Gene name - scalloped
Synonyms - spatula
Cytological map position - 13F
Function - transcription factor
Symbol - sd
Genetic map position - 1-51.5
Classification - TEF-1 family
Cellular location - nuclear
|Recent literature||Meserve, J. H. and Duronio, R. J. (2015). Scalloped and Yorkie are required in Drosophila for cell cycle re-entry of quiescent cells after tissue damage. Development [Epub ahead of print]. PubMed ID: 26160905
Regeneration of damaged tissues typically requires a population of active stem cells. How damaged tissue is regenerated in quiescent tissues lacking a stem cell population is less well understood. This study used a genetic screen in the developing Drosophila melanogaster eye to investigate the mechanisms that trigger quiescent cells to re-enter the cell cycle and proliferate in response to tissue damage. Hippo signaling was found to regulate compensatory proliferation after extensive cell death in the developing eye. Scalloped and Yorkie, transcriptional effectors of the Hippo pathway, drive Cyclin E expression to induce cell cycle re-entry in cells that normally remain quiescent in the absence of damage. Ajuba, an upstream regulator of Hippo signaling that functions as a sensor of epithelial integrity, is also required for cell cycle re-entry. Thus, in addition to its well-established role in modulating proliferation during periods of tissue growth, Hippo signaling maintains homeostasis by regulating quiescent cell populations affected by tissue damage.
|Boone, E., Colombani, J., Andersen, D. S. and Leopold, P. (2016). The Hippo signalling pathway coordinates organ growth and limits developmental variability by controlling dilp8 expression. Nat Commun 7: 13505. PubMed ID: 27874005
Coordination of organ growth during development is required to generate fit individuals with fixed proportions. Dilp8 has been identified as a key hormone in coupling organ growth with animal maturation. In addition, dilp8 mutant flies exhibit elevated fluctuating asymmetry (FA) demonstrating a function for Dilp8 in ensuring developmental stability. The signals regulating Dilp8 activity during normal development are not yet known. This study shows that the transcriptional co-activators of the Hippo (Hpo) pathway, Yorkie (Yki, YAP/TAZ) and its DNA-binding partner Scalloped (Sd), directly regulate dilp8 expression through a Hpo-responsive element (HRE) in the dilp8 promoter. It was further demonstrated that mutation of the HRE by genome-editing results in animals with increased FA, thereby mimicking full dilp8 loss of function. Therefore, these results indicate that growth coordination of organs is connected to their growth status through a feedback loop involving Hpo and Dilp8 signalling pathways.
The dorsal/ventral boundary of the developing wing imaginal disc structures the growth of the entire wing. Interacting here are apterous, expressed in the dorsal margin, and the genes scalloped and vestigial (vg), the latter coding for a novel nuclear protein. apterous expression structures the dorsal and ventral expression of the adhesive integrins. Interactions between dorsal and ventral cells in the growing imaginal disc induce vestigial gene expression through a discrete, extraordinarily conserved imaginal disc-specific enhancer. The link between dorsal/ventral compartmentalization and wing formation distinguishes the development of this sheet-like appendage from that of legs and antennae (Williams, 1994).
scalloped is also expressed in the central and peripheral nervous systems of the developing larva, where it is required for the differentiation of sensory organs. The role of scalloped and vestigial in wing and sensory organ morphogenesis recalls the role of hairy in structuring sensory organs in the wing and leg. scalloped interacts with cut, a proneural gene involved at the wing margin (Jack, 1992). Thus scalloped and vestigial are termed patterning genes, because of their effects on wing sensory organ distribution. However, these two genes are responsible as well for a more global pattern, with ramifications for all of wing development.
The scalloped gene is a downstream effector molecule in the wingless pathway of wing imaginal discs. sd is required for early expression of vestigial. In one-good-turn-deserves-another fashion, vg is later required to maintain sd expression. Early expression of vg, and presumably of sd as well, is widespread and unaltered in wingless mutants, but the phenotypic effect of mutations in both genes is the same: a disrupted wing margin.
Both apterous and wingless are required to retain vestigial. Accompanying vestigial is the presumed scalloped expression, flanking the dorso-ventral margin of the wing imaginal disc. Since early scalloped and vestigial expression do not require wingless, sd and vg are the earliest tissue specific pro-wing genes whose appearance reflects the initial specification of the wing. In this model ap and wg are subsequently required for dorso-ventral compartmentalization (Williams, 1993).
The questions continue: what is the role of the homeodomain proteins distal-less and aristaless in regulation of the early expression of scalloped in wing specification? Is scalloped dependent or independent of these organizers of the distal tip of imaginal discs?
Scalloped is required for Vg function: altering Sd and Vg cellular levels relative to one another inhibits wing formation. Whereas Vg expression is normally restricted to the wing and haltere imaginal discs, a subset of cells within almost all imaginal discs normally express sd. Thus, when Vg is ectopically produced a supply of Sd is already present in those tissues. Since Sd is required for formation of the normal wing, a test was performed to see if there is a similar requirement for Sd in the formation of Vg-induced ectopic wings. The induction of wing tissue overgrowths by ectopic Vg is partially suppressed in animals heterozygous for a strong viable allele of sd and is completely suppressed in hemizygotes for the same viable allele. These observations demonstrate that Vg requires Sd to transform cells to wing fates. This requirement does not appear to reflect a role for Sd as a downstream effector of Vg function, because the expression of Sd alone, whether under the control of dpp or other promoters, does not induce the formation of ectopic wing tissue. Instead, these observations suggest that Sd and Vg could act in parallel to induce wing cell fates (Simmonds, 1998).
In vitro, Vg binds directly to both Sd and its human homolog, Transcription Enhancer Factor-1. The interaction domains map to a small region of Vg that is essential for Vg-mediated gene activation and to the carboxy-terminal half of Sd. To map Vg-Sd and Vg-TEF-1 interaction domains, a Far Western blotting assay was used to screen 15 deleted proteins that remove terminal or internal regions of Vg. Only Vg proteins that contain amino acids 279-335 have any significant affinity for Sd. The Vg-Sd interaction appears to be limited to this 56-amino-acid domain, since Sd does not bind to a deleted Vg protein missing only these amino acids, and a construct encoding only this portion of the protein will still bind to Sd. Significantly, a duplicate panel of Vg deletion proteins probed with TEF-1 shows that TEF-1 interacts with Vg via the same protein domain. Affinity columns containing this protein fragment of Vg bind Sd and TEF-1 protein as well as does full-length Vg. This Sd/TEF-1-binding domain of Vg is serine rich and includes putative phosphorylation sites. Phosphorylation of Vg at these sites may potentially modify the Vg-Sd interaction. This region is highly conserved in Vg proteins from Drosophila virilis and Aedes aegypti. Similar sequences also occur in mammalian genomic and expressed sequence tag databases. The amino- and carboxy-terminal portions of Sd were also tested to map which region of Sd interacts with Vg. Previous studies with TEF-1 have demonstrated that regions mediating interaction with cell-specific TIFs are separable from the DNA-binding TEA/ATTS domain. The Vg-binding region of Sd maps to the carboxy-terminal half of the protein, separable from the TEA/ATTS domain in the amino-terminal half. The carboxy-terminal portion of Sd is also highly similar to TEF-1, which is consistent with the observation that TEF-1 binds to Vg with the same affinity as does Sd. To confirm the direct protein-protein interaction between Vg and Sd in a cellular environment, a yeast two-hybrid assay was used. In yeast, Vg and Sd proteins show a specific and reciprocal interaction when fused to either Gal4-binding domain (pGBDU) or Gal4 activation domain (pACT) fusion constructs. Activation of target genes sd and cut by Vg requires the Sd-Vg interaction domain identified in vitro, implying that this activation is Sd dependent. Moreover, a UAS-vg construct with the Sd-binding domain deleted is unable to induce the formation of ectopic wing tissue, consistent with the observation that this induction is sd dependent (Simmonds, 1998).
A wide variety of studies have suggested that TEA/ATTS domain proteins require tissue-specific transcriptional intermediary factors (TIFs), although relatively little progress has been made toward identifying and characterizing these TIFs . According to the definitions established by the analysis of TEF-1, a TIF for Sd would be expected to bind directly to Sd, to show a restricted pattern of expression, and to be required for Sd function in vivo. These observations presented here, together with the analysis of the coordinate regulation of downstream target genes by Sd and Vg, argue that Vg functions as a tissue-specific TIF for Sd. Although it is possible that Vg interacts with proteins other than Sd, genetic studies argue against this, because all vg mutant phenotypes are shared by sd. In contrast, sd is required for the development of other tissues in which vg is not required. Thus, it is likely that there are other trans-acting factors in Drosophila that interact with Sd. Although the DNA target sequence of Sd is as yet uncharacterized, one target of a yeast TEA/ATTS domain protein (TEC-1) is an element in the TEC-1 promoter. Likewise, in flies, one target of Sd-Vg is likely to be the sd promoter itself, since activation of sd during early development is dependent on Vg, and ectopic Vg induces elevated expression of sd. This suggests a model whereby low levels of Sd expression within wing imaginal discs are elevated in the presence of Vg by positive autoregulation. The dependence on sd of elevated levels of Vg in the wing disc suggests that vg is also a target of positive autoregulation, and direct evidence for this now been obtained. The TEA/ATTS domain protein family is involved in developmental processes as diverse as mammalian neuronal and cardiac muscle development to conidial formation in Aspergillus and pseudohyphal growth of Saccharomyces cerevisiae. Although Vg homologs have not yet been identified in these organisms, genes containing sequences related to the Sd/TEF-1 interaction domain of Vg are conserved in mammals; these genes are thus candidate TIFs for mammalian TEF-1-related proteins. One of these candidate TIFs is expressed in fetal heart tissue, which is intriguing, given that gene-targeted mutations in TEF-1 result in cardiac defects. Future challenges will be to determine whether genes encoding these putative Sd-interacting domains actually function as TIFs for TEF-1 or related proteins, and whether distinct regulatory TIFs have evolved that adapt the transcriptional activities of conserved Sd/TEF-1 homologs to specific functions in different tissues in their respective organisms (Simmonds, 1998).
In Drosophila, Scalloped (Sd) belongs to a family of evolutionarily conserved proteins characterized by the presence of a TEA/ATTS DNA-binding domain. Sd physically interacts with the product of the vestigial (vg) gene, where the dimer functions as a master gene controlling wing formation. The Vg-SD dimer activates the transcription of several specific wing genes, including sd and vg themselves. The dimer drives cell-cycle progression by inducing expression of the dE2F1 transcription factor, which regulates genes involved in DNA replication and cell-cycle progression. Yorkie (Yki) is a transcriptional coactivator that is the downstream effector of the Hippo signaling pathway, which controls cell proliferation and apoptosis in Drosophila. This study identified Sd as a partner for Yki. Interaction between Yki and Sd increases Sd transcriptional activity both ex vivo in Drosophila S2 cells and in vivo in Drosophila wing discs and promotes Yki nuclear localization. Yki overexpression induces vg and dE2F1 expression, and proliferation induced by Yki or by a dominant-negative form of Fat in wing disc is significantly reduced in a sd hypomorphic mutant context. Contrary to Yki, Sd is not required in all imaginal tissues. This indicates that Yki-Sd interaction acts in a tissue-specific fashion and that other Yki partners must exist (Goulev, 2008).
Yorkie (Yki) encodes a transcriptional coactivator that is the downstream effector of the Salvador-Warts-Hippo signaling pathway. This pathway consists of the two serine-threonine kinases, Hippo and Warts (Wts, also known as Large Tumor Suppressor), the adaptator molecules Salvador (Sav) and Mob as Tumor Suppressor (MATS), the FERM domain proteins Expanded (Ex) and Merlin (Mer), and the protocadherin Fat (Ft). In this pathway, the Hpo kinase phosphorylates Wts, which in turn phosphorylates and inactivates Yki by excluding it from the nucleus. Mer and Es colocalize at the cell cortex and act upstream of Hpo to regulate the activity of Yki. Fat acts upstream of the Hippo signaling cascade by recruiting Ex to the apical plasma membrane and modulating the abundance of Wts. Inactivation of ft, hpo, wts, and sav, inactivation of both ex and mer, and overexpression of yki all lead to increased proliferation and reduced apoptosis, This pathway acts through Yki to regulate the expression of cyclin E (cycE) and the Drosophila inhibitor of apoptosis 1 (diap-1), which are involved in cell-cycle progression and cell-death inhibition, respectively, and the microRNA bantam, which can promote growth and inhibit apoptosis. The Yki peptide shares significant homology with the mammalian Yes Associated Protein 65 (YAP) (31% identity with human YAP). A particular region near the amino terminus of Yki (amino acids 57-114) displays the highest homology with YAP (56.2% identity). This region in YAP corresponds to a domain that binds all mammalian transcription-enhancer factors (TEFs), which are the homologs of the Drosophila Sd protein (Goulev, 2008).
Conservation between mammalian YAP and Yki of the YAP domain that interacts with TEF suggests that Yki might interact with Sd. To investigate this possibility, glutathione S-transferase (GST)-pull-down experiments were conducted. 35S-labeled Yki specifically binds to GST-Sd beads (Goulev, 2008).
These results are in good accordance with a large two-hybrid screen in which Sd was identified as a binding partner for Yki. To refine the Sd domain that binds Yki and the region of Yki involved in the interaction with Sd, the binding of deleted Sd or Yki peptides was examined. Results that were obtained indicate a requirement of the N terminus of Yki and the C terminus domain of Sd. These results implicate Yki as an auxiliary protein that specifically interacts with Sd and demonstrate that this ability to interact is conserved between TEA factors (TEF-1 and Sd) and YAP factors (YAP and Yki) (Goulev, 2008).
To determine whether Sd, which possesses a putative nuclear localization signal (NLS) within the TEA domain, can modify Yki localization, transfection experiments were performed with plasmids expressing Sd fused to FLAG sequence and Yki fused to hemagglutinin (HA) sequence. In Drosophila S2 cells, Yki is mainly cytoplasmic. S2 cells were transfected with Yki and Sd and it was observed that Yki is exclusively nuclear. The nuclear translocation of Yki when Sd is expressed argues in favor of the idea that Sd and Yki interact ex vivo. Co-transfection experiments indicate that Yki interacts in a cellular context with Sd to stimulate the transcriptional activity conferred by Sd and additional experiments indicate that Vg and Yki do not compete for binding to Sd (Goulev, 2008).
It has been shown that the transcriptional activity of Yki is suppressed by coexpression of upstream components of the Hippo pathway. The transcriptional activity of Sd-Vg complex was analyzed when Hpo was overexpressed. Coexpression of GAL4db-Sd and Vg results in a powerful induction of luciferase activity. This induction was suppressed when Hpo was overexpressed. Although it cannot be excluded that Hpo used a mechanism other than Yki inhibition, the most likely expanation for these results is that the Hippo pathway, and thus Yki function, is required for Sd-Vg activity (Goulev, 2008).
It has been proposed that, in the Sd-Vg complex, Vg provides the activating component, and that the binding of Vg to Sd switches the target selectivity of Sd. If it is assumed that the Sd-Vg-Yki complex is able to form into a cell, one attractive explanation for the role of interaction between Yki and Sd is that Yki provides the main activating component and Vg undergoes a conformational change in Sd ensuring accurate DNA target selection (Goulev, 2008).
To explore the possibility that Yki function is evolutionarily conserved, the effects of Yki on TEF transcriptional activity was tested in a HeLa human cell line. Yki, similarly to YAP, stimulate TEF1 transcriptional activity. These results reveal an evolutionarily conserved regulatory mechanism. Interestingly, it has recently been shown that YAP is the effector of the mammalian Hippo pathway. It is amplified in mouse tumor models and has several oncogenic properties. Furthermore, YAP can functionally substitute for Yki in Drosophila. The relevance of the YAP-TEF interaction in this pathway, however, remains to be determined (Goulev, 2008).
To determine whether Yki-Sd interaction regulates Sd transcriptional activity in vivo in Drosophila, a sensor transgene, 'hsp70-GAL4db-sd', expressing a modified Sd protein was used, in which the TEA DNA-binding domain has been replaced by that of the yeast GAL4 DNA-binding domain, under the control of the hsp70 promoter. GAL4::Sd activity, monitored by the UAS-lacZ-responsive reporter transgene, is restricted to the wing pouch, where Sd dimerizes with Vg. A significant reduction in lacZ activity was observed in wing discs heterozygous for the amorphic ykiB5 allele compared to control wing discs. The fact that Sd activity is sensitive to yki dosage in wing imaginal discs supports the idea that Yki is required for Sd transcriptional activity in vivo (Goulev, 2008).
To better understand the functional relationship between Yki and Sd during wing development, genetic interactions between these genes was assessed. The results indicate that reduction of both vg and yki dosage enhances the sd phenotype and support the hypothesis that Yki works together with Sd-Vg during wing development (Goulev, 2008).
It has been shown that binding sites for Sd and Sd-Vg are necessary for regulation of vg expression and thus may regulate by a feedback mechanism vg expression. Results obtained in S2 cells indicate that Yki increases Sd transcriptional activity. This prompted an investigation of whether yki overexpression in imaginal discs enhances Vg-Sd activity, thus regulating vg expression and vgQE activity. yki overexpression was drived by using a UAS-GAL4 system or by flip-out (FLP-FRT) recombination in clones. In both cases, overexpression of yki was shown to induce vg and vgQE expression in a cell-autonomous manner in wing discs and in haltere discs (Goulev, 2008).
Several sets of data argue strongly in favor of an involvement of Vg-Sd in cell proliferation and cell-cycle progression in wing imaginal discs. vgnull cell clones do not proliferate in the wing pouch, whereas they can be recovered in the notum region, where vg is not expressed, indicating that vg is required for the proliferation of wing blade cells or for their survival. In contrast, vg ectopic expression induces wing outgrowths and dE2F1 expression. dE2F1 is the Drosophila homolog of the E2F transcription-factor family that plays a pivotal role during cell-cycle progression in controlling expression of different genes involved in G1-S transition (including cycE, a target gene of the Hippo pathway) and DNA replication. It was found that Yki overexpression with the ptc-GAL4 driver clearly upregulates expression of the dE2F1-LacZ reporter strain in a cell-autonomous manner. This indicates that Yki and Vg-Sd both activate dE2F1 expression and might work together in this process. It can be hypothesized either that Sd-Vg is required for dE2F1 induction by Yki or that Yki induces dE2F1 independently from Vg-Sd and reinforces dE2F1 expression through Sd-Vg induction (Goulev, 2008).
To determine more precisely the relationship between Sd and Yki in cell proliferation, the effect of Yki overexpression was tested in a mutant context for sd. Strong hypomorph alleles of sd are characterized by an absence of wing pouch cells. In contrast, sdETX4 corresponds to a weak allele of sd in which a large number of wing pouch cells are still present. This allele was used to exclude the possibility that the observed effect resulted from an absence of cell proliferation in the wing pouch because of a lack of sd expression. Wing discs overexpressing yki with the en-GAL4 driver exhibit clear increases in size of the posterior region, resulting in massive overgrowth. In contrast, the size of the posterior compartment was almost normal in wing discs from flies hemizygous for sdETX4 and overexpressing yki with the en-GAL4 driver. This result shows that Yki needs Sd to fully induce cell proliferation. Next, it was asked whether Sd-Yki interaction is acting downstream of the Hippo pathway. To test this, use was made of a dominant-negative form of Ft that lacks the intracellular domain (FTΔICD) . Indeed, Ft is an upstream component of the Hippo pathway, and overexpression of FTΔICD induces overgrowth and the expression of Hippo target genes. A significant reduction was observed of induced overgrowth in sdETX4 wing discs. This result strongly suggests that Sd-Yki interaction acts downstream of the Hippo pathway in wing discs (Goulev, 2008).
This paper has addressed the role of Sd-Yki interaction in the wing disc. The role of Sd in other imaginal tissues is poorly understood. Strong alleles of sd are associated with lethality in early larval stage. In the eye disc, clones homozygous for a strong allele of sd die or poorly survived, whereas they are associated with truncated legs when generated in the leg disc, suggesting that Sd should play a role in cell survival in these tissues. In the wing disc, sd clones die in the wing pouch but can be easily recovered in regions that will give rise to the notum, whereas Yki is required in the entire wing disc. This implies that other partners must interact with Yki to promote tissue growth in other structures. However, the induction of vg that was observed in cells overexpressing yki and the results showing that proliferation induced by Yki in the wing disc is significantly reduced in sd mutant context suggest that Sd-Vg is required for Yki function in the wing disc (Goulev, 2008).
Bases in 5' UTR - 596
Exons - 12
Bases in 3' UTR - 1021
The TEA domain, homologous to that of human TEF-1, is a DNA binding domain. (Campbell, 1992). There is an N terminal serine-rich region.
date revised: 10 February 98
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