Smoothened-mediated Hedgehog signalling is required for the maintenance of the anterior-posterior lineage restriction in the developing wing of Drosophila

The wing imaginal disc

The Drosophila wing imaginal disc is a tissue of undifferentiated cells that are precursors of the wing and most of the notum of the adult fly. The wing disc first forms during embryogenesis from a cluster of ∼30 cells located in the second thoracic segment, which invaginate to form a sac-like structure. They undergo extensive proliferation during larval stages to form a mature larval wing disc of ∼35,000 cells. During this time, distinct cell fates are assigned to different regions, and the wing disc develops a complex morphology. Finally, during pupal stages the wing disc undergoes morphogenetic processes and then differentiates to form the adult wing and notum. While the bulk of the wing disc comprises epithelial cells, it also includes neurons and glia, and is associated with tracheal cells and muscle precursor cells. The relative simplicity and accessibility of the wing disc, combined with the wealth of genetic tools available in Drosophila, have combined to make it a premier system for identifying genes and deciphering systems that play crucial roles in animal development. Studies in wing imaginal discs have made key contributions to many areas of biology, including tissue patterning, signal transduction, growth control, regeneration, planar cell polarity, morphogenesis, and tissue mechanics.

Disrupting Hedgehog Cardin-Weintraub sequence and positioning changes cellular differentiation and compartmentalization in vivo

Metazoan Hedgehog (Hh) morphogens are essential regulators of growth and patterning at significant distances from their source, despite being produced as N-terminally palmitoylated and C-terminally cholesteroylated proteins, which firmly tethers them to the outer plasma membrane leaflet of producing cells and limits their spread. One mechanism to overcome this limitation is proteolytic processing of both lipidated terminal peptides, called shedding, but molecular target site requirements for effective Hh shedding remained undefined. In this work, by using Drosophila melanogaster as a model, we show that mutagenesis of the N-terminal Cardin-Weintraub (CW) motif inactivates recombinant Hh proteins to variable degrees and, if overexpressed in the same compartment, converts them into suppressors of endogenous Hh function. In vivo, additional removal of N-palmitate membrane anchors largely restored endogenous Hh function, supporting the hypothesis that proteolytic CW processing controls Hh solubilization. Importantly, we also observed that CW repositioning impairs anterior/posterior compartmental boundary maintenance in the third instar wing disc. This demonstrates that Hh shedding not only controls the differentiation of anterior cells, but also maintains the sharp physical segregation between these receiving cells and posterior Hh-producing cells.

Hedgehog mediated degradation of Ihog adhesion proteins modulates cell segregation in Drosophila wing imaginal discs

The Drosophila Hedgehog receptor functions to regulate the essential downstream pathway component, Smoothened, and to limit the range of signaling by sequestering Hedgehog protein signal within imaginal disc epithelium. Hedgehog receptor function requires both Patched and Ihog activity, the latter interchangeably encoded by interference hedgehog (ihog) or brother of ihog (boi). Here we show that Patched and Ihog activity are mutually required for receptor endocytosis and degradation, triggered by Hedgehog protein binding, and causing reduced levels of Ihog/Boi proteins in a stripe of cells at the anterior/posterior compartment boundary of the wing imaginal disc. This Ihog spatial discontinuity may contribute to classically defined cell segregation and lineage restriction at the anterior/posterior wing disc compartment boundary, as suggested by our observations that Ihog activity mediates aggregation of otherwise non-adherent cultured cells and that loss of Ihog activity disrupts wing disc cell segregation, even with downstream genetic rescue of Hedgehog signal response.

Cytoneme-mediated cell-cell contacts for Hedgehog reception

Morphogens regulate tissue patterning through their distribution in concentration gradients. Emerging research establishes a role for specialized signalling filopodia, or cytonemes, in morphogen dispersion and signalling. Previously we demonstrated that Hedgehog (Hh) morphogen is transported via vesicles along cytonemes emanating from signal-producing cells to form a gradient in Drosophila epithelia. However, the mechanisms for signal reception and transfer are still undefined. Here, we demonstrate that cytonemes protruding from Hh-receiving cells contribute to Hh gradient formation. The canonical Hh receptor Patched is localized in these cellular protrusions and Hh reception takes place in membrane contact sites between Hh-sending and Hh-receiving cytonemes. These two sets of cytonemes have similar dynamics and both fall in two different dynamic behaviours. Furthermore, both the Hh co-receptor Interference hedgehog (Ihog) and the glypicans are critical for this cell-cell cytoneme mediated interaction. These findings suggest that the described contact sites might facilitate morphogen presentation and reception.

The Selector Gene apterous and Notch Are Required to Locally Increase Mechanical Cell Bond Tension at the Drosophila Dorsoventral Compartment Boundary

The separation of cells with distinct fates and functions is important for tissue and organ formation during animal development. Regions of different fates within tissues are often separated from another along straight boundaries. These compartment boundaries play a crucial role in tissue patterning and growth by stably positioning organizers. In Drosophila, the wing imaginal disc is subdivided into a dorsal and a ventral compartment. Cells of the dorsal, but not ventral, compartment express the selector gene apterous. Apterous expression sets in motion a gene regulatory cascade that leads to the activation of Notch signaling in a few cell rows on either side of the dorsoventral compartment boundary. Both Notch and apterous mutant clones disturb the separation of dorsal and ventral cells. Maintenance of the straight shape of the dorsoventral boundary involves a local increase in mechanical tension at cell bonds along the boundary. The mechanisms by which cell bond tension is locally increased however remain unknown. Here we use a combination of laser ablation of cell bonds, quantitative image analysis, and genetic mutants to show that Notch and Apterous are required to increase cell bond tension along the dorsoventral compartment boundary. Moreover, clonal expression of the Apterous target gene capricious results in cell separation and increased cell bond tension at the clone borders. Finally, using a vertex model to simulate tissue growth, we find that an increase in cell bond tension at the borders of cell clones, but not throughout the cell clone, can lead to cell separation. We conclude that Apterous and Notch maintain the characteristic straight shape of the dorsoventral compartment boundary by locally increasing cell bond tension.

Regulating mechanical tension at compartment boundaries in Drosophila

During animal development, cells with similar function and fate often stay together and sort out from cells with different fates. In Drosophila wing imaginal discs, cells of anterior and posterior fates are separated by a straight compartment boundary. Separation of anterior and posterior cells requires the homeodomain-containing protein Engrailed, which is expressed in posterior cells. Engrailed induces the expression of the short-range signaling molecule Hedgehog in posterior cells and confines Hedgehog signal transduction to anterior cells. Transduction of the Hedgehog signal in anterior cells is required for the separation of anterior and posterior cells. Previous work showed that this separation of cells involves a local increase in mechanical tension at cell junctions along the compartment boundary. However, how mechanical tension was locally increased along the compartment boundary remained unknown. A recent paper now shows that the difference in Hedgehog signal transduction between anterior and posterior cells is necessary and sufficient to increase mechanical tension. The local increase in mechanical tension biases junctional rearrangements during cell intercalations to maintain the straight shape of the compartment boundary. These data highlight how developmental signals can generate patterns of mechanical tension important for tissue organization.

The expression of the T-box selector gene midline in the leg imaginal disc is controlled by both transcriptional regulation and cell lineage

The Drosophila Tbx20 homologs midline and H15 act as selector genes for ventral fate in Drosophila legs. midline and H15 expression defines the ventral domain of the leg and the two genes are necessary and sufficient for the development of ventral fate. Ventral-specific expression of midline and H15 is activated by Wingless (Wg) and repressed by Decapentaplegic (Dpp). Here we identify VLE, a 5 kb enhancer that drives ventral specific expression in the leg disc that is very similar to midline expression. Subdivision of VLE identifies two regions that mediate both activation and repression and third region that only mediates repression. Loss- and gain-of-function genetic mosaic analysis shows that the activating and repressing regions respond to Wg and Dpp signaling respectively. All three repression regions depend on the activity of Mothers-against-decapentaplegic, a Drosophila r-Smad that mediates Dpp signaling, and respond to ectopic expression of the Dpp target genes optomoter-blind and Dorsocross 3. However, only one repression region is responsive to loss of schnurri, a co-repressor required for direct repression by Dpp-signaling. Thus, Dpp signaling restricts midline expression through both direct repression and through the activation of downstream repressors. We also find that midline and H15 expression are both subject to cross-repression and feedback inhibition. Finally, a lineage analysis indicates that ventral midline-expressing cells and dorsal omb-expressing cells do not mix during development. Together this data indicates that the ventral-specific expression of midline results from both transcriptional regulation and from a lack of cell-mixing between dorsal and ventral cells.

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

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

The small GTPase Rap1 is a modulator of Hedgehog signaling

During development, the evolutionarily conserved Hedgehog (Hh) morphogen provides instructional cues that influence cell fate, cell affinity and tissue morphogenesis. To do so, the Hh signaling cascade must coordinate its activity with other morphogenetic signals. This can occur through engagement of or response to effectors that do not typically function as core Hh pathway components. Given the ability of small G proteins of the Ras family to impact cell survival, differentiation, growth and adhesion, we wanted to determine whether Hh and Ras signaling might intersect during development. We performed genetic modifier tests in Drosophila to examine the ability of select Ras family members to influence Hh signal output, and identified Rap1 as a positive modulator of Hh pathway activity. Our results suggest that Rap1 is activated to its GTP-bound form in response to Hh ligand, and that the GTPase exchange factor C3G likely contributes to this activation. The Rap1 effector Canoe (Cno) also impacts Hh signal output, suggesting that a C3G-Rap1-Cno axis intersects the Hh pathway during tissue morphogenesis.

Signals and mechanics shaping compartment boundaries in Drosophila

During animal development groups of cells with similar fates and functions often stay together and separate from cells with different fates. An example for this cellular behavior is the formation of compartments, groups of cells with similar fates that are separated by sharp boundaries from neighboring groups of cells. Compartments play important roles during patterning by serving as units of growth and gene expression. Boundaries between compartments are associated with organizers that secrete signaling molecules instructing growth and differentiation throughout the tissue. The straight shape of the boundary between compartments is important for maintaining the position and shape of the organizer and thus for precise patterning. The straight shape of compartment boundaries, however, is challenged by cell divisions and cell intercalations that take place in many developing tissues. Early work established a role for selector genes and signaling pathways in setting up and keeping boundaries straight. Recent work in Drosophila has now begun to further unravel the physical and cellular mechanisms that maintain compartment boundaries. Key to the separation of compartments is a local increase of actomyosin-dependent mechanical tension at cell junctions along the boundary. Increased mechanical tension acts as a barrier to cell mixing during cell division and influences cell rearrangements during cell intercalations along the compartment boundary in a way that the straight shape of the boundary is maintained. An important question for the future is how the signaling pathways that maintain the straight shape of compartment boundaries control mechanical tension along these boundaries.

An RNA interference screen for genes required to shape the anteroposterior compartment boundary in Drosophila identifies the Eph receptor

The formation of straight compartment boundaries separating groups of cells with distinct fates and functions is an evolutionarily conserved strategy during animal development. The physical mechanisms that shape compartment boundaries have recently been further elucidated, however, the molecular mechanisms that underlie compartment boundary formation and maintenance remain poorly understood. Here, we report on the outcome of an RNA interference screen aimed at identifying novel genes involved in maintaining the straight shape of the anteroposterior compartment boundary in Drosophila wing imaginal discs. Out of screening 3114 transgenic RNA interference lines targeting a total of 2863 genes, we identified a single novel candidate that interfered with the formation of a straight anteroposterior compartment boundary. Interestingly, the targeted gene encodes for the Eph receptor tyrosine kinase, an evolutionarily conserved family of signal transducers that has previously been shown to be important for maintaining straight compartment boundaries in vertebrate embryos. Our results identify a hitherto unknown role of the Eph receptor tyrosine kinase in Drosophila and suggest that Eph receptors have important functions in shaping compartment boundaries in both vertebrate and insect development.

Ihog and Boi elicit Hh signaling via Ptc but do not aid Ptc in sequestering the Hh ligand

Hedgehog (Hh) proteins are secreted molecules essential for tissue development in vertebrates and invertebrates. Hh reception via the 12-pass transmembrane protein Patched (Ptc) elicits intracellular signaling through Smoothened (Smo). Hh binding to Ptc is also proposed to sequester the ligand, limiting its spatial range of activity. In Drosophila, Interference hedgehog (Ihog) and Brother of ihog (Boi) are two conserved and redundant transmembrane proteins that are essential for Hh pathway activation. How Ihog and Boi activate signaling in response to Hh remains unknown; each can bind both Hh and Ptc and so it has been proposed that they are essential for both Hh reception and sequestration. Using genetic epistasis we established here that Ihog and Boi, and their orthologs in mice, act upstream or at the level of Ptc to allow Hh signal transduction. In the Drosophila developing wing model we found that it is through Hh pathway activation that Ihog and Boi maintain the boundary between the anterior and posterior compartments. We dissociated the contributions of Ptc from those of Ihog/Boi and, surprisingly, found that cells expressing Ptc can retain and sequester the Hh ligand without Ihog and Boi, but that Ihog and Boi cannot do so without Ptc. Together, these results reinforce the central role for Ptc in Hh binding in vivo and demonstrate that, although Ihog and Boi are dispensable for Hh sequestration, they are essential for pathway activation because they allow Hh to inhibit Ptc and thereby relieve its repression of Smo.

A rapid genome-wide microRNA screen identifies miR-14 as a modulator of Hedgehog signaling

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression by binding to sequences within the 3' UTR of mRNAs. Because miRNAs bind to short sequences with partial complementarity, target identification is challenging. To complement the existing target prediction algorithms, we devised a systematic "reverse approach" screening platform that allows the empirical prediction of miRNA-target interactions. Using Drosophila cells, we screened the 3' untranslated regions (3' UTRs) of the Hedgehog pathway genes against a genome-wide miRNA library and identified both predicted and many nonpredicted miRNA-target interactions. We demonstrate that miR-14 is essential for maintaining the proper level of Hedgehog signaling activity by regulating its physiological target, hedgehog. Furthermore, elevated levels of miR-14 suppress Hedgehog signaling activity by cotargeting its apparent nonphysiological targets, patched and smoothened. Altogether, our systematic screening platform is a powerful approach to identifying both physiological and apparent nonphysiological targets of miRNAs, which are relevant in both normal and diseased tissues.

Distinct effects of the mesenchymal dysplasia gene variant of murine Patched-1 protein on canonical and non-canonical Hedgehog signaling pathways

Hedgehog (Hh) signaling requires regulation of the receptor Patched-1 (Ptch1), which, in turn, regulates Smoothened activity (canonical Hh signaling) as well as other non-canonical signaling pathways. The mutant Ptch1 allele mesenchymal dysplasia (mes), which truncates the Ptch1 C terminus, produces a limited spectrum of developmental defects in mice as well as deregulation of canonical Hh signaling in some, but not all, affected tissues. Paradoxically, mes suppresses canonical Hh signaling and binds to Hh ligands with an affinity similar to wild-type mouse Ptch1 (mPtch1). We characterized the distinct activities of the mes variant of mPtch1 mediating Hh signaling through both canonical and non-canonical pathways. We demonstrated that mPtch1 bound c-src in an Hh-regulated manner. Stimulation with Sonic Hedgehog (Shh) of primary mammary mesenchymal cells from wild-type and mes animals activated Erk1/2. Although Shh activated c-src in wild-type cells, c-src was constitutively activated in mes mesenchymal cells. Transient assays showed that wild-type mPtch1, mes, or mPtch1 lacking the C terminus repressed Hh signaling in Ptch1-deficient mouse embryo fibroblasts and that repression was reversed by Shh, revealing that the C terminus was dispensable for mPtch1-dependent regulation of canonical Hh signaling. In contrast to these transient assays, constitutively high levels of mGli1 but not mPtch1 were present in primary mammary mesenchymal cells from mes mice, whereas the expression of mPtch1 was similarly induced in both mes and wild-type cells. These data define a novel signal transduction pathway involving c-src that is activated by the Hh ligands and reveals the requirement for the C terminus of Ptch in regulation of canonical and non-canonical Hh signaling pathways.

Wing tips: The wing disc as a platform for studying Hedgehog signaling

Hedgehog (Hh) signal transduction is necessary for the development of most mammalian tissues and can go awry and cause birth defects or cancer. Hh signaling was initially described in Drosophila, and much of what we know today about mammalian Hh signaling was directly guided by discoveries in the fly. Indeed, Hh signaling is a wonderful example of the use of non-vertebrate model organisms to make basic discoveries that lead to new disease treatment. The first pharmaceutical to treat hyperactive Hh signaling in Basal Cell Carcinoma was released in 2012, approximately 30 years after the isolation of Hh mutants in Drosophila. The study of Hh signaling has been greatly facilitated by the imaginal wing disc, a tissue with terrific experimental advantages. Studies using the wing disc have led to an understanding of Hh ligand processing, packaging into particles for transmission, secretion, reception, signal transduction, target gene activation, and tissue patterning. Here we describe the imaginal wing disc, how Hh patterns this tissue, and provide methods to use wing discs to study Hh signaling in Drosophila. The tools and approaches we highlight form the cornerstone of research efforts in many laboratories that use Drosophila to study Hh signaling, and are essential for ongoing discoveries.

Dampening the signals transduced through hedgehog via microRNA miR-7 facilitates notch-induced tumourigenesis

Analysis of tumorigenesis in Drosophila reveals a tumor-suppressor role for Hedgehog signaling in the context of oncogenic Notch signaling.

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

Growth control mechanisms ensure that organs attain the correct final size, generally averting tumour growth. This control is often linked to spatially confined domains known as organizers (conserved signalling centres), established along the dorsal-ventral and anterior-posterior axes of the organ by the Notch and Hedgehog pathways, respectively. The organizers emit signals that dictate growth, cell fate specification, and differentiation. However, how the distinct organizing signals received are integrated by cells within a growing organ remains a mystery. By studying how Delta-Notch signalling drives tumorigenesis, we identified the conserved microRNA miR-7 as a co-operative element in tumorigenesis mediated by Delta. We found that the cooperation between the microRNA and Delta-Notch pathway converged on the silencing of two obligatory and functionally redundant Hedgehog receptors, interference hedgehog and brother of ihog. Downregulation of other hedgehog pathway genes via RNA interference or genetic mosaics revealed a tumour suppressor role for Hedgehog signalling in the context of the oncogenic Notch pathway. Given the conservation of miR-7, as well as of the Notch and Hedgehog pathways, the conclusions we have drawn from these studies on Drosophila may be applicable to some human cancers.

Differential activity of Drosophila Hox genes induces myosin expression and can maintain compartment boundaries

Compartments are units of cell lineage that subdivide territories with different developmental potential. In Drosophila, the wing and haltere discs are subdivided into anterior and posterior (A/P) compartments, which require the activity of Hedgehog, and into dorsal and ventral (D/V) compartments, needing Notch signaling. There is enrichment in actomyosin proteins at the compartment boundaries, suggesting a role for these proteins in their maintenance. Compartments also develop in the mouse hindbrain rhombomeres, which are characterized by the expression of different Hox genes, a group of genes specifying different structures along their main axis of bilaterians. We show here that the Drosophila Hox gene Ultrabithorax can maintain the A/P and D/V compartment boundaries when Hedgehog or Notch signaling is compromised, and that the interaction of cells with and without Ultrabithorax expression induces high levels of non-muscle myosin II. In the absence of Ultrabithorax there is occasional mixing of cells from different segments. We also show a similar role in cell segregation for the Abdominal-B Hox gene. Our results suggest that the juxtaposition of cells with different Hox gene expression leads to their sorting out, probably through the accumulation of non-muscle myosin II at the boundary of the different cell territories. The increase in myosin expression seems to be a general mechanism used by Hox genes or signaling pathways to maintain the segregation of different groups of cells.

Balancing Hedgehog, a retention and release equilibrium given by Dally, Ihog, Boi and shifted/DmWif

Hedgehog can signal both at a short and long-range, and acts as a morphogen during development in various systems. We studied the mechanisms of Hh release and spread using the Drosophila wing imaginal disc as a model system for polarized epithelium. We analyzed the cooperative role of the glypican Dally, the extracellular factor Shifted (Shf, also known as DmWif), and the Immunoglobulin-like (Ig-like) and Fibronectin III (FNNIII) domain-containing transmembrane proteins, Interference hedgehog (Ihog) and its related protein Brother of Ihog (Boi), in the stability, release and spread of Hh. We show that Dally and Boi are required to prevent apical dispersion of Hh; they also aid Hh recycling for its release along the basolateral part of the epithelium to form a long-range gradient. Shf/DmWif on the other hand facilitates Hh movement restrained by Ihog, Boi and Dally, establishing equilibrium between membrane attachment and release of Hh. Furthermore, this protein complex is part of thin filopodia-like structures or cytonemes, suggesting that the interaction between Dally, Ihog, Boi and Shf/DmWif is required for cytoneme-mediated Hh distribution during gradient formation.

Regulation of cranial morphogenesis and cell fate at the neural crest-mesoderm boundary by engrailed 1

The characterization of mesenchymal progenitors is central to understanding development, postnatal pathology and evolutionary adaptability. The precise identity of the mesenchymal precursors that generate the coronal suture, an important structural boundary in mammalian skull development, remains unclear. We show in mouse that coronal suture progenitors originate from hedgehog-responsive cephalic paraxial mesoderm (Mes) cells, which migrate rapidly to a supraorbital domain and establish a unidirectional lineage boundary with neural crest (NeuC) mesenchyme. Lineage tracing reveals clonal and stereotypical expansion of supraorbital mesenchymal cells to form the coronal suture between E11.0 and E13.5. We identify engrailed 1 (En1) as a necessary regulator of cell movement and NeuC/Mes lineage boundary positioning during coronal suture formation. In addition, we provide genetic evidence that En1 functions upstream of fibroblast growth factor receptor 2 (Fgfr2) in regulating early calvarial osteogenic differentiation, and postulate that it plays an additional role in precluding premature osteogenic conversion of the sutural mesenchyme.

Physical mechanisms shaping the Drosophila dorsoventral compartment boundary

BACKGROUND

Separating cells with distinct identities and fates by straight and sharp compartment boundaries is important for growth and pattern formation during animal development. The physical mechanisms shaping compartment boundaries, however, are not fully understood.

RESULTS

We combine theory and quantitative experiments to investigate the roles of different mechanisms to shape compartment boundaries. Our theoretical work shows that cell elongation created by anisotropic stress, cell proliferation rate, orientation of cell division, and cell bond tension all have distinct effects on the morphology of compartment boundaries during tissue growth. Our experiments using the developing Drosophila wing reveal that the roughness of the dorsoventral compartment boundary is dynamic and that it decreases during development. By measuring tissue relaxation in response to laser ablation of cell bonds at different developmental times, we demonstrate that decreased boundary roughness correlates with increased cell bond tension along the compartment boundary. Finally, by using experimentally determined values for cell bond tension, cell elongation and bias in orientation of cell division in simulations of tissue growth, we can reproduce the main features of the time evolution of the dorsoventral compartment boundary shape.

CONCLUSIONS

Local increase of cell bond tension along the boundary as well as global anisotropies in the tissue contribute to shaping boundaries in cell networks. We propose a simple scenario that combines time-dependent cell bond tension at the boundary, oriented cell division, and cell elongation in the tissue that can account for the main features of the dynamics of the shape of the dorsoventral compartment boundary.

Establishment and maintenance of compartmental boundaries: role of contractile actomyosin barriers

During animal development, tissues and organs are partitioned into compartments that do not intermix. This organizing principle is essential for correct tissue morphogenesis. Given that cell sorting defects during compartmentalization in humans are thought to cause malignant invasion and congenital defects such as cranio-fronto-nasal syndrome, identifying the molecular and cellular mechanisms that keep cells apart at boundaries between compartments is important. In both vertebrates and invertebrates, transcription factors and short-range signalling pathways, such as EPH/Ephrin, Hedgehog, or Notch signalling, govern compartmental cell sorting. However, the mechanisms that mediate cell sorting downstream of these factors have remained elusive for decades. Here, we review recent data gathered in Drosophila that suggest that the generation of cortical tensile forces at compartmental boundaries by the actomyosin cytoskeleton could be a general mechanism that inhibits cell mixing between compartments.

Cell-sorting at the A/P boundary in the Drosophila wing primordium: a computational model to consolidate observed non-local effects of Hh signaling

Non-intermingling, adjacent populations of cells define compartment boundaries; such boundaries are often essential for the positioning and the maintenance of tissue-organizers during growth. In the developing wing primordium of Drosophila melanogaster, signaling by the secreted protein Hedgehog (Hh) is required for compartment boundary maintenance. However, the precise mechanism of Hh input remains poorly understood. Here, we combine experimental observations of perturbed Hh signaling with computer simulations of cellular behavior, and connect physical properties of cells to their Hh signaling status. We find that experimental disruption of Hh signaling has observable effects on cell sorting surprisingly far from the compartment boundary, which is in contrast to a previous model that confines Hh influence to the compartment boundary itself. We have recapitulated our experimental observations by simulations of Hh diffusion and transduction coupled to mechanical tension along cell-to-cell contact surfaces. Intriguingly, the best results were obtained under the assumption that Hh signaling cannot alter the overall tension force of the cell, but will merely re-distribute it locally inside the cell, relative to the signaling status of neighboring cells. Our results suggest a scenario in which homotypic interactions of a putative Hh target molecule at the cell surface are converted into a mechanical force. Such a scenario could explain why the mechanical output of Hh signaling appears to be confined to the compartment boundary, despite the longer range of the Hh molecule itself. Our study is the first to couple a cellular vertex model describing mechanical properties of cells in a growing tissue, to an explicit model of an entire signaling pathway, including a freely diffusible component. We discuss potential applications and challenges of such an approach.

In developing animal tissues, cells can often re-arrange locally and mix relatively freely. However, in some stereotypic and crucially important instances during body development, cells will strictly not intermingle, and instead form sharp boundaries along which they will sort out from each other. This mechanism helps organisms to establish signaling centers and to maintain distinct cellular identities. Often, cells at such boundaries will remain in close physical contact and are morphologically alike. Thus, the boundary itself can be difficult to observe unless the expression status of specific marker genes is monitored experimentally. How are these ‘compartment boundaries’ established? Here we devise a computational model that aims to describe one such boundary in a well-studied animal tissue: the developing wing primordium of Drosophila melanogaster. We model the production, diffusion and local sensing of an essential signaling molecule, the Hedgehog protein. We reveal one possible mechanism by which Hedgehog sensing can influence the mechanical properties of cells, and compare the simulated outcome to observations in experimentally perturbed, actual wing discs. Our relatively simple model suffices to establish a straight and stable compartment boundary.

Gbx2 and Fgf8 are sequentially required for formation of the midbrain-hindbrain compartment boundary

In vertebrates, the common expression border of two homeobox genes, Otx2 and Gbx2, demarcates the prospective midbrain-hindbrain border (MHB) in the neural plate at the end of gastrulation. The presence of a compartment boundary at the MHB has been demonstrated, but the mechanism and timing of its formation remain unclear. We show by genetic inducible fate mapping using a Gbx2(CreER) knock-in mouse line that descendants of Gbx2(+) cells as early as embryonic day (E) 7.5 do not cross the MHB. Without Gbx2, hindbrain-born cells abnormally populate the entire midbrain, demonstrating that Gbx2 is essential for specifying hindbrain fate. Gbx2(+) and Otx2(+) cells segregate from each other, suggesting that mutually exclusive expression of Otx2 and Gbx2 in midbrain and hindbrain progenitors is responsible for cell sorting in establishing the MHB. The MHB organizer gene Fgf8, which is expressed as a sharp transverse band immediately posterior to the lineage boundary at the MHB, is crucial in maintaining the lineage-restricted boundary after E7.5. Partial deletion of Fgf8 disrupts MHB lineage separation. Activation of FGF pathways has a cell-autonomous effect on cell sorting in midbrain progenitors. Therefore, Fgf8 from the MHB may signal the nearby mesencephalic cells to impart distinct cell surface characteristics or induce local cell-cell signaling, which consequently prevents cell movements across the MHB. Our findings reveal the distinct function of Gbx2 and Fgf8 in a stepwise process in the development of the compartment boundary at the MHB and that Fgf8, in addition to its organizer function, plays a crucial role in maintaining the lineage boundary at the MHB by restricting cell movement.

Ihog and Boi are essential for Hedgehog signaling in Drosophila

Background

The Hedgehog (Hh) signaling pathway is important for the development of a variety of tissues in both vertebrates and invertebrates. For example, in developing nervous systems Hh signaling is required for the normal differentiation of neural progenitors into mature neurons. The molecular signaling mechanism underlying the function of Hh is not fully understood. In Drosophila, Ihog (Interference hedgehog) and Boi (Brother of Ihog) are related transmembrane proteins of the immunoglobulin superfamily (IgSF) with orthologs in vertebrates. Members of this IgSF subfamily have been shown to bind Hh and promote pathway activation but their exact role in the Hh signaling pathway has remained elusive. To better understand this role in vivo, we generated loss-of-function mutations of the ihog and boi genes, and investigated their effects in developing eye and wing imaginal discs.

Results

While mutation of either ihog or boi alone had no discernible effect on imaginal tissues, cells in the developing eye disc that were mutant for both ihog and boi failed to activate the Hh pathway, causing severe disruption of photoreceptor differentiation in the retina. In the anterior compartment of the developing wing disc, where different concentrations of the Hh morphogen elicit distinct cellular responses, cells mutant for both ihog and boi failed to activate responses at either high or low thresholds of Hh signaling. They also lost their affinity for neighboring cells and aberrantly sorted out from the anterior compartment of the wing disc into posterior territory. We found that ihog and boi are required for the accumulation of the essential Hh signaling mediator Smoothened (Smo) in Hh-responsive cells, providing evidence that Ihog and Boi act upstream of Smo in the Hh signaling pathway.

Conclusions

The consequences of boi;ihog mutations for eye development, neural differentiation and wing patterning phenocopy those of smo mutations and uncover an essential role for Ihog and Boi in the Hh signaling pathway.

Mechanosensilla in the adult abdomen of Drosophila: engrailed and slit help to corral the peripheral sensory axons into segmental bundles

The abdomen of adult Drosophila bears mechanosensory bristles with axons that connect directly to the CNS, each hemisegment contributing a separate nerve bundle. Here, we alter the amount of Engrailed protein and manipulate the Hedgehog signalling pathway in clones of cells to study their effects on nerve pathfinding within the peripheral nervous system. We find that high levels of Engrailed make the epidermal cells inhospitable to bristle neurons; sensory axons that are too near these cells are either deflected or fail to extend properly or at all. We then searched for the engrailed-dependent agent responsible for these repellent properties. We found slit to be expressed in the P compartment and, using genetic mosaics, present evidence that Slit is the responsible molecule. Blocking the activity of the three Robo genes (putative receptors for Slit) with RNAi supported this hypothesis. We conclude that, during normal development, gradients of Slit protein repel axons away from compartment boundaries - in consequence, the bristles from each segment send their nerves to the CNS in separated sets.

Neural-specific α3-fucosylation of N-linked glycans in the Drosophila embryo requires fucosyltransferase A and influences developmental signaling associated with O-glycosylation

Addition of fucose (Fuc) to glycoprotein N-linked glycans or in O-linkage directly to Ser/Thr residues modulates specific cell-cell interactions and cell signaling events. Vertebrates and invertebrates add Fuc in α6-linkage to the reducing terminal N-acetylglucosamine residue of N-glycans. In Drosophila and other invertebrates, Fuc can also be added in α3-linkage to the same residue. These difucosylated N-glycans are recognized by anti-horseradish peroxidase (anti-HRP) antisera, providing a well-established marker for insect neural tissue. To understand the mechanisms and consequences of tissue-specific glycan expression, we identified a single α3-fucosyltransferase (FucTA) that produces the anti-HRP epitope in Drosophila embryos. FucTA transcripts are temporally and spatially restricted to cells that express the anti-HRP epitope and are missing in a mutant that lacks neural α3-fucosylation. Transgenic expression of FucTA, but not of any other candidate α3-fucosyltransferase, rescues the anti-HRP epitope in the embryonic nervous system of this mutant. Mass spectrometric characterization of the N-glycans of Drosophila embryos overexpressing FucTA confirms that this enzyme is indeed responsible for the biosynthesis of difucosylated glycans in vivo. Whereas ectopic expression of FucTA in the larval wing disc produces mild wing notching, the heterochronic, pan-neural expression of FucTA in early differentiating neurons generates neurogenic and cell migration phenotypes; this latter effect is associated with reduced GDP-Fuc levels in the embryo and indicates that the diversion of fucosylation resources towards fucosylation of N-glycans has an impact on developmental signaling associated with O-fucosylation.

Compartment boundaries: sorting cells with tension

The subdivision of proliferating tissues into groups of non-intermingling sets of cells, termed compartments, is a common process of animal development. Signaling between adjacent compartments induces the local expression of morphogens that pattern the surrounding tissue. Sharp and straight boundaries between compartments stabilize the source of such morphogens during tissue growth and, thus, are of crucial importance for pattern formation. Signaling pathways required to maintain compartment boundaries have been identified, yet the physical mechanisms that maintain compartment boundaries remained elusive. Recent data now show that a local increase in actomyosin-based mechanical tension on cell bonds is vital for maintaining compartment boundaries in Drosophila.

Overexpression of smoothened activates the sonic hedgehog signaling pathway in pancreatic cancer-associated fibroblasts

PURPOSE

Accumulating evidence suggests that cancer-associated stromal fibroblasts (CAF) contribute to tumor growth by actively communicating with cancer cells. Our aim is to identify signaling pathways involved in tumor-stromal cell interactions in human pancreatic cancer.

EXPERIMENTAL DESIGN

We established primary fibroblast cultures from human pancreatic adenocarcinomas and nonneoplastic pancreas tissues. To identify differentially expressed genes in CAFs, we did gene expression profiling of human pancreatic CAFs and nonneoplastic pancreatic fibroblasts.

RESULTS

The Hedgehog receptor Smoothened (SMO) was upregulated in CAFs relative to control fibroblasts. CAFs expressing SMO could transduce the Sonic hedgehog signal to activate Gli1 expression, and small interfering RNA knockdown of SMO blocked the induction of Gli1 in these cells. Stromal fibroblasts of human primary pancreatic adenocarcinomas overexpressed Smo compared with normal pancreatic fibroblasts.

CONCLUSIONS

These findings implicate overexpression of Smo as a mechanism for the activation of Hedgehog signaling in human pancreatic CAFs and suggest that stromal cells may be a therapeutic target for Smo antagonists in pancreatic cancer.

An actomyosin-based barrier inhibits cell mixing at compartmental boundaries in Drosophila embryos

Partitioning tissues into compartments that do not intermix is essential for the correct morphogenesis of animal embryos and organs. Several hypotheses have been proposed to explain compartmental cell sorting, mainly differential adhesion, but also regulation of the cytoskeleton or of cell proliferation. Nevertheless, the molecular and cellular mechanisms that keep cells apart at boundaries remain unclear. Here we demonstrate, in early Drosophila melanogaster embryos, that actomyosin-based barriers stop cells from invading neighbouring compartments. Our analysis shows that cells can transiently invade neighbouring compartments, especially when they divide, but are then pushed back into their compartment of origin. Actomyosin cytoskeletal components are enriched at compartmental boundaries, forming cable-like structures when the epidermis is mitotically active. When MyoII (non-muscle myosin II) function is inhibited, including locally at the cable by chromophore-assisted laser inactivation (CALI), in live embryos, dividing cells are no longer pushed back, leading to compartmental cell mixing. We propose that local regulation of actomyosin contractibility, rather than differential adhesion, is the primary mechanism sorting cells at compartmental boundaries.

Wing vein patterning in Drosophila and the analysis of intercellular signaling

The positioning and elaboration of ectodermal veins in the wing of Drosophila melanogaster rely on widely utilized developmental signals, including those mediated by EGF, BMP, Hedgehog, Notch, and Wnt. Analysis of vein patterning mutants, using the molecular and genetic mosaic techniques available in Drosophila, has provided important insights into how a combination of short-range and long-range signaling can pattern a simple epidermal tissue. Moreover, venation has become a powerful system for isolating and analyzing novel components in these signaling pathways. I here review the basic events of vein patterning and give examples of how changes in venation have been used to identify important features of cell signaling pathways.

Hedgehog signal activation in gastric pit cell and in diffuse-type gastric cancer

BACKGROUND & AIMS

Sonic hedgehog (SHH) and Indian hedgehog (IHH) have cell-specific actions in some organs. Only SHH has been shown to regulate parietal cell differentiation. This study examined whether SHH, 2 other ligands IHH and Desert hedgehog, and receptors or downstream targets are expressed in normal gastric epithelium or in intestinal and diffuse-type gastric cancers. The effects of a Hedgehog (Hh) inhibitor, cyclopamine, were assessed in primary gastric epithelium cultures and gastric cancer cell lines.

METHODS

Reverse-transcription polymerase chain reaction and immunostaining compared expression and localization of Hh signaling molecules and phenotypic markers of pit, neck, and gland cells in situ and in cultured cells treated with cyclopamine. Bromodeoxyuridine staining assessed the effects of cyclopamine on proliferation.

RESULTS

Hh signaling molecules were expressed differentially in pit, neck, and gland cells. IHH co-expressed with most downstream targets in the pit. IHH, SHH, Patched (PTCH), Smoothened (SMO), and downstream targets were expressed more frequently and highly in the diffuse as compared with intestinal type cancers. In diffuse cancers, IHH was expressed in cells with an epithelial phenotype and SHH in cells with a mesenchymal phenotype. Cyclopamine reduced the number of cells with a pit phenotype but not a gland phenotype in primary cultures. Cyclopamine had particularly potent effects of inhibiting the growth of cell lines that expressed high levels of SMO.

CONCLUSIONS

Expression of IHH and downstream targets correlates with pit cells. IHH and SMO may be useful biomarkers of diffuse cancers that may show growth inhibition with Hh antagonists such as cyclopamine.

Localization and requirement for Myosin II at the dorsal-ventral compartment boundary of theDrosophila wing

As organisms develop, their tissues can become separated into distinct cell populations through the establishment of compartment boundaries. Compartment boundaries have been discovered in a wide variety of tissues, but in many cases the molecular mechanisms that separate cells remain poorly understood. In the Drosophila wing, a stripe of Notch activation maintains the dorsal-ventral compartment boundary, through a process that depends on the actin cytoskeleton. Here, we show that the dorsal-ventral boundary exhibits a distinct accumulation of Myosin II, and that this accumulation is regulated downstream of Notch signaling. Conversely, the dorsal-ventral boundary is depleted for the Par-3 homologue Bazooka. We further show that mutations in the Myosin heavy chain subunit encoded by zipper can impair dorsal-ventral compartmentalization without affecting anterior-posterior compartmentalization. These observations identify a distinct accumulation and requirement for Myosin activity in dorsal-ventral compartmentalization, and suggest a novel mechanism in which contractile tension along an F-actin cable at the compartment boundary contributes to compartmentalization.

Compartments and their boundaries in vertebrate brain development

Fifteen years ago, cell lineage restriction boundaries were discovered in the embryonic vertebrate hindbrain, subdividing it into a series of cell-tight compartments (known as rhombomeres). Compartition, together with segmentally reiterative neuronal architecture and the nested expression of Hox genes, indicates that the hindbrain has a truly metameric organization. This finding initiated a search for compartments in other regions of the developing brain. The results of recent studies have clarified where compartment boundaries exist, have shed light on molecular mechanisms that underlie their formation and have revealed an important function of these boundaries: the positioning and stabilization of local signalling centres.

Long-range Dpp signaling is regulated to restrict BMP signaling to a crossvein competent zone

The sensitivity of the crossveins of the Drosophila wing to reductions in BMP signaling provides a valuable system for characterizing members of this signaling pathway. We demonstrate here two reasons for that sensitivity. First, the initial stage of posterior crossvein development depends on BMP signaling but is independent of EGF signaling. This is the opposite of the longitudinal veins, which rely of EGF signaling for their initial specification. Second, BMP signaling in the posterior crossvein depends on Decapentaplegic (Dpp) at a stage when it is being produced in the longitudinal veins. Thus, the posterior crossvein will be especially vulnerable to reductions in the levels or range of Dpp signaling. We investigated the roles of the BMP receptor Thickveins (Tkv) and the BMP inhibitor Short gastrulation (Sog) in allowing this long-range signaling. Expression of both is downregulated in the developing posterior crossvein. The Tkv downregulation depends on BMP signaling and may provide a positive feedback by allowing the spread of Dpp. The Sog downregulation is independent of BMP signaling; Sog misexpression experiments indicate that this prepattern is essential for posterior crossvein development. However, this requirement can be overridden by co-misexpression of the BMP agonist Cv-2, indicating the presence of as yet unknown cues; we discuss possible candidates.

Cadherin Cad99C is regulated by Hedgehog signaling in Drosophila

The subdivision of the Drosophila wing imaginal disc into anterior and posterior compartments requires a transcriptional response to Hedgehog signaling. However, the genes regulated by Hedgehog signal transduction that mediate the segregation of anterior and posterior cells have not been identified. Here, we molecularly characterize the previously predicted gene cad99C and show that it is regulated by Hedgehog signaling. Cad99C encodes a transmembrane protein with a molecular weight of approximately 184 kDa that contains 11 cadherin repeats in its extracellular domain and a conserved type I PDZ-binding site at its C-terminus. The levels of cad99C RNA and protein are low throughout the wing imaginal disc. However, in the pouch region, these levels are elevated in a strip of anterior cells along the A/P boundary where the Hedgehog signal is transduced. Ectopic expression of Hedgehog, or the Hedgehog-regulated transcription factor Cubitus interruptus, induces high-level expression of Cad99C. Conversely, blocking Hedgehog signal transduction by either inactivating Smoothened or Cubitus interruptus reduces high-level Cad99C expression. Finally, by analyzing mutant clones of cells, we show that Cad99C is not essential for cell segregation at the A/P boundary. We conclude that cad99C is a novel Hedgehog-regulated gene encoding a member of the cadherin superfamily in Drosophila.

Echinoid Is a Component of Adherens Junctions That Cooperates with DE-Cadherin to Mediate Cell Adhesion

Echinoid is an immunoglobulin domain-containing transmembrane protein that modulates cell-cell signaling by Notch and the EGF receptors. We show that, in the Drosophila wing disc epithelium, Echinoid is a component of adherens junctions that cooperates with DE-Cadherin in cell adhesion. Echinoid and beta-catenin (a DE-Cadherin interacting protein) each possess a C-terminal PDZ domain binding motif that binds to Bazooka/PAR-3; these motifs redundantly position Bazooka to adherens junctions. Echinoid also links to actin filaments by binding to Canoe/AF-6/afadin. Moreover, interfaces between Echinoid- and Echinoid+ cells, like those between DE-Cadherin- and DE-Cadherin+ cells, are deficient in adherens junctions and form actin cables. These characteristics probably facilitate the strong sorting behavior of cells that lack either of these cell-adhesion molecules. Finally, cells lacking either Echinoid or DE-Cadherin accumulate a high density of the reciprocal protein, further suggesting that Echinoid and DE-Cadherin play similar and complementary roles in cell adhesion.

The role of Dpp signaling in maintaining the Drosophila anteroposterior compartment boundary

The subdivision of the developing Drosophila wing into anterior (A) and posterior (P) compartments is important for its development. The activities of the selector genes engrailed and invected in posterior cells and the transduction of the Hedgehog signal in anterior cells are required for maintaining the A/P boundary. Based on a previous study, it has been proposed that the signaling molecule Decapentaplegic (Dpp) is also important for this function by signaling from anterior to posterior cells. However, it was not known whether and in which cells Dpp signal transduction was required for maintaining the A/P boundary. Here, we have investigated the role of the Dpp signal transduction pathway and the epistatic relationship of Dpp and Hedgehog signaling in maintaining the A/P boundary by clonal analysis. We show that a transcriptional response to Dpp involving the T-box protein Optomotor-blind is required to maintain the A/P boundary. Further, we find that Dpp signal transduction is required in anterior cells, but not in posterior cells, indicating that anterior to posterior signaling by Dpp is not important for maintaining the A/P boundary. Finally, we provide evidence that Dpp signaling acts downstream of or in parallel with Hedgehog signaling to maintain the A/P boundary. We propose that Dpp signaling is required for anterior cells to interpret the Hedgehog signal in order to specify segregation properties important for maintaining the A/P boundary.

Shifted, the Drosophila Ortholog of Wnt Inhibitory Factor-1, Controls the Distribution and Movement of Hedgehog

We here identify and characterize an extracellular modulator of Hedgehog signaling in Drosophila, Shifted. Shifted is required for high levels of long-range signaling in the developing wing imaginal disc. Surprisingly, shifted encodes the only Drosophila ortholog of the secreted vertebrate protein Wnt Inhibitory Factor-1 (WIF-1), whose known role is to bind to extracellular Wnts and inhibit their activity. However, Shifted does not regulate Hedgehog signaling by affecting Wingless or Wnt signaling. We show instead that Shifted is a secreted protein that acts over a long distance and is required for the normal accumulation of Hh protein and its movement in the wing. Our data further indicate that Shf interacts with Hh and the heparan sulfate proteoglycans. Therefore, we propose that Shf stabilizes the interaction between Hh and the proteoglycans, an unexpected role for a member of the WIF-1 family.

Hedgehog signaling controls Soma-Germen interactions duringDrosophila ovarian morphogenesis

The genetic analysis of Drosophila adult oogenesis has provided insights into the molecular mechanisms that control cell proliferation, differentiation, migration, and intercellular signaling. However, little is known about the larval and pupal cellular events leading to the formation of the highly organized adult ovary, which is composed of ovarioles each containing germline cells enveloped by specialized somatic cells. We describe here the presence of ovarioles devoid of any germ cells in adult females mutant for fused, which encodes a Hedgehog signal transducing serine/threonine kinase. We show that this phenotype corresponds to a requirement for fused function for the organization of germ cells with respect to ovarian somatic cells during ovariole formation specifically during pupal stages and provide some evidence by means of clonal analysis suggesting that fused function may be necessary in the germline. hedgehog is expressed specifically in somatic terminal filament cells in pupal ovaries, and females bearing hedgehog strong loss-of-function mutations also exhibit aberrant germ cell distribution and formation of agametic ovarioles. These results indicate a positive role for Fused in the transduction of somatic Hedgehog signaling instructing ovariole morphogenesis. We also provide evidence for the use of noncanonical Hedgehog signal transducer(s) within germline cells.

Developmental Biology: Notching the Hindbrain

One of the best-characterized lineage restrictions in developing vertebrates occurs between adjacent -rhombomeres of the hindbrain. It was recently shown that cells at the boundaries of zebrafish rhombomeres also differ from non-boundary cells in their migratory abilities, a difference driven by Notch signaling.

Development of the genitalia in Drosophila melanogaster

The imaginal discs of Drosophila melanogaster are an excellent material with which to analyze how signaling pathways and Hox genes control growth and pattern formation. The study of one of these discs, the genital disc, offers, in addition, the possibility of integrating the sex determination pathway into this analysis. This disc, whose growth and shape are sexually dimorphic, gives rise to the genitalia and analia, the more posterior structures of the fruit fly. Male genitalia, which develop from the ninth abdominal segment, and female genitalia, which develop mostly from the eighth one, display a characteristic array of structures. We will review here some recent findings about the development of these organs. As in other discs, different signaling pathways establish the positional information in the genital primordia. The Hox and sex determination genes modify these signaling routes at different levels to specify the particular growth and differentiation of male and female genitalia.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

A genetic screen for hedgehog targets involved in the maintenance of the Drosophila anteroposterior compartment boundary

The development of multicellular organisms requires the establishment of cell populations with different adhesion properties. In Drosophila, a cell-segregation mechanism underlies the maintenance of the anterior (A) and posterior (P) compartments of the wing imaginal disc. Although engrailed (en) activity contributes to the specification of the differential cell affinity between A and P cells, recent evidence suggests that cell sorting depends largely on the transduction of the Hh signal in A cells. The activator form of Cubitus interruptus (Ci), a transcription factor mediating Hh signaling, defines anterior specificity, indicating that Hh-dependent cell sorting requires Hh target gene expression. However, the identity of the gene(s) contributing to distinct A and P cell affinities is unknown. Here, we report a genetic screen based on the FRT/FLP system to search for genes involved in the correct establishment of the anteroposterior compartment boundary. By using double FRT chromosomes in combination with a wing-specific FLP source we screened 250,000 mutagenized chromosomes. Several complementation groups affecting wing patterning have been isolated, including new alleles of most known Hh-signaling components. Among these, we identified a class of patched (ptc) alleles exhibiting a novel phenotype. These results demonstrate the value of our setup in the identification of genes involved in distinct wing-patterning processes.

A direct requirement for Hedgehog signaling for normal specification of all ventral progenitor domains in the presumptive mammalian spinal cord

The hedgehog signaling pathway organizes the developing ventral neural tube by establishing distinct neural progenitor fates along the dorsoventral axis. Smoothened (Smo) is essential for all Hedgehog (Hh) signaling, and genetic inactivation of Smo cells autonomously blocks the ability of cells to transduce the Hh signal. Using a chimeric approach, we examined the behavior of Smo null mutant neural progenitor cells in the developing vertebrate spinal cord, and we show that direct Hh signaling is essential for the specification of all ventral progenitor populations. Further, Hh signaling extends into the dorsal half of the spinal cord including the intermediate Dbx expression domain. Surprisingly, in the absence of Sonic hedgehog (Shh), we observe the presence of a Smo-dependent Hh signaling activity operating in the ventral half of the spinal cord that most likely reflects Indian hedgehog (Ihh) signaling originating from the underlying gut endoderm. Comparative studies of Shh, Smo, and Gli3 single and compound mutants reveal that Hh signaling acts in part to specify neural cell identity by counteracting the repressive action of Gli3 on p0, p1, p2, and pMN formation. However, whereas these cell identities are restored in Gli3/Smo compound mutants, correct stratification of the rescued ventral cell types is lost. Thus, Hh signaling is essential for organizing ventral cell pattern, possibly through the control of differential cell affinities.

Design and constraints of the Drosophila segment polarity module: robust spatial patterning emerges from intertwined cell state switches

The Drosophila segment polarity genes constitute the last tier in the segmentation cascade; their job is to maintain the boundaries between parasegments and provide positional "read-outs" within each parasegment for the entire developmental history of the animal. These genes constitute a relatively well-defined network with a relatively well-understood patterning task. In a previous publication (von Dassow et al. 2000. Nature 406:188-192) we showed that a computer model predicts the segment polarity network to be a robust boundary-making device. Here we elaborate those findings. First, we explore the constraints among parameters that govern the network model. Second, we test architectural variants of the core network, and show that the network tolerates a wide variety of adjustments in design. Third, we evaluate several topologically identical models that incorporate more or less molecular detail, finding that more-complex models perform noticeably better than simplified ones. Fourth, we discuss two instances in which the failure of the network model to behave in a life-like fashion highlights mechanistic details that need further experimental investigation. We conclude with an explanation of how the segment polarity network can be understood as an interwoven conspiracy of simple dynamical elements, several bistable switches and a homeostat. The robustness with which the network as a whole maintains a spatial regime of stable cell state emerges from generic dynamical properties of these simple elements.

A new nuclear component of the Wnt signalling pathway

The Wnt signalling pathway is pivotal in normal and malignant development. A key effector is Armadillo (Arm)/beta-catenin, which functions with TCF to transcribe Wnt target-genes. Here, we report the discovery of pygopus (pygo), whose mutant phenotypes specifically mimic loss-of-Wingless (Wg) signalling. pygo is required for dTCF-mediated transcription, but not for Wg-induced stabilization of Arm. Pygo is a nuclear protein that is found in a complex with Arm in vivo. Humans possess two Pygo proteins, both of which are required for TCF-mediated transcription in colorectal cancer cells. The presence of a PHD domain implicates Pygo proteins in a chromatin-related function, and we propose that they mediate chromatin access to TCF or Arm/beta-catenin.