Gene name - diego
Cytological map position - 41C1-2
Function - signaling
Symbol - diego
FlyBase ID: FBgn0033552
Genetic map position -
Classification - ankyrin repeat protein
Cellular location - cytoplasmic
During planar polarization of the Drosophila wing epithelium, the homophilic adhesion molecule Flamingo localizes to proximal/distal cell boundaries in response to Frizzled signaling; perturbing Frizzled signaling alters Flamingo distribution, many cell diameters distant, by a mechanism that is not well understood. diego, a tissue polarity gene, encodes a protein comprised of six ankyrin repeats that colocalizes with Flamingo at proximal/distal boundaries. Diego is specifically required for polarized accumulation of Flamingo and drives ectopic clustering of Flamingo when overexpressed. It is suggested that Frizzled acts through Diego to promote local clustering of Flamingo, and that the clustering of Diego and Flamingo in one cell nonautonomously propagates to others. Localized Frizzled signaling would modify the properties of the Diego protein at proximal-distal boundaries, increasing Diego's ability to promote clustering of itself and Flamingo. A bias in Diego localization might then be enhanced via homophilic interactions between Flamingo molecules on adjacent cells. This model is consistent with the reciprocal effects that Flamingo and Diego have on each other's localization (Feiguin, 2001).
To isolate genes whose overexpression causes tissue polarity defects, lines harboring 'EP' P elements were crossed to flies expressing GAL4 under the control of the apterous promoter. EP elements contain upstream activation sequences that drive GAL4-dependent transcription from an adjacent P element promoter through genomic DNA next to the site of insertion. Activation of transcription by GAL4 in apterous-expressing cells results in the overexpression of random genomic sequences in cells that give rise to the thorax and dorsal wing blade. A screen of 2300 EP crosses was carried out for those that produced bristle polarity defects in the thorax (33/2300), and then these EP lines were reexamined for their ability to cause defects in hair polarity or number. Six were found that produced multiple wing hairs when overexpressed, but only one, EP(2)2619, was found whose overexpression causes predominantly hair polarity defects. The DNA downstream of the EP element was cloned by plasmid rescue, and when it was sequenced, was found to be homologous to an EST corresponding to clone LD08259 (CG12342 in the GadFly database) (Feiguin, 2001).
To ask whether expression of the gene represented by this EST actually causes the polarity phenotype, transgenic flies were produced containing the cDNA under the control of the UAS promoter and its expression was driven by crossing these transgenic flies to flies that expressed GAL4 under the control of the apterous promoter. Expression of LD08259 causes polarity defects similar to those observed when EP 2619 is crossed to apterous GAL4. These data confirm that the protein encoded by LD08259 causes tissue polarity defects when overexpressed. The gene represented by this cDNA is diego (Feiguin, 2001).
Is Frizzled activity required for polarized Diego localization? Diego localization was examined in clones of frizzled null tissue. Diego protein is undetectable at the cortex between frizzled mutant cells. In contrast, Diego accumulates to a higher level at boundaries between wild-type and frizzled mutant cells, although it is not possible to resolve whether Diego is present on both sides of the boundary. These data suggest that Diego accumulates at boundaries between cells with different levels of Frizzled signaling activity (Feiguin, 2001).
Frizzled mutant clones nonautonomously reorganize the polarity of hairs distal to the mutant tissue, and the effect is most pronounced on the medial side of the clone. Correspondingly, Diego localization in cells on the distal/medial side of frizzled clones is reoriented, suggesting that perturbations in Frizzled signaling can nonautonomously repolarize the distribution of Diego (Feiguin, 2001).
To confirm that Diego localizes to boundaries between cells with different levels of Frizzled signaling activity, its distribution was examined in wings expressing Frizzled under the control of ptc:GAL4. This produces levels of Frizzled protein that change with distance from the AP boundary. In these wings, Diego relocalizes to reflect the artificial gradient of Frizzled expression generated by Ptc:GAL4. In the cells posterior to Frizzled-overexpressing cells, Diego is also relocalized to reflect the nonautonomous disruption of polarity caused by Frizzled. Taken together, these data show that apposition of cells with different levels of Frizzled causes the accumulation of Diego at the boundary between cells. These data further show that the nonautonomous disruption of polarity produced by altering Frizzled signaling results in the mispolarization of Diego (Feiguin, 2001).
Does cortical localization of Diego depend on Flamingo? The distribution of Diego was examined in clones of Flamingo mutant cells. Diego is undetectable at the cortex of flamingo mutant cells and is also absent from both proximal and distal interfaces between wild-type and flamingo mutant tissue. These data suggest that Diego is localized to the cortex via Flamingo, either directly or indirectly. Like Diego, Flamingo also fails to accumulate at either the proximal or distal interface between wild-type and Flamingo mutant clones. This has been interpreted to mean that Flamingo is normally enriched both proximally and distally and that homophilic interactions are required for its polarized accumulation. Similarly, it can be argued that Diego is normally enriched on both proximal and distal sides of the cell and that stable cortical Diego localization occurs only where homophilic Flamingo interactions are possible (Feiguin, 2001).
Clearly, Flamingo is required for cortical accumulation of Diego. To ask whether it was also sufficient, that is, whether Diego simply accumulates wherever Flamingo is present, the distribution of Diego was examined in cells that overexpressed Flamingo. Although some Flamingo can be detected at cell boundaries when the protein is overexpressed, Flamingo accumulates most obviously in bright spots inside the cell. Since Flamingo is a transmembrane protein, these spots might either represent protein that is trapped in the secretory pathway or that has accumulated in the endocytic pathway. In Flamingo-overexpressing cells, Diego protein appears less abundant on the cortex overall, and what Diego remains is no longer restricted to proximal-distal cell boundaries. Diego does not colocalize with Flamingo inside the cell where Flamingo accumulates to the highest level. This indicates that other proteins besides Flamingo must promote the cortical localization of Diego. Colocalization of Diego and Flamingo upon transfection of S2 cells could not be seen, indicating that other proteins may regulate whether Diego and Flamingo are present in the same complexes. Nevertheless, at least at the cortex, dominant mislocalization of Flamingo by overexpression can relocalize Diego (Feiguin, 2001).
In wild-type cells adjacent to Flamingo-overexpressing cells, Diego disappears from the proximal-distal boundaries and appears to accumulate to a higher level at the boundary with the Flamingo-overexpression domain, perpendicular to its normal proximal-distal pattern. Furthermore, overaccumulation of Diego is also seen on the cell boundary opposite that which is in contact with the Flamingo-overexpressing cells. In some wings, reorientation of both Diego and Flamingo occurs up to three cell diameters away from the overexpression stripe. Thus, despite the fact that loss of Flamingo activity in clones has an essentially autonomous effect, depleting Diego and Flamingo only from the directly adjacent cell boundary, flamingo overexpression causes nonautonomous relocalization of both itself and Diego over several cell diameters (Feiguin, 2001).
Is Diego required for Flamingo localization? The distribution of Flamingo in wild-type versus homozygous diego mutant wings was compared. In all of the wild-type wings examined, the Flamingo protein is enriched on proximal-distal boundaries by 27 hr after puparium formation. In contrast, Flamingo in dgo380 wings localizes uniformly around the junctional region at this time, similar to its distribution in frizzled mutant wings. The ratio of Flamingo staining intensity on proximal-distal versus anterior-posterior cell boundaries was quantified in images from four wild-type wings and four dgo380 mutant wings. In wild-type wings, Flamingo is enriched between 2.5- and 3.2-fold on proximal-distal boundaries at 27 hr after puparium formation. In contrast, the Flamingo in dgo380 mutant wings is not significantly enriched (ratios ranged between 0.8 and 1.3). These data suggest that Diego promotes Flamingo polarization (Feiguin, 2001).
Flamingo sometimes polarizes its distribution in parts of dgo380 mutant wings at later times (29-30 hr after puparium formation), but the axis of polarity is often abnormal, presaging the whorls of misoriented hairs observed in dgo380 adult wings. In all cases, the effect of diego is specific to Flamingo and does not affect the distribution of cortical actin. Clonal analysis shows that diego loss of function does not affect Flamingo localization outside of diego clones. These data suggest that Diego increases the efficiency of Flamingo polarization and promotes its correspondence with the proximal-distal axis determined by Frizzled. The fact that Flamingo is localized similarly in frizzled and diego mutant cells may indicate that Frizzled acts through Diego to mediate Flamingo polarization (Feiguin, 2001).
To assess the effects of excess Diego protein on polarity, the effect of Diego overexpression on Flamingo localization was examined. When Diego protein is overexpressed with the ptc:GAL4 driver, it still localizes exclusively to the junctional region, but is present in large aggregates with no proximal-distal polarity. Flamingo protein accumulates in these cortical clusters, and its distribution is no longer polarized along the proximal-distal axis. These data suggest that Diego specifically promotes clustering of Flamingo. It is suggested that under wild-type conditions, Diego-mediated Flamingo clustering is regulated in response to Frizzled signaling and occurs only at proximal-distal boundaries (Feiguin, 2001).
Interestingly, Diego overexpression has nonautonomous effects on Flamingo localization in adjacent cells. In the first row of wild-type cells adjacent to Diego overexpressers, Flamingo is missing from proximal-distal boundaries and presumably relocalizes to maximize homophilic interactions with the clustered Flamingo on the Diego-overexpressing cells. Interestingly, Flamingo protein levels are also modestly elevated on the cell boundary opposite that of the Diego-overexpressing cells. As a result, the Flamingo in the second row of cells seems less unambiguously polarized to proximal-distal boundaries than in the cells at a greater distance from the overexpression stripe (Feiguin, 2001).
The ability of adjacent pupal wing cells to compare the activity of different tissue polarity genes and orient hairs accordingly is intrinsic to the mechanism of planar polarization. When artificial expression gradients are produced, Frizzled and Flamingo cause opposite effects on hair polarity; hairs point away from cells with higher Frizzled expression but toward cells with higher Flamingo expression. To ask how cells with differing levels of Diego behaved, the adult wings from UAS:dgo * ptc:GAL4 crosses were examined. Wing hairs were oriented from high- to low-expressing cells in ptc:GAL4;UAS:dgo wings -- similar to the effects caused by Frizzled and the opposite of those caused by Flamingo. These data suggest that Diego responds positively to Frizzled signaling and that the subsequent Diego-mediated clustering of Flamingo has negative effects on Frizzled signaling activity (Feiguin, 2001).
The mechanism by which Frizzled proteins mediates cell polarization events is not well understood. In wing epithelial cells, one consequence of Frizzled signaling is the polarization of Flamingo. Diego protein has been shown in this study to be required for normal polarization of Flamingo. Diego is itself localized at proximal-distal boundaries in response to Frizzled signaling. In the wing, proximal-distal localization of Diego and Flamingo proteins is interdependent. Perturbing the distribution of either protein disrupts that of the other. In the absence of Flamingo, Diego fails to localize to the cortex at all. However, in diego mutant cells, Flamingo is cortically localized but fails to polarize efficiently or accurately along the proximal-distal axis. The localization of each of these proteins can also be perturbed by overexpression of the other. When Diego is overexpressed, it accumulates in large nonpolarized junctional clusters that recruit high levels of Flamingo protein as well. In contrast, overexpression of Flamingo has a dispersive rather than a clustering effect on Diego in the same cell, causing it to delocalize from proximal-distal boundaries and assume a more uniform cortical distribution. Furthermore, in cells adjacent to Flamingo overexpressers, Diego is dominantly relocalized with Flamingo to face the overexpression domain. Clearly, Flamingo and Diego each affect the localization of the other (Feiguin, 2001).
It is suggested that Diego associates with the cortical membrane and helps to polarize the cell in response to Frizzled signaling. Diego's cortical localization and ability to cluster make this model more likely than one in which Diego promotes polarized delivery of the Flamingo protein from the Golgi apparatus. How might Diego and Flamingo polarize in response to Frizzled? Although one possibility is that Diego mediates polarization by directly interacting with components of the activated Frizzled pathway, the interdependence of Flamingo and Diego localization speaks against such a linear model. A more reasonable model is that localized Frizzled signaling might modify the properties of the Diego protein at proximal-distal boundaries, increasing its ability to promote clustering of itself and Flamingo. This initial bias in localization caused by clustering might then be potentiated via homophilic interactions between Flamingo molecules on adjacent cells. This model is consistent with the reciprocal effects that Flamingo and Diego have on each other's localization (Feiguin, 2001).
The ability of polarization signals to propagate from cell to cell is one of the most intriguing aspects of tissue polarity and is one of the least understood. It has been known for some time that locally perturbing Frizzled signaling has long-range effects on the polarity of the surrounding tissue. This repolarization is reflected, at the molecular level, by the altered distribution of Flamingo and Diego. Interestingly, in the absence of any direct disruption of Frizzled, Flamingo overexpression causes nonautonomous repolarization of Diego and Flamingo that propagates over several cell diameters. Both Flamingo and Diego accumulate ectopically to a high level at the interface between frizzled mutant and wild-type tissue. The data suggest that the accumulation of Diego and Flamingo at frizzled clonal boundaries might play an important role in propagating repolarization of these proteins into the surrounding tissue (Feiguin, 2001).
How might this propagation occur? Although it is easy to understand how Flamingo-mediated homophilic interactions could recruit Flamingo and Diego from a directly adjacent cell to the boundary with the overexpression domain, it is less clear how this would result in a corresponding accumulation on the opposite side of the cell. One possibility is that when Diego and Flamingo are polarized via Flamingo-mediated homophilic interactions, a locally acting intracellular signal is generated that prevents similar interactions nearby. Flamingo and Diego might then tend to accumulate on the opposite side of the cell, where such interactions were not discouraged. Increased levels of Diego and Flamingo on the opposite side of the cell might, in turn, recruit Diego and Flamingo from the next cell in line. Such a mechanism might be sufficient to propagate polarization of these proteins over many cell diameters (Feiguin, 2001).
Members of the Frizzled (Fz) family of seven-pass transmembrane receptors are required for the transduction of both Wnt-Fz/β-catenin and Fz/planar cell polarity (PCP) signals. Although both pathways transduce signals via interactions between Fz and the cytoplasmic protein Dishevelled (Dsh), each pathway has specific and distinct effectors. One explanation for the pathway specificity is that signal-induced conformational changes result in unique Fz-Dsh interactions. Mutational analyses of Fz-Dsh activities in vivo do however not support this model, since both pathways are affected by all mutations tested. Alternatively, the interaction of Fz or Dsh with other proteins could modulate the signaling outcome. The role of a Dsh-binding PCP molecule, Diego (Dgo), was studied in both Wnt-Fz/β-catenin and Fz/PCP signaling. Both loss-of-function and gain-of-function results suggest that Dgo promotes Fz-Dsh/PCP signaling at the expense of Wnt-Fz/β-catenin signaling. The data suggest that Dgo sequesters Dsh to a functionally distinct Fz/PCP signaling compartment within the cell (Wu, 2008).
It has suggested that the KTXXXW motif in Fz C-tails is important for the activation of Wnt-Fz/β-cat signaling targets, but conversely other data implied that this motif was dispensable for Wnt-Fz/β-cat signaling. This issue was addressed in a physiological context. Expressing Fz under control of the tubulin (tub)-promoter fully rescues Fz-activity in fz, fz2 double mutant flies with respect to both Wnt-Fz/β-cat and Fz–PCP signaling. Importantly no dominant phenotypes result from tub-fz expression and thus the tubulin promoter presumably drives the Fz transgenes close to endogenous levels. All Fz C-tail isoforms defective in the Dsh interaction motifs in the C-tail and third cytoplasmic loop (M469R) failed to rescue Wnt-Fz/β-cat signaling, indicating that the Dsh interacting sites are important for Wnt-Fz/β-cat signaling in vivo (Wu, 2008).
The function of the KTXXXW C-tail motif in PCP signaling has previously not been determined. All KTXXXW mutations tested in vivo in this study failed to rescue the fz− PCP mutant phenotypes, indicating that the Dsh interacting sites are required for both signaling pathways. These data suggest that there are no obvious differences in how Fz and Dsh interact with each other in the context of either pathway, and therefore that additional factors are likely involved to modulate the signaling outcome and to provide specificity (Wu, 2008).
Dgo is a core Fz/PCP signaling factor (Feiguin, 2001: Jenny, 2005). During the interactions of the core PCP factors, Fz, Dsh, and Dgo become localized to the distal end of pupal wing cells (or the R3-side of the R3/R4 border in the eye), suggesting that they form a functional complex (Axelrod, 2001; Das, 2004; Strutt, 2001). The localization of Dsh and Dgo depends on Fz (Axelrod, 2001; Das, 2004). Dgo localization also partially depends on Dsh and Dgo and Dsh interact physically (Jenny, 2005). Taken together, these data are consistent with the notion that Fz, Dsh, and Dgo are forming a functional complex during PCP signaling that promotes Dsh PCP-activity (Wu, 2008).
Co-expression of Dgo enhances Fz-mediated inhibition of Wnt-Fz/β-cat signaling in the wing. Furthermore, overexpressed Dgo sequesters more Dsh into the subapical junctional region (where PCP signaling takes place) in a Fz dependent manner, suggesting that a Dgo-Dsh association sequesters Dsh away from canonical Wnt-signaling. Thus, a Fz–Dsh–Dgo complex selectively acts in Fz/PCP signaling and is likely not active for the Wnt-Fz/β-cat pathway (Wu, 2008).
Does Dgo affect the levels of Wnt-Fz/β-cat signaling in a loss-of-function (LOF) scenario? Although dgo LOF alleles show only minor effects on Wnt-Fz/β-cat associated phenotypes, double mutant LOF combinations of dgo and nmo, a mild inhibitor of Wnt-Fz/β-cat signaling, show more robust defects (manifest in the observation that the nmoP allele, which does not show Wg GOF defects, in combination with dgo LOF alleles frequently displayed ectopic margin bristles). This indicates that in vivo Dgo can affect the levels of Wnt-Fz/β-cat signaling, but redundantly with other Wg-signaling inhibitors. It is interesting to note that while Dgo presumably acts at the level of Dsh, Nmo phosphorylates the nuclear transcription factors of the TCF family and thus inhibits their association with β-cat and/or the DNA (Wu, 2008).
How does Dgo negatively affect Dsh in Wnt-Fz/β-cat signaling? in vivo data suggest that Dgo acts mainly by sequestering Dsh away from the cytoplasmic and/or basolateral cell regions where Wnt-Fz/β-cat signaling is thought to take place. Thus, a Dgo influenced shift in Dsh subcellular localization, caused either by loss or excess of Dgo, makes the pathway sensitive to additional changes. When Dsh itself or other Wnt-Fz/β-cat signaling factors become more limiting, alteration of Dgo levels can have effects on Wnt-Fz/β-cat signaling strength (Wu, 2008).
Does Dgo affect overall Dsh levels? Studies with the vertebrate Dgo homologue Inversin have suggested that Inversin, the vertebrate Dgo homologue, can affect Dsh levels through ubiquitination and associated degradation in HEK 293T cells (Simons, 2005). It seemed thus possible that Dgo affected the overall Dsh levels: Dgo could stabilize Dsh at the subapical membrane but cause its destabilization in the cytoplasm. However, no evidence was seen for a destabilization mechanism in vivo or in HEK 293T cells. Thus, it seems that Dgo and Inversin do not share this biochemical property (Wu, 2008).
Diversin is a second Dgo-related vertebrate factor that can act as a repressor of Wnt-Fz/β-cat signaling (Schwarz-Romond, 2002; Simons, 2005). The Diversin and Dgo sequences C-terminal to the Ankyrin repeats do not share homologous domains, although clusters of high homology are present. Diversin is thought to inhibit Wnt-Fz/β-cat signaling through its interaction with Axin and CKIε (Schwarz-Romond, 2002). Dgo does not interact with Axin. Thus, it appears that both Diversin and Inversin can inhibit Wnt-Fz/β-cat signaling by (at least partially) different mechanisms from Dgo, suggesting that these features have diverged evolutionarily (Wu, 2008).
Taken together, the in vivo and cell culture data suggest that Dgo can negatively affect Wnt-Fz/β-cat signaling by trapping Dsh in a Fz/PCP specific complex that is inactive for canonical Wnt-Fz/β-cat signaling. Comparative analyses with Dgo, Inversin, and Diversin will be interesting to shed light on conserved mechanisms of action for these three related proteins (Wu, 2008).
The sequence of the diego cDNA predicts a 106 kDa protein containing six ankyrin repeats at its N terminus. It differs in the first 22 amino acids from that of the protein predicted from the genome sequence (AAF58749 in the Gadfly database), but it is otherwise in agreement. An error report for the gene-encoding protein AAF58749 has been submitted to Flybase. To see whether Diego was homologous to a protein of known function, available databases were searched with BLAST for related sequences. Its closest relative is KIAA0957, a human cDNA of unknown function derived from a brain library, with which it shares sequence both inside and outside of the ankyrin repeat regions. Conversely, the sequence most closely related to KIAA0957 is diego, suggesting that the genes may be orthologs (Feiguin, 2001).
Wnt signals control decisive steps in development and can induce the formation of tumors. Canonical Wnt signals control the formation of the embryonic axis, and are mediated by stabilization and interaction of ß-catenin with Lef/Tcf transcription factors. An alternative branch of the Wnt pathway uses JNK to establish planar cell polarity in Drosophila and gastrulation movements in vertebrates. This study describes the vertebrate protein Diversin that interacts with two components of the canonical Wnt pathway, Casein kinase Iepsilon (CKIepsilon) and Axin/Conductin. Diversin recruits CKIepsilon to the ß-catenin degradation complex that consists of Axin/Conductin and GSK3ß and allows efficient phosphorylation of ß-catenin, thereby inhibiting ß-catenin/Tcf signals. Morpholino-based gene ablation in zebrafish shows that Diversin is crucial for axis formation, which depends on ß-catenin signaling. Diversin is also involved in JNK activation and gastrulation movements in zebrafish. Diversin is distantly related to Diego of Drosophila, which functions only in the pathway that controls planar cell polarity. These data show that Diversin is an essential component of the Wnt-signaling pathway and acts as a molecular switch, which suppresses Wnt signals mediated by the canonical ß-catenin pathway and stimulates signaling via JNK (Schwarz-Romond, 2002).
Compared with Diversin, Diego contains six ankyrin repeats instead of eight, and has 35% amino acid sequence identity within the ankyrin repeats, but little identity (18%) in the residual domains. Diego acts downstream of Frizzled and controls planar polarization of epithelial cells in the eye and wing; such polarization depends upon JNK activity. Coimmunoprecipitation experiments show that Diego interacts with mammalian CKIepsilon but not with Drosophila Axin. Further, Diversin stimulates JNK-dependent transcription in 293 cells. Diversin also promotes JNK activation that is induced by Dishevelled or Wnt11. In zebrafish embryos, injection of Diversin mRNA results in abnormal gastrulation movements, that is, the convergence and extension of injected embryos are defective. Low amounts of Diversin mRNA induce failure of gastrulation movements, but little ventralization, whereas at higher dosages, ventralization is predominant. Similarly, injection of low amounts of Diversin MOs also interfers with gastrulation movements and induces a general undulation of the embryo along its anterior-posterior axis, as revealed by in situ hybridization for myoD at the 5-10 somite stage. Thus, both activation and inhibition of the Wnt/JNK pathway perturb gastrulation movements. Injection of Diego mRNA also induces deficits in convergence and extension movements, but does not affect axis formation at any concentration tested. Diego could not rescue the Diversin MO-induced dorsalization. Taken together, these data indicate that Diversin functions both in the Wnt/ß-catenin and the Wnt/JNK pathway, and that Diego acts only in the Wnt/JNK pathway. Diego and Diversin are therefore structurally and functionally not entirely homologous (Schwarz-Romond, 2002).
Biochemical analysis allows the molecular mechanism by which Diversin functions in the canonical Wnt pathway to be assigned. Efficient ß-catenin degradation requires a two-step mechanism, a priming phosporylation at Ser 45 catalyzed by CKIepsilon or CKIalpha, and subsequent phosphorylation on three equally spaced serine/threonine residues by GSK3ß. Diversin recruits the priming kinase CKIepsilon to the Axin/Conductin-GSK3ß complex. Separate domains of Diversin, the central and C-terminal regions, mediate these two interactions. Both Diversin and GSK3ß bind simultaneously to dimeric Axin/Conductin, and they use identical binding sites. Diversin-mediated recruitment of CKIepsilon allows phosporylation of Ser 45 of ß-catenin, thus creating a classical GSK3ß recognition motif and initiating the subsequent phosphorylation cascade. A minimal fusion molecule that contains the catalytic domain of CKIepsilon and the Axin/Conductin-binding domain of Diversin is fully functional in ß-catenin signaling, showing the role of Diversin as a molecular linker. Diversin is inactive in the presence of Frat-1/GBP, which displaces GSK3ß, showing the importance of GSK3ß in the complex. Taken together, these data demonstrate that Diversin functions in the canonical Wnt pathway by engaging CKIepsilon to the ß-catenin degradation complex, and allows priming phosphorylation and degradation of ß-catenin (Schwarz-Romond, 2002).
Diversin activates the JNK branch of the Wnt-signaling pathway, that controls the establishment of planar cell polarity in Drosophila and gastrulation movements in vertebrates. In zebrafish, inhibition and overexpression of Diversin cause defects in gastrulation movements, that is, a reduction in body length and undulation of the body axis -- these defects are similar to those observed in pipetail (Wnt5a) mutants. Thus, Diversin controls gastrulation movements, as does the Drosophila protein Diego. However, Diego is only in part a functional homolog of Diversin, because it does not interact with Drosophila Axin and has not been implicated in Wnt/ß-catenin signaling. Specific to Diversin in vertebrates is its role at a branchpoint of intracellular Wnt signaling, where it represses the canonical Wnt/ß-catenin pathway and simultaneously activates the JNK pathway (Schwarz-Romond, 2002).
Cystic renal diseases are caused by mutations of proteins that share a unique subcellular localization: the primary cilium of tubular epithelial cells. Mutations of the ciliary protein inversin cause nephronophthisis type II, an autosomal recessive cystic kidney disease characterized by extensive renal cysts, situs inversus and renal failure. This study reports that inversin acts as a molecular switch between different Wnt signaling cascades. Inversin inhibits the canonical Wnt pathway by targeting cytoplasmic dishevelled (Dsh or Dvl1) for degradation; concomitantly, it is required for convergent extension movements in gastrulating Xenopus laevis embryos and elongation of animal cap explants, both regulated by noncanonical Wnt signaling. In zebrafish, the structurally related switch molecule diversin ameliorates renal cysts caused by the depletion of inversin, implying that an inhibition of canonical Wnt signaling is required for normal renal development. Fluid flow increases inversin levels in ciliated tubular epithelial cells and seems to regulate this crucial switch between Wnt signaling pathways during renal development (Simons, 2005).
The coordinated polarization of neighboring cells within the plane of the tissue, known as planar cell polarity (PCP), is a recurring theme in biology. It is required for numerous developmental processes for the form and function of many tissues and organs across species. The genetic pathway regulating PCP was first discovered in Drosophila, and an analogous but distinct pathway is emerging in vertebrates. It consists of membrane protein complexes known as core PCP proteins that are conserved across species. This study reports that the over-expression of the murine Ankrd6 (mAnkrd6) gene that shares homology with Drosophila core PCP gene diego causes a typical PCP phenotype in Drosophila, and mAnkrd6 can rescue the loss of function of diego in Drosophila. In mice, mAnkrd6 protein is asymmetrically localized in cells of the inner ear sensory organs, characteristic of components of conserved core PCP complexes. The loss of mAnkrd6 causes PCP defects in the inner ear sensory organs. Moreover, canonical Wnt signaling is significantly increased in mouse embryonic fibroblasts from mAnkrd6 knockout mice in comparison to wild type controls. Together, these results indicated that mAnkrd6 is a functional homolog of the Drosophila diego gene for mammalian PCP regulation and act to suppress canonical Wnt signaling (Jones, 2014).
date revised: 23 November 2002
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