Regulation of dally, an integral membrane proteoglycan, and its function during adult sensory organ formation of Drosophila.

In Drosophila, imaginal wing discs, Wg and Dpp, play important roles in the development of sensory organs. These secreted growth factors govern the positions of sensory bristles by regulating the expression of achaete-scute (ac-sc), genes affecting neuronal precursor cell identity. Earlier studies have shown that Dally, an integral membrane, heparan sulfate-modified proteoglycan, affects both Wg and Dpp signaling in a tissue-specific manner. Here, we show that dally is required for the development of specific chemosensory and mechanosensory organs in the wing and notum. dally enhancer trap is expressed at the anteroposterior and dorsoventral boundaries of the wing pouch, under the control of hh and wg, respectively. dally affects the specification of proneural clusters for dally-sensitive bristles and shows genetic interactions with either wg or dpp signaling components for distinct sensory bristles. These findings suggest that dally can differentially regulate Wg- or Dpp-directed patterning during sensory organ assembly. We have also determined that, for pSA, a bristle on the lateral notum, dally shows genetic interactions with iroquois complex (IRO-C), a gene complex affecting ac-sc expression. Consistent with this interaction, dally mutants show markedly reduced expression of an iro::lacZ reporter. These findings establish dally as an important regulator of sensory organ formation via Wg- and Dpp-mediated specification of proneural clusters.

Genetic dissection of proteoglycan function in Drosophila and C. elegans.

Genetic analysis of the signaling pathways that govern patterning during development in the fruitfly Drosophila melanogaster and in the nematode C. elegans have provided insight into the in vivo functions of proteoglycans and their associated glycosaminoglycans. These studies have shown that patterning events dictated by Fibroblast Growth Factor Receptors, Wnt, Transforming Growth Factor- beta(TGF- beta), and Hedgehog families of growth factors are regulated by proteoglycans. Recent biochemical and structural analyses have shown that the molecular machinery of glycosaminoglycan biosynthesis is highly conserved between these invertebrate organisms and mammals. Drosophila and C. elegans therefore provide powerful model systems for exploring the varied functions proteoglycans and their glycosaminoglycan modifications.

sqv-3, -7, and -8, a set of genes affecting morphogenesis in Caenorhabditis elegans, encode enzymes required for glycosaminoglycan biosynthesis.

sqv (squashed vulva) genes comprise a set of eight independent loci in Caenorhabditis elegans required zygotically for the invagination of vulval epithelial cells and maternally for normal oocyte formation and embryogenesis. Sequencing of sqv-3, sqv-7, and sqv-8 suggested a role for the encoded proteins in glycolipid or glycoprotein biosynthesis. Using a combination of in vitro analysis of SQV enzymatic activities, sqv(+)-mediated rescue of vertebrate cell lines, and biochemical characterization of sqv mutants, we show that sqv-3, -7, and -8 all affect the biosynthesis of glycosaminoglycans and therefore compromise the function of one specific class of glycoconjugates, proteoglycans. These findings establish the importance of proteoglycans and their associated glycosaminoglycans in epithelial morphogenesis and patterning during C. elegans development.

Structural analysis of glycosaminoglycans in animals bearing mutations in sugarless, sulfateless, and tout-velu. Drosophila homologues of vertebrate genes encoding glycosaminoglycan biosynthetic enzymes.

Mutations that disrupt developmental patterning in Drosophila have provided considerable information about growth factor signaling mechanisms. Three genes recently demonstrated to affect signaling by members of the Wnt, transforming growth factor-beta, Hedgehog, and fibroblast growth factor families in Drosophila encode proteins with homology to vertebrate enzymes involved in glycosaminoglycan synthesis. We report here the biochemical characterization of glycosaminoglycans in Drosophila bearing mutations in sugarless, sulfateless, and tout-velu. We find that mutations in sugarless, which encodes a protein with homology to UDP-glucose dehydrogenase, compromise the synthesis of both chondroitin and heparan sulfate, as would be predicted from a defect in UDP-glucuronate production. Defects in sulfateless, a gene encoding a protein with similarity to vertebrate N-deacetylase/N-sulfotransferases, do not affect chondroitin sulfate levels or composition but dramatically alter the composition of heparin lyase-released disaccharides. N-, 6-O-, and 2-O-sulfated disaccharides are absent and replaced entirely with an unsulfated disaccharide. A mutation in tout-velu, a gene related to the vertebrate Exostoses 1 heparan sulfate co-polymerase, likewise does not affect chondroitin sulfate synthesis but reduces all forms of heparan sulfate to below the limit of detection. These findings show that sugarless, sulfateless, and tout-velu affect glycosaminoglycan biosynthesis and demonstrate the utility of Drosophila as a model organism for studying the function and biosynthesis of glycosaminoglycans in vivo.

Proteoglycans and pattern formation: sugar biochemistry meets developmental genetics.

While it has been long appreciated that sugar-modified proteins coat the cell surface, their functions are poorly understood. Here, I describe recent genetic studies that demonstrate that one class of sugar-modified proteins, cell-surface proteoglycans, play crucial roles in morphogenesis, growth regulation and tumor suppression. Mutations that affect individual proteoglycans or the enzymes required for glycosaminoglycan synthesis regulate Wingless and Decapentaplegic signaling in Drosophila, and body size in mice and humans. Compromising proteoglycan function is also associated with the development of Wilm's tumors and hereditary multiple exostoses. In this review, these biological findings are placed in the context of proteoglycan biochemistry and molecular function.

Structural analysis of glycosaminoglycans in Drosophila and Caenorhabditis elegans and demonstration that tout-velu, a Drosophila gene related to EXT tumor suppressors, affects heparan sulfate in vivo.

We have devised a sensitive method for the isolation and structural analysis of glycosaminoglycans from two genetically tractable model organisms, the fruit fly, Drosophila melanogaster, and the nematode, Caenorhabditis elegans. We detected chondroitin/chondroitin sulfate- and heparan sulfate-derived disaccharides in both organisms. Chondroitinase digestion of glycosaminoglycans from adult Drosophila produced both nonsulfated and 4-O-sulfated unsaturated disaccharides, whereas only unsulfated forms were detected in C. elegans. Heparin lyases released disaccharides bearing N-, 2-O-, and 6-O-sulfated species, including mono-, di-, and trisulfated forms. We observed tissue- and stage-specific differences in both chondroitin sulfate and heparan sulfate composition in Drosophila. We have also applied these methods toward the analysis of tout-velu, an EXT-related gene in Drosophila that controls the tissue distribution of the growth factor Hedgehog. The proteins encoded by the vertebrate tumor suppressor genes EXT1 and 2, show heparan sulfate co-polymerase activity, and it has been proposed that tout-velu affects Hedgehog activity via its role in heparan sulfate biosynthesis. Analysis of total glycosaminoglycans from tout-velu mutant larvae show marked reductions in heparan sulfate but not chondroitin sulfate, consistent with its proposed function as a heparan sulfate co-polymerase.

The cell-surface proteoglycan Dally regulates Wingless signalling in Drosophila.

Wingless (Wg) is a member of the Wnt family of growth factors, secreted proteins that control proliferation and differentiation during development. Studies in Drosophila have shown that responses to Wg require cell-surface heparan sulphate, a glycosaminoglycan component of proteoglycans. These findings suggest that a cell-surface proteoglycan is a component of a Wg/Wnt receptor complex. We demonstrate here that the protein encoded by the division abnormally delayed (dally) gene is a cell-surface, heparan-sulphate-modified proteoglycan. dally partial loss-of-function mutations compromise Wg-directed events, and disruption of dally function with RNA interference produces phenotypes comparable to those found with RNA interference of wg or frizzled (fz)/Dfz2. Ectopic expression of Dally potentiates Wg signalling without altering levels of Wg and can rescue a wg partial loss-of-function mutant. We also show that dally, a regulator of Decapentaplegic (Dpp) signalling during post-embryonic development, has tissue-specific effects on Wg and Dpp signalling. Dally can therefore differentially influence signalling mediated by two growth factors, and may form a regulatory component of both Wg and Dpp receptor complexes.

dally, a Drosophila glypican, controls cellular responses to the TGF-beta-related morphogen, Dpp.

Decapentaplegic (Dpp) is a Drosophila member of the Transforming Growth Factor-beta (TGF-beta)/Bone Morphogenetic Protein (BMP) superfamily of growth factors. Dpp serves as a classical morphogen, where concentration gradients of this secreted factor control patterning over many cell dimensions. Regulating the level of Dpp signaling is therefore critical to its function during development. One type of molecule proposed to modulate growth factor signaling at the cell surface are integral membrane proteoglycans. We show here that division abnormally delayed (dally), a Drosophila member of the glypican family of integral membrane proteoglycans is required for normal Dpp signaling during development, affecting cellular responses to this morphogen. Ectopic expression of dally+ can alter the patterning activity of Dpp, suggesting a role for dally+ in modulating Dpp signaling strength. These findings support a role for members of the glypican family in controlling TGF-beta/BMP activity in vivo by affecting signaling at the cell surface.

Regulation of cell cycle synchronization by decapentaplegic during Drosophila eye development.

In the developing Drosophila eye, differentiation is coordinated with synchronized progression through the cell cycle. Signaling mediated by the transforming growth factor-beta-related gene decapentaplegic (dpp) was required for the synchronization of the cell cycle but not for cell fate specification. DPP may affect cell cycle synchronization by promoting cell cycle progression through the G2-M phases. This synchronization is critical for the precise assembly of the eye.

The division abnormally delayed (dally) gene: a putative integral membrane proteoglycan required for cell division patterning during postembryonic development of the nervous system in Drosophila.

We have devised a genetic screen to obtain mutants affecting cell division patterning in the developing central nervous system of Drosophila. The division abnormally delayed (dally) locus was identified using a combination of "enhancer trap" and behavioral screening methods. The ordered cell cycle progression of lamina precursor cells, which generate synaptic target neurons for photoreceptors, is disrupted in dally mutants. The first of two lamina precursor cell divisions shows a delayed entry into mitosis. The second division, one that is triggered by an intercellular signal from photoreceptor axons, fails to take place. Similar to lamina precursors, cells that generate the ommatidia of the adult eye show two synchronized divisions found along the morphogenetic furrow in the eye disc and the first division cycle in dally mutants displays a delayed progression into M phase like that found in the first lamina precursor cell division. dally mutations also affect viability and produce morphological defects in several adult tissues, including the eye, antenna, wing and genitalia. Sequencing of a dally cDNA reveals a potential open reading frame of 626 amino acids with homology to a family of Glypican-related integral membrane proteoglycans. These heparan sulfate-containing proteins are attached to the external leaflet of the plasma membrane via a glycosylphosphatidylinositol linkage. Heparan sulfate proteoglycans may serve as co-receptors for a variety of secreted proteins including fibroblast growth factor, vascular endothelial growth factor, hepatocyte growth factor and members of the Wnt, TGF-beta and Hedgehog families. The cell division defects found in dally mutants implicate the Glypican group of integral membrane proteoglycans in the control of cell division during development.

Regulation of the G1-S transition in postembryonic neuronal precursors by axon ingrowth.

In the newly cellularized Drosophila embryo, progress through the cell cycle is regulated at the G2-M transition. We have examined cell-cycle regulation later in Drosophila development, in a group of postembryonic neuronal precursors. The S-phase precursor cells, which generate photoreceptor target neurons (lamina neurons) in the central nervous system, are not present in the absence of photoreceptor innervation. Here we report that axons selectively approach G1-phase precursors. Without axon ingrowth, lamina precursors do not enter their final S phase and by several criteria, arrest in the preceding G1 phase. These findings provide evidence that at this stage in development the control of cell division can occur at the G1-S transition.

The influence of retinal innervation on neurogenesis in the first optic ganglion of Drosophila.

We have examined the influence of retinal innervation on the development of target neurons in the first optic ganglion, the lamina, of D. melanogaster. Mitotically active lamina precursor cells (LPCs), which normally produce lamina neurons, are absent in mutants that lack retinal innervation, while other proliferative centers appear unaffected. Reducing the number of innervating photoreceptor axons results in fewer mitotic LPCs. In glass mutants photoreceptors project to abnormal locations and LPCs are found adjacent to these aberrant projections. We conclude that the arrival of photoreceptor axons in the larval brain initiates, directly or indirectly, cell division to produce lamina neurons. Our results provide an explanation for how the synchronous development of these two interacting systems is coordinated.

In vivo "photofootprint" changes at sequences between the yeast GAL1 upstream activating sequence and "TATA" element require activated GAL4 protein but not a functional TATA element.

Transcription of the yeast GAL1 and GAL10 genes is induced by growth on galactose. Using the technique of photofootprinting in vivo, we previously documented equivalent transcription-dependent footprints within the putative "TATA" elements of both genes. To explore the functional significance of these observations, we created a 3-base-pair substitution mutation within the GAL1 promoter TATA element, which disrupted the ATATAA consensus sequence but left intact the photomodification targets. The mutation reduced galactose-induced RNA levels by a factor of 100. The mutant promoter no longer displayed the characteristic TATA sequence footprint, supporting the hypothesis that transcription activation involves the binding of a TATA box factor. We also observed a collection of transcription-correlated alterations in the modification pattern at sites between the UASG and the GAL1 TATA element, within sequences that are not required for inducible transcription. These patterns, characteristic of the induced wild-type GAL1 gene, were still galactose inducible with the TATA mutant GAl1 promoter, despite the low level of transcription from this promoter. We conclude that the GAL4-dependent protein/DNA structure responsible for the altered pattern within nonessential sequences is therefore not strictly coupled to an active TATA element or to high levels of expression. Nonetheless, the patterns probably reflect a stable protein-dependent structure that accompanies assembly of the transcription initiation complex.

In vivo DNA-binding properties of a yeast transcription activator protein.

UV light can serve as a molecular probe to identify DNA-protein interactions at nucleotide level resolution from intact yeast cells. We have used the photofootprinting technique to determine during which of three regulated states (uninduced, induced, and catabolite repressed) the transcriptional activator protein encoded by GAL4 binds to its recognition sites within the GAL1-GAL10 upstream activating sequence (UASG). GAL4 protein is bound to at least four, and probably five, related sequence blocks within UASG under both induced and uninduced states. GAL4-dependent photofootprints are lost under conditions of catabolite repression. We observed no footprint patterns unique to catabolite-repressed cells, which suggests that binding of a repressor to the UASG is not involved in this process. Photofootprints of the GAL10 TATA element are strictly correlated with transcription: uninduced, catabolite-repressed, and delta gal4 cells exhibit footprints characteristic of the inactive promoter; induced and delta gal80 cells, which express GAL10 constitutively, display footprints unique to the actively transcribed gene.

Photofootprinting in vivo detects transcription-dependent changes in yeast TATA boxes.

To elucidate critical steps in the transcription initiation process, we have devised a protocol for obtaining information about DNA structure and DNA-protein interactions at nucleotide level resolution from intact yeast cells. Our procedure combines the ultraviolet light 'footprinting' method developed by Becker and Wang with the 'genomic sequencing' technique described by Church and Gilbert. Using this approach we were able to detect the binding of GAL 4 protein at sites within the upstream activating sequence (UASG) previously mapped using other in vivo and in vitro foot-printing procedures. We also observed transcription-dependent changes in sensitivity of DNA to ultraviolet-induced covalent modification at several positions between the upstream activating sequence and the transcription initiation sites of the GAL 1 and GAL 10 genes. The most prominent of these changes occurs at a common site within the putative 'TATA' boxes of the two genes. Ultraviolet modification at this site is enhanced only in transcriptionally active promoters.

Supercoil-dependent features of DNA structure at Drosophila locus 67B1.

We have analyzed the pattern of supercoil-dependent, single strand-specific nuclease cleavage sites across 11.6 kb (11.6 X 10(3) base-pairs) of cloned Drosophila melanogaster DNA from locus 67B1. This region contains coding sequences for the heat shock proteins hsp23, hsp26 and hsp28 as well as for a 1.6 kb developmentally regulated transcript (R). Two major sites are detected on digestion with S1 nuclease or mung bean nuclease. The most prominent site maps 100 base-pairs upstream of hsp26 in a very pyrimidine-rich region adjacent to a known region of chromatin DNAase I hypersensitivity. The other site is located approximately 800 base-pairs upstream of hsp28 in an area devoid of such chromatin-specific features. BAL31 nuclease produces a different array, with three to six strong cleavages located in the spacer DNA approx. 0.1 to 1.0 kb upstream of the DNAase I hypersensitive sites of hsp28, hsp23 and R. Thus, for each gene in the cluster a localized sequence sensitive to the winding state of the DNA is observed 5' to the gene. However, there is no precise coincidence of any of the major sites sensitive to BAL31 nuclease in the supercoiled plasmid with the sequences sensitive to DNAase I in chromatin. While all of the enzymes utilized in this study have prominent single strand-specific endonucleolytic activity, it is clear that they recognize different variants in the DNA structure induced by supercoiling. At least two classes of DNA perturbation have been detected.