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[S] The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3. 1 Both authors contributed equally to this work. 2 Recipient of Fondo Nacional de Ciencìa y Tecnología Postdoctoral Grant 3070015. 3 Consejo Nacional de Investigación, Ciencìa y Tecnología Ph.D. fellows.
Dynamic regulation of cell adhesion receptors is required for proper cell migration in embryogenesis, tissue repair, and cancer. Integrins and Syndecan4 (SDC4) are the main cell adhesion receptors involved in focal adhesion formation and are required for cell migration. SDC4 interacts biochemically and functionally with components of the Wnt pathway such as Frizzled7 and Dishevelled. Non-canonical Wnt signaling, particularly components of the planar cell polarity branch, controls cell adhesion and migration in embryogenesis and metastasic events. Here, we evaluate the effect of this pathway on SDC4. We have found that Wnt5a reduces cell surface levels and promotes ubiquitination and degradation of SDC4 in cell lines and dorsal mesodermal cells from Xenopus gastrulae. Gain- and loss-of-function experiments demonstrate that Dsh plays a key role in regulating SDC4 steady-state levels. Moreover, a SDC4 deletion construct that interacts inefficiently with Dsh is resistant to Wnt5a-induced degradation. Non-canonical Wnt signaling promotes monoubiquitination of the variable region of SDC4 cytoplasmic domain. Mutation of these specific residues abrogates ubiquitination and results in increased SDC4 steady-state levels. This is the first example of a cell surface protein ubiquitinated and degraded in a Wnt/Dsh-dependent manner.
Cell adhesion and migration are key processes for embryogenesis, inflammatory response, tissue repair, and cancer. Cell migration is a very dynamic process that requires the continuous assembly and disassembly of the cell surface complexes mediating cell-cell and cell-extracellular matrix adhesion (
). Which extracellular cues regulate the stability and turnover of FA receptors, in particular of SDC4, is a key question to understand cell migration.
Wnt proteins constitute a large family of secreted glycoproteins that can activate diverse intracellular signaling pathways in a context-dependent manner. Wnt5a is known to promote polarized cell migration by activating a non-canonical Wnt pathway through the tyrosine kinase Ror2 receptor and c-Jun N-terminal kinase (
). Modulation of FA receptor turnover remains a possible mechanism for Wnt5a-induced migration.
Gastrulation, in particular studies of convergence and extension (CE) in Xenopus and zebrafish embryos, has been a fertile ground to understand how growth factor signaling regulates cell migration in vivo. CE is a process of cell-cell intercalation driven by polarized protrusive activity of dorsal mesodermal cells during Xenopus gastrulation, which allows proper narrowing and elongation of the vertebrate embryo (
), supporting the importance of cell-extracellular matrix interaction in embryonic morphogenesis. Non-canonical Wnt signaling, particularly components of the planar cell polarity signaling pathway, regulates gastrulation in Xenopus and zebrafish embryos (
). SDC4 interacts biochemically and functionally with the Wnt receptor Frizzled7 (Fz7) and the intracellular multidomain protein Dishevelled (Dsh) and modulates this signaling pathway in a fibronectin-dependent manner (
). This prompted us to evaluate the effect of non-canonical Wnt signaling in SDC4. We have found that Wnt5a reduces cell surface levels and promotes degradation of SDC4 in a Dsh-dependent manner. We also demonstrate that SDC4 can be modified by monoubiquitin moieties that are linked to lysine residues located in SDC4 cytoplasmic domain. Importantly, Wnt5a and Dsh regulate ubiquitination of SDC4. Therefore, this is the first example of a cell surface protein ubiquitinated and degraded in a Wnt/Dsh-dependent manner. These results underscore a novel role for non-canonical Wnt signaling in regulating the stability and turnover of FA receptors.
Cell Surface Biotinylation Assay
Transfected HEK293T cells were treated with pure recombinant Wnt5a (R&D Systems) for different times and then incubated with a solution of 0.5 mg/ml EZ-LinkTM NHS-biotin. The biotin reaction was quenched for 10 min with 50 mm NH4Cl followed by two PBS washes. Cells were lysed in buffer A (50 mm Hepes, pH 7.4, 150 mm NaCl, 1 mm EGTA, 2 mm MgCl2, 10% glycerol, 1% Triton X-100) in the presence of protease inhibitors. Homogenates were bound and precipitated with neutroavidin-agarose beads, and proteins were eluted and analyzed by α-FLAG Western blotting.
HEK293T cells were transfected with FLAG-xSDC4, and 24 h later they were incubated in serum-free medium for 30 min. [35S]Met and cycloheximide were added for 1 h, and then cells were incubated in the presence or absence of pure recombinant Wnt5a (100 ng/ml). Cells were lysed in buffer B (10 mm Tris, pH 7.5, 150 mm NaCl, 0.5% Triton X-100), and homogenates were immunoprecipitated with α-FLAG antibody and analyzed by fluorography. SDC4 half-life was estimated by fitting an exponential trendline to the experimental values obtained.
HEK293T cells were transfected and after 24 h were lysed in buffer B and subjected to two different protocols. For double-immunoprecipitation (see Fig. 5a, lane 4), cell homogenates were processed for α-FLAG immunoprecipitation, and after three washes with buffer B the Sepharose beads containing the precipitated immunocomplexes were boiled in 1% SDS for 5 min, diluted with TNTE (50 mm Tris-HCl, pH 7.4, 150 mm NaCl, 1 mm EDTA, 0.1% Triton X-100), subjected to a second α-FLAG immunoprecipitation, and immunocomplexes eluted as below. For single-immunoprecipitation (see Fig. 5a, lanes 1–3, b–d, and supplemental Fig. S3, a and b), cell extracts were immunoprecipitated with α-FLAG antibody and the immunocomplexes eluted in SDS-PAGE loading buffer. Immunoprecipitated proteins were analyzed by Western blotting using an α-HA-HRP-conjugated antibody.
To generate FLAG-xSDC4ΔPBM or FLAG-xSDC4-mutK, site-directed mutagenesis was performed by using the QuikChange Site-Directed Mutagenesis kit (Stratagene, La Jolla, CA), with FLAG-xSDC4 as a template DNA (
). CHX (350 μm) was added for 4 h prior to cell lysis. siDvl2 (sc-35230, Santa Cruz Biotechnology) was used for Fig. 3e.
For co-immunoprecipitation experiments HEK293T cells were grown to 80% confluence in 6-well plates and were transiently transfected with 1 μg of FLAG-xSDC4, FLAG-xSDC4ΔC, or FLAG-xSDC4ΔPBM vectors using the Lipofectamine 2000 reagent (Invitrogen). After transfection (24 h), the cells were lysed with 250 μl of immunoprecipitation buffer (150 mm NaCl, 20 mm Tris, pH 7.5, 1.5 mm CaCl2, 1.5 mm MgCl2, and 0.5% Triton X-100) containing protease inhibitors. Homogenates were precleared with 30 μl of protein A-Sepharose beads (Santa Cruz Biotechnology) at 4 °C for 1 h with rocking. FLAG-xSDC4 proteins were immunoprecipitated with α-FLAG mouse antibody (Stratagene) and incubated overnight at 4 °C with rocking followed by protein A-Sepharose bead pulldown at 4 °C for 2 h. After three washes with the immunoprecipitation buffer, immune complexes were released in 65 μl of SDS-PAGE loading buffer by boiling at 100 °C for 5 min, separated by SDS-PAGE, transferred to PVDF membranes, and then subjected to immunoblot analysis, with α-FLAG mouse antibody (Stratagene) 1:2000 or α-myc (Santa Cruz Biotechnology) 1:500.
Embryos and Explants
All mRNAs were synthesized in vitro with SP6 polymerase using the Message Machine kit (Ambion, Austin, TX). Synthetic mRNAs encoding xWnt5a (60 pg/blastomere), xWnt11 (10–20 pg/blastomere), FLAG-EGFP-xSDC4, and FLAG-EGFP-xSDC4-mutK (30–60 pg/blastomere), FLAG-xSDC4 and FLAG-xSDC4ΔPBM (6–60 pg/blastomere), mRFP (30 pg/blastomere), dnWnt11 and sFz7 (20–30 pg/blastomere), Ub-HA (60 pg/blastomere), myc-xDsh (10–30 pg/blastomere) and xDsh-DEP+ (1–120 pg/blastomere) were microinjected 2–4 times at the animal pole or B tiers at the four-cell stage. Embryos were grown to stage 9 or 10+, and animal caps or dorsal marginal zones (DMZs) were dissected. To detect FLAG-xSDC4, 10 explants were homogenized in 100 μl of lysis buffer A (10 mm Tris, pH 7.5, 150 mm NaCl, 0.5% Triton X-100) containing protease inhibitors. Samples were separated by SDS-PAGE and developed using the α-FLAG antibody.
HEK293T cells were growth on fibronectin-coated coverslips (10 mg/ml) in DMEM containing 10% FBS in an atmosphere of 5% CO2 at 37 °C. Cells were incubated in the presence or absence of Wnt5a protein for 3 h and analyzed by immunofluorescence. Cells were washed twice with PBS and fixed with 3% paraformaldehyde for 10 min at room temperature followed by blocking with PBS containing 5% non-fat powdered milk for 1 h at room temperature. Mouse monoclonal α-SDC4 antibody (5G9, sc-12766; Santa Cruz Biotechnology) was diluted at 1:100 and incubated in a humid chamber at room temperature for 1 h in PBS-blotto. After three washes cells, were incubated with α-mouse conjugated to Alexa Fluor 488 (Molecular Probes) and incubated for 1 h at room temperature in a humid chamber. Nuclear counterstaining was performed using Hoechst 33258 (1 mg/ml) for 5 min at room temperature. Samples were washed, mounted, and analyzed, and representative cells were selected using a Nikon Diaphot inverted microscope equipped for epifluorescence.
HEK293T cells were cultured in 6-well plates and transfected by the calcium phosphate method with FLAG-SDC4 or FLAG-SDC4mutK. At 24 h after transfection, cells were plated onto poly-d-lysine-coated glass coverslips. 24 h later, the cells were incubated with binding medium (DMEM, 20 mm Hepes, pH 7.5, 0.1% BSA p/v) for 30 min at 4 °C, and further incubated with 500 ng/ml anti-FLAGM2® (Stratagene) at 4 °C for 1 h. The cells were then washed three times with cold PBS and incubated with 250 ng/ml Wnt5a (R&D Systems) or DMEM without serum (control medium) at 37 °C. Time 0 corresponds to cells that were always maintained at 4 °C. After that, the cells were processed for immunofluorescense as depicted above. A blind observer did the counting of the cells.
Non-canonical Wnt Signaling Reduces Cell Surface Levels and Promotes Degradation of Syndecan4
The effect of Wnt5a/11 in SDC4 cell surface levels was evaluated by different approaches. Xenopus embryos were microinjected with synthetic mRNA for EGFP-xSDC4 alone or together with mRNA for xWnt5a or xWnt11. Explants were isolated at blastula or gastrula stages (animal caps or DMZs containing cells undergoing CE) and visualized by confocal microscopy. Overexpression of Wnt5a or Wnt11 resulted in a strong reduction in the cell surface levels of SDC4 (Fig. 1, a–f and supplemental Fig. S1, a–f). In addition, incubation with pure recombinant Wnt5a for 3 h resulted in a strong reduction in the cell surface levels of endogenous SDC4 in HEK293T cells (Fig. 1, g and h). Wnt5a was also able to reduce the levels of biotinylated cell surface FLAG-xSDC4 rapidly (Fig. 1i). Therefore, Wnt5a and Wnt11 reduce SDC4 cell surface levels in cultured cells and Xenopus gastrulae.
The experiments depicted above also suggested that Wnt5a/11 could have an effect on SDC4 total levels (Fig. 1i, compare SDC4 total protein in lanes 1 and 5). To study with more detail the effects of Wnt5a/11 on SDC4 steady-state levels the following experimental paradigm was set up and used along this work. FLAG-xSDC4 was overexpressed in HEK293T cells (by DNA transfection with FLAG-xSDC4 under the control of a CMV promoter) or in Xenopus explants (by RNA microinjection) followed by incubation with pure recombinant Wnt5a (100–250 ng/ml) for different times (1–3 h), and total levels of FLAG-xSDC4 were evaluated by Western blotting. Importantly, after 3 h of incubation, the steady-state levels of FLAG-xSDC4 protein were down-regulated in cells and embryos (Fig. 2, a and b). This effect is even more remarkable considering that FLAG-xSDC4 was constantly being overproduced under these conditions. Similar results were obtained when Wnt11 was co-expressed with FLAG-xSDC4 (see supplemental Fig. S1, g and h) and also when Wnt5a was co-expressed with mouse SDC4 (see supplemental Fig. S1i).
Because SDC4 protein was produced from synthetic mRNA in microinjected embryos, the possibility that the effects of Wnt ligands occurred at the transcriptional level was ruled out. Due to the kinetics of Wnt5a effects and the experimental layout the most plausible explanation was that Wnt5a had a post-translational effect. To address this possibility, we tested the effect of Wnt5a in the presence of the protein synthesis inhibitor, CHX. Wnt5a was able to reduce the steady-state levels of SDC4 in the presence of CHX (Fig. 2c, compare lanes 1 and 2), indicating that Wnt5a regulates SDC4 at a post-translational level. Of note, the effect of Wnt5a was specific because the steady-state levels of other cell surface receptors were not affected (Fig. 2c, lanes 3-8). Together, these experiments suggested that Wnt5a promotes degradation of SDC4 protein. Accordingly, a pulse-chase experiment showed that in the absence of exogenous Wnt5a, the half-life of FLAG-xSDC4 was ∼4.7 h, whereas it was reduced to 2.3 h when Wnt5a was added (Fig. 2, d and e). Altogether, these findings demonstrated that Wnt5a accelerates SDC4 degradation.
To evaluate the consequences of reducing non-canonical Wnt signaling on SDC4 steady-state levels, we tested the effect of dnWnt11 or sFz7, two constructs that block this signaling pathway in HEK293T cells and dorsal mesodermal cells (
). In line with the results depicted above, overexpression of dnWnt11 or sFz7 in Xenopus DMZ explants or HEK293T cells increased the steady-state levels of FLAG-xSDC4 protein (Fig. 2f and supplemental Fig. S1, j and k). These results indicate that SDC4 is actively degraded in cultured cells and dorsal mesodermal cells by endogenous levels of non-canonical Wnt signaling and blocking this activity is sufficient to counteract SDC4 degradation. In conclusion, increasing non-canonical Wnt signaling is sufficient to promote SDC4 degradation and reduce its steady-state levels.
To determine how Wnt5a induces SDC4 degradation we tested the effect of lysosome or proteasome inhibition on Wnt5a-induced degradation. We found that SDC4 degradation was insensitive to lysosome inhibition by methylamine and chloroquine (Fig. 2g and supplemental Fig. S2a). However, SDC4 degradation was blocked after proteasome inhibition with lactacystin (Fig. 2h). The same concentration of methylamine was able to block TrkA degradation induced by NGF, corroborating the effectiveness of the drug in cell surface receptors degraded by the lysosome (supplemental Fig. S2b). In conclusion, Wnt5a induces degradation of SDC4 by a lysosome-independent and proteasome-dependent pathway. A similar mechanism has been described previously for other cell surface proteins, including the opioid receptors, urea transporter, amiloride-sensitive epithelial Na+ channel, and the glutamate transporter (
), we decided to evaluate the role of Dsh in regulating SDC4 steady-state levels. In gain-of-function experiments we found that Dsh reduces SDC4 steady-state levels (Fig. 3a). More importantly, suboptimal amounts of Dsh and Wnt5a that have almost no effect when used alone (Fig. 3b, compare lane 2 with lanes 3–5) have a strong cooperative effect in reducing SDC4 steady-state levels (Fig. 3b, lanes 6 and 7). Conversely, reduction of Dsh levels/activity by morpholino and siRNA and overexpression of Dsh dominant-negative constructs (xDsh-DEP+, Xdd1), which specifically block non-canonical Wnt signaling, results in increased SDC4 steady-state levels (Fig. 3, c–e, and supplemental Fig. S2, c and d). The effect of Dsh knockdown was rescued by microinjection of myc-xDsh mRNA supporting the specificity of this results (Fig. 3c, lanes 4–6). These experiments demonstrated that Dsh is sufficient and necessary to regulate SDC4 levels in cells and embryos.
Then we studied the relevance of the interaction between Dsh and SDC4 for SDC4 stability. For this, mutants of SDC4 cytoplasmic domain were used. Deletion of the last three amino acids of SDC4 cytoplasmic domain, corresponding to a putative PDZ binding motif (PBM, FLAG-xSDC4ΔPBM, Fig. 4a) diminished co-immunoprecipitation of Dsh (Fig. 4b, compare lanes 2 and 4) without decreasing SDC4 cell surface localization (Fig. 4c). This deletion construct provided an excellent tool to correlate the disruption of Dsh-SDC4 interaction in SDC4 steady-state levels and Wnt5a-induced degradation. Overexpression of equal amounts of the RNA or DNA for xSDC4ΔPBM resulted in the production of significantly higher steady-state levels of the SDC4 mutant proteins compared with wild type (Figs. 4d, compare lanes 2–4 with 5–7 and supplemental Fig. S2e). This suggests that the interaction with Dsh makes SDC4 more suitable for degradation. To investigate this directly, we evaluated the effect of Wnt5a on SDC4ΔPBM. Importantly, FLAG-xSDC4ΔPBM is completely resistant to the presence of Wnt5a (Fig. 4e, compare lanes 3 and 4 with 6 and 7). Furthermore, SDC4ΔPBM steady-state levels are insensitive to overexpression of xDsh-DEP+ (Fig. 4f). These results demonstrated that the interaction with Dsh is required for Wnt5a-induced degradation of SDC4.
Syndecan4 Is Ubiquitinated in a Wnt5a- and Dsh-dependent Manner
Ubiquitin (Ub) can covalently modify proteins as a monomer (monoubiquitination) or by forming Ub chains (polyubiquitination). Monoubiquitination has been involved in a variety of cellular processes, including internalization and endocytosis of cell surface receptors, whereas polyubiquitination regulates protein degradation (
). The four conserved lysines in SDC4 cytoplasmic domain (Fig. 4a) were candidate amino acids for ubiquitination. Importantly, these lysine residues are located in the variable region of SDC4 cytoplasmic domain, which is unique for this proteoglycan among the syndecan family and is conserved along evolution (
To test SDC4 ubiquitination, FLAG-xSDC4 and HA-tagged ubiquitin (Ub-HA) were overexpressed in HEK293T cells and embryos. Cell homogenates were analyzed through α-FLAG immunoprecipitation followed by α-HA Western blotting. Using this assay, we have found that SDC4 was covalently modified by Ub in HEK293T cells and Xenopus embryos (Fig. 5a, lane 2, and supplemental Fig. S3a). The possibility that a ubiquitinated SDC4-interacting protein co-precipitates with SDC4 was excluded because no ubiquitination was detected when the lysine residues of SDC4 cytoplasmic domain were mutated to alanines (SDC4-mutK, see Fig. 4a and 5a, lane 3). Moreover, the same bands were obtained when the experiments were performed under stringent denaturing conditions (Fig. 5a, lane 4).
Ub shifted SDC4 from an apparent molecular mass of 46 kDa (corresponding approximately to a SDC4 homodimer) to bands running with an apparent molecular mass between 55 and 70 kDa (see arrows in Fig. 5a), suggesting that SDC4 could receive approximately one to three Ub moieties (Ub molecular mass is ∼8.5 kDa). This shift could result from the addition of multiple monoubiquitins moieties or a short Ub chain. To address this issue directly, we made use of a Ub mutant that only supports monoubiquitination because their seven lysines residues that allow chain formation have been mutated to arginine (UbK0-HA) (
). Importantly, the same pattern of SDC4 ubiquitination was detected when Ub-HA and UbK0-HA were used (Fig. 5b, compare lanes 2 and 4), indicating that SDC4 is monoubiquitinated.
Considering that non-canonical Wnt signaling promotes degradation of SDC4 and that SDC4 is ubiquitinated, we hypothesized that this signaling pathway could regulate SDC4 ubiquitination. To prove this, we tested the effect of modifying the levels of non-canonical Wnt signaling on SDC4 ubiquitination. To avoid the effect of Wnt5a in SDC4 steady-state levels, we performed experiments in the presence of the proteasome inhibitor lactacystin, which blocks Wnt5a-induced degradation of SDC4 (Fig. 2h). Addition of Wnt5a for 3 h resulted in increased levels of ubiquitinated SDC4 (Fig. 5c). Conversely, overexpression of xDsh-DEP+ resulted in a reduction in the levels of SDC4 ubiquitination in HEK293T cells and Xenopus gastrula (Fig. 5d and supplemental Fig. S3b). In conclusion, non-canonical Wnt signaling regulates SDC4 monoubiquitination.
To determine the relevance of monoubiquitination in SDC4 homeostasis we tested its effect at two steps: internalization and steady-state levels. To evaluate the role of ubiquitination on SDC4 internalization we measured the effect of Wnt5a in the uptake of anti-FLAG antibody on cells overexpressing FLAG-xSDC4 and FLAG-xSDC4mutk. We found that Wnt5a was able to induce internalization of the ubiquitination mutant as efficiently as the wild type (Fig. 5, e–i). To determine the effect of ubiquitination in the protein levels of SDC4 we microinjected equal amounts of synthetic mRNAs for FLAG-EGFP-xSDC4 or FLAG-EGFP-xSDC4-mutK into Xenopus embryos, and isolated explants were visualized by confocal microscopy. Higher levels of SDC4-mutK compared with wild type SDC4 were detected by immunofluorescense (Fig. 5, j–o), indicating that ubiquitination regulates SDC4 steady-state levels in Xenopus embryos. Intriguingly, we detected very low amounts of FLAG-xSDC4mutk by Western blot analysis of Xenopus explants and HEK293T cells (Fig. 5p, compare lanes 1 and 4 and supplemental Fig. S3c). As we discuss below, we propose that this paradox could be explained if the absence of ubiquitin modification in the intracellular domain favors the SDC4 tendency to form oligomers that are unable to enter SDS-PAGE gels. In these conditions Wnt5a and Wnt11 reduced the steady-state levels of wild type FLAG-xSDC4 but not of the ubiquitination mutant (Fig. 5p, compare lanes 1–3 with 4 and 5 and supplemental Fig. S3d). We conclude from these results that ubiquitination regulates Wnt-induced degradation of SDC4 but not its internalization. A similar role for monoubiquitination has been described for other cell surface receptors (
This work introduces non-canonical Wnt signaling as a new extracellular cue involved in regulation of SDC4 internalization and stability. We have demonstrated that non-canonical Wnt signaling reduces cell surface levels and promotes degradation and monoubiquitination of SDC4 in Xenopus gastrulae and cultured cells. Dsh plays an important role on regulating SDC4 steady-state levels. Increasing Dsh levels enhanced Wnt5a-induced degradation of SDC4, and blocking Dsh activity increased SDC4 steady-state levels and diminished its ubiquitination. Mutation of SDC4 cytoplasmic lysine residues abolished ubiquitination and resulted in up-regulation of SDC4 cell surface levels in Xenopus gastrulae, indicating that ubiquitination regulates SDC4 in vivo. This represents the first example of a cell surface protein that is ubiquitinated and degraded in a Wnt5a/11-dependent manner.
The key role of Dsh in ubiquitination and degradation of SDC4 is particularly relevant because Dsh also binds to important components of the endocytic and degradative cellular machinery. Dsh interacts with the clathrin AP-2 adaptor in part via its DEP domain and promotes endocytosis and degradation of Fz4 (
). Moreover, Wnt5a induces the formation of a complex between Dsh and the E3 ubiquitin ligases Smurf1 and Smurf2, promoting ubiquitination and degradation of Prickle, a cytoplasmic component of the non-canonical Wnt signaling pathway (
). We suggest that Dsh could bridge the binding of Smurf1 or 2 and the AP2 adaptor to SDC4 and regulate its ubiquitination, endocytosis, and degradation. Furthermore, SDC4 and Smurfs regulate CE in Xenopus and mouse embryos, respectively (
). Based on this and our finding that SDC4 is mainly mono- and multiubiquitinated, it is plausible to propose that Wnt/Dsh can promote SDC4 degradation by favoring sorting to the endosomal degradation route. It is important to note that similar results have been described for cell surface proteins such as the opioid receptors, urea transporter, amiloride-sensitive epithelial Na+ channel, and the glutamate transporter (
). We have found that Dsh promotes SDC4 degradation through binding to its PBM, suggesting a model whereby Syntenin and Dsh binding to the C-terminal domain of SDC4 could have different effects in its sorting within the endosomal compartments.
Of note, ubiquitination of SDC4 occurs in the variable region of its cytoplasmic domain containing the highly conserved motif KKPIYKK. This peptide binds the membrane lipid phosphatidylinositol 4,5-bisphosphate, allowing clustering of SDC4 followed by activation of PKCα (
). Cell surface receptor clustering, such as when SDC4 is incorporated into FA, is a common mechanism involved in signaling. The addition of Ub moieties to this motif could be a mechanism to disrupt SDC4 clustering with the consequent signaling termination and degradation of this cell surface proteoglycan. In such a case, ubiquitination-deficient mutants should have a strong tendency for oligomerization. Formation of SDC4mutk oligomers that are resistant to reducing conditions used in SDS-PAGE analysis could explain the paradoxical observation that higher levels of SDC4mutk than SDC4 were visualized by immunofluorescense (Fig. 5, j–o) but the opposite was detected by Western blot analysis (Fig. 5p).
Our findings raised the possibility that Wnt5a-induced ubiquitination could be a general mechanism to modulate the stability and turnover of cell adhesion complexes. Non-canonical Wnt signaling regulates turnover, ubiquitination, and proteasome-dependent degradation of Paxillin (
) and that together with SDC4, they are the main FA receptors, it is plausible that non-canonical Wnt signaling could also regulate integrin turnover.
Furthermore, our findings are relevant for the current understanding of the molecular and cellular mechanism underneath the metastasic behavior of tumorigenic cells. It has been reported that abnormally high levels of Wnt5a correlate with malignancy (
). The finding that Wnt5a regulates ubiquitination and degradation of the FA receptor SDC4 suggests a possible mechanism to explain the increased invasiveness of tumorigenic cells when Wnt5a activity is augmented.
We thank Dr. O. Wessely and members of the Larraín laboratory for critical reading of the manuscript and Drs. A. Ciechanover, L. Davidson, M. Tada, S. Wilson, O. Wessely, M. P. Marzolo, A. Gonzalez, H. Steinbeisser, R. Moon, S. Sokol, and S. Fraser for constructs.