scalloped: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

Gene name - scalloped

Synonyms - spatula

Cytological map position - 13F

Function - transcription factor

Keyword(s) - sensory organ and wing morphogenesis

Symbol - sd

FlyBase ID:FBgn0003345

Genetic map position - 1-51.5

Classification - TEF-1 family

Cellular location - nuclear

NCBI links: Precomputed BLAST | Entrez Gene
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.
Shu, Z. and Deng, W.M. (2017). Differential Regulation of Cyclin E by Yorkie-Scalloped Signaling in Organ Developmen G3 (Bethesda) [Epub ahead of print]. PubMed ID: 28143945
Tissue integrity and homeostasis are accomplished through strict spatial and temporal regulation of cell growth and proliferation during development. Various signaling pathways have emerged as major growth regulators across metazoans; yet, how differential growth within a tissue is spatiotemporally coordinated remains largely unclear. This study reports a role of a growth modulator Yorkie (Yki), the Drosophila homolog of Yes-associated protein (YAP), which interacts with its transcriptional partner, Scalloped (Sd), the homolog of the TEAD/TEF family transcription factor in mammals, to control an essential cell-cycle regulator Cyclin E (CycE) in wing imaginal discs. Interestingly, when Yki is coexpressed with Fizzy-related (Fzr), a Drosophila endocycle inducer and homolog of Cdh1 in mammals, surrounding hinge cells display larger nuclear size than distal pouch cells. The observed size difference is attributable to differential regulation of CycE, a target of Yki and Sd, the latter of which can directly bind to CycE regulatory sequences, and is expressed only in the pouch region of the wing disc starting from the late second-instar larval stage. During earlier stages of larval development, when Sd expression is not detected in the wing disc, coexpression of Fzr and Yki does not cause size differences between cells along the proximal-distal axis of the disc. Ectopic CycE promotes cell proliferation and apoptosis, and inhibits transcriptional activity of Yki targets. These findings suggest that spatiotemporal expression of transcription factor Sd induces differential growth regulation by Yki during wing disc development, highlighting coordination between Yki and CycE to control growth and maintain homeostasis.

Ferguson, G. B. and Martinez-Agosto, J. A. (2017). The TEAD family transcription factor Scalloped regulates blood progenitor maintenance and proliferation in Drosophila through PDGF/VEGFR receptor (Pvr) signaling. Dev Biol. PubMed ID: 28322737
The Drosophila lymph gland is a well-characterized hematopoietic organ in which a population of multipotent stem-like progenitors is maintained by a combination of signals from different cellular populations within the organ. This study demonstrates a requirement for the TEAD transcription factor Scalloped in the maintenance and proliferation of hematopoietic progenitors. A novel population of hemocytes in the early lymph gland was identified by the expression of Hand, Scalloped, and the PVR ligand PVF2. In this unique population, Scalloped maintains PVF2 expression, which is required for hemocyte proliferation and achievement of normal lymph gland size. STAT signaling was demonstrated to marks actively proliferating hemocytes in the early lymph gland, and inhibition of this pathway causes decreased lymph gland growth similar to loss of Scalloped and PVF2, demonstrating a requirement for PVR/STAT signaling in the regulation of lymph gland size. Finally, it was demonstrate that Scalloped regulates PVR expression and the maintenance of progenitors downstream of PVR/STAT/ADGF signaling. These findings further establish the role of the TEAD family transcription factors in the regulation of important signaling molecules, and expand mechanistic insight into the balance between progenitor maintenance and proliferation required for the regulation of lymph gland homeostasis.
Rohith, B. N. and Shyamala, B. V. (2017). Scalloped a member of the Hippo tumor suppressor pathway controls mushroom body size in Drosophila brain by non-canonical regulation of neuroblast proliferation.Dev Biol 432(2):203-214. PubMed ID: 29080790
Cell proliferation, growth and survival are three different basic processes which converge at determining a fundamental property -the size of an organism. Scalloped (Sd) is the first characterised transcriptional partner to Yorkie (Yki), the downstream effector of the Hippo pathway which is a highly potential and evolutionarily conserved regulator of organ size. The hypomorphic effect of sd on the development of Mushroom Bodies (MBs) in Drosophila brain was studied. sd non-function results in an increase in the size of MBs. sd regulation on MB size operates through multiple routes. Sd expressed in the differentiated MB neurons, imposes non-cell autonomous repression on the proliferation of MB precursor cells, and Sd expression in the MB neuroblasts (NB) cell autonomously represses mushroom body neuroblast (MBNB) proliferation. Further Sd in Kenyon cells (KCs) imparts a cell autonomous restriction on their growth. These findings are distinctive because, while the classical sd loss of function phenotypes in eye, wing and lymph gland are reported as loss of tissue or reduced organ size, the present study shows that, Sd inactivation in the developing MB, promotes precursor cell proliferation and results in an increase in the organ size.
Zhang, P., Pei, C., Wang, X., Xiang, J., Sun, B. F., Cheng, Y., Qi, X., Marchetti, M., Xu, J. W., Sun, Y. P., Edgar, B. A. and Yuan, Z. (2017). A balance of Yki/Sd activator and E2F1/Sd repressor complexes controls cell survival and affects organ size. Dev Cell 43(5): 603-617.e605. PubMed ID: 29207260
The Hippo/Yki and RB/E2F pathways both regulate tissue growth by affecting cell proliferation and survival, but interactions between these parallel control systems are poorly defined. This study demonstrates that interaction between Drosophila E2F1 and Sd disrupts Yki/Sd complex formation and thereby suppresses Yki target gene expression. RBF modifies these effects by reducing E2F1/Sd interaction. This regulation has significant effects on apoptosis, organ size, and progenitor cell proliferation. Using a combination of DamID-seq and RNA-seq, this study identified a set of Yki targets that play a diversity of roles during development and are suppressed by E2F1. Further, it was found that human E2F1 competes with YAP for TEAD1 binding, affecting YAP activity, indicating that this mode of cross-regulation is conserved. In sum, this study uncovers a previously unknown mechanism in which RBF and E2F1 modify Hippo signaling responses to modulate apoptosis, organ growth, and homeostasis.
Rohith, B. N. and Shyamala, B. V. (2019). Developmental deformity due to scalloped non-function in Drosophila brain leads to cognitive impairment. Dev Neurobiol. PubMed ID: 30676700
Neural identity and wiring specificity are fundamental to brain function. Factors affecting proliferation of the progenitor cells leading to an expansion or regression of specific neuronal clusters are expected to challenge the process of formation of precise synaptic connections with their partners and their further integration to result in proper functional neural circuitry. This study has investigated the role of scalloped, a Hippo pathway gene in Drosophila brain development and has shown that its function is critical to regulate proliferation of Mushroom Body Neuroblasts and to limit the neuronal cluster size to normal in the fly brain. The consequent effect of the anatomical phenotype of mutant flies on the brain function was tested, as exemplified by their cognitive performance. The neural expansion in important neural clusters of the olfactory pathway, caused by Scalloped inactivation, imparts severe disabilities in learning, short-term memory and long-term memory. Scalloped knockdown in alphabeta Kenyon Cell clusters drastically reduces long-term memory performance. Scalloped deficiency induced neural expansion in antennal lobe and ellipsoid body neurons, bring down short-term memory performance significantly. It was also demonstrate that the cognitive impairments observed in this stdy are not due to a problem in memory formation or execution in the adult, but are due to the developmental deformities caused in the respective class of neurons. These results strongly indicate that the additional neurons generated by Scalloped inactivation are not synergistically integrated into, but rather perturb the formation of precise functional circuitry.

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).

Scalloped interacts with Yorkie, the nuclear effector of the hippo tumor-suppressor pathway in Drosophila

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).

The Scalloped and Nerfin-1 transcription factors cooperate to maintain neuronal cell fate

The ability of cells to stably maintain their fate is governed by specific transcription regulators. This study shows that the Scalloped (Sd) and Nervous fingers-1 (Nerfin-1) transcription factors physically and functionally interact to maintain medulla neuron fate in the Drosophila melanogaster CNS. Using Targeted DamID, Sd and Nerfin-1 were found to occupy a highly overlapping set of target genes, including regulators of neural stem cell and neuron fate, and signaling pathways that regulate CNS development such as Notch and Hippo. Modulation of either Sd or Nerfin-1 activity causes medulla neurons to dedifferentiate to a stem cell-like state, and this is mediated at least in part by Notch pathway deregulation. Intriguingly, orthologs of Sd and Nerfin-1 have also been implicated in control of neuronal cell fate decisions in both worms and mammals. These data indicate that this transcription factor pair exhibits remarkable biochemical and functional conservation across metazoans (Vissers, 2018).

When cells differentiate, they must maintain their fate in a stable manner and repress their ability to adopt alternate cell fates. This is essential for the function of differentiated cells and, when aberrant, can result in pathological consequences. The mechanism by which neuronal cell fate is stably maintained is incompletely understood, with only a handful of factors being linked to this process. This study demonstrates that Nerfin-1 maintains the fate of medulla neurons in the optic lobes of the D. melanogaster CNS, in partnership with the TEA domain transcription factor Sd. The data are consistent with the idea that these proteins operate as a transcription factor pair, given that they form a physical complex, and bind to a highly overlapping set of genomic loci. Putative Sd/Nerfin-1 targets were enriched for genes that are functionally associated with the fate of neurons and neuroblasts, cellular metabolism, as well as developmental signaling pathways such as Notch and Hippo. Given that forced activation of Sd target genes induced reversion of medulla neurons to neural stem cells (NSCs), it is hypothesized that aberrant activation of genes that Sd and Nerfin-1 regulate, is the primary driver of neuronal dedifferentiation in their absence (Vissers, 2018).

The Notch pathway was identified as a key target for regulation by Nerfin-1 and Sd, because (1) expression of multiple Notch pathway members was elevated when either Sd or Nerfin-1 function was perturbed; (2) Notch activity was required for dedifferentiation caused by Sd or Nerfin-1 deregulation; and (3) expression of a hyperactive Notch transgene was sufficient to induce dedifferentiation of medulla neurons. Recently, a requirement of Nerfin-1 has been identified in maintaining the differentiated status of neurons in the ventral nerve cord, central brain, and medulla lineages (Froldi, 2015, Xu, 2017). The latter study also demonstrated a requirement for Notch hyperactivity in mediating medulla neuron dedifferentiation following nerfin-1 loss. The current study extend these studies by showing that Nerfin-1 regulates neuronal maintenance in partnership with Sd. Furthermore, this study demonstrates that these transcription factors promote neuronal fate by regulating the expression of multiple Notch pathway genes (Vissers, 2018).

These data further demonstrate that neuronal cell fate is maintained by distinct factors in different regions of the CNS. In contrast to Nerfin-1, which is required to maintain neuronal differentiation in several neuroblast lineages, Sd is specifically required to maintain the fate of medulla neurons but not neurons derived from ventral nerve cord or central brain neuroblast lineages. The CNS region-specific function of Sd, versus the general requirement for Nerfin-1, is reminiscent of that described for Lola and Prospero, where Lola is required to maintain medulla neuronal fate, but acts redundantly with Prospero in other regions of the CNS. Future studies will elucidate the cooperative transcriptional networks that govern neuronal fate maintenance in different regions of the CNS (Vissers, 2018).

Sd function has been best studied in the context of Hippo pathway-dependent tissue growth, where it serves as the key transcription factor of the Yki transcriptional co-activator. Sd has also been linked to regulation of transcription with other proteins, such as Vestigial, Tondu-domain-containing Growth Inhibitor (Tgi), and in this study, Nerfin-1. Interestingly, the Hippo pathway was among the top signaling pathways identified in KEGG analyses on putative Sd/Nerfin-1 target genes. However, Sd promotes medulla neuronal fate independent of Hippo and Yki, as Yki is not obviously expressed in medulla neurons and cannot induce dedifferentiation. Interestingly, this contrasts with the reported role of Hippo, Yki, and Sd in other neurons. For example, these proteins operate together to control the fate of R8 photoreceptor neurons of the D. melanogaster eye. In one class of these light-sensing neurons, Yki and Sd are required to adopt a fate that allows the sensing of blue light, whereas in the other subtype, which senses green light, the Hippo pathway represses Yki and Sd activity (Vissers, 2018).

Orthologs of Sd and Nerfin-1 have been functionally linked in both C. elegans and vertebrates. In C. elegans, EGL-44 and EGL-46 form a physical complex and are both required to specify neuronal cell fate and Q neuroblast differentiation. By characterizing both the biochemical interaction of Sd and Nerfin-1 and their role in maintenance of neuronal fate, this study shows that they cooperate to perform similar functions in flies and worms. The vertebrate orthologs of Sd and Nerfin-1 (TEAD1-4 and INSM1) have also been implicated in various aspects of neural and neuroendocrine development. Murine INSM1 is required for the development of endocrine and neuroendocrine cells of the pancreas, intestine, pituitary, and lung, while in the CNS, it is required for differentiation of neural progenitor cells. Furthermore, zebrafish insm1a has been implicated in dedifferentiation in the context of Müller glia regeneration. Similarly, expression of a TEAD gain-of-function allele caused a marked expansion of the neural progenitor pool in the developing chick neural tube. Interestingly, preliminary studies suggest that the TEAD/INSMI pair might also operate together in vertebrates; a motif corresponding to the TEAD binding site was enriched in INSM1 target genes, as determined by INSM1 ChIP-seq performed in murine pancreatic beta cells. These studies of Sd and Nerfin-1 in the Drosophila CNS, coupled with the finding that human TEAD1 and INSM1 form a physical complex, further strengthen the idea that these proteins represent an evolutionarily conserved transcription factor pair (Vissers, 2018).


Genomic length - 14 kb

Bases in 5' UTR - 596

Exons - 12

Bases in 3' UTR - 1021


Amino Acids - 440

Structural Domains

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.

scalloped: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 10 February 98

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