The Interactive Fly

Zygotically transcribed genes

Segment Polarity Genes

  • Segment polarity in Drosophila: Cell-cell signaling and the origins of patterning
  • A local difference in Hedgehog signal transduction increases mechanical cell bond tension and biases cell intercalations along the Drosophila anteroposterior compartment boundary
  • Hedgehog signaling regulates the ciliary transport of odorant receptors in Drosophila
  • The APC/C coordinates retinal differentiation with G1 arrest through the Nek2-dependent modulation of Wingless signaling
  • Wingless signaling regulates winner/loser status in Minute cell competition
  • Alternative direct stem cell derivatives defined by stem cell location and graded Wnt signalling
  • Coupling optogenetics and light-sheet microscopy, a method to study Wnt signaling during embryogenesis
  • Dampening the signals transduced through Hedgehog via microRNA miR-7 facilitates Notch-induced tumourigenesis
    Transcription factors of the posterior compartment
    Genes regulated by engrailed and invected
    Secreted proteins acting in segment polarity
    Genes downstream of hedgehog
    Genes downstream of wingless

    Other

    Segmentation in Drosophila: Cell-Cell Signaling and the Origin of Patterning

    Pattern formation takes place through a series of logical steps, reiterated many times over during the development of an organism. Viewed from a broader evolutionary perspective, across species, the same sorts of reiterative pattern formations are seen. The central dogma of pattern formation has been described by Lawrence and Struhl (1996) in an article entitled "Morphogens, Compartments and Pattern: Lessons from Drosophila." Three interlocking and overlapping steps are defined: first, positional information in the form of morphogen gradients allocate cells into nonoverlapping sets, each set founding a compartment. Second, each of these compartments acquires a genetic address, as a result of the function of active "selector" genes that specify cell fate within a compartment and also instruct cells and their descendents how to communicate with cells in neighboring compartments. The third step involves interactions between cells in adjacent compartments, initiating new morphogen gradients, which directly organize the pattern.

    Taking these steps in greater detail, one finds the first step in patterning to be the definition of sets of cells in each primordium. Cells are allocated according to their positions with respect to both dorsoventral and anterior/posterior axes by morphogen gradients. Allocation of cells in the dorsoventral axis constitutes the germ layers, such as mesoderm or neurectoderm. This function is carried out by the gene dorsal and its targets. Subdivision of the anterior/posterior axis into segmental units known as parasegments is carried out by gap and pair rule genes.

    In segmentation, the second step (the specification of cell fate in each compartment) is carried out by the gene engrailed and elements of the bithorax complex. engrailed defines anterior and posterior compartments both in segmentation and in limb specification.

    The wing disc provides an excellent example of pattern formation in Drosophila. During the larval period, both anterior and posterior compartments are subdivided by the apterous selector gene, which is activated in dorsal and repressed in ventral cells. Selector genes do much more than specify the pattern and structures that the compartments will eventually make - they also specify, indirectly, a surface property termed cell affinity. Cells that share the same affinity can intermingle during growth, while cells in the neighboring compartment, with a different basis for affinity, also self-associate but minimize contact with cells in adjacent compartments; in this way, a well defined boundary forms between adjacent compartments. In wing compartment definition, alternative integrins function in different compartments determining mutual and exclusive affinity.

    The third step in pattern formation, secretion of morphogens, functions to differentiate patterns within compartments (and thereby establish segment polarity). Initially all cells within a compartment are equipotent, but they become diversified to form pattern. Pattern formation depends on gradients of morphogens, gradients initiated along compartment boundaries. How are these gradients established? A short-range signal is induced in all the cells of the compartment in which a selector gene (engrailed) is active. For segment polarity this signal is Hedgehog. In the adjacent compartment the selector gene is inactive, ensuring that the cells are sensitive to the signal. The Hedgehog signal range is probably only a few rows of cells wide; responding cells become a linear source of a long-range morphogen that diffuses outward in all directions.

    The long range signal in wing segment polarity is Decapentaplegic. Two targets of DPP are spalt and optomotor blind, both transcription factors activated by DPP. A graded distribution of DPP outside of cells organizes a graded distribution of the domains of spalt and omb, which in turn generate the patterning of elements such as bristles, arranged according to transcription factor concentration.

    In the wing disc apterous functions as a selector gene that makes the dorsal surface distinct from the ventral surface. Apterous has at least two functions: first it is responsible for making the dorsal cell type distinct from the ventral, a property that may be due to its activation of gene Dorsal wing; second, it directs the expression of fringe and Serrate in the dorsal compartment and by its absence, Notch in the ventral compartment. It could be that wingless is the long-range morphogen induced by Serrate action on Notch. Fringe functions as a short range secreted signal. In embryonic segmentation, the long range signal is unknown, but may again be wingless.

    A local difference in Hedgehog signal transduction increases mechanical cell bond tension and biases cell intercalations along the Drosophila anteroposterior compartment boundary

    Tissue organization requires the interplay between biochemical signaling and cellular force generation. The formation of straight boundaries separating cells with different fates into compartments is important for growth and patterning during tissue development. In the developing Drosophila wing disc, maintenance of the straight anteroposterior (AP) compartment boundary involves a local increase in mechanical tension at cell bonds along the boundary. The biochemical signals that regulate mechanical tension along the AP boundary, however, remain unknown. This study shows that a local difference in Hedgehog signal transduction activity between anterior and posterior cells is necessary and sufficient to increase mechanical tension along the AP boundary. This difference in Hedgehog signal transduction is also required to bias cell rearrangements during cell intercalations to keep the characteristic straight shape of the AP boundary. Moreover, severing cell bonds along the AP boundary does not reduce tension at neighboring bonds, implying that active mechanical tension is upregulated, cell bond by cell bond. Finally, differences in the expression of the homeodomain-containing protein Engrailed also contribute to the straight shape of the AP boundary, independently of Hedgehog signal transduction and without modulating cell bond tension. The data reveal a novel link between local differences in Hedgehog signal transduction and a local increase in active mechanical tension of cell bonds that biases junctional rearrangements. The large-scale shape of the AP boundary thus emerges from biochemical signals inducing patterns of active tension on cell bonds (Rudolf, 2015).

    This study has analyzed the links between the determination of cell fate and the physical and mechanical mechanisms shaping the AP boundary of larval Drosophila wing discs. Previous work has shown a role for the transcription factors Engrailed and Invected and the Hedgehog signal transduction pathway in organizing the segregation of anterior and posterior cells of the wing disc. This study now shows that a difference in Hedgehog signal transduction between anterior and posterior cells significantly contributes to the straight shape of the AP boundary by autonomously and locally increasing mechanical cell bond tension that in turn biases the asymmetry of cell rearrangements during cell intercalations. Furthermore, Engrailed and Invected also contribute to maintaining the characteristic straight shape of the AP boundary by mechanisms that are independent of Hedgehog signal transduction and do not appear to modulate cell bond tension (Rudolf, 2015).

    In the wild-type wing disc, anterior cells transducing the Hedgehog signal are juxtaposed to posterior cells that do not transduce the Hedgehog signal. Three cases were genereated to test whether this difference in Hedgehog signal transduction is important for the straight shape of the AP boundary, the morphological and molecular signature of cells along the AP boundary, and the local increase in cell bond tension. In case I, Hedgehog signal transduction was low (or absent) in both A and P cells. In case II, Hedgehog signal transduction was high in both A and P cells. And in case III, Hedgehog signal transduction was high in P cells, but low in A cells, reversing the normal situation. In cases I and II the AP boundary was no longer as straight as in the wild-type situation. Moreover, the increased apical cross-section area of cells along the AP boundary that is characteristic for the wild type was no longer seen. Finally, the levels of F-actin and cell bond tension were no longer increased along the AP boundary. In case III, it was found that the difference in Hedgehog signal transduction is sufficient to maintain the characteristic straight shape of the AP boundary, to induce the morphological signatures of cells along the AP boundary and to increase F-actin and mechanical tension. Taken together, these experiments establish that the difference in Hedgehog signal transduction between anterior and posterior cells plays a key role in increasing cell bond tension along the AP boundary, in maintaining the characteristic shape of the AP boundary, and in defining the molecular and morphological signatures of cells along the AP boundary. These findings account for the observation that while Hedgehog signal transduction is active within the strip of anterior cells, the increase in mechanical tension is confined to cell bonds along the AP boundary, where cells with highly different Hedgehog signal transduction activities are apposed. The small differences in Hedgehog signal transduction activity that might exist between neighboring rows of anterior cells in the vicinity of the AP boundary appear to be insufficient to increase cell bond tension. Importantly, Hedgehog signal transduction per se does not increase cell bond tension along the AP boundary. The role of Hedgehog signal transduction along the AP boundary thus differs from its roles during other morphogenetic processes in which all cells that transduce the Hedgehog signal, for example, respond by accumulation of F-actin and a change in shape. It will be interesting to elucidate the molecular mechanisms by which cells perceive a difference in Hedgehog signal transduction, and how such a difference in Hedgehog signal transduction results in increased cell bond tension (Rudolf, 2015).

    F-actin and Myosin II are enriched along the AP boundary. Based on similar observations, the existence of actomyosin cables has been proposed for several compartment boundaries, including the AP boundary in the Drosophila embryonic epidermis, the DV boundary of Drosophila wing discs and the rhombomeric boundaries in zebrafish embryos. Actomyosin cables have been proposed to maintain the straight shape of compartment boundaries by acting as barriers of cell mixing between cells of the adjacent compartments. Actomyosin cables are also characteristic of additional processes, e.g. dorsal closure and germband extension in the Drosophila embryo, tracheal tube invagination and neural plate bending and elongation. During Drosophila germ band extension, it has been shown that mechanical tension is higher at cell bonds that are part of an actomyosin cable compared with isolated cell bonds, indicating that cell bond tension is influenced by higher-order cellular organization during this process. The results, based on laser ablation experiments, show that the increased cell bond tension along the AP boundary can be induced by single cells and does not depend on the integrity of the actomyosin cable. Thus, these data instead indicate that increased cell bond tension is autonomously generated cell bond by cell bond along the AP boundary. This suggests that differences in Hedgehog signal transduction activity regulate the structure and mechanical properties of cell junctions between adjacent cells and in particular upregulate an active mechanical tension, mediated by actomyosin contractility (Rudolf, 2015).

    The cell cortex is a thin layer of active material that is under mechanical tension. In addition to viscous and elastic stresses, active stresses generated by actomyosin contractility are an important contribution. Adherens junctions are adhesive structures that include elements of the cell cortices of the adhering cells. Locally generated active tension, therefore, can largely determine the cell bond tension as long as cell bonds do not change length or rearrange. As a consequence, locally generated active tension also sets the cell bond tension at the actomyosin cable along the AP boundary. This view is consistent with experiments in which cell bond tension remains high even if the integrity of the actomyosin cable is lost. These mechanical properties of cell junctions along the AP boundary are thus different from those of a conventional string or cable in which elastic stresses are associated with stretching deformations. Such elastic stresses relax and largely disappear when the cable is severed. Thus, this work suggests that the mechanical properties of the actomyosin cable along the AP boundary are very different from those of a conventional cable, but fit well in the concepts of active tension studied in the cell cortex, e.g., in Caenorhabditis elegans. This active tension is a local property that can be set by local signals irrespective of the local force balances. Force balances rather determine movements and rearrangements, e.g. upon laser ablation (Rudolf, 2015).

    How does a local increase in actively generated cell bond tension contribute to the straight shape of the AP boundary? Previous work showed that cell intercalations promote irregularities in the shape of compartment boundaries. The local increase in active cell bond tension enters the force balances during cell rearrangements. During cell intercalation, differences in active cell bond tension between junctions along the AP boundary and neighboring junctions are balanced by frictional forces associated with vertex movements. As a result, vertex movements are biased such that the AP boundary remains straight and cell mixing between neighboring compartments is suppressed. The observation that a local difference in Hedgehog signal transduction upregulates active cell bond tension leads to the prediction that cell rearrangements along the AP boundary should not be biased if there is no difference in Hedgehog signal transduction. This is indeed what was found in case II (Rudolf, 2015).

    It has been previously suggested that the engrailed and invected selector genes play a role in maintaining the separation of anterior and posterior cells that is independent of Hedgehog signal transduction. Quantitative analysis of clone shapes in this study supports this notion. It is speculated that this Hedgehog-independent pathway contributes to the remarkably straight shape of the AP boundary in cases I and II, in which Hedgehog signal transduction activities between anterior and posterior cells have been nearly equalized. Two lines of evidence indicate that the Hedgehog-independent pathway shapes the AP boundary without modulating cell bond tension. First,several cases have been generated in which neighboring cell populations differed in the expression of Engrailed and Invected, but not in Hedgehog signal transduction activity. In none of these cases was an increase in cell bond tension detected along the interface of these two cell populations. Second, in cases in which a difference was created in Hedgehog signal transduction between two cell populations in the absence of differences in Engrailed and Invected expression, the same increase was detected in cell bond tension between these cell populations compared with the wild-type compartment boundary (Rudolf, 2015).

    Previously studies have described several physical mechanisms that shape the DV boundary of wing discs. In addition to a local increase in mechanical tension along the DV boundary, evidence was provided that oriented cell division and cell elongation created by anisotropic stress contribute to the characteristic shape of the DV boundary. It is therefore conceivable that the Hedgehog-independent pathway influences the shape of the AP boundary by one or more of these mechanisms (Rudolf, 2015).

    It is proposed that the AP boundary is shaped by mechano-biochemical processes that integrate signaling pathways with patterns of cell mechanical properties. In tjos model, Engrailed and Invected shape the AP boundary with the help of two different mechanisms. (1) Engrailed and Invected result in a difference in Hedgehog signal transduction between anterior and posterior cells. This difference leads to a cell-autonomous increase in F-actin and active cell bond tension along the AP boundary. The local increase in active cell bond tension then biases the asymmetry of cell rearrangements during cell intercalations and thereby contributes to maintaining the straight shape of the AP boundary. (2) Engrailed and Invected contribute independently of Hedgehog signal transduction to the straight shape of the AP boundary by an as yet unknown mechanism not involving the modulation of cell bond tension. The first mechanism uses biochemical signals to create mechanical patterns that subsequently guide junctional dynamics to organize a straight compartment boundary. It is speculated that the second mechanism also involves a mechano-chemical process, even though the nature of this process is currently unknown. The current work suggests that the large-scale shape of the AP boundary thus emerges from the collective behavior of many cells that locally exchange biochemical signals and regulate active mechanical tension (Rudolf, 2015).

    Hedgehog signaling regulates the ciliary transport of odorant receptors in Drosophila

    Hedgehog (Hh) signaling is a key regulatory pathway during development and also has a functional role in mature neurons. This study shows that Hh signaling regulates the odor response in adult Drosophila olfactory sensory neurons (OSNs). This is achieved by regulating odorant receptor (OR) transport to and within the primary cilium in OSN neurons. Regulation relies on ciliary localization of the Hh signal transducer Smoothened (Smo). This study further demonstrates that the Hh- and Smo-dependent regulation of the kinesin-like protein Cos2 acts in parallel to the intraflagellar transport system (IFT) to localize ORs within the cilium compartment. These findings expand knowledge of Hh signaling to encompass chemosensory modulation and receptor trafficking (Sanchez, 2016).

    This study demonstrates that the Hh pathway modulates the magnitude of the odorant response in adult Drosophila. The results show that the Hh pathway determines the level of the odorant response because it regulates the response in both the positive and negative directions. Loss of Ptc function increases the odorant response and the risk for long sustained responses, which shows that the Hh pathway limits the response potential of the OSNs and is crucial for maintaining the response at a physiological level. In addition, it was shown that the OSNs produce Hh protein, which regulates OR localization, which is interesting because autoregulation is one of the prerequisites for an adaptive mechanism. It was further shown that Hh signaling regulates the responses of OSNs that express different ORs, which demonstrates that the regulation is independent of OSN class and suggests that Hh signaling is a general regulator of the odorant response. It has been shown previously that Hh tunes nociceptive responses in both vertebrates and Drosophila (Babcock, 2011). It is not yet understood how Hh regulates the level of nociception. However, the regulation is upstream of the nociceptive receptors, which indicates that the Hh pathway is a general regulator of receptor transport and the level of sensory signaling (Sanchez, 2016).

    The results show that OSN cilia have two separate OR transport systems, the Hh-regulated Cos2 and the intraflagellar transport complex B (IFT-B) together with the kinesin II system. The results show that Cos2 is required for OR transport to or within the distal cilium domain and suggest that the IFT system regulates the inflow to the cilium compartment. The two transport systems also are required for Smo cilium localization (Kuzhandaivel, 2014). This spatially divided transport of one cargo is similar to the manner in which Kif3a and Kif17 regulate distal and proximal transport in primary cilia in vertebrates. However, Cos2 is not required for the distal location of Orco or tubulin (Kuzhandaivel, 2014), indicating that, for some cargos, the IFT system functions in parallel to Cos2 (Sanchez, 2016).

    Interestingly, the vertebrate Cos2 homolog Kif7 organizes the distal compartment of vertebrate primary cilia (He, 2014). Similar to the current results, Kif7 does so without affecting the IFT system, and its localization to the cilia is dependent on Hh signaling. However, the Kif7 kinesin motor function has been questioned (He, 2014). Therefore, it will be interesting to analyze whether Kif7-mediated transport of ORs and other transmembrane proteins occurs within the primary cilium compartment and whether the ciliary transport of ORs is also regulated by Hh and Smo signaling in vertebrates. To conclude, these results place the already well-studied Hh signaling pathway in the post-developmental adult nervous system and also provide an exciting putative role for Hh as a general regulator of receptor transport to and within cilia (Sanchez, 2016).

    The APC/C coordinates retinal differentiation with G1 arrest through the Nek2-dependent modulation of Wingless signaling

    The cell cycle is coordinated with differentiation during animal development. This study reports a cell-cycle-independent developmental role for a master cell-cycle regulator, the anaphase-promoting complex or cyclosome (APC/C), in the regulation of cell fate through modulation of Wingless (Wg) signaling. The APC/C controls both cell-cycle progression and postmitotic processes through ubiquitin-dependent proteolysis. Through an RNAi screen in the developing Drosophila eye, this study found that partial APC/C inactivation severely inhibits retinal differentiation independently of cell-cycle defects. The differentiation inhibition coincides with hyperactivation of Wg signaling caused by the accumulation of a Wg modulator, Drosophila Nek2 (dNek2). The APC/C degrades dNek2 upon synchronous G1 arrest prior to differentiation, which allows retinal differentiation through local suppression of Wg signaling. Evidence is provided that Decapentaplegic signaling may posttranslationally regulate this APC/C function. Thus, the APC/C coordinates cell-fate determination with the cell cycle through the modulation of developmental signaling pathways (Martins, 2017).

    Wingless signaling regulates winner/loser status in Minute cell competition

    Cells heterozygously mutant for a ribosomal protein gene, called Minute/+ mutants, are eliminated from epithelium by cell competition when surrounded by wild-type cells. Whereas several factors that regulate Minute cell competition have been identified, the mechanisms how winner/loser status is determined and thereby triggers cell competition are still elusive. To address this, two assay systems were establised for Minute cell competition, namely (i) the CORE (competitive elimination of RpS3-RNAi-expressing cells) system in which RpS3-RNAi-expressing wing pouch cells are eliminated from wild-type wing disc and (ii) the SURE (supercompetition of RpS3-expressing clones in RpS3/+ tissue) system in which RpS3-over-expressing clones generated in RpS3/+ wing disc outcompete surrounding RpS3/+ cells. An ectopic over-expression screen using the CORE system identified Wg signaling as a critical regulator of Minute cell competition. Activation of Wg signaling in loser cells suppressed their elimination, whereas down-regulation of Wg signaling in loser cells enhanced their elimination. Furthermore, using the SURE system, it was found that down-regulation of Wg signaling in winner cells suppressed elimination of neighboring losers. These observations suggest that cellular Wg signaling activity is crucial for determining winner/loser status and thereby triggering Minute cell competition (Akai, 2018).

    Coupling optogenetics and light-sheet microscopy, a method to study Wnt signaling during embryogenesis

    Optogenetics allows precise, fast and reversible intervention in biological processes. Light-sheet microscopy allows observation of the full course of Drosophila embryonic development from egg to larva. Bringing the two approaches together allows unparalleled precision into the temporal regulation of signaling pathways and cellular processes in vivo. To develop this method,the regulation of canonical Wnt signaling was investigated during anterior-posterior patterning of the Drosophila embryonic epidermis. Cryptochrome 2 (CRY2; see Drosophila Crptochrome) from Arabidopsis Thaliana was fused to mCherry fluorescent protein and Drosophila beta-catenin to form an easy to visualize optogenetic switch. Blue light illumination caused oligomerization of the fusion protein and inhibited downstream Wnt signaling in vitro and in vivo. Temporal inactivation of beta-catenin confirmed that Wnt signaling is required not only for Drosophila pattern formation, but also for maintenance later in development. It is anticipate that this method will be easily extendable to other developmental signaling pathways and many other experimental systems (Kaur, 2017).

    Alternative direct stem cell derivatives defined by stem cell location and graded Wnt signalling

    Adult stem cells provide a renewable source of differentiated cells for a wide variety of tissues and generally give rise to multiple cell types. Basic principles of stem cell organization and regulation underlying this behaviour are emerging. Local niche signals maintain stem cells, while different sets of signals act outside the niche to diversify initially equivalent stem cell progeny. This study shows that Drosophila ovarian follicle stem cells (FSCs) produced two distinct cell types directly. This cell fate choice was determined by the anterior-posterior position of an FSC and by the magnitude of spatially graded Wnt pathway activity. These findings reveal a paradigm of immediate diversification of stem cell derivatives according to stem cell position within a larger population, guided by a graded niche signal. It was also found that FSCs strongly resemble mammalian intestinal stem cells in many aspects of their organization, including population asymmetry and dynamic heterogeneity (Reilein, 2017).

    SMOC can act as both an antagonist and an expander of BMP signaling

    The matricellular protein SMOC (Secreted Modular Calcium binding protein) is conserved phylogenetically from vertebrates to arthropods. It has been previously shown that SMOC inhibits bone morphogenetic protein (BMP) signaling downstream of its receptor via activation of mitogen-activated protein kinase (MAPK) signaling. In contrast, the most prominent effect of the Drosophila orthologue, pentagone (pent), is expanding the range of BMP signaling during wing patterning. Using SMOC deletion constructs this study found that SMOC-∆EC, lacking the extracellular calcium binding (EC) domain, inhibits BMP2 signaling, whereas SMOC-EC (EC domain only) enhances BMP2 signaling. The SMOC-EC domain binds HSPGs with a similar affinity to BMP2 and can expand the range of BMP signaling in an in vitro assay by competition for HSPG-binding. Together with data from studies in vivo the study proposes a model to explain how these two activities contribute to the function of Pent in Drosophila wing development and SMOC in mammalian joint formation (Thomas, 2017).

    Dampening the signals transduced through Hedgehog via microRNA miR-7 facilitates Notch-induced tumourigenesis

    Fine-tuned Notch and Hedgehog signalling pathways via attenuators and dampers have long been recognized as important mechanisms to ensure the proper size and differentiation of many organs and tissues. This notion is further supported by identification of mutations in these pathways in human cancer cells. However, although it is common that the Notch and Hedgehog pathways influence growth and patterning within the same organ through the establishment of organizing regions, the cross-talk between these two pathways and how the distinct organizing activities are integrated during growth is poorly understood. An unbiased genetic screen in the Drosophila melanogaster eye has found that tumour-like growth was provoked by cooperation between the microRNA miR-7 and the Notch pathway. Surprisingly, the molecular basis of this cooperation between miR-7 and Notch converged on the silencing of Hedgehog signalling. In mechanistic terms, miR-7 silenced the interference hedgehog (ihog) Hedgehog receptor, while Notch repressed expression of the brother of ihog (boi) Hedgehog receptor. Tumourigenesis was induced co-operatively following Notch activation and reduced Hedgehog signalling, either via overexpression of the microRNA or through specific down-regulation of ihog, hedgehog, smoothened, or cubitus interruptus or via overexpression of the cubitus interruptus repressor form. Conversely, increasing Hedgehog signalling prevented eye overgrowth induced by the microRNA and Notch pathway. Further, it was shown that blocking Hh signal transduction in clones of cells mutant for smoothened also enhance the organizing activity and growth by Delta-Notch signalling in the wing primordium. Together, these findings uncover a hitherto unsuspected tumour suppressor role for the Hedgehog signalling and reveal an unanticipated cooperative antagonism between two pathways extensively used in growth control and cancer (Da Ros, 2013).

    A challenge to understand oncogenesis produced by pleiotropic signalling pathways, such as Notch, Hh, and Wnts, is to unveil the complex cross-talk, cooperation, and antagonism of these signalling pathways in the appropriate contexts. Studies in flies, mice, and in human cell cultures have provided critical insights into the contribution of Notch to tumourigenesis. These studies highlighted that Notch when acting as an oncogene needs additional mutations or genes to initiate tumourigenesis and for tumour progression, identifying several determinants for such co-operation. The identification of these co-operative events has often been knowledge-driven, although unbiased genetic screens also identified known unanticipated tumour-suppressor functions. In this sense, this study describes a conserved microRNA that cooperates with Notch-induced overproliferation and tumour-like overgrowth in the D. melanogaster eye, miR-7. Alterations in microRNAs have been implicated in the initiation or progression of human cancers, although such roles of microRNAs have rarely been demonstrated in vivo. In addition, by identifying and validating functionally relevant targets of miR-7 in tumourigenesis, this study also exposed a hitherto unsuspected tumour suppressor role for the Hh signalling pathway in the context of the oncogenic Notch pathway. Given the conservation of the Notch and Hh pathways, and the recurrent alteration of microRNAs in human cancers, it is speculated that the genetic configuration of miR-7, Notch, and Hh is likely to participate in the development of certain human tumours (Da Ros, 2013).

    In human cancer cells, miR-7 has been postulated to have an oncogene or a tumour suppressor functions that may reflect the participation of the microRNA in distinct pathways, due to the regulation of discrete target genes in different cell types, such as Fos, IRS-2, EGFR, Raf-1, CD98, IGFR1, bcl-2, PI3K/AKT, and YY1 in humans (Da Ros, 2013).

    In Drosophila, multiple, cell-specific, targets for miR-7 have been previously validated via luciferase or in vivo eGFP-reporter sensors or less extensively via functional studiest. Although microRNAs are thought to regulate multiple target genes, when tested in vivo it is a subset or a given target that predominates in a given cellular context. Indeed, of the 39 predicted miR-7 target genes tested by direct RNAi, only downregulating ihog with several RNAi transgenes (UAS-ihog-IR) fully mimicked the effect of miR-7 overexpression in the transformation of Dl-induced mild overgrowth into severe overgrowth and even tumour-like growth. Moreover, it was confirmed that endogenous ihog is directly silenced by miR-7 and that this silencing involves direct binding of the microRNA to sequences in the 3'UTR of ihog both in vivo and in vitro (Da Ros, 2013).

    Nevertheless, other miR-7 target genes may contribute to the cooperation with Dl-Notch pathway along with ihog, such as hairy and Tom. While miR-7 can directly silence hairy in the wing, this effect has been shown to be very modest, and thus, it is considered that while hairy may contribute to such effects, it is unlikely to be instrumental in this tumour model. Indeed, the loss of hairy is inconsequential in eye development, although retinal differentiation is accelerated by genetic mosaicism of loss of hairy and extramacrochaetae that negatively sets the pace of MF progression. It is unclear how Hairy might contribute to Dl-induced tumourigenesis (Da Ros, 2013).

    The RNAi against Tom produced overgrowth with the gain of Dl albeit inconsistently and with weak penetrance, where one RNAi line did not modify the Dl-induced overgrowth and the other RNAi line caused tumours in less than 40% of the progeny. Tom is required to counteract the activity of the ubiquitin ligase Neuralized in regulating the Notch extracellular domain, and Dl in the signal emitting cells. These interactions are normally required to activate Notch signalling in the receiving cells through lateral inhibition and cell fate allocation. However, although it remains to be shown whether similar interactions are active during cell proliferation and growth, the moderate enhancement of Dl that is induced when Tom is downregulated by RNAi suggests that miR-7-mediated repression of Tom may contribute to the oncogenic effects of miR-7 in the context of Dl gain of function, along with other targets such as ihog (Da Ros, 2013).

    Conversely, while the target genes of the Notch pathway, E(spl)m3 and E(spl)m4 as well as E(spl)mγ, Bob, E(spl)m5, and E(spl)mδ, have been identified as direct targets of miR-7 in the normal wing disc via analysis of 3'UTR sensors, there was no evidence that HLHm3, HLHm4, HLHm5, Bob, and HLHmγ are biological relevant targets of miR-7 in the Dl overexpression context. HLHmδ RNAi produced inconsistent phenotypes in the two RNAi transgenic lines available, causing tumour-like growth at very low frequency in only one of the lines. No evidence was obtained that miR-7 provoked overgrowth by targeting the ETS transcription factor in the EGFR pathway AOP/Yan, a functionally validated target of the microRNA miR-7 during retinal differentiation. Neither was any evidence obtained that RNAi of atonal provoked eye tumours with Dl overexpression, although a strong inhibition via expression of a fusion protein Atonal::EN that converts Atonal into a transcriptional repressor has been shown to be sufficient to trigger tumorigenesis together with Dl. Thus, it was reasoned that given that microRNA influenced target genes only subtly (even when using ectopic expression), it is possible that downregulation of atonal contributes to the phenotype along with the other targets (Da Ros, 2013).

    In conclusion, this study has identified cooperation between the microRNA miR-7 and Notch in the D. melanogaster eye and identified and validated ihog as a direct target of the miR-7 in this context and have identified boi as a target of Notch-mediated activity at the DV eye organizer, although it remains whether this regulation is direct or indirect. A hitherto unanticipated tumour suppressor activity was uncovered of the endogenous Hh signalling pathway in the context of gain of Dl-Notch signalling that is also apparent during wing development (Da Ros, 2013).

    Hh tumour suppressor role is revealed when components of the Hh pathway were lost in conjunction with a gain of Dl expression in both the eye and wing discs. Hh and Notch establish signalling centres along the AP and DV axes, respectively, of the disc to organize global growth and patterning. Where the organizer domains meet, the Hh and Notch conjoined activities specify the position of the MF in the eye disc and the proximodistal patterning in the wing disc. This study also unvailed that in addition antagonistic interaction between the Hh and Notch signalling might help to ensure correct disc growth. Thus, it was shown that Hh signalling limits the organizing activity of Dl-Notch signalling. Although it is often confounded whether Dl-Notch signalling instructs overgrowth by autonomous or nonautonomous (i.e., DV organizers) mechanisms, these findings uncover that loss of Hh signalling enhances a non-cell autonomous oncogenic role of Dl-Notch pathway (Da Ros, 2013).

    To date, Hh has not yet to be perceived as a tumour suppressor, although it is noteworthy that human homologs of ihog, CDO, and BOC were initially identified as tumour suppressors. Importantly, both CDO and BOC are downregulated by RAS oncogenes in transformed cells and their overexpression can inhibit tumour cell growth in vitro. Since human RAS regulates tumourigenesis in the lung by overexpressing miR-7 in an ERK-dependent manner, it is possible that RAS represses CDO and BOC via this microRNA. Indeed, the 3'UTR of both CDO and BOC like Drosophila ihog contains predicted binding sites for miR-7. There is additional clinical and experimental evidence connecting elements of the Hedgehog pathway with tumour-suppression. The function of Growth arrest specific gene 1 (GAS1), a Hh ligand-binding factor, overlaps that of CDO and BOC, while its overexpression inhibits tumour growth . More speculative is the association of some cancer cells with the absence of cilium, a structure absolutely required for Hh signal transduction in vertebrate cells (Da Ros, 2013).

    Given the pleiotropic nature of Notch, Wnts, BMP/TGFβ, Ras, and Hh signalling pathways in normal development in vivo, it is speculated that competitive interplay as that described in this study between Notch and Hh may not be uncommon among core growth control and cancer pathways that act within the same cells at the same or different time to exert multiple outputs (such as growth and cell differentiation). Moreover, context-dependent tumour suppressor roles could explain the recurrent, unexplained, identification of somatic mutations in Hh pathway in human cancer samples. Indeed, the current findings stimulate a re-evaluation of the signalling pathways previously considered to be exclusively oncogenic, such as the Hh pathway (Da Ros, 2013).

    References

    Akai, N., Igaki, T. and Ohsawa, S. (2018). Wingless signaling regulates winner/loser status in Minute cell competition. Genes Cells 23(3): 234-240. PubMed ID: 29431244

    Babcock, D. T., Shi, S., Jo, J., Shaw, M., Gutstein, H. B. and Galko, M. J. (2011). Hedgehog signaling regulates nociceptive sensitization. Curr Biol 21: 1525-1533. PubMed ID: 21906949

    Da Ros, V. G., Gutierrez-Perez, I., Ferres-Marco, D. and Dominguez, M. (2013). Dampening the signals transduced through Hedgehog via microRNA miR-7 facilitates Notch-induced tumourigenesis. PLoS Biol 11(5): e1001554. PubMed ID: 23667323

    He, M., Subramanian, R., Bangs, F., Omelchenko, T., Liem, K. F., Jr., Kapoor, T. M. and Anderson, K. V. (2014). The kinesin-4 protein Kif7 regulates mammalian Hedgehog signalling by organizing the cilium tip compartment. Nat Cell Biol 16: 663-672. PubMed ID: 24952464

    Kaur, P., Saunders, T. E. and Tolwinski, N. S. (2017). Coupling optogenetics and light-sheet microscopy, a method to study Wnt signaling during embryogenesis. Sci Rep 7(1): 16636. PubMed ID: 29192250

    Kuzhandaivel, A., Schultz, S. W., Alkhori, L. and Alenius, M. (2014). Cilia-mediated hedgehog signaling in Drosophila. Cell Rep 7: 672-680. PubMed ID: 24768000

    Lawrence, P. A. and Struhl, G. (1996). Morphogens, compartments, and pattern: lessons from drosophila? Cell 85: 951-961. PubMed ID: 8674123

    Martins, T., Meghini, F., Florio, F. and Kimata, Y. (2017). The APC/C coordinates retinal differentiation with G1 arrest through the Nek2-dependent modulation of Wingless signaling. Dev Cell 40(1): 67-80. PubMed ID: 28041905

    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

    Rudolf, K., Umetsu, D., Aliee, M., Sui, L., Julicher, F. and Dahmann, C. (2015). A local difference in Hedgehog signal transduction increases mechanical cell bond tension and biases cell intercalations along the Drosophila anteroposterior compartment boundary. Development 142: 3845-3858. PubMed ID: 26577205

    Sanchez, G. M., Alkhori, L., Hatano, E., Schultz, S. W., Kuzhandaivel, A., Jafari, S., Granseth, B. and Alenius, M. (2016). Hedgehog signaling regulates the ciliary transport of odorant receptors in Drosophila. Cell Rep 14: 464-470. PubMed ID: 26774485

    Thomas, J.T., Eric Dollins, D., Andrykovich, K.R., Chu, T., Stultz, B.G., Hursh, D.A. and Moos, M. (2017). SMOC can act as both an antagonist and an expander of BMP signaling. Elife 6. PubMed ID: 28323621

    Zygotically transcribed genes

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