Clonal analysis of a genetically caused duplication of the anterior wing in Drosophila melanogaster

Hedgehog signaling

Hedgehog (Hh) proteins constitute one family of a small number of secreted signaling proteins that together regulate multiple aspects of animal development, tissue homeostasis and regeneration. Originally uncovered through genetic analyses in Drosophila, their subsequent discovery in vertebrates has provided a paradigm for the role of morphogens in positional specification. Most strikingly, the Sonic hedgehog protein was shown to mediate the activity of two classic embryonic organizing centers in vertebrates and subsequent studies have implicated it and its paralogs in a myriad of processes. Moreover, dysfunction of the signaling pathway has been shown to underlie numerous human congenital abnormalities and diseases, especially certain types of cancer. This review focusses on the genetic studies that uncovered the key components of the Hh signaling system and the subsequent, biochemical, cell and structural biology analyses of their functions. These studies have revealed several novel processes and principles, shedding new light on the cellular and molecular mechanisms underlying cell-cell communication. Notable amongst these are the involvement of cholesterol both in modifying the Hh proteins and in activating its transduction pathway, the role of cytonemes, filipodia-like extensions, in conveying Hh signals between cells; and the central importance of the Primary Cilium as a cellular compartment within which the components of the signaling pathway are sequestered and interact.

From Drosophila segmentation to human cancer therapy

First described in Drosophila, Hedgehog signalling is a key regulator of embryonic development and tissue homeostasis and its dysfunction underlies a variety of human congenital anomalies and diseases. Although now recognised as a major target for cancer therapy as well as a mediator of directed stem cell differentiation, the unveiling of the function and mechanisms of Hedgehog signalling was driven largely by an interest in basic developmental biology rather than clinical need. Here, I describe how curiosity about embryonic patterning led to the identification of the family of Hedgehog signalling proteins and the pathway that transduces their activity, and ultimately to the development of drugs that block this pathway.

Drosophila Segment Polarity Mutants and the Rediscovery of the Hedgehog Pathway Genes

The Nüsslein-Volhard and Wieschaus screen for mutations disrupting the segmentation of the Drosophila embryo revolutionized developmental genetics, leading the way to the identification of many of the transcription factors and signaling pathways that orchestrate development, not just in the fruit fly but across the animal kingdom. The Hedgehog signaling pathway is a case in point: yet remarkably, all but one of the genes encoding the Hedgehog pathway components-including Hedgehog itself-had previously been discovered, in some cases decades earlier. Here I review the original identification of these genes and consider why their significance remained obscure until the Nobel Prize winning screen.

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.

Hedgehog signalling: Kif7 is not that fishy after all

Recent reports examining the mammalian kinesin relative Kif7 highlight the conserved role for microtubule motor proteins in Drosophila and vertebrate Hedgehog signalling. Mammalian Kif7 action centres at the primary cilium, an organelle absent from Drosophila. These studies raise interesting questions about the coupling of microtubule trafficking to the Hedgehog pathway.

Modulation of the Suppressor of fused protein regulates the Hedgehog signaling pathway in Drosophila embryo and imaginal discs

The Suppressor of fused (Su(fu)) protein is known to be a negative regulator of Hedgehog (Hh) signal transduction in Drosophila imaginal discs and embryonic development. It is antagonized by the kinase Fused (Fu) since Su(fu) null mutations fully suppress the lack of Fu kinase activity. In this study, we overexpressed the Su(fu) gene in imaginal discs and observed opposing effects depending on the position of the cells, namely a repression of Hh target genes in cells receiving Hh and their ectopic expression in cells not receiving Hh. These effects were all enhanced in a fu mutant context and were suppressed by cubitus interruptus (Ci) overexpression. We also show that the Su(fu) protein is poly-phosphorylated during embryonic development and these phosphorylation events are altered in fu mutants. This study thus reveals an unexpected role for Su(fu) as an activator of Hh target gene expression in absence of Hh signal. Both negative and positive roles of Su(fu) are antagonized by Fused. Based on these results, we propose a model in which Su(fu) protein levels and isoforms are crucial for the modulation of the different Ci states that control Hh target gene expression.

Differential regulation of Hedgehog target gene transcription by Costal2 and Suppressor of Fused

The mechanism by which the secreted signaling molecule Hedgehog (Hh) elicits concentration-dependent transcriptional responses from cells is not well understood. In the Drosophila wing imaginal disc, Hh signaling differentially regulates the transcription of target genes decapentaplegic (dpp), patched (ptc) and engrailed (en) in a dose-responsive manner. Two key components of the Hh signal transduction machinery are the kinesin-related protein Costal2 (Cos2) and the nuclear protein trafficking regulator Suppressor of Fused [Su(fu)]. Both proteins regulate the activity of the transcription factor Cubitus interruptus (Ci) in response to the Hh signal. We have analyzed the activities of mutant forms of Cos2 in vivo and found effects on differential target gene transcription. A point mutation in the motor domain of Cos2 results in a dominant-negative form of the protein that derepresses dpp but not ptc. Repression of ptc in the presence of the dominant-negative form of Cos2 requires Su(fu), which is phosphorylated in response to Hh in vivo. Overexpression of wild-type or dominant-negative cos2 represses en. Our results indicate that differential Hh target gene regulation can be accomplished by differential sensitivity of Cos2 and Su(Fu) to Hh.

A gain-of-function mutant of patched dissects different responses to the hedgehog gradient

The Hedgehog (Hh) signal has an inductive role during Drosophila development. Patched is part of the Hedgehog-receptor complex and shows a repressive function on the signaling cascade, which is alleviated in the presence of Hh. Herein, we identify the first dominant gain-of-function allele of patched, Confused (patched(Con)). Analysis of the patched(Con) allele led us to uncover novel features of the reception and function of the Hh signal. At least three different regions of gene expression were identified and a gradient of cell affinities was established in response to Hh. A new state of Cubitus interruptus activity responsible for the activation of araucan and caupolican genes of the iroquois complex, independent of Fused kinase function, was shown. In the disc, patched(Con) behaved like fused mutants and was rescued by Suppressor of fused mutations. However, fused mutants are embryonic lethal while patched(Con) is not, suggesting that Patched could interpret Hedgehog signaling differently in the embryo and in the adult.

Transducing Hedgehog: the story so far

The secreted proteins of the Hedgehog family have been implicated in many different processes in vertebrate development including cartilage differentiation, myotome and sclerotome specification, hair follicle development, limb morphogenesis and the specification of different neuronal cell types. In addition, the aberrant activation of the Hedgehog pathway has been identified as the likely cause of a number of tumours in humans including basal cell carcinomas (BCCs) and primitive neurectodermal tumours (PNETs). Elucidating the mechanisms by which Hedgehog signals are transduced will thus have widespread implications for our understanding of both normal development and disease.

Costal2, a novel kinesin-related protein in the Hedgehog signaling pathway

The Hedgehog (HH) signaling proteins control cell fates and patterning during animal development. In Drosophila, HH protein induces the transcription of target genes encoding secondary signals such as DPP and WG proteins by opposing a repressor system. The repressors include Costal2, protein kinase A, and the HH receptor Patched. Like HH, the kinase Fused and the transcription factor Cubitus interruptus (CI) act positively upon targets. Here we show that costal2 encodes a kinesin-related protein that accumulates preferentially in cells capable of responding to HH. COS2 is cytoplasmic and binds microtubules. We find that CI associates with COS2 in a large protein complex, suggesting that COS2 directly controls the activity of CI.

The fu gene discriminates between pathways to control dpp expression in Drosophila imaginal discs

The genes decapentaplegic (dpp) and wingless (wg), which encode secreted factors of the TGF-beta and Wnt families, respectively, are required for the proper development of the imaginal discs. The expression of these genes must be finely regulated since their ectopic expression induces overgrowth and pattern alterations in wings and legs. Genes like patched (ptc) and costal-2 (cos-2), and the gene encoding the catalytic subunit of the protein kinase A gene (pkA) are required to restrict dpp and wg expression in their proper positions. We show here that some mutations in the cubitus interruptus (ci) gene also show ectopic dpp expression in the wing disc. We have also analyzed the functional hierarchy between these genes and the gene fused (fu), in the activation of dpp by the hedgehog (hh) signal. fu is required to transmit the hh signal in imaginal discs, since fu mutations rescue the phenotype due to the ectopic hh expression or to the lack of ptc activity. fu is also required for the activation of engrailed (en) caused when hh is ectopically activated in the wing disc. By contrast, fu mutations do not rescue the phenotypic consequences of the abnormal ci, cos-2 or pkA activity. Although fu, cos-2 and ci probably form part of the same pathway that controls dpp expression, pkA probably controls dpp transcription by a different pathway.

The Drosophila segment polarity gene patched interacts with decapentaplegic in wing development

The decapentaplegic (dpp) gene of Drosophila melanogaster encodes a polypeptide of the transforming growth factor-beta family of secreted factors. It is required for the proper development of both embryonic and adult structures, and may act as a morphogen in the embryo. In wing imaginal discs, dpp is expressed and required in a stripe of cells near the anterior-posterior compartment boundary. Here we show that viable mutations in the segment polarity genes patched (ptc) and costal-2 (cos2) cause specific alterations in dpp expression within the anterior compartment of the wing imaginal disc. The interaction between ptc and dpp is particularly interesting; both genes are expressed with similar patterns at the anterior-posterior compartment boundary of the disc, and mis-expressed in a similar way in segment polarity mutant backgrounds like ptc and cos2. This mis-expression of dpp could be correlated with some of the features of the adult mutant phenotypes. We propose that ptc controls dpp expression in the imaginal discs, and that the restricted expression of dpp near the anterior-posterior compartment boundary is essential to maintain the wild-type morphology of the wing disc.

The segment polarity gene costal-2 in Drosophila. II. The origin of imaginal pattern duplications

Imaginal pattern duplications caused by hypomorphic expression of the segment polarity gene costal-2 are described. These affect the anteroposterior coordinate of the imaginal disc. A very small part of the pattern is deleted and a large number of additional pattern elements arise in a progressive order, anterior-most first followed by more and more posterior structures. Mosaic analyses show that the duplications arise nonautonomously in the larval stages but that the costal-2 gene is not required after early embryogenesis. Arguments that the duplications are the result of cell interactions and intercalary growth that themselves arise from an abnormal polarity of the embryonic segment are presented.

The segment polarity gene costal-2 in Drosophila. I. The organization of both primary and secondary embryonic fields may be affected

A series of loss of function alleles at the costal-2 locus is described. Embryos mutant for lethal alleles that are derived from a mutant female germ line display polarity defects on the larval segments. A posterior part of the segmental denticle belt is missing and in its place is a mirror-image duplication of the anterior part including the segment boundary. Maternally rescued embryos are lethal but have normal morphology. Hypomorphic alleles escape to adults that display pattern duplications on the wings and halteres. Dominant gain of function alleles at the Costal-1 locus are also described and data are presented that argue that these are neomorphic and act in trans to impair functioning of costal-2. Some wild-type isoalleles of costal-2 are particularly sensitive to interference from Costal-1 mutations and different combinations of these alleles with Costal-1 can lead to embryos in which the primary embryonic field is disrupted (bicaudal phenotype) and adults with pattern duplications on the anterior compartment of most body segments.

Cell determination boundaries as organizing regions for secondary embryonic fields

A model is proposed for pattern formation in secondary embryonic fields. It is stipulated that the boundaries, resulting from the primary embryonic organization of a developing organism, act as organizing regions for secondary embryonic fields, e.g., imaginal discs in insects. This boundary mechanism would allow very reliable pattern formation in the course of development: Primary positional information leads to cells of different determination, separated by sharp borders. At these borders, in turn, positional information would be generated for the next finer subdivision, and so on. This occurs if two or more differently determined cell types (e.g., compartments) cooperate for the production of a morphogenetic substance. A high concentration of the morphogen would appear at the common boundary of the cell types involved. Many experiments reported in the literature, for instance, the formation of duplicated and triplicated insect legs and the regeneration-duplication phenomenon of imaginal disc fragments can be explained under this assumption. The proposed boundary mechanism provides a molecularly feasible basis for the polar coordinate model.

Distal regeneration and symmetry

A revision of the "polar coordinate model" shows how pattern formation in diverse regenerating systems can be understood in terms of strictly local cell interactions.