Expression and function of decapentaplegic and thick veins during the differentiation of the veins in the Drosophila wing

Function of the spalt/spalt-related gene complex in positioning the veins in the Drosophila wing

Spalt and Spalt-related encode conserved Zn-finger proteins that mediate the function of the TGF-beta molecule Decapentaplegic during the positioning of veins in the Drosophila wing. Here we show that Spalt and Spalt-related regulate the vein-specific expression of the transcription factors of the knirps and iroquois gene complexes, delimiting their domains of expression in the wing pouch. The effects of spalt/spalt-related mutations on knirps and iroquois expression are cell-autonomous, suggesting that they could be direct. The regulation of iroquois involves transcriptional repression by Spalt and Spalt-related, whereas the regulation of knirps involves a combination of transcriptional activation and repression mediated by the same genes. We suggest that the regulation of the iroquois and knirps gene complexes by Spalt and Spalt-related translates the Decapentaplegic morphogenetic gradient into precisely spaced pattern elements.

Drosophila bunched integrates opposing DPP and EGF signals to set the operculum boundary

The Drosophila BMP homolog DPP can function as a morphogen, inducing multiple cell fates across a developmental field. However, it is unknown how graded levels of extracellular DPP are interpreted to organize a sharp boundary between different fates. Here we show that opposing DPP and EGF signals set the boundary for an ovarian follicle cell fate. First, DPP regulates gene expression in the follicle cells that will create the operculum of the eggshell. DPP induces expression of the enhancer trap reporter A359 and represses expression of bunched, which encodes a protein similar to the mammalian transcription factor TSC-22. Second, DPP signaling indirectly regulates A359 expression in these cells by downregulating expression of bunched. Reduced bunched function restores A359 expression in cells that lack the Smad protein MAD; ectopic expression of BUNCHED suppresses A359 expression in this region. Importantly, reduction of bunched function leads to an expansion of the operculum and loss of the collar at its boundary. Third, EGF signaling upregulates expression of bunched. We previously demonstrated that the bunched expression pattern requires the EGF receptor ligand GURKEN. Here we show that activated EGF receptor is sufficient to induce ectopic bunched expression. Thus, the balance of DPP and EGF signals sets the boundary of bunched expression. We propose that the juxtaposition of cells with high and low BUNCHED activity organizes a sharp boundary for the operculum fate.

Hedgehog creates a gradient of DPP activity in Drosophila wing imaginal discs

Hedgehog (HH) and Decapentaplegic (DPP) direct anteroposterior patterning in the developing Drosophila wing by functioning as short- and long-range morphogens, respectively. Here, we show that the activity of DPP is graded and is directly regulated by a novel HH-dependent mechanism. DPP activity was monitored by visualizing the activated form of Mothers against dpp (MAD), a cytoplasmic transducer of DPP signaling. We found that activated MAD levels are highest near the source of DPP but are unexpectedly low in the cells that express dpp. HH induces dpp in these cells; it also attenuates their response to DPP by downregulating expression of the DPP receptor thick veins (tkv). We suggest that regulation of tkv by HH is a key part of the mechanism that controls the level and distribution of DPP.

Lateral induction by juxtacrine signaling is a new mechanism for pattern formation

Many signaling molecules in epithelia are now known to function in a membrane-bound form, binding to receptors on immediately neighbouring cells. This "juxtacrine" mode of communication has been well studied in the case of lateral inhibition, where ligand binding at the cell surface downregulates ligand and receptor expression, and is known to generate spatial patterns with a wavelength of exactly two cells. However, recent evidence shows that a number of juxtacrine signals can lead to the opposite phenomenon of lateral induction. Here, we use mathematical modeling to show that such positive feedback, in combination with juxtacrine communication, provides a novel mechanism for the generation of spatial patterns, with wavelengths that vary with parameters and can be many cell lengths.

A temporal switch in DER signaling controls the specification and differentiation of veins and interveins in the Drosophila wing

The Drosophila EGF receptor (DER) is required for the specification of diverse cell fates throughout development. We have examined how the activation of DER controls the development of vein and intervein cells in the Drosophila wing. The data presented here indicate that two distinct events are involved in the determination and differentiation of wing cells. (1) The establishment of a positive feedback amplification loop, which drives DER signaling in larval stages. At this time, rhomboid (rho), in combination with vein, initiates and amplifies the activity of DER in vein cells. (2) The late downregulation of DER activity. At this point, the inactivation of MAPK in vein cells is necessary for the maintenance of the expression of decapentaplegic (dpp) and becomes essential for vein differentiation. Together, these temporal and spatial changes in the activity of DER constitute an autoregulatory network that controls the definition of vein and intervein cell types.

Engineered truncations in the Drosophila mastermind protein disrupt Notch pathway function

The phenotypes and genetic interactions associated with mutations in the Drosophila mastermind (mam) gene have implicated it as a component of the Notch signaling pathway. However, its function and site of action within many tissues requiring Notch signaling have not been thoroughly investigated. To address these questions, we have constructed truncated versions of the Mam protein that elicit dominant phenotypes when expressed in imaginal tissues under GAL4-UAS regulation. By several criteria, these effects appear to phenocopy loss of function for the Notch pathway. When expressed in the notum, truncated Mam results in failure of lateral inhibition within proneural clusters and perturbations in cell fate specification within the sensory organ precursor cell lineage. Expression in the wing is associated with vein thickening and margin defects, including nicking and bristle loss. The truncation-associated wing margin phenotypes are modified by mutations in Notch and Wg pathway genes and are correlated with depressed expression of wg, cut, and vg. These data support the idea that Mam truncations have lost key effector domains and therefore behave as dominant-negative proteins. Coexpression of Delta or an activated form of Notch suppresses the effects of the Mam truncation, suggesting that Mam can function upstream of ligand-receptor interaction in the Notch pathway. This system should prove useful for the investigation of the role of Mam within the Notch pathway.

Differential effects of transforming growth factor-beta(s) and glial cell line-derived neurotrophic factor on gene expression of presenilin-1 in human post-mitotic neurons and astrocytes

Mutations in the presenilin-1 gene are linked to the majority of early-onset familial Alzheimer's disease cases. We have previously shown that the expression of transforming growth factor-beta is altered in Alzheimer's patients, compared to controls. Here we examine presenilin- expression in human post-mitotic neurons (hNT cells), normal human astrocytes, and human brain tumor cell lines following treatment with three isoforms of transforming growth factor-beta, or glial cell line-derived neurotrophic factor, a member of the transforming growth factor-beta superfamily. As the NT2/D1 teratocarcinoma cell line is treated with retinoic acid to induce differentiation to hNT cells, presenilin-1 messenger RNA expression is dramatically increased. Furthermore, there is a 2-3-fold increase in presenilin-1 messenger RNA expression following treatment of hNT cells with growth factors and similar results are found by Western blotting and with immunohistochemical staining for presenilin-1 protein. However, treatment of normal human astrocytes with cytokines results in minimal changes in presenilin-1 messenger RNA and protein. Interestingly, the expression of presenilin-1 in human U87 MG astrocytoma and human SK-N-SH neuroblastoma cells is only increased when cells are treated with glial cell line-derived neurotrophic factor or transforming growth factor-beta3. These findings suggest that endogenous presenilin-1 gene expression in human neurons can be induced by growth factors present in normal and diseased brain tissue. Cytokines may play a major role in regulating expression of presenilin-1 which may affect its biological actions in physiological and pathological conditions.

Wingless, decapentaplegic and EGF receptor signaling pathways interact to specify dorso-ventral pattern in the adult abdomen of Drosophila

Adult abdominal segments of Drosophila are subdivided along the dorso-ventral axis into a dorsal tergite, a ventral sternite and ventro-lateral pleural cuticle. We report that this pattern is largely specified during the pupal stage by Wingless (Wg), Decapentaplegic (Dpp) and Drosophila EGF Receptor (DER) signaling. Expression of wg and dpp is activated at the posterior edge of the anterior compartment by Hedgehog signaling. Within this region, wg and dpp are expressed in domains that are mutually exclusive along the dorso-ventral axis: wg is expressed in the sternite and medio-lateral tergite, whereas dpp expression is confined to the pleura and the dorsal midline. Neither gene is expressed in the lateral tergite. Shirras and Couso (1996, Dev. Biol. 175, 24–36) have shown that tergite and sternite cell fates are specified by Wg signaling. We find that DER acts synergistically with Wg to promote tergite and sternite identities, and that Wg and DER activities are opposed by Dpp signaling, which promotes pleural identity. Wg and Dpp interact antagonistically at two levels. First, their expression is confined to complementary domains by mutual transcriptional repression. Second, Wg and Dpp compete directly with one another by exerting opposite effects on cell fate. DER signaling does not affect the expression of wg or dpp, indicating that it interacts with Wg and Dpp at the level of cell fate determination. Within the tergite, the requirements for Wg and DER function are roughly complementary: Wg is required mainly in the medial region, whereas DER is most important laterally. Finally, we show that Dpp signaling at the dorsal midline controls dorso-ventral patterning within the tergite by promoting pigmentation in the medial region.

Distortion of proximodistal information causes JNK-dependent apoptosis in Drosophila wing

Distinct and evolutionarily conserved signal-transduction cascades mediate the survival or death of cells during development. The c-Jun amino-terminal kinases (JNKs) of the mitogen-activated protein kinase superfamily are involved in apoptotic signalling in various cultured cells. However, the role of the JNK pathway in development is less well understood. In Drosophila, Decapentaplegic (Dpp; a homologue of transforming growth factor-beta) and Wingless (Wg; a Wnt homologue) proteins are secretory morphogens that act cooperatively to induce formation of the proximodistal axis of appendages. Here we show that either decreased Dpp signalling in the distal wing cells or increased Dpp signalling in the proximal wing cells causes apoptosis. Inappropriate levels of Dpp signalling lead to aberrant morphogenesis in the respective wing zones, and these apoptotic zones are also determined by the strength of the Wg signal. Our results indicate that distortion of the positional information determined by Dpp and Wg signalling gradients leads to activation of the JNK apoptotic pathway, and the consequent induction of cell death thereby maintains normal morphogenesis.

TGF-beta family signal transduction in Drosophila development: from Mad to Smads

The transforming growth factor-beta (TGF-beta) superfamily encompasses a large group of soluble extracellular proteins that are potent regulators of development in both vertebrates and invertebrates. Drosophila TGF-beta family members include three proteins with homology to vertebrate bone morphogenetic proteins (BMPs): Decapentaplegic (Dpp), Screw, and Glass bottom boat-60A. Genetic studies of Dpp signaling led to the identification of Smad proteins as central mediators of signal transduction by TGF-beta family members. Work in mammalian tissue culture has elucidated a biochemical model for signal transduction, in which activation of receptor serine-threonine kinase activity leads to phosphorylation of specific Smad proteins and translocation of heteromeric Smad protein complexes to the nucleus. Once in the nucleus Smad proteins interact with other DNA binding proteins to regulate transcription of specific target genes. Dissection of Dpp-response elements from genes expressed during embryonic mesoderm patterning and midgut morphogenesis provides important insights into the contributions of Smad proteins and tissue-specific transcription factors to spatial regulation of gene expression. Genetic studies in Drosophila are now expanding to include multiple BMP ligands and receptors and have uncovered activities not explained by the current signal transduction model. Identification of more ligand sequences and demonstration of a functional Drosophila activin-like signal transduction pathway suggest that all TGF-beta signal transduction pathways are present in flies.

The Drosophila melanogaster Suppressor of deltex gene, a regulator of the Notch receptor signaling pathway, is an E3 class ubiquitin ligase

During development, the Notch receptor regulates many cell fate decisions by a signaling pathway that has been conserved during evolution. One positive regulator of Notch is Deltex, a cytoplasmic, zinc finger domain protein, which binds to the intracellular domain of Notch. Phenotypes resulting from mutations in deltex resemble loss-of-function Notch phenotypes and are suppressed by the mutation Suppressor of deltex [Su(dx)]. Homozygous Su(dx) mutations result in wing-vein phenotypes and interact genetically with Notch pathway genes. We have previously defined Su(dx) genetically as a negative regulator of Notch signaling. Here we present the molecular identification of the Su(dx) gene product. Su(dx) belongs to a family of E3 ubiquitin ligase proteins containing membrane-targeting C2 domains and WW domains that mediate protein-protein interactions through recognition of proline-rich peptide sequences. We have identified a seven-codon deletion in a Su(dx) mutant allele and we show that expression of Su(dx) cDNA rescues Su(dx) mutant phenotypes. Overexpression of Su(dx) also results in ectopic vein differentiation, wing margin loss, and wing growth phenotypes and enhances the phenotypes of loss-of-function mutations in Notch, evidence that supports the conclusion that Su(dx) has a role in the downregulation of Notch signaling.

Genetic analysis of the bone morphogenetic protein-related gene, gbb, identifies multiple requirements during Drosophila development

We have isolated mutations in the Drosophila melanogaster gene glass bottom boat (gbb), which encodes a TGF-beta signaling molecule (formerly referred to as 60A) with highest sequence similarity to members of the bone morphogenetic protein (BMP) subgroup including vertebrate BMPs 5-8. Genetic analysis of both null and hypomorphic gbb alleles indicates that the gene is required in many developmental processes, including embryonic midgut morphogenesis, patterning of the larval cuticle, fat body morphology, and development and patterning of the imaginal discs. In the embryonic midgut, we show that gbb is required for the formation of the anterior constriction and for maintenance of the homeotic gene Antennapedia in the visceral mesoderm. In addition, we show a requirement for gbb in the anterior and posterior cells of the underlying endoderm and in the formation and extension of the gastric caecae. gbb is required in all the imaginal discs for proper disc growth and for specification of veins in the wing and of macrochaete in the notum. Significantly, some of these tissues have been shown to also require the Drosophila BMP2/4 homolog decapentaplegic (dpp), while others do not. These results indicate that signaling by both gbb and dpp may contribute to the development of some tissues, while in others, gbb may signal independently of dpp.

Control of growth and differentiation by Drosophila RasGAP, a homolog of p120 Ras-GTPase-activating protein

Mammalian Ras GTPase-activating protein (GAP), p120 Ras-GAP, has been implicated as both a downregulator and effector of Ras proteins, but its precise role in Ras-mediated signal transduction pathways is unclear. To begin a genetic analysis of the role of p120 Ras-GAP we identified a homolog from the fruit fly Drosophila melanogaster through its ability to complement the sterility of a Schizosaccharomyces pombe (fission yeast) gap1 mutant strain. Like its mammalian homolog, Drosophila RasGAP stimulated the intrinsic GTPase activity of normal mammalian H-Ras but not that of the oncogenic Val12 mutant. RasGAP was tyrosine phosphorylated in embryos and its Src homology 2 (SH2) domains could bind in vitro to a small number of tyrosine-phosphorylated proteins expressed at various developmental stages. Ectopic expression of RasGAP in the wing imaginal disc reduced the size of the adult wing by up to 45% and suppressed ectopic wing vein formation caused by expression of activated forms of Breathless and Heartless, two Drosophila receptor tyrosine kinases of the fibroblast growth factor receptor family. The in vivo effects of RasGAP overexpression required intact SH2 domains, indicating that intracellular localization of RasGAP through SH2-phosphotyrosine interactions is important for its activity. These results show that RasGAP can function as an inhibitor of signaling pathways mediated by Ras and receptor tyrosine kinases in vivo. Genetic interactions, however, suggested a Ras-independent role for RasGAP in the regulation of growth. The system described here should enable genetic screens to be performed to identify regulators and effectors of p120 Ras-GAP.

p38 mitogen-activated protein kinase can be involved in transforming growth factor beta superfamily signal transduction in Drosophila wing morphogenesis

p38 mitogen-activated protein kinase (p38) has been extensively studied as a stress-responsive kinase, but its role in development remains unknown. The fruit fly, Drosophila melanogaster, has two p38 genes, D-p38a and D-p38b. To elucidate the developmental function of the Drosophila p38's, we used various genetic and pharmacological manipulations to interfere with their functions: expression of a dominant-negative form of D-p38b, expression of antisense D-p38b RNA, reduction of the D-p38 gene dosage, and treatment with the p38 inhibitor SB203580. Expression of a dominant-negative D-p38b in the wing imaginal disc caused a decapentaplegic (dpp)-like phenotype and enhanced the phenotype of a dpp mutant. Dpp is a secretory ligand belonging to the transforming growth factor beta superfamily which triggers various morphogenetic processes through interaction with the receptor Thick veins (Tkv). Inhibition of D-p38b function also caused the suppression of the wing phenotype induced by constitutively active Tkv (TkvCA). Mosaic analysis revealed that D-p38b regulates the Tkv-dependent transcription of the optomotor-blind (omb) gene in non-Dpp-producing cells, indicating that the site of D-p38b action is downstream of Tkv. Furthermore, forced expression of TkvCA induced an increase in the phosphorylated active form(s) of D-p38(s). These results demonstrate that p38, in addition to its role as a transducer of emergency stress signaling, may function to modulate Dpp signaling.

Dpp receptor levels contribute to shaping the Dpp morphogen gradient in the Drosophila wing imaginal disc

Axis formation in the Drosophila wing depends on the localized expression of the secreted signaling molecule Decapentaplegic (Dpp). Dpp acts directly at a distance to specify discrete spatial domains, suggesting that it functions as a morphogen. Expression levels of the Dpp receptor thick veins (tkv) are not uniform along the anterior-posterior axis of the wing imaginal disc. Receptor levels are low where Dpp induces its targets Spalt and Omb in the wing pouch. Receptor levels increase in cells farther from the source of Dpp in the lateral regions of the disc. We present evidence that Dpp signaling negatively regulates tkv expression and that the level of receptor influences the effective range of the Dpp gradient. High levels of tkv sensitize cells to low levels of Dpp and also appear to limit the movement of Dpp outside the wing pouch. Thus receptor levels help to shape the Dpp gradient.

Synergistic signaling by two BMP ligands through the SAX and TKV receptors controls wing growth and patterning in Drosophila

In Drosophila wing discs, a morphogen gradient of DPP has been proposed to determine the transcriptional response thresholds of the downstream genes sal and omb. We present evidence that the concentration of the type I receptor TKV must be low to allow long-range DPP diffusion. Low TKV receptor concentrations result, however, in low signaling activity. To enhance signaling at low DPP concentrations, we find that a second ligand, GBB, augments DPP/TKV activity. GBB signals primarily through the type I receptor SAX, which synergistically enhances TKV signaling and is required for proper OMB expression. We show that OMB expression in wing discs requires synergistic signaling by multiple ligands and receptors to overcome the limitations imposed on DPP morphogen function by receptor concentration levels.

TGF-beta/BMP superfamily members, Gbb-60A and Dpp, cooperate to provide pattern information and establish cell identity in the Drosophila wing

Within a developing organism, cells receive many signals which control their proliferation, fate specification and differentiation. One group of such proteins is the TGF-beta/BMP class of related signaling molecules. Based on expression studies, multiple members of this class of ligands must impinge upon the same cells of a developing tissue; however, the role that multiple TGF-beta/BMP ligands may play in directing the development of such a tissue is not understood. Here we provide evidence that multiple BMPs are required for growth and patterning of the Drosophila wing. The Drosophila BMP gene, gbb-60A, exhibits a requirement in wing morphogenesis distinct from that shown previously for dpp, a well-characterized Drosophila BMP member. gbb-60A mutants exhibit a loss of pattern elements from the wing, particularly those derived from cells in the posterior compartment, consistent with the gbb-60A RNA and protein expression pattern. Based on genetic analysis and expression studies, we conclude that Gbb-60A must signal primarily as a homodimer to provide patterning information in the wing imaginal disc. We demonstrate that gbb-60A and dpp genetically interact and that specific aspects of this interaction are synergistic while others are antagonistic. We propose that the positional information received by a cell at a particular location within the wing imaginal disc depends on the balance of Dpp to Gbb-60A signaling. Furthermore, the critical ratio of Gbb-60A to Dpp signaling appears to be mediated by both Tkv and Sax type I receptors.

Feedback regulation is central to Delta-Notch signalling required for Drosophila wing vein morphogenesis

Delta and Notch are required for partitioning of vein and intervein cell fates within the provein during Drosophila metamorphosis. We find that partitioning of these fates is dependent on Delta-mediated signalling from 22 to 30 hours after puparium formation at 25 degrees C. Within the provein, Delta is expressed more highly in central provein cells (presumptive vein cells) and Notch is expressed more highly in lateral provein cells (presumptive intervein cells). Accumulation of Notch in presumptive intervein cells is dependent on Delta signalling activity in presumptive vein cells and constitutive Notch receptor activity represses Delta accumulation in presumptive vein cells. When Delta protein expression is elevated ectopically in presumptive intervein cells, complementary Delta and Notch expression patterns in provein cells are reversed, and vein loss occurs because central provein cells are unable to stably adopt the vein cell fate. Our findings imply that Delta-Notch signalling exerts feedback regulation on Delta and Notch expression during metamorphic wing vein development, and that the resultant asymmetries in Delta and Notch expression underlie the proper specification of vein and intervein cell fates within the provein.

The homeobox gene Distal-less induces ventral appendage development in Drosophila

This study investigates the role of the homeobox gene Distal-less (Dll) in the development of the legs, antennae, and wings of Drosophila. Lack of Dll function causes a change in the identity of ventral appendage cells (legs and antennae) that often results in the loss of the appendage. Ectopic Dll expression in the proximal region of ventral appendages induces nonautonomous duplication of legs and antennae by the activation of wingless and decapentaplegic. Ectopic Dll expression in dorsal appendages produces transformation into corresponding ventral appendages; wings and halteres develop ectopic legs and the head-eye region develops ectopic antennae. In the wing, the exogenous Dll product induces this transformation by activating the endogenous Dll gene and repressing the wing determinant gene vestigial. It is proposed that Dll induces the development of ventral appendages and also participates in a genetic address that specifies the identity of ventral appendages and discriminates the dorsal versus the ventral appendages in the adult. However, unlike other homeotic genes, Dll expression and function is not defined by a cell lineage border. Dll also performs a secondary and late function required for the normal patterning of the wing.