Wnt oncogene analog 6: Biological Overview | References
Gene name - Wnt oncogene analog 6
Cytological map position - 27F1-27F2
Function - ligand
Keywords - required for maxillary palp formation, oogenesis, anterior escort cells are maintained by Wnt6 signaling - required for germline stem cell maintenance
Symbol - Wnt6
FlyBase ID: FBgn0031902
Genetic map position - chr2L:7,333,714-7,352,524
NCBI classification - Wnt family
Cellular location - secreted
|Recent literature||Waghmare, I., Wang, X. and Page-McCaw, A. (2020). Dally-like protein sequesters multiple Wnt ligands in the Drosophila germarium. Dev Biol. PubMed ID: 32473955
Cells in multicellular organisms rely on secreted ligands for development and morphogenesis. Several mechanisms modulate the availability and distribution of secreted ligands, determining their ability to signal locally and at long range from their source. One of these mechanisms is Dally-like protein (Dlp), a cell-surface glypican that exhibits biphasic functions in Drosophila wing discs, promoting Wg signaling at long-range from Wg source cells and inhibiting Wg signaling near source cells. In the germarium at the tip of the ovary, Dlp promotes long-range distribution of Wg from cap cells to follicle stem cells. However, the germarium also expresses other Wnts - Wnt2, Wnt4, and Wnt6 - that function locally in escort cells to promote oogenesis. Whether and how local functions of these Wnts are regulated remains unknown. This study shows that the dlp overexpression phenotype is multifaceted and phenocopies multiple Wnt loss-of-function phenotypes. Each aspect of dlp overexpression phenotype is suppressed by co-expression of individual Wnts, and the suppression pattern exhibited by each Wnt suggests that Wnts have functional specificity in the germarium. Further, dlp knockdown phenocopies Wnt gain-of-function phenotypes. Together these data show that Dlp inhibits the functions of each Wnt. All four Wnts co-immunoprecipitate with Dlp in S2R+ cells, suggesting that in the germarium, Dlp sequesters Wnts to inhibit local paracrine Wnt signaling. These results indicate that Dlp modulates the availability of multiple extracellular Wnts for local paracrine Wnt signaling in the germarium.
|Wang, M., Luan, X., Yan, Y., Zheng, Q., Chen, W. and Fang, J. (2021). Wnt6 regulates the homeostasis of the stem cell niche via Rac1-and Cdc42-mediated noncanonical Wnt signalling pathways in Drosophila testis. Exp Cell Res: 112511. PubMed ID: 33582096
The homeostasis of the stem cell niche is regulated by both intrinsic and extrinsic factors, and the complex and ordered molecular and cellular regulatory mechanisms need to be further explored. In Drosophila testis, germline stem cells (GSCs) rely on hub cells for self-renewal and physical attachment. GSCs are also in contact with somatic cyst stem cells (CySCs). Utilizing genetic manipulation in Drosophila, this study investigated the role of Wnt6 in vivo and in vitro. In Drosophila testis, Wnt6 was found to be required for GSC differentiation and CySC self-renewal. In Schneider 2 (S2) cells, Wnt6 was found to regulate cell proliferation and apoptosis. Mechanistically, it was demonstrated that Wnt6 can downregulate the expression levels of Arm, Rac1 and Cdc42 in S2 cells. Notably, Rac1 and Cdc42, which act downstream of the noncanonical Wnt signalling pathway, imitated the phenotypes of Wnt6 in Drosophila testis. Thus, the newly discovered Wnt6-Rac1/Cdc42 signal axis is required for the homeostasis of the stem cell niche in the Drosophila testis.
Stem cells reside in a niche, a complex cellular and molecular environment. In Drosophila ovaries, germline stem cells depend on cap cells for self-renewing signals and physical attachment. Germline stem cells also contact the anterior escort cells, and this study reports that anterior escort cells are absolutely required for germline stem cell maintenance. When escort cells die from impaired Wnt signaling or hid expression, the loss of anterior escort cells causes consequent loss of germline stem cells. Anterior escort cells function as an integral niche component by promoting DE-cadherin anchorage and by transiently expressing the Dpp ligand to promote full-strength BMP signaling in germline stem cells. Anterior escort cells are maintained by Wnt6 ligands produced by cap cells; without Wnt6 signaling, anterior escort cells die leaving vacancies in the niche, leading to loss of germline stem cells. These data identify anterior escort cells as constituents of the germline stem cell niche, maintained by a cap-cell produced Wnt6 survival signal (Wang, 2018).
Adult tissues are maintained by stem cells that self-renew and differentiate into functional cells. Stem cells reside within a specialized microenvironment known as the niche, and their self-renewal, numbers and activities are regulated by extrinsic cues from the niche. Understanding the niche structure is fundamental to harnessing stem cells in applications such as regenerative medicine. The cellular organization of the stem cell niche is complex, and it can include stem cells themselves, their progeny, nearby mesenchymal cells or stromal cells, muscles, extracellular matrix, and distant sources within or even outside the tissue. How different niche components interact with each other remains elusive (Wang, 2018).
Studies on Drosophila ovarian germline stem cells (GSCs) have provided an archetypal example of a stem cell niche composed of adjacent support cells. In the Drosophila ovary, two or three GSCs are located at the apex of each ovariole in a structure known as the germarium. GSCs form direct contact on their anterior side with a cluster of five to seven disc-shaped cap cells via adherens junctions. This anchorage is essential for GSC self-renewal. Furthermore, cap cells secrete bone morphogenetic protein (BMP) ligands including Decapentaplegic (Dpp) and Glass bottom boat (Gbb) to repress differentiation of GSCs. As a GSC divides, it produces a self-renewing GSC daughter that remains in contact with cap cells, and a cystoblast daughter positioned away from the niche. Without continuous BMP signaling, the cystoblast differentiates into a germline cyst and eventually an egg. For these reasons, the cap cells are considered to be the GSC niche (Wang, 2018).
Escort cells are a population of 30-40 squamous cells that line the basement membrane of the anterior half of the germarium, and they extend cytoplasmic processes to encase each GSC, cystoblast and developing germline cyst. Escort cells play an essential role in germline differentiation, as many studies have shown that escort cell disruptions result in an accumulation of undifferentiated, stem-like germline cells. Over the last decade, scattered observations have suggested a role for unspecified escort cells in maintaining GSCs, but this role has not been probed in depth (Wang, 2018).
This study demonstrates that anterior escort cells, which contact the GSCs, are essential for GSC maintenance. Like cap cells, the most anterior escort cells anchor GSCs through DE-cadherin-based junctions, and these anterior escort cells produce Dpp ligand necessary for full-strength BMP signaling in GSCs. Furthermore, these anterior escort cells are maintained specifically by cap cell-secreted Wnt6 ligands: when Wnt6 is knocked down in cap cells, anterior escort cells frequently die and are not replaced, resulting in a loss of Dpp signaling and GSC loss from the niche. Altogether, these data provide direct evidence that anterior escort cells are an essential cell type within the stem cell niche, and they indicate that cap cells maintain anterior escort cells in the niche by promoting anterior escort cell survival through Wnt6 signaling (Wang, 2018).
Previously, it was held that the GSC niche was composed of cap cells located at the anterior tip of the germaria. Cap cells produce BMP ligands to inhibit differentiation, and they anchor GSCs via DE-cadherin-mediated adherens junctions for continuous self-renewal. This study demonstrates that, in addition to cap cells, the anterior-most escort cells are required to maintain GSCs in the niche. Although these anterior escort cells have not been identified with a specific cell marker, multiple lines of evidence point to anterior escort cells having a crucial niche function. First, like cap cells, anterior escort cells form adherens junctions with GSCs via DE-cadherin, and when DE-cadherin is knocked down in all escort cells, GSCs are lost; this requirement suggests that anterior escort cells participate with cap cells in physically attaching GSCs in the niche. Second, when all escort cells are challenged and dying, as a result of either impaired Wnt signaling or direct killing with hid, remaining escort cells cluster in the anterior around the GSCs. GSC loss is evident only after nearly all escort cells have died, leaving visible anterior vacancies around the GSCs. Third, when all escort cells are dying, GSCs lose the full-strength BMP signaling that is necessary to maintain the stem-cell state; in control germaria, the BMP ligand Dpp is expressed exclusively in escort cells of Region 1, primarily in the anterior-most escort cells, in an apparently transient manner. Fourth, Wnt6 ligand is required specifically in cap cells and not in escort cells for maintaining anterior escort cell survival, for maintaining anterior escort cell architecture within the niche, for full-strength BMP signaling in GSCs, and for maintaining GSCs in the niche. Together, these data demonstrate that anterior escort cells are crucial components of the GSC niche. Furthermore, anterior escort cells share the niche hallmarks of dpp expression and DE-cadherin attachments to GSCs, both of which are required in escort cells as well as cap cells for GSC maintenance in the niche (Wang, 2018).
This model of escort cell participation in the GSC niche is consistent with and extends some previous observations. One study (Wang, 2011) showed that when escort cells were knocked down for the histone modifier eggless, escort cells slowly died with a concomitant loss of GSCs, but this phenotype was not quantified or further investigated. Several labs have shown by RT-PCR or by a conventional and challenging in situ hybridization method that escort cells contribute Dpp ligand to the germarium environment. Importantly, when dpp was knocked down in all escort cells with adult-specific expression of ptc-Gal4, GSC loss was observed. These results are all consistent with the current data and model of anterior escort cell function (Wang, 2018).
Escort cells are better known as the 'differentiation niche', because they are required for the proper differentiation of GSC progeny. Indeed, several studies have shown that escort cells, and specifically Wnt signaling in escort cells, are essential for germline differentiation. Like these groups, this study observed a germline differentiation phenotype when Wnt signaling was compromised in escort cells in addition to the GSC-loss phenotype, but, interestingly, the two phenotypes were inversely correlated: manipulations that resulted in the greatest number of undifferentiated germ cells (such as sggS9A overexpression or moderate induction of hid) were those that maintained a moderate escort cell number, and these displayed the lowest level of GSC loss; reciprocally, manipulations that resulted in the greatest loss of GSC (such as Axn overexpression or high induction of hid) were those that induced a severe loss of escort cells, and these displayed the lowest levels of undifferentiated germ cells. It is concluded that the earliest phenotype caused by escort cell death is a failure of germline differentiation, appearing as a germline tumor. The loss of GSCs from the niche is a later phenotype, appearing only after nearly all the escort cells have been lost from the germarium, which happens when Wnt signaling is strongly impaired or when hid is highly expressed. The inverse correlation makes sense because when GSCs are lost from the niche, fewer of their cystoblast progeny are born to populate a germline tumor. It is expectrf that studies analyzing the role of Wnt in germ cell differentiation might not have detected the weak loss of GSCs in their strong differentiation mutants, and further, weak GSC loss is hard to detect in the presence of many undifferentiated germ cells because of the large number of spectrosomes. These two phenotypes represent two distinct functions of escort cells: promoting germline stemness in the GSC niche at the anterior of the germarium, and promoting germline differentiation in the differentiation niche in more posterior positions. Both Wnt6 and Hh, signaling from cap cells to anterior escort cells, are positioned appropriately to signal this switch in escort cell function (Wang, 2018 and references therein).
Intriguingly, this study found that cap cells signal via Wnt6 to anterior escort cells to promote their survival. This signaling between two different niche cell types is crucial for niche function, as without Wnt6, niche escort cells die, dpp expression in anterior escort cells is lost, BMP signaling in GSCs is decreased, and GSCs are lost. It seems likely that the loss of dpp expression is an indirect effect of losing the anterior escort cells themselves, rather than a direct effect of the loss of Wnt signaling, as it has been reported that cap cell-derived Wnt ligands limit rather than promote dpp signaling. Also, it has been previously shown that cap cell-derived Hh ligands promote dpp expression in escort cells (Rojas-Ríos, 2012). Thus, a model is favored in this study in which Wnt6 is important for anterior escort cell survival and recruitment. In support of this model, it was observed that in the presence of intact Wnt6 signaling, when escort cells were killed by hid, surviving escort cells routinely clustered at the GSC niche, even though escort cell death occurred evenly across the germarium. Indeed, GSCs were maintained in the niche until virtually all escort cells had died, when there were few or no remaining escort cells to fill vacancies in the niche. Escort cells behaved very differently, however, when Wnt6 was knocked down in cap cells. Without Wnt6, an increase was observed in cell death specifically in the anterior of the germarium, and lost cells were not replaced, leaving functional vacancies in the GSC niche. Thus, cap cell-produced Wnt6 seems to ensure continued occupancy of escort cells in the GSC niche. It is also possible that Wnt6 could coordinate the niche cell types during changes in niche size, as previous studies have shown that both the numbers of GSCs and cap cells decrease in response to a poor diet and increase under rich food conditions (Wang, 2018).
Anterior escort cell replacements appear to derive from the more posterior cycling somatic cells, labeled by FUCCI (fluorescence ubiquitination-based cell cycle indicator). Based on recent work by Reilein (2017), it appears that these cycling cells are stem cells from which both follicle and escort cells derive. The anterior migration of stem cell daughters into escort cell territory has been captured by live imaging ex vivo (Reilein, 2017), strong evidence that anterior movement occurs also in vivo. Furthermore, this study observed some BrdU-labeled cells that probably migrated from this cycling area into Region 1. Thus, Wnt6 might act as a homing signal for newly born escort cells, attracting them to the anterior-most location in the GSC niche (Wang, 2018).
It has been proposed that a stem cell niche acts as an 'interlocutor' or interpreter, relaying information about the status of the organism or tissue to the stem cells. Because of this interpreter role, it is expected that niches would be composed of multiple cell types to report different types of information. Indeed, some mammalian somatic stem cell niches are known to be composed of multiple cell types. The bone marrow niche for hematopoietic stem cells (HSCs), one of the best understood mammalian stem cell niches, is composed of multiple cell types, including different endothelial cells in the circulatory system and cells in the nervous and immune systems. Another example is the mammalian intestinal stem cell niche, composed of paneth cells, pericryptic fibroblasts and smooth muscle cells. This study has demonstrated that escort cells are an essential and non-redundant niche cell type, acting in concert with the cap cells to form the Drosophila ovarian GSC niche. Following the interlocutor model, what could each of these two cell types be communicating to the GSCs? Germline differentiation and the development of gametes need to be coordinated with at least two types of information: nutritional status of the organism, and the level of threat to the genome from transposable elements. The cap cells are known to gather information on the nutritional status of the organism, as they change their number or alter the availability of signaling ligands in response to diet. Interestingly, a recent study has shown that escort cells respond to transposable element activation by downregulating Wnt4 levels, a potentially direct mechanism by which escort cells communicate the level of transposon threat to the germline. In this scenario, increased transposon activity leads to reduced Wnt4 signaling, and the data shows that reduced Wnt4 results in potentially corrupted GSCs being lost from the perpetuity of the niche. Thus, both cap cells and escort cells are poised to transmit crucial information relevant to gamete development through the GSC niche (Wang, 2018).
Many organisms lose the capacity to regenerate damaged tissues as they mature. Damaged Drosophila imaginal discs regenerate efficiently early in the third larval instar (L3) but progressively lose this ability. This correlates with reduced damage-responsive expression of multiple genes, including the WNT genes wingless (wg) and Wnt6. This study demonstrates that damage-responsive expression of both genes requires a bipartite enhancer whose activity declines during L3. Within this enhancer, a damage-responsive module stays active throughout L3, while an adjacent silencing element nucleates increasing levels of epigenetic silencing restricted to this enhancer. Cas9-mediated deletion of the silencing element alleviates WNT repression, but is, in itself, insufficient to promote regeneration. However, directing Myc expression to the blastema overcomes repression of multiple genes, including wg, and restores cellular responses necessary for regeneration. Localized epigenetic silencing of damage-responsive enhancers can therefore restrict regenerative capacity in maturing organisms without compromising gene functions regulated by developmental signals (Harris, 2016).
Many organisms lose the ability to regenerate damaged tissues as they mature. This change often occurs concurrently with a slowing of the growth of the organism, or a major transformation in its developmental state, e.g. metamorphosis in Drosophila and Xenopus. The loss of regenerative capacity is likely an important mechanism to balance the successful progression to reproductive adulthood at the cost of forming functionally complete tissue. Very few 'true' regeneration-specific genes have been identified (i.e. genes that are not required at any other time throughout the organism's life), but rather developmentally required pathways are often re-used during regeneration. Thus, how regenerative growth can be selectively inhibited without compromising cell proliferation or differentiation remains unknown. Here this study has shown that in the Drosophila wing disc this loss of regenerative capacity is achieved in part by the localized epigenetic inactivation of a damage-responsive enhancer that regulates the expression of wg and potentially Wnt6. This mechanism allows an organism to continue with its normal developmental program while shutting down its regenerative response to tissue damage (Harris, 2016).
Previous studies have demonstrated that the JNK pathway is robustly activated following tissue damage and has an important role in regenerative growth. The current data confirm that JNK is strongly activated following damage, but furthermore, it appears similarly activated in both day 7 and day 9 discs, as assessed by the expression of an AP-1 reporter. Thus, the loss of regeneration that occurs between day 7 and day 9 cannot be attributed to a failure to activate JNK. Despite the similar levels of AP-1 activity, the cellular responses and changes in gene expression elicited by tissue damage differ considerably as the disc matures. Importantly, genes that are known to be downstream targets of the JNK/AP-1 pathway such as Mmp1 have reduced expression on day 9 when compared to day 7. These changes in gene expression are likely to account for many of the differences in the cellular responses to tissue damage that we observe (Harris, 2016).
In addition to the aforementioned genes, the WNT genes wg and Wnt6 also exhibit a significant decline in damage-induced expression with disc maturity. The data shows this is due to the highly localized epigenetic silencing of a damage-responsive WNT enhancer, BRV118, that prevents their expression specifically in response to injury in mature discs, but still allows expression from nearby developmentally regulated enhancers. This mechanism ensures that the contribution of both genes to a regeneration program can be shut off in mature tissues independently of their essential roles in growth and development of the disc. An inability to detect expression of the BRV118-GFP reporter in unablated discs suggests that the BRV118 enhancer does not have a role in normal development. However, the wg1 allele, which results in an incompletely-penetrant phenotype characterized by a failure to specify the wing pouch, is a deletion whose breakpoints are very close to the boundaries of the BRV118 fragment that we have studied. This suggests that a separate element, possibly very close to, but not fully contained within the boundaries of BRV118, may also be disrupted by the wg1 deletion (Harris, 2016).
The expression profile of regenerating discs suggests the regulation of multiple genes is required during regeneration, and that a significant number of these genes are also involved in developmental processes. Thus, equivalent regeneration specific enhancers, like BRV118, might also exist for these genes, such as DILP8 and Mmp1. Both genes are known to be activated by JNK, although damage-responsive enhancers have not yet been characterized. Notably though, the Mmp1-lacZ reporter we used to investigate Mmp1 activation, which accurately reflects Mmp1 protein expression following injury, consists of a ~5 kb intronic region upstream of a lacZ reporter, which, based on its pattern of expression on days 7 and 9, must possess regulatory regions that allow both damage-induced activation and maturity-dependent silencing. Sequence comparison with BRV118 reveals several AP-1 binding sites that are identical to those found in BRV118, and multiple consensus sites for PcG repression. This combination of regulatory motifs could therefore reflect a molecular signature of genes that function in regeneration, and thus could potentially be used to identify genes that comprise a regeneration program through genome-wide analyses in the future (Harris, 2016).
These studies of the regulation of wg expression have shown that, despite similar levels of JNK activation, increased levels of PcG-mediated epigenetic silencing can override the effect of JNK activation and suppress gene expression in late L3. PcG-mediated silencing is best characterized for its role in the epigenetic silencing of Hox genes during embryonic development in Drosophila, but also has important functions in imaginal disc development and during regeneration. Indeed inappropriate cell fate switching following damage in imaginal discs (transdetermination) is associated with changes in PcG gene expression, and in one instance JNK signaling reduced the extent of PcG mediated repression. A key property of epigenetic regulation by PcG is the ability to simultaneously silence multiple regions across the genome via the activity of a single master regulator complex, and, moreover, this silencing is heritable and thus its activation can maintain the locus in a repressed state through subsequent cell generations. Such a mechanism is ideally suited to the sustained and progressive silencing of a regeneration program during the ongoing growth and development of imaginal discs. However, unlike Hox genes, silencing of wg and Wnt6 does not involve the entire transcription unit, but rather, is restricted to a damage-responsive enhancer. A similar local mode of epigenetic regulation has been described for the Drosophila rpr locus, in which epigenetic blocking of an irradiation-responsive enhancer region through enrichment of H3K27me3 prevents rpr expression following irradiation in late embryogenesis. Importantly, the remainder of the rpr locus itself remains accessible, and is thus responsive to developmental signals required for programmed cell death to occur in the nervous system in late embryogenesis. Localized epigenetic silencing of individual regulatory elements is therefore likely an important and potentially pervasive mechanism by which gene expression can be selectively activated or repressed by distinct inputs (Harris, 2016).
But how is this epigenetic silencing limited to just the enhancer? Elements that are responsible for expression of the 'inner circle' of wg expression at the edge of the pouch and for expression in the leg disc are immediately adjacent to the BRV118 enhancer. Thus, while the BRV-C fragment nucleates PcG-mediated repression that then spreads over the remainder of the BRV118 enhancer, mechanisms must exist that limit spread beyond the borders of the enhancer and thus preserve the activity of the adjacent developmentally-regulated enhancers. Chromatin boundary elements that are able to block the spread of heterochromatin formation have previously been described and are found in a variety of organisms including Drosophila. Unlike other boundary elements such as insulators that inhibit enhancer-promoter interactions, these ‘chromatin barrier' elements can prevent the propagation of repressive histone marks separately from a role in enhancer blocking. Thus, a similar barrier element might be present within or near BRV118 to limit chromatin modifications to the damage responsive region, yet allow nearby developmental enhancers to remain active (Harris, 2016).
If multiple genes that function in regeneration have a similar bipartite mode of regulation, it is unlikely that expressing just one of these genes at a later stage of development can restore the ability to regenerate. Indeed, this study found that restoring wg expression in day 9 discs did not promote regeneration. In contrast, expression of Myc, which is able to increase the levels of expression of both wg and Mmp1, and possibly the expression of other genes that are similarly regulated, was able to enhance regeneration. However, it is likely that Myc does not promote the expression of all genes that have been silenced in late L3. Indeed, unlike wg and Mmp1, this study found that the JAK/STAT reporter is not reactivated in mature discs by the presence of Myc. In addition, the delay in pupariation is not restored, which possibly results from a failure to restore the damage-responsive DILP8 expression level to that of a younger disc. While this study has shown that Myc functions cell autonomously to reactivate BRV118-mediated expression of WNT genes, it is unclear whether Myc reverses the PcG-mediated repression of BRV118 or bypasses it completely. However, since the BRV-B-Myc transgene is only expressed in a small region of the disc, it is not easy to detect a change in the overall level of H3K27 methylation at the WNT locus in these cells with confidence. Additionally, even increasing Myc levels has little effect by day 10, suggesting that the silencing mechanism has become even more effective. It might be necessary to combine Myc overexpression with other manipulations to restore regeneration at even later stages. Previous studies have implicated Myc as a regulator of chromatin organization and also as a regulator of cellular reprogramming, and therefore studying the role of Myc in reactivating BRV118-mediated expression might provide a tractable way of understanding the role of Myc in these processes (Harris, 2016).
Overall, this investigation has revealed a mechanism by which genes required for both regeneration and development can be regulated to allow the age-dependent restriction of a regenerative response without affecting normal organismal growth and patterning of tissues. As PcG proteins are highly conserved from flies to vertebrates, as indeed are the targets they regulate, it would be of considerable interest to determine whether the loss of regenerative capacity in vertebrates also results from the selective epigenetic silencing of damage-responsive enhancers that regulate orthologs of Drosophila genes , such as matrix metalloproteases and WNT genes (Harris, 2016).
Wnt6 is an evolutionarily ancient member of the Wnt family. In Drosophila, Wnt6 loss-of-function animals have not yet been reported, hence information about fly Wnt6 function is lacking. In wing discs, Wnt6 is expressed at the dorsal/ventral boundary in a pattern similar to that of wingless, an important regulator of wing size. To test whether Wnt6 also contributes towards wing size regulation, Wnt6 knockout flies were generated. Wnt6 knockout flies are viable and have no obvious defect in wing size or planar cell polarity. Surprisingly, Wnt6 knockouts lack maxillary palps. Interestingly, Wnt6 is absent from the genome of hemipterans, correlating with the absence of maxillary palps in these insects. It is concluded Wnt6 is important for maxillary palp development in Drosophila, and phylogenetic analysis indicates that loss of Wnt6 may also have led to loss of maxillary palps on an evolutionary time scale (Doumpas, 2018).
Wnt6 appears to have a specific role during Drosophila development, promoting maxillary palp formation. This is surprising, given that Wnt6 is quite ancient and present in most bilaterians. One possible interpretation is that the Wnt6 function might be redundant in most parts of the animal, perhaps due to overlapping expression with wingless, whereas Wnt6 expression in the maxillary palp might have been acquired in insects in a non-redundant fashion. This specific function in promoting maxillary palp formation might serve as a useful tool for studying the contribution of maxillary palps to olfaction and behavior. The maxillary palp contribution is currently assayed by surgical removal of the palps, whereas this could now also be accomplished genetically (Doumpas, 2018).
The Wnt6 gene is located directly adjacent to the wingless gene, raising the possibility that it arose as a genomic duplication of wingless. Accordingly, Wnt6 expression overlaps with that of wingless in numerous places (Janson, 2001). One possible reason for the overlapping expression patterns could be that Wnt6 expression is induced by wingless signaling; however, the data suggest this is not the case. Instead, it is likely that they either share enhancer elements or that regulatory elements were also duplicated alongside the open reading frame. Since wingless and Wnt6 have similar expression patterns and presumably transcriptional regulation, and since the anti-Wingless monoclonal antibody 4D4, the most widely-used in the field to detect wingless, appears to cross-react with Wnt6, some caution might be warranted in interpreting results with this antibody (Doumpas, 2018).
Given that Wnt6 is able to induce canonical wingless signaling in S2 cells, it was surprising that Wnt6 is quite poor at inducing wingless signaling in the wing disc. Consistent with this observation, expression of UAS-Wnt6 with various GAL4 drivers such as patchedts-GAL4 (with GAL80ts) and nubbin-GAL4 cause pupal lethality; however, this does not yield obvious morphological defects in the resulting wings, suggesting that the lethality is likely due to expression in other parts of the body. In contrast, Wnt6 expression in the central nervous system, including the maxillary palp, induces obvious significant morphological effects. One possible explanation could be that a component required for Wnt6 signaling might be expressed at higher levels in the nervous system compared to wing discs (Doumpas, 2018).
The specific absence of Wnt6 from the aphid A. pisum and the plant lice D. citri, both belonging to the order Hemiptera, a group that has lost maxillary palps, suggests that Wnt6-loss could have been the underlying genetic alteration leading to this morphological change. In hemipterans, the mouthparts are modified to form a tube-like structure for piercing. The tube, formed by the labrum and labium, comprises piercing-sucking structures formed by the modified mandible and the maxilla. The evolutionary loss of the maxillary palps was one of many structural modifications leading to the specialized hemipteran mouthparts. The loss of the maxillary palps could have compromised the sense of smell in hemipteran ancestors, but this may have been compensated by the elaboration of sensory structures on the labium. The specific phenotype of the Wnt6 knock-out in Drosophila contrasts with the pleiotropic effects of other secreted signaling molecules including wingless. This means that the deletion of the gene, rather than tinkering with its regulatory regions, could have resulted in a subtle morphological change, the loss of the maxillary palp, contributing to the morphological evolution of the beak-like hemipteran mouthparts (Doumpas, 2018).
Although Wnt6 expression overlaps substantially with that of wingless, it appears to play a critical role in maxillary palp growth, but not wing growth. Phylogenetic analysis suggests that loss of Wnt6 also correlates with loss of maxillary palps on an evolutionary timescale (Doumpas, 2018).
Stem cell self-renewal versus differentiation is regulated by the niche, which provides localized molecules that favor self-renewal. In the Drosophila melanogaster female germline stem cell (GSC) niche, Decapentaplegic (Dpp), a fly transforming growth factor β molecule and well-established long-range morphogen, acts over one cell diameter to maintain the GSCs. This study shows that Thickveins (Tkv; a type I receptor of Dpp) is highly expressed in stromal cells next to Dpp-producing cells and functions to remove excess Dpp outside the niche, thereby spatially restricting its activity. Interestingly, Tkv expression in these stromal cells was regulated by multiple Wnt ligands, including Wnt6, that were produced by the niche. These data demonstrate a self-restraining mechanism by which the Drosophila ovarian GSC niche acts to define its own boundary (Luo, 2015).
Members of the Wnt gene family encode secreted proteins that signal through the Frizzled family of receptors to function in many aspects of development. This study has analyzed the expression of two Wnt genes and one Frizzled family member that were recently identified through the Drosophila genome sequencing project. DWnt6 is only weakly expressed in developing embryos, with transcripts faintly detected in the gut. By late third instar however, this gene is expressed in a pattern that is identical to that of wingless (wg) in the imaginal discs. DWnt10 is expressed in the embryonic mesoderm, central nervous system and gut, whereas its expression is below the levels of detection in third instar imaginal discs. DFz4 is also expressed in a dynamic pattern in the mesoderm, gut, and central nervous system (Jason, 2001).
Search PubMed for articles about Drosophila Wnt6
Doumpas, N., Jekely, G. and Teleman, A. A. (2013). Wnt6 is required for maxillary palp formation in Drosophila. BMC Biol 11: 104. PubMed ID: 24090348
Harris, R.E., Setiawan, L., Saul, J. and Hariharan, I.K. (2016). Localized epigenetic silencing of a damage-activated WNT enhancer limits regeneration in mature Drosophila imaginal discs. Elife [Epub ahead of print]. PubMed ID: 26840050
Janson, K., Cohen, E. D. and Wilder, E. L. (2001). Expression of DWnt6, DWnt10, and DFz4 during Drosophila development. Mech Dev 103(1-2): 117-120. PubMed ID: 11335117
Luo, L., Wang, H., Fan, C., Liu, S. and Cai, Y. (2015). Wnt ligands regulate Tkv expression to constrain Dpp activity in the Drosophila ovarian stem cell niche. J Cell Biol 209(4): 595-608. PubMed ID: 26008746
Reilein, A., Melamed, D., Park, K. S., Berg, A., Cimetta, E., Tandon, N., Vunjak-Novakovic, G., Finkelstein, S. and Kalderon, D. (2017). Alternative direct stem cell derivatives defined by stem cell location and graded Wnt signalling. Nat Cell Biol 19(5): 433-444. PubMed ID: 28414313
Rojas-Rios, P., Guerrero, I. and Gonzalez-Reyes, A. (2012). Cytoneme-mediated delivery of hedgehog regulates the expression of bone morphogenetic proteins to maintain germline stem cells in Drosophila. PLoS Biol 10(4): e1001298. PubMed ID: 22509132
Wang, X., Pan, L., Wang, S., Zhou, J., McDowell, W., Park, J., Haug, J., Staehling, K., Tang, H. and Xie, T. (2011). Histone H3K9 trimethylase Eggless controls germline stem cell maintenance and differentiation. PLoS Genet 7(12): e1002426. PubMed ID: 22216012
Wang, X. and Page-McCaw, A. (2018). Wnt6 maintains anterior escort cells as an integral component of the germline stem cell niche. Development 145(3). PubMed ID: 29361569
date revised: 20 July 2018
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