Arrow (LRP6) and Frizzled2 cooperate to degrade Wingless in Drosophila imaginal discs

Endocytosis: A Positive or a Negative Influence on Wnt Signalling?

Endocytosis, with subsequent targeting to lysosomes for degradation, is traditionally seen as a way for cells to terminate signalling. However, in a few instances, endocytosis has been demonstrated to contribute positively to signalling. Here we review recent work on the role of endocytosis in Wnt signalling. Biochemical evidence suggests that the branch of Wnt signalling that controls planar cell polarity (PCP) does require endocytosis, although how endocytosis of Frizzled receptors is translated into PCP in vivo remains unknown. With respect to the main signalling branch (called the canonical or beta-catenin pathway), the literature is divided as to whether endocytosis is required. Results of in vivo experiments are inconclusive because of the toxic side-effects of blocking endocytosis. Some results with cultured cells suggest the need for endocytosis in canonical signalling; however, it remains unclear whether the ligand-receptor complex must enter the cell by clathrin-mediated or caveolae-mediated endocytosis in order to signal. Means of specifically altering Wnt trafficking as well as of tracking the internalization route in different cell types are needed.

Frizzled-7 turnover at the plasma membrane is regulated by cell density and the Ca2+-dependent protease calpain-1

Frizzled are seven-transmembrane domain G-protein coupled receptors involved in cell polarity and Wnt signaling. The mechanisms regulating their turnover at the plasma membrane remain unclear. We have identified a regulated C-terminus cleavage of Frizzled-7 in endothelial cells using ectopic expression of N- and C-termini-tagged Frizzled-7 proteins. This specific cleavage produced a 10 kDa C-terminus fragment that remained associated with intracellular vesicles and was localized within the 3rd intracytoplasmic loop using N-terminal sequencing and targeted mutagenesis. Frizzled-7 mutated forms displaying reduced C-terminus cleavage were also defective for dvl2 translocation at the plasma membrane. PMA, an activator of PKC and endocytosis, but not Wnt13A and Wnt5A, increased the appearance of Frizzled-7 C-terminus-containing vesicles and Frizzled-7 cleavage. Concanavalin-A, an inhibitor of receptor internalization decreased both constitutive and PMA-induced Frizzled-7 cleavage, while inhibition of the endocytic pathway with Delta95-295-Eps15 dominant-negative prevented only PMA-induced Frizzled-7 cleavage. Frizzled-7 C-terminus cleavage was increased with cell density and by the Ca(2+) ionophore ionomycin and was decreased by specific calpain inhibitors, by the expression of DN-calpain-1 and the down-regulation of calpain-1 levels by siRNAs. Altogether, our findings pinpoint calpain-1 as a regulator of Frizzled-7 turnover at the plasma membrane and reveal a link between Frizzled-7 cleavage and its activity.

Association of Dishevelled with the clathrin AP-2 adaptor is required for Frizzled endocytosis and planar cell polarity signaling

Upon activation by Wnt, the Frizzled receptor is internalized in a process that requires the recruitment of Dishevelled. We describe a novel interaction between Dishevelled2 (Dvl2) and micro2-adaptin, a subunit of the clathrin adaptor AP-2; this interaction is required to engage activated Frizzled4 with the endocytic machinery and for its internalization. The interaction of Dvl2 with AP-2 requires simultaneous association of the DEP domain and a peptide YHEL motif within Dvl2 with the C terminus of micro2. Dvl2 mutants in the YHEL motif fail to associate with micro2 and AP-2, and prevent Frizzled4 internalization. Corresponding Xenopus Dishevelled mutants show compromised ability to interfere with gastrulation mediated by the planar cell polarity (PCP) pathway. Conversely, a Dvl2 mutant in its DEP domain impaired in PCP signaling exhibits defective AP-2 interaction and prevents the internalization of Frizzled4. We suggest that the direct interaction of Dvl2 with AP-2 is important for Frizzled internalization and Frizzled/PCP signaling.

Kinetics of morphogen gradient formation

In the developing fly wing, secreted morphogens such as Decapentaplegic (Dpp) and Wingless (Wg) form gradients of concentration providing positional information. Dpp forms a longer-range gradient than Wg. To understand how the range is controlled, we measured the four key kinetic parameters governing morphogen spreading: the production rate, the effective diffusion coefficient, the degradation rate, and the immobile fraction. The four parameters had different values for Dpp versus Wg. In addition, Dynamin-dependent endocytosis was required for spreading of Dpp, but not Wg. Thus, the cellular mechanisms of Dpp and Wingless spreading are different: Dpp spreading requires endocytic, intracellular trafficking.

The vacuolar-ATPase inhibitor bafilomycin and mutant VPS35 inhibit canonical Wnt signaling

Endosomal acidification and transport are essential functions in signal transduction. Recent data suggest that Wnt signaling requires intact endosomal transport machinery but the effects of endosomal acidification on Wnt signal transduction have not been evaluated. Here we report that bafilomycin, a specific inhibitor of the vacuolar proton ATPase that blocks endosomal acidification, inhibits canonical Wnt signal transduction initiated by Wnt ligand and partially inhibits signaling initiated by disheveled. Bafilomycin does not affect Tcf promoter activation by beta-catenin. These data indicate that endosomal acidification is necessary for Wnt signaling. To identify interactions between endosomal transport proteins and Wnt receptors, we performed a GST fusion protein pulldown experiment and identified a possible indirect interaction between the LRP6 intracellular domain and vacuolar protein sorting protein 35 (VPS35). We show that an N-terminal deletion mutant of VPS35 reduces canonical Wnt signaling in HEK-293 cells expressing exogenous Wnt-1. These data suggest that endosomal V-type ATPase activity and retromer trafficking proteins are functionally important in Wnt signal transduction.

Endocytosis, endosome trafficking, and the regulation of Drosophila development

Endocytosis and endosome trafficking regulate cell signaling in unexpected ways. Here we review the contribution that Drosophila research has made to this exciting field. In addition to attenuating signaling, endocytosis shapes morphogen gradients, activates ligands, and regulates spatially receptor activation within a single cell. Moreover, some receptors signal from within endosomes, and the ability of a specific type of endosome to form controls the ability of cells to signal. Experiments in Drosophila reveal that through regulation of a variety of cell signaling pathways, endocytosis controls cell patterning and cell fate.

Sprinter: a novel transmembrane protein required for Wg secretion and signaling

Wingless (Wg) is a secreted ligand that differentially activates gene expression in target tissues. It belongs to the Wnt family of secreted signaling molecules that regulate cell-to-cell interactions during development. Activation of Wg targets is dependent on the ligand concentration in the extracellular milieu; cellular mechanisms that govern the synthesis,delivery and receipt of Wg are elaborate and complex. We have identified sprinter (srt), which encodes a novel, evolutionarily conserved transmembrane protein required for the transmission of the Wg signal. Mutations in srt cause the accumulation of Wg in cells that express it, and retention of the ligand prevents activation of its target genes in signal-receiving cells. In the absence of Srt activity, levels of Wg targets (including Engrailed in embryos lacking maternal and zygotic srt, and Senseless and Achaete in wing discs) are reduced. Activation of Wg targets in the receiving cells does not require srt. Hence, the function of Srt is restricted to events occurring within the Wg-producing cells. We show that srt is not required for any aspect of Hedgehog(Hh) signal transduction, suggesting specificity of srt for the Wg pathway. We propose that srt encodes a protein required for Wg secretion that regulates maturation, membrane targeting or delivery of Wg. Loss of srt function in turn diminishes Wg-pathway activation in receiving cells.

Endocytosis in signalling and development

After a long period of neglect, endocytosis in plants is finally coming of age. The constitutive recycling of plasma membrane proteins has been well established in the past few years, and recent studies report the ligand-induced endocytosis of receptors and other plasma membrane proteins. Signalling by ligand-bound receptors from endosomes has not, however, been demonstrated in plants. Although novel markers have been used to map endocytic pathways, the functional compartmentalisation of endosomes is still controversial. It is thus not clear where and how cargo proteins such as receptors are sorted towards either recycling to the plasma membrane or targeting to the vacuole for degradation.

Fab1 phosphatidylinositol 3-phosphate 5-kinase controls trafficking but not silencing of endocytosed receptors

The trafficking of endocytosed receptors through phosphatidylinositol 3-phosphate [PtdIns(3)P]-containing endosomes is thought to attenuate their signaling. Here, we show that the PtdIns(3)P 5-kinase Fab1/PIKfyve controls trafficking but not silencing of endocytosed receptors. Drosophila fab1 mutants contain undetectable phosphatidylinositol 3,5-bisphosphate levels, show profound increases in cell and organ size, and die at the pupal stage. Mutant larvae contain highly enlarged multivesicular bodies and late endosomes that are inefficiently acidified. Clones of fab1 mutant cells accumulate Wingless and Notch, similarly to cells lacking Hrs, Vps25, and Tsg101, components of the endosomal sorting machinery for ubiquitinated membrane proteins. However, whereas hrs, vps25, and tsg101 mutant cell clones accumulate ubiquitinated cargo, this is not the case with fab1 mutants. Even though endocytic receptor trafficking is impaired in fab1 mutants, Notch, Wingless, and Dpp signaling is unaffected. We conclude that Fab1, despite its importance for endosomal functions, is not required for receptor silencing. This is consistent with the possibility that Fab1 functions at a late stage in endocytic receptor trafficking, at a point when signal termination has occurred.

Internalization is required for proper Wingless signaling in Drosophila melanogaster

The Wnt–Wingless (Wg) pathway regulates development through precisely controlled signaling. In this study, we show that intracellular trafficking regulates Wg signaling levels. In Drosophila melanogaster cells stimulated with Wg media, dynamin or Rab5 knockdown causes reduced Super8XTOPflash activity, suggesting that internalization and endosomal transport facilitate Wg signaling. In the wing, impaired dynamin function reduces Wg transcription. However, when Wg production is unaffected, extracellular Wg levels are increased. Despite this, target gene expression is reduced, indicating that internalization is also required for efficient Wg signaling in vivo. When endosomal transport is impaired, Wg signaling is similarly reduced. Conversely, the expression of Wg targets is enhanced by increased transport to endosomes or decreased hepatocyte growth factor–regulated tyrosine kinase substrate– mediated transport from endosomes. This increased signaling correlates with greater colocalized Wg, Arrow, and Dishevelled on endosomes. As these data indicate that endosomal transport promotes Wg signaling, our findings suggest that the regulation of endocytosis is a novel mechanism through which Wg signaling levels are determined.

Secretion of Wnt Ligands Requires Evi, a Conserved Transmembrane Protein

Wnt signaling pathways are important for multiple biological processes during development and disease. Wnt proteins are secreted factors that activate target-gene expression in both a short- and long-range manner. Currently, little is known about how Wnts are released from cells and which factors facilitate their secretion. Here, we identify a conserved multipass transmembrane protein, Evenness interrupted (Evi/Wls), through an RNAi survey for transmembrane proteins involved in Drosophila Wingless (Wg) signaling. During development, evi mutants have patterning defects that phenocopy wg loss-of-function alleles and fail to express Wg target genes. evi's function is evolutionarily conserved as depletion of its human homolog disrupts Wnt signaling in human cells. Epistasis experiments and clonal analysis place evi in the Wg-producing cell. Our results show that Wg is retained by evi mutant cells and suggest that evi is the founding member of a gene family specifically required for Wg/Wnt secretion.

Endocytic trafficking of Wingless and its receptors, Arrow and DFrizzled-2, in the Drosophila wing

During animal development, Wnt/Wingless (Wg) signaling is required for the patterning of multiple tissues. While insufficient signal transduction is detrimental to normal development, ectopic activation of the pathway can be just as devastating. Thus, numerous controls exist to precisely regulate Wg signaling levels. Endocytic trafficking of pathway components has recently been proposed as one such control mechanism. Here, we characterize the vesicular trafficking of Wg and its receptors, Arrow and DFrizzled-2 (DFz2), and investigate whether trafficking is important to regulate Wg signaling during dorsoventral patterning of the larval wing. We demonstrate a role for Arrow and DFz2 in Wg internalization. Subsequently, Wg, Arrow and DFz2 are trafficked through the endocytic pathway to the lysosome, where they are degraded in a hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs)-dependent manner. Surprisingly, we find that Wg signaling is not attenuated by lysosomal targeting in the wing disc. Rather, we suggest that signaling is dampened intracellularly at an earlier trafficking step. This is in contrast to patterning of the embryonic epidermis, where lysosomal targeting is required to restrict the range of Wg signaling. Thus, signal modulation by endocytic routing will depend on the tissue to be patterned and the goals during that patterning event.

The endocytic pathway and formation of the Wingless morphogen gradient

Controlling the spread of morphogens is crucial for pattern formation during development. In the Drosophila wing disc, Wingless secreted at the dorsal-ventral compartment boundary forms a concentration gradient in receiving tissue, where it activates short- and long-range target genes. The glypican Dally-like promotes Wingless spreading by unknown mechanisms, while Dynamin-dependent endocytosis is thought to restrict Wingless spread. We have utilized short-term expression of dominant negative Rab proteins to examine the polarity of endocytic trafficking of Wingless and its receptors and to determine the relative contributions of endocytosis, degradation and recycling to the establishment of the Wingless gradient. Our results show that Wingless is internalized via two spatially distinct routes: one on the apical, and one on the basal, side of the disc. Both restrict the spread of Wingless, with little contribution from subsequent degradation or recycling. As previously shown for Frizzled receptors, depleting Arrow does not prevent Wingless from entering endosomes. We find that both Frizzled and Arrow are internalized mainly from the apical membrane. Thus, the basal Wingless internalization route must be independent of these proteins. We find that Dally-like is not required for Wingless spread when endocytosis is blocked, and propose that Dally-like promotes the spread of Wingless by directing it to lateral membranes, where its endocytosis is less efficient. Thus, subcellular localization of Wingless along the apical-basal axis of receiving cells may be instrumental in shaping the Wingless gradient.