Interactions among Delta, Serrate and Fringe modulate Notch activity during Drosophila wing development

The Notch signalling pathway plays an important role during the development of the wing primordium, especially of the wing blade and margin. In these processes, the activity of Notch is controlled by the activity of the dorsal specific nuclear protein Apterous, which regulates the expression of the Notch ligand, Serrate, and the Fringe signalling molecule. The other Notch ligand, Delta, also plays a role in the development and patterning of the wing. It has been proposed that Fringe modulates the ability of Serrate and Delta to signal through Notch and thereby restricts Notch signalling to the dorsoventral boundary of the developing wing blade. Here we report the results of experiments aimed at establishing the relationships between Fringe, Serrate and Delta during wing development. We find that Serrate is not required for the initiation of wing development but rather for the expansion and early patterning of the wing primordium. We provide evidence that, at the onset of wing development, Delta is under the control of apterous and might be the Notch ligand in this process. In addition, we find that Fringe function requires Su(H). Our results suggest that Notch signalling during wing development relies on careful balances between positive and dominant negative interactions between Notch ligands, some of which are mediated by Fringe.     

Notch signalling mediates segmentation of the Drosophila leg

The legs of Drosophila are divided into segments along the proximodistal axis by flexible structures called joints. The separation between segments is already visible in the imaginal disc as folds of the epithelium, and cells at segment boundaries have different morphology during pupal development. We find that Notch is locally activated in distal cells of each segment, as demonstrated by the restricted expression of the Enhancer of split mbeta gene, and is required for the formation of normal joints. The genes fringe, Delta, Serrate and Suppressor of Hairless, also participate in Notch function during leg development, and their expression is localised within the leg segments with respect to segment boundaries. The failure to form joints when Notch signalling is compromised leads to shortened legs, suggesting that the correct specification of segment boundaries is critical for normal leg growth. The requirement for Notch during leg development resembles that seen during somite formation in vertebrates and at the dorsal ventral boundary of the wing, suggesting that the creation of boundaries of gene expression through Notch activation plays a conserved role in co-ordinating growth and patterning.     

Genes of the Enhancer of split and achaete-scute complexes are required for a regulatory loop between Notch and Delta during lateral signalling in Drosophila

Diabetic retinopathy is the most common chronic complication associated with diabetes mellitus. It affects 20% to 30% of diabetic women in the reproductive age group. This article reviews the course and treatment of pregnancy complicated by diabetic retinopathy.     

The dynamics of neurogenic signalling underlying bristle development in Drosophila melanogaster

We have examined expression of the neurogenic gene, Delta (Dl), and the regulatory relationships between the Delta-Notch signalling pathway and the proneural gene, achaete, during microchaeta development in Drosophila. Delta is expressed in all microchaeta proneural cells and microchaeta sensory organ precursors (SOPs) and is expressed dynamically in SOP progeny. We find that Delta expression in microchaeta proneural cells is detected prior to the onset of achaete expression and arises normally in the absence of achaete/scute function, indicating that initial Delta expression in the notum is not dependent on proneural gene function. Activation of the Delta-Notch pathway results in loss of Delta protein accumulation, suggesting that Delta expression is regulated, in part, by Delta-Notch signalling activity. We find that Delta signalling is required for correct delineation of early proneural gene expression in developing nota. Within microchaeta proneural stripes, we demonstrate that Delta-Notch signalling prohibits adoption of the SOP fate by repressing expression of proneural genes.     

Primary neurogenesis in Xenopus embryos regulated by a homologue of the Drosophila neurogenic gene Delta

X-Delta-1, a Xenopus homologue of the Drosophila Delta gene, is expressed in the early embryonic nervous system in scattered cells that appear to be the prospective primary neurons. Ectopic X-Delta-1 activity inhibits production of primary neurons and interference with endogenous X-Delta-1 activity results in overproduction of primary neurons. These results indicate that the X-Delta-1 protein mediates lateral inhibition delivered by prospective neurons to adjacent cells, and that commitment to a neural fate in vertebrates is regulated by Delta-Notch signalling as in Drosophila.     

The Notch signalling pathway in hair growth

The Notch signalling pathway is an important mediator of cell fate selection whose involvement in epidermal appendage formation is now becoming recognised. Hair follicle development and hair formation involve the co-ordinated differentiation of several different cell types in which Notch appears to have a role. We report intricate expression patterns for the Notch-1 receptor and three ligands, Delta-1, Jagged-1 and Jagged-2 in the hair follicle. Notch-1 is expressed in ectodermal-derived cells of the follicle, in the inner cells of the embryonic placode and the follicle bulb, and in the suprabasal cells of the mature outer root sheath. Delta-1 is only expressed during embryonic follicle development and is exclusive to the mesenchymal cells of the pre-papilla located beneath the follicle placode. Expression of Jagged-1 or Jagged-2 overlaps Notch-1 expression at all stages. In mature follicles, Jagged-1 and Jagged-2 are expressed in complementary patterns in the follicle bulb and outer root sheath, Jagged-1 in suprabasal cells and Jagged-2 predominantly in basal cells. In the follicle bulb, Jagged-2 is localised to the inner (basal) bulb cells next to the dermal papilla which do not express Notch-1, whereas Jagged-1 expression in the upper follicle bulb overlaps Notch-1 expression and correlates with bulb cell differentiation into hair shaft cortical and cuticle keratinocytes.     

Complex proteolytic processing acts on Delta, a transmembrane ligand for Notch, during Drosophila development

Delta functions as a cell nonautonomous membrane-bound ligand that binds to Notch, a cell-autonomous receptor, during cell fate specification. Interaction between Delta and Notch leads to signal transduction and elicitation of cellular responses. During our investigations to further understand the biochemical mechanism by which Delta signaling is regulated, we have identified four Delta isoforms in Drosophila embryonic and larval extracts. We have demonstrated that at least one of the smaller isoforms, Delta S, results from proteolysis. Using antibodies to the Delta extracellular and intracellular domains in colocalization experiments, we have found that at least three Delta isoforms exist in vivo, providing the first evidence that multiple forms of Delta exist during development. Finally, we demonstrate that Delta is a transmembrane ligand that can be taken up by Notch-expressing Drosophila cultured cells. Cell culture experiments imply that full-length Delta is taken up by Notch-expressing cells. We present evidence that suggests this uptake occurs by a nonphagocytic mechanism.     

Conservation of the Notch signalling pathway in mammalian neurogenesis

PURPOSE: To determine whether there is an association between epidermal growth factor (EGF)-induced activation of phosphatidylinositol 3-kinase (PI 3-kinase) and stimulation of wound closure in rabbit corneal epithelial cells. METHODS: Immortalized rabbit corneal epithelial cells were cultured in 24-well plates until they became confluent. Circular wounds were created in confluent cultures by cell denudation and then incubated in the absence and presence of EGF for varying intervals. Wound closure was monitored by staining the cells with Giemsa and quantifying the wound area with SigmaS can computer program. Cell proliferation during wound repair was estimated by measuring the incorporation of [3H]thymidine into nuclear DNA. Changes in PI 3-kinase activity were assessed by measuring the production of phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P3] in 32P-labeled cells as well as by immunoprecipitating and assaying PI 3-kinase activity with phosphatidylinositol 4,5-bisphosphate and [gamma-32P]ATP as substrates. The enzyme product, PIP3, was analyzed by a combination of thin-layer and high-pressure liquid chromatography. RESULTS: Addition of 10 ng/ml EGF to the wounded corneal epithelial cells stimulated wound closure in a time-dependent manner, and the wound closed completely within 48 hours. The effect of EGF was dose dependent, and maximal wound closure occurred at 10 ng/ml EGF. As the epithelial cells were undergoing EGF-stimulated wound closure, there was a time-dependent increase in PI 3-kinase activity. The enzyme activity increased maximally at 24 hours and then decreased gradually as the incubation was continued to 48 hours. When the cells were treated with wortmannin, a PI 3-kinase inhibitor, the EGF-stimulated PIP3 formation as well as the wound closure were inhibited significantly. Treatment of the cells with genistein or tyrphostin B42 also decreased both EGF-stimulated PIP3 formation and wound closure in a dose-dependent manner. Concomitant with stimulation of wound repair, the growth factor increased [3H]thymidine incorporation into nuclear DNA, and this effect was inhibited by pretreatment of the cell with wortmannin. CONCLUSIONS: The data suggest a close correlation between EGF-stimulated wound closure and activation of PI 3-kinase in corneal epithelial cells. It can be concluded that PI 3-kinase might be an important component in signal transduction cascade initiated by EGF-receptor interaction, which leads to mitosis and cell proliferation during wound closure in corneal epithelial cells.     

A functional analysis of Notch mutations in Drosophila

In 1900, the world population was less than 1.7 billion people; the United Nations projects that in 2000 it will be 6.2, and in 2020, 7.9 billion. The proportion of the elderly (65 years and over), will increase from 5.1% (1950) to 6.8% by the year 2000 and to 8.8% by 2020, when out of an elderly population of 796 million people, 124 million are projected to be 80 years and over. Due to an increasing gender inequality in life expectation, the majority of the elderly will be women. An aged population is a basically new feature in the history of humanity, the implications of which are-as yet-incompletely understood. It is clear, however, that the last years of life are accompanied by an increase in disability and sickness, with very high demands for health and social services. Hence, the soaring elderly population will raise major social, economic and ethical issues worldwide and may strain to the limit the ability of health, social and economic infrastructures of many countries. It may also result in an increasingly large proportion of humanity (the elderly in general and elderly women, in particular) living in absolute poverty. The demographic, health, socioeconomic and ethical dimensions of the problem are discussed with particular emphasis on the situation of elderly women and a plea is made for greatly increased medical and socioeconomic research.     

Notch signalling in development: on equivalence groups and asymmetric developmental potential

Notch-mediated lateral signalling takes place between equivalent cells that then adopt alternative fates. An example is the epidermal-neural choice in the Drosophila neuro-ectoderm. Sometimes this choice is random. Sometimes it is biased to a preferred cell by an extrinsic signal. Sometimes the neural precursor is predetermined. A stochastic outcome is probably older in evolutionary terms.     

Wingless and Notch regulate cell-cycle arrest in the developing Drosophila wing

In developing organs, the regulation of cell proliferation and patterning of cell fates is coordinated. How this coordination is achieved, however, is unknown. In the developing Drosophila wing, both cell proliferation and patterning require the secreted morphogen Wingless (Wg) at the dorsoventral compartment boundary. Late in wing development, Wg also induces a zone of non-proliferating cells at the dorsoventral boundary. This zone gives rise to sensory bristles of the adult wing margin. Here we investigate how Wg coordinates the cell cycle with patterning by studying the regulation of this growth arrest. We show that Wg, in conjunction with Notch, induces arrest in both the G1 and G2 phases of the cell cycle in separate subdomains of the zone of non-proliferating cells. Wg induces G2 arrest in two subdomains by inducing the proneural genes achaete and scute, which downregulate the mitosis-inducing phosphatase String (Cdc25). Notch activity creates a third domain by preventing arrest at G2 in wg-expressing cells, resulting in their arrest in G1.     

Intercellular signalling in Drosophila segment formation reconstructed in vitro

Genetic studies show that intercellular signalling is involved in key steps in Drosophila melanogaster development, but it has not previously been possible to investigate these processes in simplified in vitro systems. Analysis of engrailed (en) and wingless (wg) and other segment polarity genes suggests that two or more intercellular signalling processes may be involved in intrasegmental patterning. Expression of en and wg begins about three hours after egg laying, in adjacent rows of cells in the posterior half of each segmental primordium. In wg- embryos and in conditional mutants in which wg function is inactivated during a critical period between three and five hours after egg laying, early en expression begins normally but then disappears within several hours. The wg gene encodes a protein highly similar to the product of the mouse Wnt-1 proto-oncogene, a secreted glycoprotein; wg protein is proposed to function as an extracellular signal, maintaining en expression and activating other molecular and morphogenetic processes in nearby cells. Several lines of evidence support the model, including the secretion of wg protein in the embryo, genetic mosaic experiments and cell lineage studies. We tested this model using purified embryonic cells isolated by whole animal cell sorting, and validated three key predictions: (1) when en-expressing cells from early embryos are grown alone in culture, they rapidly and selectively lose en expression; (2) purified wg-expressing cells provide a locally active signal that prevents this loss; (3) heterologous cells engineered to express wg also show signalling activity, indicating that wg protein alone, or in conjunction with more generally expressed factors, is the signal.     

Different spatial and temporal interactions between Notch, wingless, and vestigial specify proximal and distal pattern elements of the wing in Drosophila

The wing of Drosophila is composed of a proximal element, the hinge, which attaches it to the thorax, and a distal one, the wing blade. The development of the wing is a complex process that requires the integration of cellular responses to two signaling systems centered along the anteroposterior and the dorsoventral axes. The genes Notch (N) and wingless (wg) play an important role in generating the information from the dorsoventral axis. The vestigial (vg) gene is necessary for the development of the wing and is a target of these signaling systems during the growth of the wing. Here we examine the roles that N, wg, and vg play during the initial stages of wing development. Our results reveal that vg is involved in the specification of the wing primordium under the combined control of Notch and wingless signaling. Furthermore, we show that once cells are assigned to the wing fate, their development relies on a sequence of regulatory loops that involve N, wg, and vg. During this process, cells that are exposed to the activity of both wg and vg will become wing blade and those that are continuously under the influence of wg alone will develop as hinge. Our results also indicate that the growth of the cells in the wing blade results from a synergistic effect of the three genes N, wg, and vg on the cells that have been specified as wing blade.     

Heartbroken is a specific downstream mediator of FGF receptor signalling in Drosophila

Drosophila possesses two FGF receptors which are encoded by the heartless and breathless genes. HEARTLESS is essential for early migration and patterning of the embryonic mesoderm, while BREATHLESS is required for proper branching of the tracheal system. We have identified a new gene, heartbroken, that participates in the signalling pathways of both FGF receptors. Mutations in heartbroken are associated with defects in the migration and later specification of mesodermal and tracheal cells. Genetic interaction and epistasis experiments indicate that heartbroken acts downstream of the two FGF receptors but either upstream of or parallel to RAS1. Furthermore, heartbroken is involved in both the HEARTLESS- and BREATHLESS-dependent activation of MAPK. In contrast, EGF receptor-dependent embryonic functions and MAPK activation are not perturbed in heartbroken mutant embryos. A strong heartbroken allele also suppresses the effects of hyperactivated FGF but not EGF receptors. Thus, heartbroken may contribute to the specificity of developmental responses elicited by FGF receptor signalling.     

Cooperative signalling between opponents in fish fights

Cichlids of the species Nannacara anomala employ several colour displays during fights which do not seem to signal either fighting ability or motivation. How should these colour displays be interpreted when winning is reliably predicted by weight asymmetries? Medial Line colour displays were associated with, and predicted, tail-beating, while Vertical Bar colour displays were associated with mouth-wrestling. I suggest that these colour displays are used to facilitate the transmission of assessment information within a fight, and that they are an example of cooperative signalling between opponents. The results support the idea that the structure of fights contains strong cooperative aspects.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour