Indirect autoregulation of a homeotic Drosophila gene mediated by extracellular signaling

BMP-gated cell-cycle progression drives anoikis during mesenchymal collective migration

Tissue homeostasis involves the elimination of abnormal cells to avoid compromised patterning and function. Although quality control through cell competition is well studied in epithelial tissues, it is unknown if and how homeostasis is regulated in mesenchymal collectives. Here, we demonstrate that collectively migrating Drosophila muscle precursors utilize both fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) signaling to promote homeostasis via anoikis, a form of cell death in response to substrate de-adhesion. Cell-cycle-regulated expression of the cell death gene head involution defective is responsible for caudal visceral mesoderm (CVM) anoikis. The secreted BMP ligand drives cell-cycle progression via a visceral mesoderm-specific cdc25/string enhancer to synchronize collective proliferation, as well as apoptosis of cells that have lost access to substrate-derived FGF. Perturbation of BMP-dependent cell-cycle progression is sufficient to confer anoikis resistance to mismigrating cells and thus facilitate invasion of other tissues. This BMP-gated cell-cycle checkpoint defines a quality control mechanism during mesenchymal collective migration.

Insect Gut Regeneration

In adult insects, as in vertebrates, the gut epithelium is a highly regenerative tissue that can renew itself rapidly in response to changing inputs from nutrition, the gut microbiota, ingested toxins, and signals from other organs. Because of its cellular and genetic similarities to the mammalian intestine, and its relevance as a target for the control of insect pests and disease vectors, many researchers have used insect intestines to address fundamental questions about stem cell functions during tissue maintenance and regeneration. In Drosophila, where most of the experimental work has been performed, not only are intestinal cell types and behaviors well characterized, but numerous cell signaling interactions have been detailed that mediate gut epithelial regeneration. A prevailing model for regenerative responses in the insect gut invokes stress sensing by damaged enterocytes (ECs) as a principal source for signaling that activates the division of intestinal stem cells (ISCs) and the growth and differentiation of their progeny. However, extant data also reveal alternative mechanisms for regeneration that involve ISC-intrinsic functions, active culling of healthy epithelial cells, enhanced EC growth, and even cytoplasmic shedding by infected ECs. This article reviews current knowledge of the molecular mechanisms involved in gut regeneration in several insect models (Drosophila and Aedes of the order Diptera, and several Lepidoptera).

Genes regulating wing patterning in Drosophila melanogaster show reduced expression under exposure of Daminozide, the fruit ripening retardant

In our previous study we demonstrated that the fruit ripening retardant Daminozide or Alar causes change in life history traits, distortion of adult wing structure, DNA damage in brain cells and mutagenic effects in fruit fly Drosophila melanogaster. As a continuation of the previous study the present work is designed to explore the metabolic modification of Daminozide following ingestion, the effects of Daminozide on the expression of genes which are pivotal for wing development and molecular interactions of Daminozide with those proteins involved in wing patterning. We demonstrated through reporter gene construct assay using X-gal staining method and transgenic Drosophila melanogaster stocks that the vestigial, wingless and decapentaplegic genes in wing imaginal disc from 3^rd instar larvae exhibited reduced expression when exposed to Daminozide in compare to control larvae. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) of those genes confirmed that exposure to Daminozide reduces the transcription level of those genes. In silico approach with molecular docking study revealed Daminozide may bind and interfere with the optimal functioning of expressed wing signaling proteins.

Wingless Signaling: A Genetic Journey from Morphogenesis to Metastasis

This FlyBook chapter summarizes the history and the current state of our understanding of the Wingless signaling pathway. Wingless, the fly homolog of the mammalian Wnt oncoproteins, plays a central role in pattern generation during development. Much of what we know about the pathway was learned from genetic and molecular experiments in Drosophila melanogaster, and the core pathway works the same way in vertebrates. Like most growth factor pathways, extracellular Wingless/Wnt binds to a cell surface complex to transduce signal across the plasma membrane, triggering a series of intracellular events that lead to transcriptional changes in the nucleus. Unlike most growth factor pathways, the intracellular events regulate the protein stability of a key effector molecule, in this case Armadillo/β-catenin. A number of mysteries remain about how the "destruction complex" destabilizes β-catenin and how this process is inactivated by the ligand-bound receptor complex, so this review of the field can only serve as a snapshot of the work in progress.

JAK/STAT and Hox Dynamic Interactions in an Organogenetic Gene Cascade

Organogenesis is controlled by gene networks activated by upstream selector genes. During development the gene network is activated stepwise, with a sequential deployment of successive transcription factors and signalling molecules that modify the interaction of the elements of the network as the organ forms. Very little is known about the steps leading from the early specification of the cells that form the organ primordium to the moment when a robust gene network is in place. Here we study in detail how a Hox protein induces during early embryogenesis a simple organogenetic cascade that matures into a complex gene network through the activation of feedback and feed forward interaction loops. To address how the network organization changes during development and how the target genes integrate the genetic information it provides, we analyze in Drosophila the induction of posterior spiracle organogenesis by the Hox gene Abdominal-B (Abd-B). Initially, Abd-B activates in the spiracle primordium a cascade of transcription factors and signalling molecules including the JAK/STAT signalling pathway. We find that at later stages STAT activity feeds back directly into Abd-B, initiating the transformation of the Hox cascade into a gene-network. Focusing on crumbs, a spiracle downstream target gene of Abd-B, we analyze how a modular cis regulatory element integrates the dynamic network information set by Abd-B and the JAK/STAT signalling pathway during development. We describe how a Hox induced genetic cascade transforms into a robust gene network during organogenesis due to the repeated interaction of Abd-B and one of its targets, the JAK/STAT signalling cascade. Our results show that in this network STAT functions not just as a direct transcription factor, but also acts as a "counter-repressor", uncovering a novel mode for STAT directed transcriptional regulation.

Organogenesis is controlled by gene networks activated by upstream selector genes. To address how the network organization changes during development and how the target genes integrate the genetic information it provides, we analyze in Drosophila the induction of posterior spiracle organogenesis by the Hox gene Abdominal-B (Abd-B). Initially, Abd-B activates in the spiracle primordium a cascade of transcription factors and signalling molecules including the JAK/STAT pathway. We find that at later stages STAT activity feeds back into Abd-B, initiating the transformation of the Hox cascade into a gene-network. Focusing on a spiracle downstream target gene of Abd-B, we analyze how its cis regulatory elements integrate the dynamic network information set by Abd-B and the JAK/STAT signalling pathway during development. Our results also show that the well known transcription factor STAT can control gene expression as a “counter-repressor”, uncovering an alternative novel mode for STAT directed transcriptional regulation.

Photoperiodic flowering regulation in Arabidopsis thaliana

Photoperiod, or the duration of light in a given day, is a critical cue that flowering plants utilize to effectively assess seasonal information and coordinate their reproductive development in synchrony with the external environment. The use of the model plant, Arabidopsis thaliana, has greatly improved our understanding of the molecular mechanisms that determine how plants process and utilize photoperiodic information to coordinate a flowering response. This mechanism is typified by the transcriptional activation of FLOWERING LOCUS T (FT) gene by the transcription factor CONSTANS (CO) under inductive long-day conditions in Arabidopsis. FT protein then moves from the leaves to the shoot apex, where floral meristem development can be initiated. As a point of integration from a variety of environmental factors in the context of a larger system of regulatory pathways that affect flowering, the importance of photoreceptors and the circadian clock in CO regulation throughout the day has been a key feature of the photoperiodic flowering pathway. In addition to these established mechanisms, the recent discovery of a photosynthate derivative trehalose-6-phosphate as an activator of FT in leaves has interesting implications for the involvement of photosynthesis in the photoperiodic flowering response that were suggested from previous physiological experiments in flowering induction.

Interlocking feedback loops govern the dynamic behavior of the floral transition in Arabidopsis

During flowering, primordia on the flanks of the shoot apical meristem are specified to form flowers instead of leaves. Like many plants, Arabidopsis thaliana integrates environmental and endogenous signals to control the timing of reproduction. To study the underlying regulatory logic of the floral transition, we used a combination of modeling and experiments to define a core gene regulatory network. We show that FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1) act through FD and FD PARALOG to regulate the transition. The major floral meristem identity gene LEAFY (LFY) directly activates FD, creating a positive feedback loop. This network predicts flowering behavior for different genotypes and displays key properties of the floral transition, such as signal integration and irreversibility. Furthermore, modeling suggests that the control of TFL1 is important to flexibly counterbalance incoming FT signals, allowing a pool of undifferentiated cells to be maintained despite strong differentiation signals in nearby cells. This regulatory system requires TFL1 expression to rise in proportion to the strength of the floral inductive signal. In this network, low initial levels of LFY or TFL1 expression are sufficient to tip the system into either a stable flowering or vegetative state upon floral induction.

Org-1 is required for the diversification of circular visceral muscle founder cells and normal midgut morphogenesis

The T-Box family of transcription factors plays fundamental roles in the generation of appropriate spatial and temporal gene expression profiles during cellular differentiation and organogenesis in animals. In this study we report that the Drosophila Tbx1 orthologue optomotor-blind-related-gene-1 (org-1) exerts a pivotal function in the diversification of circular visceral muscle founder cell identities in Drosophila. In embryos mutant for org-1, the specification of the midgut musculature per se is not affected, but the differentiating midgut fails to form the anterior and central midgut constrictions and lacks the gastric caeca. We demonstrate that this phenotype results from the nearly complete loss of the founder cell specific expression domains of several genes known to regulate midgut morphogenesis, including odd-paired (opa), teashirt (tsh), Ultrabithorax (Ubx), decapentaplegic (dpp) and wingless (wg). To address the mechanisms that mediate the regulatory inputs from org-1 towards Ubx, dpp, and wg in these founder cells we genetically dissected known visceral mesoderm specific cis-regulatory-modules (CRMs) of these genes. The analyses revealed that the activities of the dpp and wg CRMs depend on org-1, the CRMs are bound by Org-1 in vivo and their T-Box binding sites are essential for their activation in the visceral muscle founder cells. We conclude that Org-1 acts within a well-defined signaling and transcriptional network of the trunk visceral mesoderm as a crucial founder cell-specific competence factor, in concert with the general visceral mesodermal factor Biniou. As such, it directly regulates several key genes involved in the establishment of morphogenetic centers along the anteroposterior axis of the visceral mesoderm, which subsequently organize the formation of midgut constrictions and gastric caeca and thereby determine the morphology of the midgut.

Wnt/Wingless signaling in Drosophila

The Wingless (Wg) pathway represents one of the best-characterized intercellular signaling networks. Studies performed in Drosophila over the last 30 years have contributed to our understanding of the role of Wg signaling in the regulation of tissue growth, polarity, and patterning. These studies have revealed mechanisms conserved in the vertebrate Wnt pathways and illustrate the elegance of using the Drosophila model to understand evolutionarily conserved modes of gene regulation. In this article, we describe the function of Wg signaling in patterning the Drosophila embryonic epidermis and wing imaginal disc. As well, we present an overview of the establishment of the Wg morphogen gradient and discuss the differential modes of Wg-regulated gene expression.

Intestinal stem cells in the adult Drosophila midgut

Drosophila has long been an excellent model organism for studying stem cell biology. Notably, studies of Drosophila's germline stem cells have been instrumental in developing the stem cell niche concept. The recent discovery of somatic stem cells in adult Drosophila, particularly the intestinal stem cells (ISCs) of the midgut, has established Drosophila as an exciting model to study stem cell-mediated adult tissue homeostasis and regeneration. Here, we review the major signaling pathways that regulate the self-renewal, proliferation and differentiation of Drosophila ISCs, discussing how this regulation maintains midgut homeostasis and mediates regeneration of the intestinal epithelium after injury.

Tissue deformation modulates twist expression to determine anterior midgut differentiation in Drosophila embryos

Mechanical deformations associated with embryonic morphogenetic movements have been suggested to actively participate in the signaling cascades regulating developmental gene expression. Here we develop an appropriate experimental approach to ascertain the existence and the physiological relevance of this phenomenon. By combining the use of magnetic tweezers with in vivo laser ablation, we locally control physiologically relevant deformations in wild-type Drosophila embryonic tissues. We demonstrate that the deformations caused by germ band extension upregulate Twist expression in the stomodeal primordium. We find that stomodeal compression triggers Src42A-dependent nuclear translocation of Armadillo/beta-catenin, which is required for Twist mechanical induction in the stomodeum. Finally, stomodeal-specific RNAi-mediated silencing of Twist during compression impairs the differentiation of midgut cells, resulting in larval lethality. These experiments show that mechanically induced Twist upregulation in stomodeal cells is necessary for subsequent midgut differentiation.

Wingless-independent association of Pygopus with dTCF target genes

The Wnt signaling pathway controls numerous cell fates during animal development. Its inappropriate activity can lead to cancer in many human tissues. A key effector of the canonical Wnt pathway is beta-catenin (or Drosophila Armadillo), a highly unstable phosphorylated protein that shuttles rapidly between nucleus and cytoplasm. Wnt signaling inhibits its phosphorylation and degradation; this allows it to associate with TCF/LEF factors bound to Wnt target genes and to stimulate their transcription by recruiting chromatin modifying and remodeling factors. The transcriptional activity of Armadillo/beta-catenin also depends on Pygopus (Pygo), a nuclear protein with which it associates through the Legless/BCL9 adaptor. It has been proposed that Pygo associates with TCF target genes during Wnt signaling through Armadillo and Legless to recruit a transcriptional coactivator through its Nbox motif. Here, we report that Pygo is associated constitutively with dTCF target genes in Drosophila salivary glands and tissue-culture cells. Our evidence indicates that this association depends on dTCF and on the Nbox motif of Pygo, but not on Legless. We thus propose an alternative model according to which Pygo functions at the onset of Wnt signaling, or at low signaling levels, to capture Armadillo at dTCF target genes, thus enabling the interaction between Armadillo and dTCF and, consequently, the Armadillo-mediated recruitment of transcriptional coactivators.

Biological significance of autoregulation through steady state analysis of genetic networks

Autoregulation of regulatory proteins is a recurring theme in genetic networks. Autoregulation is an important component of a genetic regulatory network besides protein-protein and protein-DNA interactions, stoichiometry, multiple binding sites and cooperativity. Although the biological significance of autoregulation has been studied before, its significance in presence of other mechanisms is not clearly enumerated. We have analyzed at steady state the significance of autoregulation in presence of other molecular mechanisms by considering hypothetical genetic networks. We demonstrate that autoregulation of a regulatory protein can impart amplification to the response. Further, autoregulation of an activator binding to the DNA as a dimer can introduce bistability, thus forcing the system to reside in two distinct steady states. In combination with autoregulation, cooperative binding can further increase the sensitivity and can yield a highly ultrasensitive response. We conclude that autoregulation with the help of other molecular mechanisms can impart distinct system level properties such as amplification, sensitivity and bistability. The results are further discussed in relation to various examples of genetic networks that exist in biological systems.

Transcriptional activation by extradenticle in the Drosophila visceral mesoderm

decapentaplegic (dpp) is a direct target of Ultrabithorax (Ubx) in parasegment 7 (PS7) of the embryonic visceral mesoderm. We demonstrate that extradenticle (exd) and homothorax (hth) are also required for dpp expression in this location, as well as in PS3, at the site of the developing gastric caecae. A 420 bp element from dpp contains EXD binding sites necessary for expressing a reporter gene in both these locations. Using a specificity swap, we demonstrate that EXD directly activates this element in vivo. Activation does not require Ubx, demonstrating that EXD can activate transcription independently of homeotic proteins. Restoration is restricted to the domains of endogenous dpp expression, despite ubiquitous expression of altered specificity EXD. We demonstrate that nuclear EXD is more extensively phosphorylated than the cytoplasmic form, suggesting that EXD is a target of signal transduction by protein kinases.

Functional specification in the Drosophila endoderm

The discovery of homeobox gene clusters led us to realize that the mechanisms for body patterning and other developmental programs are evolutionally-conserved in vertebrates and invertebrates. The endoderm contributes to the lining of the gut and associated organs such as the liver and pancreas, which are critical for physiological functions. Our knowledge of endoderm development is limited; however, recent studies suggest that cooperation between the HNF3/Fork head and GATA transcription factors is crucial for endoderm specification. It is necessary to further understand the mechanism through which cells become functionally organized. Molecular genetic analyses of the Drosophila endoderm would provide insights into this issue. During proventriculus morphogenesis, a simple epithelial tube is folded into a functional multilayered structure, while two functions of midgut copper cells (i.e. copper absorption and acid secretion) can be easily visualized. The homeobox gene defective proventriculus (dve) plays key roles in these functional specifications.

The evolution of signalling pathways in animal development

Despite the bewildering number of cell types and patterns found in the animal kingdom, only a few signalling pathways are required to generate them. Most cell-cell interactions during embryonic development involve the Hedgehog, Wnt, transforming growth factor-beta, receptor tyrosine kinase, Notch, JAK/STAT and nuclear hormone pathways. Looking at how these pathways evolved might provide insights into how a few signalling pathways can generate so much cellular and morphological diversity during the development of individual organisms and the evolution of animal body plans.

The transcriptional repressor Brinker antagonizes Wingless signaling

In the embryonic midgut of Drosophila, Wingless (Wg) signaling elicits threshold-specific transcriptional response, that is, low-signaling levels activate target genes, whereas high-signaling levels repress them. Wg-mediated repression of the HOX gene Ultrabithorax (Ubx) is conferred by a response sequence within the Ubx B midgut enhancer, called WRS-R. It further depends on the Teashirt (Tsh) repressor, which acts through the WRS-R without binding to it. Here, we show that Wg-mediated repression of Ubx B depends on Brinker, which binds to the WRS-R. Furthermore, Brinker blocks transcriptional activation by ubiquitous Wg signaling. Brinker binds to Tsh in vitro, recruits Tsh to the WRS-R, and we find mutual physical interactions between Brinker, Tsh, and the corepressor dCtBP. This suggests that the three proteins may form a ternary repressor complex at the WRS-R to quench the activity of the nearby-bound dTCF/Armadillo transcription complex. Finally, brinker and tsh produce similar mutant phenotypes in the ventral epidermis, and double mutants mimic overactive Wg signaling in this tissue. This suggests that Brinker may have a widespread function in antagonizing Wg signaling.

Direct competition between Brinker and Drosophila Mad in Dpp target gene transcription

Brinker is a nuclear protein that antagonizes Dpp signalling in Drosophila. Its expression is negatively regulated by Dpp. Here, we show that Brinker represses Ultrabithorax (Ubx) in the embryonic midgut, a HOX gene that activates, and responds to, the localized expression of Dpp during endoderm induction. We find that the functional target for Brinker repression coincides with the Dpp response sequence in the Ubx midgut enhancer, namely a tandem of binding sites for the Dpp effector Mad. We show that Brinker efficiently competes with Mad in vitro, preventing the latter from binding to these sites. Brinker also competes with activated Mad in vivo, blocking the stimulation of the Ubx enhancer in response to simultaneous Dpp signalling. These results indicate how Brinker acts as a dominant repressor of Dpp target genes, and explain why Brinker is a potent antagonist of Dpp.

Cross-regulation of Hox genes in the Drosophila melanogaster embryo

Cross-regulation of Homeotic Complex (Hox) genes by ectopic Hox proteins during the embryonic development of Drosophila melanogaster was examined using Gal4 directed transcriptional regulation. The expression patterns of the endogenous Hox genes were analyzed to identify cross-regulation while ectopic expression patterns and timing were altered using different Gal4 drivers. We provide evidence for tissue specific interactions between various Hox genes and demonstrate the induction of endodermal labial (lab) by ectopically expressed Ultrabithorax outside the visceral mesoderm (VMS). Similarly, activation and repression of Hox genes in the VMS from outside tissues seems to be mediated by decapentaplegic (dpp) gene activation. Additionally, we find that proboscipedia (pb) is activated in the epidermis by ectopically driven Sex combs reduced (Scr) and Deformed (Dfd); however, mesodermal pb expression is repressed by ectopic Scr in this tissue. Mutant analyses demonstrate that Scr and Dfd regulate pb in their normal domains of expression during embryogenesis. Ectopic Ultrabithorax and Abdominal-A repress only lab and Scr in the central nervous system (CNS) in a timing dependent manner; otherwise, overlapping expression in the CNS in tolerated. A summary of Hox gene cross-regulation by ectopically driven Hox proteins is tabulated for embryogenesis.

Homeotic Complex (Hox) gene regulation and homeosis in the mesoderm of the Drosophila melanogaster embryo: the roles of signal transduction and cell autonomous regulation

In this paper we evaluate homeosis and Homeotic Complex (Hox) regulatory hierarchies in the somatic and visceral mesoderm. We demonstrate that both Hox control of signal transduction and cell autonomous regulation are critical for establishing normal Hox expression patterns and the specification of segmental identity and morphology. We present data identifying novel regulatory interactions associated with the segmental register shift in Hox expression domains between the epidermis/somatic mesoderm and visceral mesoderm. A proposed mechanism for the gap between the expression domains of Sex combs reduced (Scr) and Antennapedia (Antp) in the visceral mesoderm is provided. Previously, Hox gene interactions have been shown to occur on multiple levels: direct cross-regulation, competition for binding sites at downstream targets and through indirect feedback involving signal transduction. We find that extrinsic specification of cell fate by signaling can be overridden by Hox protein expression in mesodermal cells and propose the term autonomic dominance for this phenomenon.

Notch signaling targets the Wingless responsiveness of a Ubx visceral mesoderm enhancer in Drosophila

BACKGROUND

Members of the Notch family of receptors mediate a process known as lateral inhibition that plays a prominent role in the suppression of cell fates during development. This function is triggered by a ligand, Delta, and is implemented by the release of the intracellular domain of Notch from the membrane and by its interaction with the protein Suppressor of Hairless [Su(H)] in the nucleus. There is evidence that Notch can also signal independently of Su(H). In particular, in Drosophila, there is evidence that a Su(H)-independent activity of Notch is associated with Wingless signaling.

RESULTS

We report that Ubx(VM)B, a visceral mesoderm-specific enhancer of the Ubx gene of Drosophila, is sensitive to Notch signaling. In the absence of Notch, but not of Su(H), the enhancer becomes activated earlier and over a wider domain than in the wild type. Furthermore, the removal of Notch reduces the requirement for Disheveled-mediated Wingless signaling to activate this enhancer. This response to Notch is likely to be mediated by the dTcf binding sites in the Ubx(VM)B enhancer.

CONCLUSIONS

Our results show that, in Drosophila, an activity of Notch that is likely to be independent of Su(H) inhibits Wingless signaling on Ubx(VM)B. A possible target of this activity is dTcf. As dTcf has been shown to be capable of repressing Wingless targets, our results suggest that this repressive activity may be regulated by Notch. Finally, we suggest that Wingless signaling is composed of two steps, a down-regulation of a Su(H)-independent Notch activity that modulates the activity of dTcf and a canonical Wingless signaling event that regulates the activity of Armadillo and its interaction with dTcf.

Teashirt is required for transcriptional repression mediated by high Wingless levels

During development, extracellular signals often act at multiple thresholds to specify distinct transcriptional and cellular responses. For example, in the embryonic midgut of Drosophila, low Wingless levels stimulate the transcription of homeotic genes whereas high Wingless levels repress these genes. Wingless- mediated transcriptional activation is conferred by Drosophila: T-cell factor (dTCF) and its co-activator Armadillo, but the nuclear factors mediating transcriptional repression are unknown. Here we show that teashirt is required for Wingless-mediated repression of Ultrabithorax: in the midgut. Teashirt is also a repressor of the homeotic gene labial in this tissue. Furthermore, the target sequence for Tsh within the Ultrabithorax: midgut enhancer coincides with the response sequence for Wingless-mediated repression. Finally, we demonstrate that the zinc finger protein Teashirt behaves as a transcriptional repressor in transfected mammalian cells. It thus appears that the response to high Wingless levels in the Drosophila: midgut is indirect and based on transcriptional activation of the Teashirt repressor.

Feedback control of intercellular signalling in development

The intercellular communication that regulates cell fate during animal development must be precisely controlled to avoid dangerous errors. How is this achieved? Recent work has highlighted the importance of positive and negative feedback loops in the dynamic regulation of developmental signalling. These feedback interactions can impart precision, robustness and versatility to intercellular signals. Feedback failure can cause disease.

The homeodomain transcription factor Xvent-2 mediates autocatalytic regulation of BMP-4 expression in Xenopus embryos

Like other genes of the transforming growth factor-beta family, the BMP-4 gene is regulated by an autocatalytic loop. In Xenopus embryos this loop can be ectopically induced by injection of BMP-2 RNA. However, cycloheximide treatment subsequent to BMP-2 overexpression revealed that BMP signaling is not direct but requires additional factor(s). As putative mediator we have identified Xvent-2 which is activated by BMP-2/4 signaling and, in turn, activates BMP-4 transcription. Using promoter/reporter assays we have delineated Xvent-2 responsive elements within the BMP-4 gene. We further demonstrate that Xvent-2 which has recently been characterized as a transcriptional repressor can also act, context dependent, as an activator binding two copies of a 5'-CTAATT-3' motif in the second intron of the BMP-4 gene. Replacement of Xvent-2 target sites within the goosecoid (gsc) promoter by the BMP-4 enhancer converts Xvent-2 caused repression of gsc to strong activation. This switch is obviously due to adjacent nucleotides probably binding a transcriptional co-activator interacting with Xvent-2. A model is presented describing the mechanism of BMP-4 gene activation in Xenopus embryos at the early gastrula stage.

An autoregulatory function of Dfos during Drosophila endoderm induction

The endoderm of Drosophila is patterned during embryogenesis by an inductive cascade emanating from the adhering mesoderm. An immediate-early endodermal target gene of this induction is Dfos whose expression is upregulated in the middle midgut by Dpp signalling. Previous evidence based on a dominant-negative Dfos construct indicated that Dfos may cooperate with Dpp signalling to induce the HOX gene labial, the ultimate target gene of the inductive cascade. Here, we examine kayak mutants that lack Dfos to establish that Dfos is indeed required for labial induction. We provide evidence that Dfos acts through a CRE-like sequence, previously identified to be a target for signalling by Dpp and by the Epidermal growth factor receptor (Egfr) in the embryonic midgut. We show that Dfos expression is stimulated by Egfr signalling. Finally, we find that Dfos function is required for its own upregulation. Thus, endoderm induction is based on at least four tiers of positive autoregulatory feedback loops.

c-Jun (AP-1) activates BMP-4 transcription in Xenopus embryos

Zygotic expression of the BMP-4 gene in Xenopus embryos is regulated by an auto-regulatory loop. Since AP-1 is known as a mediator of auto-regulatory loops both in the case of the Drosophila dpp and the mammalian TGF-beta genes, we have analysed the potential of Xenopus c-Jun (AP-1) as a mediator of BMP-4 expression during Xenopus development. RNA injection experiments revealed that both heteromeric c-Fos/c-Jun and homodimeric c-Jun/c-Jun strongly activate BMP-4 transcription, whereas BMP signaling was found to activate the Xenopus c-Jun gene only at a rather low extent. In addition, the lack of zygotic c-Jun transcripts until the end of gastrulation should exclude a role of AP-1 in the activation and the early expression of BMP-4 during gastrulation in vivo. However, at later stages of Xenopus development, we find a spatial overlap of c-Jun and BMP-4 transcripts which suggests that AP-1 might serve as an additional activatory component for the auto-regulation of BMP-4. Promoter/reporter and gel mobility shift assays demonstrate multiple responsive sites for AP-1 in the 5' flanking region and two in the second intron of the BMP-4 gene. We further demonstrate that AP-1 acts independently of Xvent-2 which has recently been shown to mediate the early expression of BMP-4 in gastrula stage embryos.

decapentaplegic is a direct target of dTcf repression in the Drosophila visceral mesoderm

Drosophila T cell factor (dTcf) mediates transcriptional activation in the presence of Wingless signalling and repression in its absence. Wingless signalling is required for the correct expression of decapentaplegic (dpp), a Transforming Growth Factor (beta) family member, in parasegments 3 and 7 of the Drosophila visceral mesoderm. Here we demonstrate that a dpp enhancer element, which directs expression of a reporter gene in the visceral mesoderm in a pattern indistinguishable from dpp, has two functional dTcf binding sites. Mutations that reduce or eliminate Wingless signalling abolish dpp reporter gene expression in parasegment 3 and reduce it in parasegment 7 while ectopic expression of Wingless signalling components expand reporter gene expression anteriorly in the visceral mesoderm. However, mutation of the dTcf binding sites in the dpp enhancer results in ectopic expression of reporter gene expression throughout the visceral mesoderm, with no diminution of expression in the endogenous sites of expression. These results demonstrate that the primary function of dTcf binding to the dpp enhancer is repression throughout the visceral mesoderm and that activation by Wingless signalling is probably not mediated via these dTcf binding sites to facilitate correct dpp expression in the visceral mesoderm.

LexA chimeras reveal the function of Drosophila Fos as a context-dependent transcriptional activator

The transcriptional activation potential of proteins can be assayed in chimeras containing a heterologous DNA-binding domain that mediates their recruitment to reporter genes. This approach has been widely used in yeast and in transient mammalian cell assays. Here, we applied it to assay the transactivation potential of proteins in transgenic Drosophila embryos. We found that a chimera between the DNA-binding bacterial LexA protein and the transactivation domain from yeast GAL4 behaved as a potent synthetic activator in all embryonic tissues. In contrast, a LexA chimera containing Drosophila Fos (Dfos) required an unexpected degree of context to function as a transcriptional activator. We provide evidence to suggest that this context is provided by Djun and Mad (a Drosophila Smad), and that these partner factors need to be activated by signaling from Jun N-terminal kinase and decapentaplegic, respectively. Because Dfos behaves as an autonomous transcriptional activator in more artificial assays systems, our data suggest that context-dependence of transcription factors may be more prevalent than previously thought.

Negative regulation of dorsoventral signaling by the homeotic gene Ultrabithorax during haltere development in Drosophila

Growth and patterning during Drosophila wing development are mediated by signaling from its dorsoventral (D/V) organizer. In the metathorax, wing development is essentially suppressed by the homeotic selector gene Ultrabithorax (Ubx) to mediate development of a pair of tiny balancing organs, the halteres. Here we show that expression of Ubx in the haltere D/V boundary down-regulates its D/V organizer signaling compared to that of the wing D/V boundary. Somatic loss of Ubx from the haltere D/V boundary thus results in the formation of a wing-type D/V organizer in the haltere field. Long-distance signaling from this organizer was analyzed by assaying the ability of a Ubx(-) clone induced in the haltere D/V boundary to effect homeotic transformation of capitellum cells away from the boundary. The clonally restored wing D/V organizer in mosaic halteres not only enhanced the homeotic transformation of Ubx(-) cells in the capitellum but also caused homeotic transformation of even Ubx(+) cells in a genetic background known to induce excessive cell proliferation in the imaginal discs. In addition to demonstrating a non-cell-autonomous role for Ubx during haltere development, these results reveal distinct spatial roles of Ubx during maintenance of cell fate and patterning in the halteres.

Mutant alleles of the Drosophila trithorax gene produce common and unusual homeotic and other developmental phenotypes

trithorax (trx) encodes chromosome-binding proteins required throughout embryogenesis and imaginal development for tissue- and cell-specific levels of transcription of many genes including homeotic genes of the ANT-C and BX-C. trx encodes two protein isoforms that contain conserved motifs including a C-terminal SET domain, central PHD fingers, an N-terminal DNA-binding homology, and two short motifs also found in the TRX human homologue, ALL1. As a first step to characterizing specific developmental functions of TRX, I examined phenotypes of 420 combinations of 21 trx alleles. Among these are 8 hypomorphic alleles that are sufficient for embryogenesis but provide different levels of trx function at homeotic genes in imaginal cells. One allele alters the N terminus of TRX, which severely impairs larval and imaginal growth. Hypomorphic alleles that alter different regions of TRX equivalently reduce function at affected genes, suggesting TRX interacts with common factors at different target genes. All hypomorphic alleles examined complement one another, suggesting cooperative TRX function at target genes. Comparative effects of hypomorphic genotypes support previous findings that TRX has tissue-specific interactions with other factors at each target gene. Some hypomorphic genotypes also produce phenotypes that suggest TRX may be a component of signal transduction pathways that provide tissue- and cell-specific levels of target gene transcription.

A function of CBP as a transcriptional co-activator during Dpp signalling

CBP/p300 is a transcriptional co-activator that is recruited to enhancers by various DNA-binding proteins, including proteins whose activity is controlled by extracellular signals. Here, we report that Drosophila CBP loss-of-function mutants show specific defects which mimic those seen in mutants that lack the extracellular signal Dpp or its effector Mad. Furthermore, we find that CBP loss severely compromises the ability of Dpp target enhancers to respond to endogenous or exogenous Dpp. Finally, we show that CBP binds to the C-terminal domain of Mad. Our results provide evidence that CBP functions as a co-activator during Dpp signalling, and they suggest that Mad may recruit CBP to effect the transcriptional activation of Dpp-responsive genes during development.

Transcriptional repression due to high levels of Wingless signalling

Extracellular signals can act at different threshold levels to elicit distinct transcriptional and cellular responses. Here, we examine the transcriptional regulation of the Wingless target gene Ultrabithorax (Ubx) in the embryonic midgut of Drosophila. Our previous work showed that Ubx transcription is stimulated in this tissue by Dpp and by low levels of Wingless signalling. We now find that high levels of Wingless signalling can repress Ubx transcription. The response sequence within the Ubx midgut enhancer required for this repression coincides with a motif required for transcriptional stimulation of Dpp, namely a tandem of binding sites for the Dpp-transducing protein, Mad. Indeed, Wingless-mediated repression depends on low levels of Dpp, although apparently not on Mad itself. In contrast, high levels of Dpp signalling antagonize Wingless-mediated repression. This suggests that transcriptional activation of Ubx is subject to competition between Dpp-activated Mad and another Smad whose function as a transcriptional repressor depends on high Wg signalling. Finally, we show that Wingless can repress its own expression via an autorepressive feedback loop that results in a change of the Wingless signalling profile during development.

Hedgehog and wingless induce metameric pattern in the Drosophila visceral mesoderm

The Drosophila visceral mesoderm (VM) is a favorite system for studying the regulation of target genes by Hox proteins. The VM is formed by cells from only the anterior subdivision of each mesodermal parasegment (PS). We show here that the VM itself acquires modular anterior-posterior subdivisions similar to those found in the ectoderm. As VM progenitors merge to form a continuous band running anterior to posterior along the embryo, expression of connectin (con) in 11 metameric patches within the VM reveals VM subdivisions analagous to ectodermal compartments. The VM subdivisions form in response to ectodermal production of secreted signals encoded by the segment polarity genes hedgehog (hh) and wingless (wg) and are independent of Hox gene activity. A cascade of induction from ectoderm to mesoderm to endoderm thus subdivides the gut tissues along the A-P axis. Induction of VM subdivisions may converge with Hox-mediated information to refine spatial patterning in the VM. Con patches align with ectodermal engrailed stripes, so the VM subdivisions correspond to PS 2-12 boundaries in the VM. The PS boundaries demarcated by Con in the VM can be used to map expression domains of Hox genes and their targets with high resolution. The resultant map suggests a model for the origins of VM-specific Hox expression in which Hox domains clonally inherited from blastoderm ancestors are modified by diffusible signals acting on VM-specific enhancers.

Functional intertwining of Dpp and EGFR signaling during Drosophila endoderm induction

Endoderm induction in Drosophila is mediated by the extracellular signals Decapentaplegic (Dpp) and Wingless (Wg). We discovered a secondary signal with a permissive role in this process, namely Vein, a neuregulin-like ligand that stimulates the epidermal growth factor receptor (EGFR) and Ras signaling. Dpp and Wg up-regulate vein expression in the midgut mesoderm in two regions overlapping the Dpp sources. Experiments based on lack of function and ectopic stimulation of Dpp and EGFR signaling show that these two pathways are functionally interdependent and that they synergize with each other, revealing functional intertwining. The transcriptional response elements for the Dpp signal in midgut enhancers from homeotic target genes are bipartite, comprising CRE sites as well as binding sites for the Dpp signal-transducing protein Mad. Of these sites, the CRE seems to function primarily in the response to Ras, the secondary signal of Dpp. We discuss the potential significance of why an inductive process might use a secondary signal whose function is intertwined with that of the primary signal.

Overexpression of zeste white 3 blocks wingless signaling in the Drosophila embryonic midgut

The extracellular signals encoded by the Wnt family of genes regulate growth and differentiation in several developmental processes in both vertebrates and invertebrates. Genetic studies of the signaling pathway of the Drosophila Wnt homologue, Wingless, have identified a number of genes, including zeste white 3, which function to transduce the Wingless signal. zeste white 3 encodes a serine/threonine kinase. We have previously proposed that the Wingless signal is mediated by repression of this kinase activity [E. Siegfried, E.L. Wilder, and N. Perrimon (1994) Nature 367, 76-80]. Here we have tested this hypothesis by overexpressing zeste white 3 in a tissue-specific fashion using the UAS/GAL4 binary expression system. We demonstrate that elevated levels of zeste white 3 in the ectoderm and mesoderm result in phenotypes that resemble a loss of wingless. Overexpression of zeste white 3 in the mesoderm disrupts several Wingless-dependent processes including the specification of a unique cell type in the larval midgut, the formation of the second midgut constriction, and the expression of Wingless target genes Ultrabithorax and decapentaplegic in the mesoderm and labial in the endoderm. Zeste white 3 regulates the stability of Armadillo which is essential for transducing the Wingless signal to the nucleus. We show that zeste white 3 overexpression blocks Wingless signaling through the modulation of Armadillo since expression of a constitutively active form of Armadillo, which is independent of Zeste white 3 regulation, is epistatic to overexpression of zeste white 3.

Wnt and TGFbeta signals subdivide the AbdA Hox domain during Drosophila mesoderm patterning

Hox genes have large expression domains yet control the formation of fine pattern elements at specific locations. We have examined the mechanism underlying subdivision of the abdominal-A (abdA) Hox domain in the visceral mesoderm. AbdA directs formation of an embryonic midgut constriction at a precise location within the broad and uniform abdA expression domain. The constriction divides the abdA domain of the midgut into two chambers, the anterior one producing the Pointed (Pnt) ETS transcription factors and the posterior one the Odd-paired (Opa) zinc finger protein. Transcription of both pnt and opa is activated by abdA but the adjacent non-overlapping patterns are not due to mutual opa-pnt regulation. Near the anterior limit of the abdA domain, two signals, Dpp (a TGFbeta) and Wg (a Wnt), are produced, in adjacent non-overlapping patterns, under Hox control in mesoderm cells. The two signals are known to regulate local mesodermal cell fates and to signal to the endoderm. We find that, in addition, they precisely subdivide the abdA domain: Wg acts upon anterior abdA domain cells to activate pnt transcription, while Dpp is essential in the same region to prevent abdA from activating opa transcription. pnt activation is required to determine the appropriate numbers of mesodermal cells in the third midgut chamber.

A genetic screen for modifiers of Drosophila decapentaplegic signaling identifies mutations in punt, Mothers against dpp and the BMP-7 homologue, 60A

decapentaplegic (dpp) is a Transforming Growth Factor beta (TGF-beta)-related growth factor that controls multiple developmental processes in Drosophila. To identify components involved in dpp signaling, we carried out a genetic screen for dominant enhancer mutations of a hypomorphic allele of thick veins (tkv), a type I receptor for dpp. We recovered new alleles of tkv, punt, Mothers against dpp (Mad) and Medea (Med), all of which are known to mediate dpp signaling. We also recovered mutations in the 60A gene which encodes another TGF-beta-related factor in Drosophila. DNA sequence analysis established that all three 60A alleles were nonsense mutations in the prodomain of the 60A polypeptide. These mutations in 60A caused defects in midgut morphogenesis and fat body differentiation. We present evidence that when dpp signaling is compromised, lowering the level of 60A impairs several dpp-dependent developmental processes examined, including the patterning of the visceral mesoderm, the embryonic ectoderm and the imaginal discs. These results provide the first in vivo evidence for the involvement of 60A in the dpp pathway. We propose that 60A activity is required to maintain optimal signaling capacity of the dpp pathway, possibly by forming biologically active heterodimers with Dpp proteins.

The Hox gene lin-39 is required during C. elegans vulval induction to select the outcome of Ras signaling

The Ras signaling pathway specifies a variety of cell fates in many organisms. However, little is known about the genes that function downstream of the conserved signaling cassette, or what imparts the specificity necessary to cause Ras activation to trigger different responses in different tissues. In C. elegans, activation of the Ras pathway induces cells in the central body region to generate the vulva. Vulval induction takes place in the domain of the Hox gene lin-39. We have found that lin-39 is absolutely required for Ras signaling to induce vulval development. During vulval induction, the Ras pathway, together with basal lin-39 activity, up-regulates lin-39 expression in vulval precursor cells. We find that if lin-39 function is absent at this time, no vulval cell divisions occur. Furthermore, if lin-39 is replaced with the posterior Hox gene mab-5, then posterior structures are induced instead of a vulva. Our findings suggest that in addition to permitting vulval cell divisions to occur, lin-39 is also required to specify the outcome of Ras signaling by selectively activating vulva-specific genes.

Cadherins, catenins and APC protein: interplay between cytoskeletal complexes and signaling pathways

Cadherins play important roles in cell-cell adhesion during tissue differentiation. Cadherins are linked to the actin cytoskeleton by catenins (beta-catenin/armadillo, plakoglobin, and alpha-catenin). Recent results show that beta-catenin also binds to another cytoskeletal complex containing the adenomatous polyposis coli protein and microtubules, and interacts with several signaling pathways that include tyrosine kinases and phosphatases and Wnt/Wingless. Interplay between these cytoskeletal complexes and signaling pathways may regulate morphogenesis.

A CREB-binding site as a target for decapentaplegic signalling during Drosophila endoderm induction

Decapentaplegic (Dpp) is an extracellular signal of the transforming growth factor-beta family with multiple functions during Drosophila development. For example, it plays a key role in the embryo during endoderm induction. During this process, Dpp stimulates transcription of the homeotic genes Ultrabithorax in the visceral mesoderm and labial in the subjacent endoderm. Here, we show that a cAMP response element (CRE) from an Ultrabithorax enhancer mediates Dpp-responsive transcription in the embryonic midgut, and that endoderm expression from a labial enhancer depends on multiple CREs. Furthermore, the Drosophila CRE-binding protein dCREB-B binds to the Ultrabithorax CRE, and ubiquitous expression of a dominant-negative form of dCREB-B suppresses CRE-mediated reporter gene expression and reduces labial expression in the endoderm. Therefore, a CREB protein may act as a nuclear target, or as a partner of a nuclear target, for Dpp signalling in the embryonic midgut.

Armadillo coactivates transcription driven by the product of the Drosophila segment polarity gene dTCF

The vertebrate transcription factors TCF (T cell factor) and LEF (lymphocyte enhancer binding factor) interact with beta-catenin and are hypothesized to mediate Wingless/Wnt signaling. We have cloned a maternally expressed Drosophila TCF family member, dTCF. dTCF binds a canonical TCF DNA motif and interacts with the beta-catenin homolog Armadillo. Previous studies have identified two regions in Armadillo required for Wingless signaling. One of these interacts with dTCF, while the other constitutes a transactivation domain. Mutations in dTCF and expression of a dominant-negative dTCF transgene cause a segment polarity phenotype and affect expression of the Wingless target genes engrailed and Ultrabithorax. Epistasis analysis positions dTCF downstream of armadillo. The Armadillo-dTCF complex mediates Wingless signaling as a bipartite transcription factor.

LEF-1, a nuclear factor coordinating signaling inputs from wingless and decapentaplegic

wingless and decapentaplegic signal during endoderm induction in Drosophila to regulate expression of the homeotic gene Ultrabithorax. Here, we define a minimal wingless response sequence in the midgut enhancer of Ultrabithorax. We show that this sequence is recognized by the murine transcription factor LEF-1 (lymphocyte enhancer binding factor 1) in a ternary complex with armadillo protein, the cytoplasmic target of the wingless signaling pathway. In stable transformants, transcriptional stimulation of the Ultrabithorax enhancer by LEF-1 depends on armadillo. Furthermore, overexpression of LEF-1 bypasses the need for wingless signaling and causes phenotypes in the midgut, notum, and wing that mimic wingless hyperstimulation. Finally, efficient transcriptional stimulation by LEF-1 in the midgut depends also on the decapentaplegic response sequence and is limited spatially by decapentaplegic signaling. Thus, LEF-1 coordinates inputs from multiple positional signals, consistent with its architectural role in regulating the assembly of multiprotein enhancer complexes.

Ectopic expression of wingless in imaginal discs interferes with decapentaplegic expression and alters cell determination

We have expressed the segment polarity gene wingless (wg) ectopically in imaginal discs to examine its regulation of both ventral patterning and transdetermination. By experimentally manipulating the amount of Wg protein, we show that different thresholds of Wg activity elicit different outcomes, which are mediated by regulation of decapentaplegic (dpp) expression and result in alterations in the expression of homeotic genes. A high level of Wg activity leads to loss of all dorsal pattern elements and the formation of a complete complement of ventral pattern elements on the dorsal side of legs, and is correlated with repression of dpp expression. wg expression in dorsal cells of each disc also leads to dose-dependent transdetermination in those cells in homologous discs such as the labial, antennal and leg, but not in cells of dorsally located discs. When dpp expression is repressed by high levels of Wg, transdetermination does not occur, confirming that dpp participates with wg to induce transdetermination. These and other experiments suggest that dorsal expression of wg alters disc patterning and disc cell determination by modulating the expression of dpp. The dose-dependent effects of wg on dpp expression, ventralization of dorsal cells and transdetermination support a model in which wg functions as a morphogen in imaginal discs.

Mothers against dpp encodes a conserved cytoplasmic protein required in DPP/TGF-beta responsive cells

The proteins necessary for signal transduction in cells responding to ligands of the TGF-beta family are largely unknown. We have previously identified Mad (Mothers against dpp), a gene that interacts with the TGF-beta family member encoded by decapentaplegic (dpp) in Drosophila. Assay of Mad's role in the DPP-dependent events of embryonic midgut development demonstrates that Mad is required for any response of the visceral mesoderm or endoderm to DPP signals from the visceral mesoderm. Replacement of the normal DPP promoter with a heterologous (hsp70) promoter fails to restore DPP-dependent responses in Mad mutant midguts. Experiments utilizing Mad transgenes regulated by tissue-specific promoters show that MAD is required specifically in cells responding to DPP. Immunohistochemical studies localize MAD to the cytoplasm in all tissues examined. Experiments in Xenopus embryos demonstrate that Drosophila MAD can function in the signaling pathway of BMP-4, a vertebrate homolog of dpp. Based on these results, we propose that Mad is a highly conserved and essential element of the DPP signal transduction pathway.

A test for cell autonomy, based on di-cistronic messenger translation

We have devised a test for cell autonomy of a gene that is switched on ectopically in a clone of cells, allowing us to ask whether the wild-type activity of this gene can influence neighbouring cells. To switch on the test gene, we used the yeast FRT system, and marked the FRT-generated cell clone by co-expressing beta-galactosidase. Co-expression is achieved by a stretch of 5′ untranslated mRNA from the homeotic gene Ultrabithorax (Ubx), which is inserted between the two coding sequences. We show that this Ubx sequence mediates efficient and reliable di-cistronic mRNA translation in wing imaginal discs of Drosophila. Applying our test to Ubx, we find that ectopic Ubx in wing discs strictly coincides with beta-galactosidase expression. Consequently, wing cells are transformed into cells that appear to be intermediates between wing and haltere cells, contesting the view that homeotic genes act as binary switches.

decapentaplegic, a target gene of the wingless signalling pathway in the Drosophila midgut

dishevelled, shaggy/zeste-white 3 and armadillo are required for transmission of the wingless signal in the Drosophila epidermis. We show that these genes act in the same epistatic order in the embryonic midgut to transmit the wingless signal. In addition to mediating transcriptional stimulation of the homeotic genes Ultrabithorax and labial, they are also required for transcriptional repression of labial by high wingless levels. Efficient labial expression thus only occurs within a window of intermediate wingless pathway activity. Finally, the shaggy/zeste-white 3 mutants revealed that wingless signalling can stimulate decapentaplegic transcription in the absence of Ultrabithorax, identifying decapentaplegic as a target gene of wingless. As decapentaplegic itself is required for wingless expression in the midgut, this represents a positive feed-back loop between two cell groups signalling to each other to stimulate each other's signal production.

Homeobox genes and growth factors in regulation of craniofacial and tooth morphogenesis

Homeobox genes encode a special group of transcription factors that regulate gene expression in the developing embryo. The so-called Hox-cluster genes were first discovered in the Drosophila (fruit fly). They specify the identity of body segments and their patterning along the anteroposterior axis. Other homeobox-containing genes appear to regulate patterning of the head and face. The function of the Msx-1 homeobox gene has been shown to be necessary for tooth development. In general, it is thought that special combinations of homeobox genes specify the patterning of individual structures. Bone morphogenetic proteins (BMPs) are growth factors belonging to the family of transforming growth factor-beta (TGF-beta). BMPs regulate bone and cartilage development, and individual BMPs have been shown to contribute to the shaping of various skeletal elements. BMPs regulate bone and dentin formation also postnatally, and they have therapeutic potential in reparative osteogenesis and odontogenesis. BMPs also act as inductive signals between tissue layers in the embryo, and they regulate the expression of several transcription factors, including homeobox-containing genes. BMP-4 has been identified as an epithelial inductive signal in tooth development. As it is produced by early dental epithelium and regulates tooth-specific gene expression in the dental mesenchyme, including Msx-1 expression, it may be an important signal for the initiation of tooth development.

Wingless induces transdetermination in developing Drosophila imaginal discs

Drosophila imaginal discs, the precursors of the adult fly appendages, have been the subject of intensive developmental studies, particularly on cell determination. Cultured disc fragments are recognized not only for the ability to maintain their determined state through extra cell divisions but also for the ability to transdetermine, or switch to the determined state of a different disc. An understanding of transdetermination at a molecular level will provide further insight into the requirements for maintaining cell determination. We find that ectopic expression of the Drosophila gene wingless induces transdetermination of foreleg imaginal disc cells to wing cells. This transdetermination occurs in foreleg discs of developing larvae without disc fragmentation. The in situ-transdetermining cells localize to the dorsal region of the foreleg disc. This wingless-induced transdetermination event is remarkably similar to the leg-to-wing switch that occurs after leg disc culture. Thus we have identified a new approach to a molecular dissection of transdetermination.