Wingless (Wg) signaling regulates expression of its target genes via Pangolin and Armadillo, and their interacting cofactors. In the absence of Wg, Pangolin mediates transcriptional repression. In the presence of Wg, Pangolin, Armadillo, and a cohort of coactivators mediate transcriptional activation. This study uncovered Coop (Corepressor of Pan) as a Pangolin-interacting protein. Coop and Pangolin form a complex on DNA containing a Pangolin/TCF-binding motif. Overexpression of Coop specifically represses Wg target genes, while loss of Coop function causes derepression. Coop is shown to antagonize the binding of Armadillo to Pangolin, providing a mechanism for Coop-mediated repression of Wg target gene transcription (Song, 2010).

Coop (CG1621) was found in an attempt to identify novel Pan-interacting proteins by a mass spectrometry-based proteomic approach. It contains an N-terminal MADF domain and a C-terminal BESS domain (Bhaskar, 2002). This architecture is conserved in 16 Drosophila proteins, one C. elegans protein, and one zebrafish protein. The putative dimeric BESS domain does not exist in mammals, and there is one uncharacterized MADF-containing protein in humans (ZSCAN29/ZNF690). Earlier studies on other members of this family indicated that they might be involved in transcriptional regulation. The interaction between Coop and Pan was confirmed in a series of pull-down experiments. In lysates of Drosophila Kc cells, Pan was coimmunoprecipitated by Coop. Consistent with a role in transcription, Coop was localized in the nucleus in these cells. To determine if this interaction was direct, GST pull-down experiments were performed. A GST fusion of Coop could pull down in vitro translated Pan. In the reverse experiment, Coop was pulled down by a Pan central fragment (247-362) harboring its DNA-binding domain, but not by a Pan N-terminal fragment (1-160). This interaction was further mapped to the C-terminal part of Coop (161-358), which contains its BESS domain (Song, 2010).

Since the DNA-binding domain of Pan may be involved in the interaction with Coop, it was asked whether Coop affected the DNA affinity of Pan. This question was addressed in a gel shift assay. Pan (247-362) induced the mobility shift of a DNA oligonucleotide containing a consensus Pan/TCF-binding site, but did not affect that of a control oligonucleotide in which the binding site was mutated. Coop alone did not bind to this DNA probe. However, in the presence of Pan, Coop induced a supershift of this oligonucleotide. This suggests that the interaction between Coop and Pan does not interfere with Pan binding to DNA. Instead, it suggests that Coop and Pan form a complex on Pan targets (Song, 2010).

To test whether the interaction between Coop and Pan affects Wg signaling in vivo, Coop was overexpressed in wing imaginal discs and expression of Wg targets was monitored, including Distal-less (Dll), senseless (sens), and Notum/wingful (wf). The expression of Coop in the dorsal compartment of wing discs by apterous-Gal4 (apGal4) resulted in the loss of Dll protein in this domain. This repression occurred at the transcriptional level, as the expression of a Dll-lacZ (DllZ) reporter transgene was similarly repressed in the same assay. The expression of Coop in the posterior compartment of wing discs by engrailed-Gal4 (enGal4) also repressed Dll expression, but in a different pattern: Dll staining was lost in the domain away from the dorsal-ventral (D/V) boundary, and was only weakened in the domain close to the D/V boundary. The Wg morphogen is expressed at the D/V boundary and forms a concentration gradient in the wing disc. It is possible that the observations reflected a dosage-dependent effect of Coop on different levels of Wg signaling. To test this idea, Coop was also expressed at lower levels, either in clones or in the center of wing discs by spalt enhancer-Gal4 (salEGal4). Indeed, the repression of Dll expression was observed preferentially in the domain with lower levels of Wg signaling. These results implied that Coop functions to antagonize incoming Wg signaling (Song, 2010).

sens is a high-threshold target of Wg in wing discs and is expressed in two to three rows of cells flanking the D/V boundary. The expression of sens starts at the center of the wing disc and extends to the periphery at late larval stage. The expression of Coop by enGal4 resulted in the repression of sens in the posterior compartment of wing discs, while the expression of Coop by apGal4 prevented Sens from extending to the periphery in the dorsal compartment. Consistent with observations with Dll, lower levels of Coop expression by act>CD2>Gal4 or salEGal4 did not affect sens expression (Song, 2010).

wf is another known Wg target gene. A 4-kb upstream element has been isolated from the wf locus, which responded to Wg signaling in cell culture. A lacZ transgene under its control (wfZ) mimicked wf expression in wing discs. The expression of Coop repressed this wfZ in wing discs and wf reporter in cultured Drosophila cells. In addition to Wg targets in the wing disc, the expression of Coop also repressed H15, a Wg target gene in the leg disc. Thus, in the case of the four Wg target genes analyzed, Coop functioned consistently as a negative transcriptional regulator (Song, 2010).

Whether Coop generally affects transcription, or specifically affects Wg signaling was tested. The effect of Coop on the expression of Notch targets wg and cut was examined. The expression of Coop by enGal4 or other drivers did not affect expression of either gene, confirming that the effect observed on Wg targets was not due to the interference in the upstream level of Wg signaling. Then Coop was expressed by apGal4, which strongly repressed Wg targets, and the expression of other non-Wg targets was monitored. Lgs plays a key role in transducing Wg signal. No effect on lgs expression by Coop was observed. Similarly, the expression of Coop had no effect on the Hedgehog (Hh) target gene decapentaplegic (dpp), or the Dpp targets optomotor blind (omb) and spalt. Similarly, hh, patched (ptc), and en expression were unaffected. In this study it was noticed that overexpression of Coop had a weak negative effect on the Gal4/UAS system. It is therefore possible that Coop expression was reduced, leading to an underestimation of the strength of Coop's activity. Whatever the case, the effect does not compromise any of these conclusions, as none of the targets monitored in this study was driven by the Gal4/UAS system. Taken together, these results suggest that the repressive effect of Coop is probably mediated by its interaction with Pan, and not an indirect effect on wg expression or components of the Wg signaling cascade. Importantly, Coop does not appear to affect Hh or Dpp signaling, suggesting its effect on the Wg pathway is fairly specific (Song, 2010).

In the next step, it was asked whether Coop was required for the proper regulation of Wg target gene expression. The effect of Coop RNAi on the basal transcriptional levels of Wg target genes was examined in Drosophila Kc cells. For comparison, RNAi against Groucho or CtBP, two known corepressors of the Wg pathway, was also performed. Surprisingly, the knockdown of Coop mRNA in Kc cells by dsRNA treatment was very inefficient. The levels of most mRNAs could be knocked down to 10%-15% after 4 d; however, the levels of Coop mRNA remained at 60%-70% after 4 d and 40%-50% after 7 d. Even so, under this condition, the knockdown of Coop mRNA caused a twofold to threefold increase in Arm-independent basal expression of Wg target gene nkd and CG6234, similar to the effect observed with Groucho RNAi, but slightly weaker than the effect seen with CtBP RNAi. In addition, the combination of Coop and CtBP RNAi showed an additive effect (Song, 2010).

Whether reducing Coop function affected the activation of Wg target genes was examined in vivo. This was tested by expressing dsRNA against Coop in the wing disc and monitoring expression of Wg targets. Coop is expressed ubiquitously in imaginal discs. The knockdown of Coop enhanced expression of Dll and wf, suggesting that endogenous Coop also affects Wg-mediated target activation. Starting from an enhancer P element (EP) line, two Coop alleles (hereafter referred to as coop) were generated that encode truncated proteins. coop mutant flies seemed normal, but showed enhanced Wg signaling in a sensitized background: Loss of Coop enhanced a rough eye phenotype caused by ectopic Wg signaling (sev-Wg). In coop mutant clones, moderately enhanced expression of Dll was also observed in the wing disc. This is consistent with in vivo RNAi results, indicating that Coop is a repressor of Wg target genes. Although coop clones had a weaker effect than RNAi, this difference might be due to the perdurance of Coop protein. The RNAi was induced earlier than the clones of coop, thus probably eliminating Coop more thoroughly (Song, 2010).

There are 15 sequence homologs of Coop in Drosophila, and it is possible that one or several of them also had similar roles in Wg signaling. Some Coop family members were examined, and overexpression of CG6854 and Adf1 were found to also repressed Wg signaling in Drosophila cultured cells. As a next step, the possibility that CG6854 or Adf1 acts like Coop in vivo was examined. Overexpression of CG6854 strongly repressed expression of Dll and sens. However, in contrast to Coop, it also repressed expression of wg and targets of other pathways, suggesting the repressive effect of CG6854 may be less specific than that of Coop. Adf1 behaved like CG6854. Taking these results together, it is proposed that, unlike Coop, Adf1 and CG6854 are not specific repressors of the Wg pathway (Song, 2010).

Having established that Coop has a defined role in Wg signaling, in contrast to CG6854 and Adf1, the mechanism by which Coop represses Wg target genes was examined. As the interaction between Arm and Pan is essential for activation of Wg target genes, whether Coop functions by preventing this process was tested. In cultured Drosophila Kc cells, Pan and Arm were overexpressed in the absence or presence of overexpressed Coop, and Pan was coimmunoprecipitated via Arm. The presence of Coop greatly reduced the amount of Pan coimmunoprecipitated. Similarly, when Pan was coimmunoprecipitated via Coop, the presence of overexpressed Arm also prevented this interaction. These results suggest that the binding of Coop to Pan and the binding of Arm to Pan are mutually exclusive (Song, 2010).

As Coop can interact with a domain in Pan that is conserved in other TCFs, whether Coop could also affect Wnt signaling was examined. Interestingly, activation of a Wnt signaling reporter was repressed by ectopic expression of Coop in HEK293T cells. It is likely that Coop achieved this by interfering with the conserved interaction between β-catenin and TCF. These results indicate a way in which Wnt signaling could be additionally regulated by a functional homolog of Coop. Since BESS domain proteins apparently do not exist in mammals, it is postulated that a Coop-like repressor function is carried out by a TCF-interacting protein that is not necessarily structurally related to Coop. Proteomic analysis of TCF interaction partners may help to identify functional homologs of Coop in mammals (Song, 2010).

It has been shown in vitro that β-catenin and TLE1 compete for binding to Lef1. By competing for an overlapping binding site adjacent to the DNA-binding domain of Lef1, TLE1 prevents the recruitment of β-catenin. This study also shows that Coop and Arm also compete to bind Pan. Thus, the levels of nuclear β-catenin/Arm, determined by the levels of Wnt/Wg signaling, decide the transcriptional activity of TCF/Lef/Pan. This dosage-dependent, reversible mechanism helps to shape Wnt/Wg gradient-induced expression of downstream targets (Song, 2010).

Ectopic Wnt signaling, transduced via interaction between β-catenin and TCF, is often detected in human cancers. Several β-catenin-binding proteins, including ICAT and Chibby, can interfere with the interaction of these two proteins. This study reports the identification of Coop as another potential blocker of the β-catenin-TCF interaction. As β-catenin has divergent functions in more than Wnt signaling, TCF-binding proteins may help to specifically decrease the transcriptional outputs of ectopic Wnt signaling. Given the specific effect of Coop in the Wg pathway, it is believed Coop may function as a specific inhibitor of Arm-Pan interaction in Drosophila. Further studies to map the Coop-Pan interaction may uncover novel ways to prevent the interaction between β-catenin and TCF (Song, 2010).