditions, wild-type transplanted cells internalize at an average of 0.55 cell per minute in lateral and axial mesoderm (Fig. 3I, blue dots/circles), while morphant cells move inward at an average of 0.85 cell per minute in lateral mesoderm and 1.8 cell per minute in the axial mesendoderm (Fig. 3I, red dots/circles). The AP length of morphant clones at 90% epiboly is slightly bigger than wild type, strongly suggesting that convergence/extension (CE) movements are not impaired in the absence of Dkk1 function (Fig. 3J). However, the position of the leading cell inside the clones is significantly shifted anteriorly in morphants compared with wild type (Fig. 3K), indicating that rostral progression is increased when Dkk1 is lacking. Indirectly, this finding (similar clone shape and length but shifted rostrally) suggests that more cells are internalized in the morphants. Lack of Dkk1 function therefore favors mesendodermal internalization and rostral progression, indicating that Dkk1 protein normally negatively regulates this type of movement.

We next assessed the effect the overexpression of dkk1 has on cell movement, using the same approaches. A lag in progression of the axial mesendoderm is observed (Fig. 2W–Y), and a caudal displacement of the anterior limit of the neural plate (Fig. 2M,P, arrowheads) typical of embryos defective in extension movements. Clonal analysis of cell motility shows that dkk1 overexpressing clones present an increase in cell dispersion and a slowing down of internalization, rostral progression, and convergence (n = 39/39) (Fig. 3,H; green graph in J,K). All together, these data indicate that Dkk1 modulates gastrulation movements.

Gain of Wnt/βcatenin function does not lead to acceleration of mesendoderm internalization

As Dkk1 regulates negatively the Wnt/βcatenin pathway, we tested whether the acceleration of gastrulation movements in the morphant embryos is the consequence of up-regulation of this pathway. We first addressed whether mesendodermal progression was affected in the mbl mutant embryos, lacking axin1 function (Heisenberg et al. 2001) and therefore unable to degrade βcatenin. We find no visible difference, either by measuring the extent of the lefty1 expression in axial mesendoderm or when monitoring cell movements of labeled mesendoderm (data not shown). To ascertain that the absence of modulation of movement was not caused by a relatively mild increase in βcatenin activity in mbl, we tested whether any change was detectable in embryos lacking both mbl and tcf3 function (injection of tcf3 and tcf3b morpholino [Dorsky et al. 2003] in mbl) (Fig. 4). In these embryos, we have been unable to observe any increase in cell movement. Conversely, we observed a slight reduction of axial mesendoderm progression in 29% of the double mbl^−/−; tcf3MO embryos (n = 8/28) (Fig. 4D). Finally, ΔNWnt8 overexpression, obtained by RNA injection at the one-cell stage (25 pg/embryo, n = 37) (Fig. 4J) is unable to rescue the acceleration observed in the Dkk1 morphants. All together, these data strongly suggest that the influence of Dkk1 on gastrulation movements is at least in part Wnt/βcatenin independent. However, this pathway seems to regulate some aspects of cell cohesion, as we observed that cells from clones lacking both mbl and tcf3 function were losing touch with each other faster than wild type after transplantation (clone shape at 75%–80% epiboly) (Fig. 4G,H).

Figure 4.

Gain of Wnt/βcatenin activity does not phenocopy the gastrulation movement defect seen in the Dkk1 morphants. (A–F) Lateral views of live prim5 (A,C,E) or dorsal views of lefty1 exression in fixed 90%–95% epiboly (B,D,F), wild-type (A,B), and mbl mutant injected with tcf3 + 3bMO (C,D), and dkk1MO (E,F) embryos. (G,H) Clone shape of transplanted wild type (G) or mbl mutant injected with tcf3 + 3bMO (H) in the host embryo at 75%–80% epiboly. (I,J) Lateral views of 75%–80% epiboly gscGFP transgenic live embryo (I) or dkk1MO coinjected with 20 pg of dominant-negative wnt8 RNA (J), showing the GFP-expressing axial mesendoderm in green under UV illumination. (K–N) Lateral views of 90% epiboly embryos showing the distribution of the wnt8-expressing cells (blue) transplanted at shield stage. The dose injected per embryo is indicated in the bottom right corner. (O) Timing of internalization of 25 wild-type (blue) and wnt8-expressing (10 pg, pink; 25 pg red; 40 pg purple) cells transplanted in the margin of early gastrula hosts. X represents time and Y represents the number of cells. (P) Measure of the distance of the leading transplanted cell from the embryo margin at 100% epiboly in microns (Y axis). Wild-type clones are shown in blue and wnt8-expressing clones are shown in pink (10 pg), red (25 pg), and purple (40 pg).

To quantify further the changes in cell movements imposed by gain of Wnt/βcatenin activity, we used our transplantation approach again and transplanted 15–20 wnt8 mRNA-injected donor cells into the margin (35° from the shield) of early shield stage wild-type host embryos and compared with wild-type transplants in the same conditions (either transplanted on the other side of the same hosts or in different host embryos). A series of wnt8 doses has been tested (Fig. 4K–N). The wnt8-expressing cells internalized as well as the wild-type cells, except for the cells expressing the highest dose, for which delay in progression and CE defect begins to be observed (Fig. 4N, graph in O,P), reminiscent of the defects observed in Xenopus (Kühl et al. 2001). In most cases, the clones are more dispersed (Fig. 4L–N). Finally, a very small number of wnt8-expressing cells (from set of 3 pg of transcripts/donor) are very rarely detected slightly further away from the margin than in wild type (n = 1/74) (data not shown). These results indicate that Wnt/βcatenin activity is not readily able to induce acceleration of mesendoderm internalization and progression, but confirm the less cohesive nature of cells exposed to high level of Wnt/βcatenin signaling. This observation nicely correlates with the distinct cell adhesion properties found in the lateral (noncohesive) (Fig. 3M) and dorsal (cohesive) (Fig. 3N) mesendoderm, which are exposed to low and high levels of Dkk1, respectively (Fig. 3L). Dkk1 is therefore likely to regulate cohesion through modulation of the Wnt/βcatenin pathway.

The glypican4/6 Knypek is able to bind to Dkk1 and potentiates its activity in vivo

In vertebrates, a subset of gastrulation movements are regulated by the Wnt/PCP pathway (for reviews, see Tada et al. 2002; Wallingford 2005). Both Wnt5 and Wnt11 are signaling molecules required for proper convergence–extension (Heisenberg et al. 2000; Kilian et al. 2003) through activation of pathways including the JNK and Ca⁺⁺. One possible way Dkk1 may regulate cell movements is through modulation of the PCP pathway. Such a modulation may be indirect, through repression of the Wnt/βcatenin (Kühl et al. 2001). Alternatively, it may be direct, by interaction with PCP pathway components. Since Dkk1 binds heparan (Fedi et al. 1999), we tested if a related molecule, Knypek, a member of the HSPG family, required for Wnt/PCP activity (Topczewski et al. 2001), is able to interact with Dkk1.

We tested whether Dkk1 and Knypek are able to bind each other, using immunoprecipitation techniques. We tested membrane colocalization and binding between these two molecules in a cell culture system, by transfection of an expression vector containing a Flag-tagged knypek c-DNA sequence (kny-Flg) (Topczewski et al. 2001) and/or another containing a dkk1GFP fusion molecule, coding for an active fluorescent Dkk1 protein (see Fig. 1G, G′). We assessed localization of the two proteins in these transfected cells. Protein extracts from transfected cells were precipitated with either an anti-Flag or an anti-GFP antibody. The precipitates were run on acrylamide gel and Western blot immunostained either with GFP or Flag antibody. The blots show that in extracts from cells cotransfected with both dkk1GFP and kny-Flg DNA (or from a mixed culture of dkk1GFP-expressing and kny-Flg-expressing cells), Kny-Flg proteins are detected in the anti-GFP precipitates (Fig. 5A), unequivocally showing that Dkk1 is able to bind to Knypek in cell culture conditions. By the same technique, we also show this binding in gastrulae extracts (Fig. 7J, below), opening the possibility that Dkk1 may directly modulate Knypek-dependent signaling events in vivo.

Figure 5.

Knypek binds to Dkk1, is required for its propagation, and potentiates its effects. (A) Western blot of an immunoprecipitation assay. Protein extracts from cells transfected with different combinations of DNA (+) are either run untreated (last three lanes) or first immunoprecipitated with an anti-GFP antibody (first five lanes), then run on a gel. The gel is then transferred on filter and stained with an anti-Flag antibody (detecting the Flag-tagged Knypek protein) or an anti-GFP antibody (detecting the GFP [1] or dkkGFP fusion [2] proteins). (Lane 4) Red arrowshows the presence, after GFP immunoprecipitation, of Kny-Flag proteins in extracts from cells expressing both Dkk1GFP and KnyFlag. (B–E′) Lateral views, rostral to the left, of prim5 live wild-type (B), kny RNA-injected (C), dkk1RNA-injected (D), and kny + dkk1RNA-injected (E,E′) embryos. In E, the quantity of RNA injected is identical to the that used for the single injections in C and D. The phenotype shown in E and E′ are found in 31% and 64% of the injected embryos, respectively. (F–I) Animal pole views (insets) and lateral views of live or fixed (insets) wild type (F,G) and kny homozygous (H,I) injected with control (F,H) or dkk1 morpholino (G,I). Insets show expression patterns of hgg1 (rostral mesendoderm, hg) and brachyury (in the notochord, n). Arrowhead shows rostral end of the body axis. (J–M) Visualization of the dkk1GFP molecules (in green) secreted by the transplanted dkk1GFP-expressing wild-type cells (yellow) in wild-type (J,K) and kny^−/− (L,M) hosts. K and M are high-magnification views of the distribution of secreted proteins away from the graft (or its absence at a distance from the clone in M). Note that the cell nuclei are visible as dark discs in unlabeled cells. The wild-type dkk1GFP-expressing cells are very intensely fluorescent when placed in the kny^−/− mutant, masking the nucleus in these cells. (N–P) Lateral views, rostral to the left, of wild-type (N) or kny^−/− (O,P) embryos, untreated (inset in N,O) or in which cells from a dkk1 RNA-injected donor embryo (in brown) have been transplanted in the shield at 50% epiboly (N,P). Note the shorter tail and big eye and brain in N. (P) No change in eye size is ever observed in transplanted kny^−/− embryos; the brain size is generally comparable to untransplanted mutants. (Q–Z) Lateral views of live embryos (Q–V) or dorsal views of fixed gastrulae (W–Z). All embryos come from a kny^+/− cross injected with either 1.8 pg of kny transcripts (Q–S,W,X) or 1.8 pg of kny and 2 pg of dkk1 transcripts (T–V,Y,Z). The number of embryos showing each illustrated phenotype is given in the bottom right corner of each picture. In W–Z, hgg1 expression shows rostral mesendoderm (hg) and brachyury, the notochord (n).

Figure 7.

Dkk1 may act as a switch between the βcatenin and PCP pathways activated by the Wnt/Fz complex. (A–C) Lateral views of 90% epiboly embryos showing the distribution of the dsh-DEP+ (A), wnt8 (B), and dsh-DEP + wnt8-expressing cells (blue) transplanted in wild-type hosts at shield stage. The dose injected per donor embryo is 5 pg for dsh-DEP+ and 12 pg for wnt8. (D) Seventy-five percent epiboly wild-type host embryo showing the rostral progression of wild-type (brown) and wnt8 + dshDEP+ (5 pg + 12 pg, blue) transplanted cells. The dashed line indicates the midline. (E) Model of the proposed mechanism by which Dkk1 may both down-regulate the Wnt/βcatenin and up-regulate the Wnt/PCP pathways. (F–I′) Live embryos injected with 8 pg of dkk1GFP (F,F′), 1.8 pg of kny-flg (G,G′), 4 pg of dkkGFP + 1.8 pg kny-flg (H,H′), and 4 pg of dkkGFP + 1.8pg of kny-flg + 1.5 pg of LRP6 (I,I′). Confocal images have been taken at 60% epiboly (F′–I′) of the GFP localization and the embryos have been left to develop until 48 hpf (F–I) to check activity of the transcripts injected. (F″–I″) Dorsal view, anterior to the top, of embryos injected with dkk1GFP (F″), kny-flg (G″), dkkGFP + kny-flg (H″), and dkkGFP + kny-flg + LRP6 (I″) at the same doses as in F–I′, showing expression of hgg (in the hatching gland, hg), ntl (in the notochord, n), and dlx3b (in the neural plate border). (J) Western blot of the immunoprecipitation assay done on extracts of 30 late-gastrula injected embryos for each condition tested. Protein extracts from the four combinations of injection (+) are either run untreated (last four lanes) or first immunoprecipitated with an anti-Flg antibody (first four lanes), then run on a gel. The gel is then transferred on filter and incubated with an anti-GFP antibody (detecting the GFP-tagged Dkk1 proteins) or an anti-Flg antibody (detecting the Flg-tagged Kny proteins. Lanes 3 and 4 show the presence, after Flg immuno-precipitation, of Dkk1-GFP proteins in extracts from embryos expressing both Dkk1GFP and KnyFlag, regardless of LRP6 overexpression. Note that a same set of injected embryos was split into one half for phenotype analysis (F–I) and the other for the coimmunoprecipitation assays (J), thereby controlling that the proteins tested for binding were active in the injected embryos.

The defects in cell movements observed in Dkk1 gain or loss of function may therefore be due to modulation of a Knypek-dependent signaling activity. We tested this hypothesis by assessing whether embryos carrying a null mutation in the kny gene are losing the ability to respond to the absence of Dkk1 function. If Dkk1 acts through a kny independent way, Dkk1 lack of function may rescue part of the movement defects observed in the kny^−/− embryos. We observed that the embryos lacking both Dkk1 and Knypek show delay in movements identical to the ones observed in single kny^−/− embryos (n = 31/31 fixed kny^−/− embryos and n = 15/15 kny^−/− live embryos) (Fig. 5F–I). This suggests that either Dkk1 influence on motility may depend on its direct binding to Knypek or that Knypek is functionally downstream from Dkk1 in the same developmental process.

To attempt to discriminate between these two possibilities, we tested whether Kny could increase Dkk1 activity when misexpressed in wild-type embryos. We injected kny or dkk1 transcripts alone or together in wild-type embryos, choosing a quantity of transcripts that gives no (for kny) (Fig. 5C) or weak (for dkk1) (Fig. 5D) phenotype when injected alone. Embryos injected with the same or half the dose of both transcripts (Supplementary Table 1), develop an enormous head and the short body axis typical of a severe dkk1 overexpression (Fig. 5E, E′; Supplementary Table 1). These results indicate that Dkk1 and Kny can act together in the same developmental process. Finally, we analyzed the localization of Dkk1 protein in wild-type and kny^−/− embryos, using the dkk1GFP fusion molecule described above. We transplanted 20–30 cells from dkk1GFP-injected wild-type donor embryos into shield stage wild-type embryos or progenies of kny^−/+ fish. We then followed the expression of the fusion proteins under the confocal microscope. kny^−/− hosts are identified based on their phenotype at 24 h post-fertilization (hpf). We first observed that propagation of the Dkk1GFP proteins is extremely fast (an average of 21 cell diameters after 45 min) in wild-type embryos (data not shown). Moreover, very intense small and highly dynamic fluorescent dots are visible inside the host cells, strongly suggesting an endocytic pathway (Fig. 5K; also suggested by Mao et al. 2002). More importantly, we found extensive propagation of the secreted molecule in 23/24 of the wild-type embryos while no propagation has been detected in 8/9 kny^−/− analyzed (Fig. 5J–M). In the absence of propagation, the fluorescence is detected weakly around the cells and highly inside the cytoplasm. Kny is therefore required for propagation of Dkk1.

To assess whether this propagation is necessary for Dkk1 function, we asked if propagation of the Dkk1 molecules was required for its well-described anteriorizing function. We therefore transplanted donor dkk1-expressing cells (from dkk1 mRNA-injected donors embryos) into the shield (Spemann Organizer) of early gastrula wild-type or kny hosts. We observed that grafted dkk1-expressing cells are able to induce a “big head/short trunk” phenotype in wild type (n = 19/21) (Fig. 5N). Introduction of Dkk1-overexpressing cells in the wild-type shield is therefore able to affect the general AP patterning such that transplanted embryos always show reduction of the trunk and the giant eyes typical of the Dkk1-injected individuals. We were unable to observe an increase of the eye and overall brain in kny embryos (n = 0/23) (Fig. 5P), but we sometimes observed a slight increase in the size of the telencephalon (n = 6/23), probably due to the effect of Dkk1 from donor cells when in close vicinity of the forebrain.

Finally, we directly assess whether Dkk1 is able to cooperate with Kny in regulating gastrulation movements. To this aim, we tested whether Dkk1 is able to help Kny in rescuing the kny mutant phenotype. We injected, at the one-cell stage, the progeny of kny^+/− parents with a suboptimal quantity of kny full-length RNA alone or accompanied by a low dose (alone not inducing a phenotype) of dkk1 transcript. To our surprise, the coinjected embryos showed a robust rescue (Fig. 5Q–Z), strongly suggesting that Kny and Dkk1 act similarly on the same gastrulation movements. As Dkk1 alone is unable to rescue kny mutant embryos (n = 32) (data not shown), the “double” rescue indicates that Dkk1 and Kny control cell motility together through regulation of the same pathway(s).

All together, the results in this section show that (1) Dkk1 is able to bind to Kny and requires Kny for its propagation, (2) propagation is necessary for Dkk1 function, and (3) Dkk1 and Kny can act together to regulate gastrulation movements.

Dkk1 promotes the activity of the Wnt/PCP pathway

As Dkk1 is able to bind to Knypek and enhance its function, it may directly regulate the Wnt/PCP pathway. We first tested whether Dkk1 negatively modulates Wnt/PCP activity. To address this possibility, we examined if the acceleration of rostral progression observed in the Dkk1 morphants may rescue the gastrulation phenotype observed in slb embryos, homozygous for a null mutation in the wnt11 gene (Heisenberg et al. 2000). CE defects were not rescued in slb^−/− embryos (obtained from slb^−/− homozygous parents) injected with dkk1MO (n = 41) (Fig. 6A–C′; data not shown). However, we observed some change in the AP extent of the prechordal territory marked by gsc (51% of double lack of function embryos) (Fig. 6C, C′). This deformation of the prechordal territory points to a slight increase of the CE defect in slb embryos and suggests that Dkk1 may activate the Wnt/PCP pathway. We therefore set out to address this, first, at the biochemical level in Xenopus, in which Dkk1, has been thoroughly studied and biochemical assays are routinely done. We first found that, as in fish, Dkk1 overexpression is inducing CE defects in Xenopus (Fig. 6D–G). Animal caps, from embryos injected with activin mRNA, are able to form mesoderm and characteristically elongate due to CE movements. After injection of both activin and wnt8, animal cap elongation is slightly enhanced compared with activin alone. Contrasting with this observation, caps injected with activin and dkk1 are unable to elongate, and this is accompanied by induction of gsc at the expense of the mesodermal marker Xbra (Fig. 6M). This loss of elongation is reminiscent of the phenotype induced by gain or loss of Wnt/PCP activity. Indeed, the cells of explants of dorsal blastopore lip (DMZ) in culture normally elongate and orient along the medio-lateral axis (Fig. 6H,L). Cells in explants from Wnt3a-injected embryos show similar or greater orientation relative to the medio-lateral axis (Fig. 6J,L), while dkk1 reduces elongation and loss of orientation relative to the medio-lateral axis (Fig. 6K,L). This cell shape change phenocopies the loss of polarization observed in explants after overexpression of Wnt11 and its receptor Fz7 (Fig. 6I,L). Our data in frog and fish therefore strongly suggest that Dkk1 is able to promote Wnt/PCP activity. To test this possibility at a molecular level, we quantified Wnt/PCP activation by measuring the level of phosphorylation of one of its major transducers, JNK (Boutros et al. 1998). We compared the level of phosphorylated JNK in embryos injected with a HA epitope-tagged JNK mRNA alone (control) or together with either wnt11 or dkk1 mRNA or both. After specific precipitation of HA-JNK, its level of activation was measured by detection of its phosphorylated form, using a phospho-specific antibody. Wnt11 or Dkk1 expression alone is able to enhance JNK phosphorylation, and expression of both of them increases dramatically this phosphorylation, while Wnt8 blocks phosporylation (Fig. 6N). These results demonstrate that Dkk1 directly or indirectly activates the Wnt/PCP pathway.

Figure 6.

Dkk1 up-regulates the Wnt/PCP pathway. (A–C′) Lateral (A–C) and dorsal (A′–B′) views of bud stage wild type (A,A′), slb (B,B′), and slb injected with dkk1MO (C,C′) embryos, expressing hgg (in the hatching gland, hg), ntl (in the notochord, n), and dlx3b (in the neural plate border, nb). (D–G) Dkk1 blocks activin-induced animal cap elongation. Xenopus embryos were uninjected (D) or injected animally at the four- to eight-cell stage with activin (E), activin and wnt8 (F), or activin and dkk1. Animal caps were dissected from stage 9 embryos and cultured until sibling embryos had reached stage 17. (H–L) Dkk1 disrupts cell polarity. Xenopus embryos were injected at the two- to four-cell stage equatorially with mRFP mRNA, and either preprolactin (ctl) or the indicated mRNAs (top right corner). Explants of the dorsal upper blastopore lip were cut at stage 10.5, and cells were imaged by confocal microscopy. The yellow line indicates orientation of the medio-lateral axis. (L, left panel) Cell elongation was determined by the mean length-to-width ratio (LWR). (Right panel) Cell orientation was calculated as the percentage of cells with their long axis tilted >20° relative to the medio-lateral axis. (M) RT–PCR analysis of animal caps injected as in D–G for the indicated genes. (N) Dkk1 activates JNK. HA epitope-tagged JNK mRNA was injected either alone or in combination with mRNA of the indicated genes into two- to four-cell-stage embryos. HA-JNK was immunoprecipitated from extracts of stage 11 embryos and JNK phosphorylation was monitored using a phospho-specific antibody on Western blot. (ni) Noninjected control. (O–R) Dorsal views of bud stage wild-type (O,Q) and mbl (P,R) embryos uninjected (O,P) or injected with 30 pg of dkk1 RNA at the one-cell stage (Q,R), expressing hgg (in the hatching gland, hg), ntl (in the notochord, n), and dlx3b (in the neural plate border, nb).

The interaction found between Kny and Dkk1 suggests the possibility of a direct regulation of the Wnt/PCP pathway. However, it may stimulate Wnt/PCP indirectly since canonical Wnt signaling is able to block the PCP pathway in vitro (Fig. 6N). A direct interaction with the Wnt/PCP is, however, very likely. Indeed, not only is moderate DNwnt8 overexpression unable to rescue the gastrulation movement phenotype in the Dkk1 morphants (Fig. 4J), but we also found that injection of Dkk1 mRNA in mbl is inducing a CE defect comparable to the one seen in wild type (Fig. 6O–R). This last result therefore shows that Dkk1 can act on CE movements in embryos with a constitutively active Wnt/βcatenin pathway. Furthermore, Dkk1 activation of the Wnt/PCP pathway is not due to indirect up-regulation of wnt11 expression, as no increase of wnt11 transcripts is detected after dkk1 RNA injection (Supplementary Fig. S2A–C).

As Dkk1 is able to up-regulate the PCP pathway, we tested whether it could rescue the CE defect in slb/wnt11 mutant embryos. Injection of various doses of dkk1 RNA in slb homozygous embryos did not lead to a general rescue (Supplementary Fig. S3). However, high dose of transcripts is able to rescue the shape of the anterior-most axial mesendoderm although not other aspects of the CE defect in slb (Supplementary Fig. S3D–E″). This observation shows that Dkk1 is able to partially correct a subset of the cell movement defect induced by loss of Wnt11, namely the rostral progression of the anterior mesendoderm.

Dkk1 may act as a switch between the βcatenin and PCP pathways activated by the Wnt/Fz complex

One of our first observations is still unresolved by the results so far. Indeed, the ability of Dkk1 to bind to Kny and activate the Wnt/PCP pathway is not explaining the acceleration of gastrulation movements observed in the Dkk1 morphant embryos. As our data indicate that Dkk1 simultaneously represses the Wnt/βcatenin and activates the Wnt/PCP pathway, we tested whether acceleration of gastrulation movements in the Dkk1 morphants may be generated by concomitant up-regulation of the Wnt/βcatenin pathway and down-regulation of the Wnt/PCP activity. We injected one- to two-cell-stage donor embryos with either wnt8 RNA, a low dose of dshDEP+ (coding for a version of Dishevelled unable to activate the PCP pathway) (Axelrod et al. 1998; Heisenberg et al. 2000), or both transcripts together. We then transplanted 15–20 cells in the margin of shield stage wild-type host embryos and watched internalization of the donor cells (Fig. 7A–D). The cells coming from the double-injected donors do indeed progress faster than the wild-type controls in a majority of the cases (25 of 43) (Fig. 7C,D). Single wnt8-expressing cells display the disperse distribution described above (n = 22) (Fig. 7B), while the DSH-DEP+ cells show difficulty in progression and in CE (n = 25) (Fig. 7A; data not shown). Cells in which the Wnt/βcatenin activity is up-regulated and the Wnt/PCP reduced are therefore able to mimic the cellular behavior observed in embryos deprived of Dkk1 activity.

In light of all our results, we therefore propose that Dkk1 acts at the level of the Wnt receptors, concomitantly repressing Wnt/βcatenin and increasing Wnt/PCP reception, via interaction with both LRP5/6 and Kny (Fig. 7E). Such a model suggests that LRP5/6 and Kny may either compete for binding to Dkk1 or differentially regulate localization of Dkk1. We tested these possibilities by monitoring Dkk1GFP subcellular localization and binding to Kny-Flg in embryos in the presence or absence of exogenous LRP6. We found that, in gastrula embryos expressing both Dkk1GFP and KnyFlg, the distribution of Dkk1GFP is shifted from being found both in cytoplasmic and extracellular location (n = 16; Fig. 7F′) to almost exclusively extracellular (n = 15) (Fig. 7H′). This distribution is completely reversed in embryos that also mildly overexpress LRP6 (1.5 pg of RNA injected at the one-cell stage), where the dkkGFP proteins are mostly found in the cytoplasm (n = 19) (Fig. 7I′). This result shows that Kny alone promotes extracellular localization of Dkk1, while LRP6 drives its endocytosis. We also monitored gastrulation movements in the same four experimental conditions. As expected, injection of dkk1 alone or in combination with kny both induce CE defects. These are more pronounced in the double-injected embryos, highlighting again the cooperation of Dkk1 and Kny in modulation of gastrulation movements (Fig. 7 F″, H″). More importantly, mild expression of LRP6 rescues this CE defect (Fig. 7I″). This rescue is not due to direct competition between LRP and Kny for binding to Dkk1. Indeed, our coimmunoprecipitation assay on protein extracts of these embryos (Fig. 7J) shows that LRP6 and Kny do not compete for binding to Dkk1 (Fig. 7J, cf. third and fourth lanes). This finding therefore strongly suggests that Kny is endocytosed with Dkk1 in embryos overexpressing LRP6 and, more importantly, that Dkk1 is able to act concomitantly on both Kny and LRP proteins, thereby modulating both patterning and cell motility.

Discussion

Dkk1 is a Wnt/βcatenin antagonist acting through binding to Kremen and the Frizzled coreceptors LRP5/6 (Mao et al. 2001, 2002). It plays a crucial role in early neural patterning, as its activity is required for maintenance of forebrain identity in the anterior neural plate in both frog and mouse (Mukhopadhyay et al. 2001). Our study uncovered a novel involvement for Dkk1 in modulation of gastrulation movements through binding to the Glypican4-like Knypek and direct regulation of both Wnt/βcatenin and PCP signaling pathways.

Dkk1 is modulating gastrulation movements

When we first observed the changes in cell motility of the internalizing mesendoderm in the Dkk1 morphants, Dkk1 had not yet been associated with regulation of cell movement. Since then, a very recent study has shown that the rostral movement of the AVE just preceding gastrulation in mouse requires Dkk1 (Kimura-Yoshida et al. 2005). Moreover, this study also shows that Wnt/βcatenin activity represses such movement and proposes a role of attractant for Dkk1 and repellent for the Wnt ligands. However, when looking at gastrulation movements, the three different approaches taken in our study to up-regulate the Wnt/βcatenin pathway in early gastrula embryos all fail to mimic the acceleration in motility observed in the Dkk1 morphants, pointing here to a novel Wnt/βcatenin-independent function for Dkk1. Moreover, none of our transplant experiments show cell behavior compatible with an attractant/repellent model. Interestingly, the cell behavior described in the mouse AVE in the presence or absence of Dkk1 activity is also compatible with a fluctuation of the level of both Wnt/βcatenin and Wnt/PCP activity, a possibility not tested by the study, as the authors did not question the nature of the pathway modulated by Dkk1. Future analysis of the molecular pathways required for AVE movements in mice is needed to elucidate whether Dkk1 may act on the same set of signaling pathways while regulating the movements of the AVE and those of the mesendoderm.

A puzzling observation is that acceleration of gastrulation movements has only been reported once before this study, in zebrafish embryos lacking Lefty function (Feldman et al. 2002), an antagonist of the Nodal signaling pathway. Nodal, like the Wnt/βcatenin pathway, is able to inhibit head organizer activity (Piccolo et al. 1999). As Nodal is able to repress the expression of Dkk1, the phenotype observed in the zebrafish Lefty morphant embryos may therefore be caused by loss of Dkk1 activity. Another possible option is that Dkk1 negatively modulates the Nodal pathway, this latter positively regulating internalization and rostral progression. Future thorough biochemical assays will be needed to test the interaction of Dkk1 with receptors of the main signaling pathways active during gastrulation.

The Wnt/βcatenin pathway modulates cell cohesion

Although not accelerating rostral mesendoderm progression, gain of Wnt/βcatenin activity does influence some aspects of cell–cell interaction and motility, as we observed that mesendoderm cells, exposed to high Wnt/βcatenin activity, internalize and progress rostrally at a similar speed as wild type cells but are less cohesive. This echoes results obtained in tumors showing that an increase in Wnt/βcatenin activity induces a loss in cell adhesion by reducing the pool of βcatenin proteins normally used at the cell membrane in the E-cadherin/αcatenin/βcatenin complex of the adherens junction (Brembeck et al. 2006) and provides indirect evidence supporting the idea of integration of patterning and modification in cell adhesion. Both βcatenin and APC regulate concomitantly gene expression and cell adhesion specifically in the context of cancer and metastasis (for review, see Willert and Jones 2006). The very recent study done in mice AVE shows that the Wnt/β-catenin activity represses, directly or indirectly, the rostral movement of this tissue (Kimura-Yoshida et al. 2005). This may suggest a possible need for adherens junction-regulated cohesion for the AVE to migrate properly.

Dkk1 binds to Glypican 4/6

Our study identifies a new binding partner for Dkk1: the Glypican4 homolog Knypek. The fact that Dkk1 is able to bind to the glypican molecule Knypek opens the possibility that Dkk1 may modulate directly a set of signals requiring Glypican4 for their activity. Glypican4 is crucial for reception of the Fgf signaling both in cell culture and in vivo (Richard et al. 2000; Zhang et al. 2001; Galli et al. 2003). Interestingly, we observed a dramatic increase of Fgf signaling in the germ ring of Dkk1 morphant gastrula embryos (data not shown). Lack or gain of Dkk1 function in mice limb buds has a spectacular effect over Fgf signaling inside the forming limb, which may account for most of the limb defects subsequently observed (Mukhopadhyay et al. 2001). In the limb, expression of Wnt signals precedes the induction of Fgf signaling and is required for such induction (Tickle and Munsterberg 2001). Fgf signaling in the AER is required for cell movement inside the developing limb (Li and Muneoka 1999) and the gene snail is expressed in the bud in response to Fgf (Isaac et al. 2000). In this context, Dkk1 may directly modulate the level of Wnt activity, thereby indirectly regulating Fgf expression and, through it, modulating cell movement inside the forming limb bud. However, temporal and spatial expression of Wnt and Fgf signals in the limb bud suggests the intriguing possibility that Dkk1 may act over the Fgf pathway in a Wnt-independent fashion.

Glypicans, HSPG (Kramer and Yost 2003), control the distribution of numerous other signaling molecules such as Hedgehog, Wingless, and TGFβ members during development in Drosophila (Lin and Perrimon 2002; Kreuger et al. 2004), Xenopus (Galli et al. 2003), and zebrafish (Myers et al. 2002). HSPGs are now perceived as major modulators of morphogen gradients. In Drosophila, the glypicans Dally and Dally-like are essential to the formation of the morphogenetic gradient of Dpp in the wing disc (Belenkaya et al. 2004). In this system, the glypicans are strictly required for the movement of the Dpp proteins away from their source. Interestingly, a Drosophila Wnt antagonist, called Notum (Giraldez et al. 2002), negatively modulates the Wnt signaling pathway by specifically inducing cleavage of Dally-like from the membrane (Kreuger et al. 2004). Dkk1 may thus be acting on the Fgf and Wnt pathways by modulating interaction between glypicans and signaling effectors of these pathways. If so, the next challenge will be to unravel the mechanisms by which Dkk1 is able to interact with glypican-dependent pathways

Dkk1 activates the Wnt/PCP pathway

We show here that Dkk1 is activating the Wnt/PCP pathway. The molecular interaction between Kny and Dkk1, the lack of rescue of the gastrulation movement defect by DNwnt8 overexpression, the ability of Dkk1 overexpression to induce CE defects in mutant embryos in which the Wnt/βcatenin pathway is constitutively active, and the capacity of Dkk1 to cooperate with Kny in rescue of the kny mutants all suggest a direct interaction between Dkk1 and this pathway. This direct interaction is not likely to act at the level of the ligands. Indeed, although Dkk1 is able to potentiate the rescue of the kny mutants by Kny, it is unable to help Wnt11 in the rescue of the slb/wnt11 mutants (data not shown). In light of our data, we therefore propose a mechanism by which Dkk1 concomitantly represses Wnt/βcatenin and activates Wnt/PCP pathways through a shift in the intracellular property of the Fz receptor (Fig. 7). Dissociation of the LRP/Fz complex induced by Dkk1 may change the receptor such that it now has more affinity to the Wnt ligands defined as PCP activators (Wnt2, Wnt4, and Wnt5) and/or is now able to impose a switch in Dsh activity. Two very recent studies provided evidence for the presence of such a switch between the two pathways. Casein kinase 1 ε (Strutt et al. 2006) and metastasis-associated kinase (MAK) (Kibardin et al. 2006) are both able to transform the activity of the Wnt/βcatenin pathway into a Wnt/PCP function by inducing change in pathway specificity at the level of Dsh. Dkk1 may therefore act as a catalyzer of a PCP activity of Dsh by inhibiting LRP5/6 and providing Kny molecules in the vicinity of the LRP-depleted receptor complex.

Dkk1 establishes a connection between patterning and cell movements

Although patterning and gastrulation movements have been suggested to be fairly independent events (Myers et al. 2002), some recent findings reopen the possibility of a complex relationship between these two key mechanisms in development of the vertebrate nervous system. The ability of a modulator such as Dkk1 protein to act simultaneously on patterning and gastrulation movements via coordinated modulation of both Wnt/βcatenin and PCP pathways provides a means to achieve coordination between cell movements and fate specification. One interesting possibility is that Dkk1 may partly regulate patterning by controlling the progression of the cells carrying the patterning signals. The gradient of Fgf protein inside the presomitic mesoderm is formed by the progressive degradation of an initial synthesis inside a moving involuting population (Dubrulle and Pourquie 2004). If this mechanism may concern not only the Fgf but also the Wnt/βcatenin pathway, one can predict that any change in the speed at which a signaling population progresses will have a direct impact on the patterning that signal regulates. So, as lack of Dkk1 accelerates rostral progression, under the developing neural plate, of the Wnt-expressing internalizing mesendoderm, it therefore also exposes prematurely part of the neural epithelium to the underlying Wnt/βcatenin and Fgf signals. It has been shown recently that not only does the spatial graded distribution of a morphogen establish positional information but, as importantly, timing of exposure to the morphogen shapes this information (Ahn and Joyner 2004; Harfe et al. 2004). Modification of the timing at which a given neural area is exposed to mesoderm vertical signaling may therefore be sufficient to perturb significantly the patterning decision of this region.

In conclusion, this work unraveled the involvement of Dkk1 in regulation of cell movements, via binding to the HSPG Knypek, driving coordinated modulation of the Wnt/βcatenin and PCP signaling pathways. Modulation of movements may be a crucial part of Dkk1’s influence on patterning, not only during gastrulation but also during limb development, forebrain organization, and organogenesis. This report therefore opens new avenues in the understanding of the mechanisms governing spatial and temporal regulation of signaling centers during development.

Materials and methods

Constructs

The construct for the in vitro synthesis of dkk1GFP mRNA was generated by cloning the zebrafish dkk1 cDNA into the pcDNA3.1/CT-GFP vector (LifeTechnology). The Kny-Flag expression construct is a gift from Lila Solnica-Krezel (Vanderbilt University, Nashville, TN) and Jacek Topczewski (Northwestern University, Chicago, IL) (Topczewski et al. 2001). The LRP6 expression construct is provided by Tamai et al. (2000).

Whole-mount in situ hybridization and immunohistochemistry

In situ hybridization and immunohistochemistry were performed as previously described (Macdonald et al. 1994). For immunohistochemistry, the following antibodies were used: anti-fluorescein-AP (Roche), anti-biotin (Vectastain), anti-GFP (TorreyPines Biolabs), and anti-Flag (Sigma).

RNA injections and MO experiments

Capped RNAs were transcribed with SP6 RNA polymerase using the mMessage mMachine Kit (Ambion). dkk1GFP RNA was injected at 50 ng/μL. MO antisense oligonucleotides (GeneTools) were designed against 25 bases around the AUG of the zebrafish dkk1 and tcf3 transcripts (against dkk1, 5′-AATTG TAGGATGTATTCCCTGGGTG-3′ and 5′-TAGAGAGCATG GCGATGTGCATCAT-3′; against tcf3, 5′-CTCCGTTTAACT GAGGCATGTTGGC-3′). Injections of MOs were done at concentrations between 2 and 0.4 mg/mL. Routine controls involved injection of four nucleotide-modified versions of the MO tested. We tested whether the phenotype observed in the morphants was due to specific loss of Dkk1 function by rescue assay, injecting dkk1MO together with a dkk1 transcript lacking the MO target sequence (Supplementary Fig. S1). We also assessed the specificity of the MO phenotypes by testing whether the phenotype observed when overexpressing dkk1 RNA could be rescued by dkk1MO. Two MOs have been used, both rescuing dkk1 RNA overexpression and giving the same phenotype when injected alone. We also showed that dkk1MOs inhibit in vivo translation and activity of the GFP-tagged version of Dkk1 proteins (Fig. 1). Overexpression of dkk1GFP alone leads to an increase in anterior brain size when injected in one- to two-cell-stage embryos, mimicking the activity of early acting Wnt antagonists such as Cerberus, Dkk1, and Frzb1 (Yamaguchi 2001). Coinjection with dkk1MO decreases fluorescence and led to concentration-dependent rescue of the dkk1GFP overexpression phenotype (Fig. 1A–H). When dkk1GFP RNA was coinjected with a high amount of dkk1MO, the embryos began to show a decrease in telencephalon and eye size typical of dkk1MO-injected embryos (Fig. 1I–J) indicating that the dkk1MO was reducing both exogenous and endogenous Dkk1 proteins.

Injections and transplantations

Injections of RNA or MOs were done at the one-cell stage, with the exception of Figure 1O,P, where the MO was injected in the newly formed YSL at the 1000-cell stage. For transplantations, donor embryos were injected with dkk1 or wnt11or wnt8 or GFP-RNA or fluorescein-tagged MOs or the tracer rhodamine biotinylated dextran shortly after fertilization. Donor cells were taken from germ ring late blastula embryos and transplanted to the germ ring of early gastrula hosts. For reproducibility, it was crucial to keep the initial position of the clones identical. We chose to place them at 35°–45° from the shield, inside the embryo margin. Transplantations were performed on embryos mounted in 4% methyl-cellulose in embryo medium and were viewed with a fixed-stage Nikon Optiphot microscope. Cells were moved by suction using a mineral oil-filled glass micropipette attached to a 50-μL Hamilton syringe (Houart et al. 1998).

Immunoprecipitation and blotting

HEG cells were used as cellular hosts to test molecular interaction between Dkk1 and Knypek. The cells were plated at a concentration of 2 × 10⁶ to 3 × 10⁶ cells in a 10-cm-diameter dish. The cells were transfected the next day by adding a total volume of 500 μL containing 25 μg of DNA, 50 μL of CaCl_2,, and 2.5 M in BES buffer, and incubation for 16 h at 37°C in 5% CO₂. The cells were then left to recover in DMEM culture medium for 48 h. The efficiency of transfection was determined by analysis of the GFP fluorescence under UV light (generally ∼90%–95%). Cells were lysed and 1:300 GFP polyclonal antibody was added to the lysate for immunoprecipitation overnight at 4°C. The immunoprecipitate was purified on A-Sepharose and G-Sepharose columns. Interaction was also tested in gastrula embryos. For each condition tested, extracts of 30 embryos injected at the one-cell stage were used for the control extracts (1/10 of the total volume) and both anti-GFP and anti-Flg immunoblotting of the anti-Flg (Sigma) immunoprecipitation (4.5/10 of total extract each).

The samples were run on polyacrylamide gels and transferred to nitrocellulose filters. The filters were incubated overnight with the anti-Flag (Sigma) and anti-GFP (TorreyPines Biolabs) at 4°C. After washes and incubation with secondary antibody, the filter was developed on photographic film (Kodak), using a chemo-luminescence detection kit (Perkin Elmer).

Time-lapse analysis

The movements of cells transplanted in the germ ring of shield stage host embryos were followed by time-lapse imaging using the Nikon C1 confocal microscope. Analysis of the movies was done using the Imaris software (Bitplane).