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

Lactobacillus melliventris promotes hive productivity and immune functionality in Bombus terrestris performance in the greenhouse

Bumblebees are important pollinators in agricultural ecosystems, but their abundance is declining globally. There is an urgent need to protect bumblebee health and their pollination services. Bumblebees possess specialized gut microbiota with potential to be used as probiotics to help defend at-risk bumblebee populations. However, evidence for probiotic benefits on bumblebees is lacking. Here, we evaluated how supplementation with Lactobacillus melliventris isolated from bumblebee gut affected the colony development of Bombus terrestris. This native strain colonized robustly and persisted long-term in bumblebees, leading to a significantly higher quality of offspring. Subsequently, the tyrosine pathway was upregulated in the brain and fat body, while the Wnt and mTOR pathways of the gut were downregulated. Notably, the field experiment in the greenhouse revealed the supplementation of L. melliventris led to a 2.5-fold increase in the bumblebee survival rate and a more than 10% increase in the number of flowers visited, indicating a better health condition and pollination ability in field conditions. Our study represents a first screening for the potential use of the native gut member, L. melliventris, as probiotic strains in hive supplement for bumblebee breeding, which may be a practical approach to improve immunity and hive health.

Bone morphogenetic protein signaling: the pathway and its regulation

In the mid-1960s, bone morphogenetic proteins (BMPs) were first identified in the extracts of bone to have the remarkable ability to induce heterotopic bone. When the Drosophila gene decapentaplegic (dpp) was first identified to share sequence similarity with mammalian BMP2/BMP4 in the late-1980s, it became clear that secreted BMP ligands can mediate processes other than bone formation. Following this discovery, collaborative efforts between Drosophila geneticists and mammalian biochemists made use of the strengths of their respective model systems to identify BMP signaling components and delineate the pathway. The ability to conduct genetic modifier screens in Drosophila with relative ease was critical in identifying the intracellular signal transducers for BMP signaling and the related transforming growth factor-beta/activin signaling pathway. Such screens also revealed a host of genes that encode other core signaling components and regulators of the pathway. In this review, we provide a historical account of this exciting time of gene discovery and discuss how the field has advanced over the past 30 years. We have learned that while the core BMP pathway is quite simple, composed of 3 components (ligand, receptor, and signal transducer), behind the versatility of this pathway lies multiple layers of regulation that ensures precise tissue-specific signaling output. We provide a sampling of these discoveries and highlight many questions that remain to be answered to fully understand the complexity of BMP signaling.

Dally is not essential for Dpp spreading or internalization but for Dpp stability by antagonizing Tkv-mediated Dpp internalization

Dpp/BMP acts as a morphogen to provide positional information in the Drosophila wing disc. Key cell-surface molecules to control Dpp morphogen gradient formation and signaling are heparan sulfate proteoglycans (HSPGs). In the wing disc, two HSPGs, the glypicans Division abnormally delayed (Dally) and Dally-like (Dlp) have been suggested to act redundantly to control these processes through direct interaction of their heparan sulfate (HS) chains with Dpp. Based on this assumption, a number of models on how glypicans control Dpp gradient formation and signaling have been proposed, including facilitating or hindering Dpp spreading, stabilizing Dpp on the cell surface, or recycling Dpp. However, how distinct HSPGs act remains largely unknown. Here, we generate genome-engineering platforms for the two glypicans and find that only Dally is critical for Dpp gradient formation and signaling through interaction of its core protein with Dpp. We also find that this interaction is not sufficient and that the HS chains of Dally are essential for these functions largely without interacting with Dpp. We provide evidence that the HS chains of Dally are not essential for spreading or recycling of Dpp but for stabilizing Dpp on the cell surface by antagonizing receptor-mediated Dpp internalization. These results provide new insights into how distinct HSPGs control morphogen gradient formation and signaling during development.

The spatiotemporal pattern of glypican coordinates primordial germ cell differentiation with ovary development

The establishment, proliferation, and differentiation of stem cells are coordinated with organ development and regulated by the signals in the microenvironment. Prior to gonad formation, how primordial germ cells (PGC) differentiate spatiotemporally to coordinate with gonadogenesis is unclear. In adult ovary, drosophila extracellular glypican Dally in germline stem cell (GSC) niche promotes BMP signaling to inhibit germline differentiation. Here we investigated the relation between the fate of PGC and the spatiotemporal pattern of glypican during ovary development. We found that Dally in ovarian soma assisted BMP signaling to prevent PGC from precocious differentiation. Dally's presence raises the "hurdle" for ecdysone peaks to eventually remove the transcription factor Kr and de-repress pro-differentiation factor, temporally postponing PGC differentiation until GSC niche establishment. The spatiotemporal glypican in somatic matrix assists PGC to integrate the ovarian local BMP and organismal steroid signals that coordinate PGC's program with organ/body development to maximize reproductive potential.

Differential heparan sulfate dependency of the Drosophila glypicans

Heparan sulfate proteoglycans (HSPGs) are composed of a core protein and glycosaminoglycan (GAG) chains and serve as coreceptors for many growth factors and morphogens. To understand the molecular mechanisms by which HSPGs regulate morphogen gradient formation and signaling, it is important to determine the relative contributions of the carbohydrate and protein moieties to the proteoglycan function. To address this question, we generated ΔGAG alleles for dally and dally-like protein (dlp), two Drosophila HSPGs of the glypican family, in which all GAG-attachment serine residues are substituted to alanine residues using CRISPR/Cas9 mutagenesis. In these alleles, the glypican core proteins are expressed from the endogenous loci with no GAG modification. Analyses of the dallyΔGAG allele defined Dally functions that do not require heparan sulfate (HS) chains and that need both core protein and HS chains. We found a new, dallyΔGAG-specific phenotype, the formation of a posterior ectopic vein, which we have never seen in the null mutants. Unlike dallyΔGAG, dlpΔGAG mutants do not show most of the dlp null mutant phenotypes, suggesting that HS chains are dispensable for these dlp functions. As an exception, HS is essentially required for Dlp's activity at the neuromuscular junction. Thus, Drosophila glypicans show strikingly different levels of HS dependency. The ΔGAG mutant alleles of the glypicans serve as new molecular genetic toolsets highly useful to address important biological questions, such as molecular mechanisms of morphogen gradient formation.

Hedgehog signaling guides migration of primordial germ cells to the Drosophila somatic gonad

In addition to inducing nonautonomous specification of cell fate in both Drosophila and vertebrates, the Hedgehog pathway guides cell migration in a variety of different tissues. Although its role in axon guidance in the vertebrate nervous system is widely recognized, its role in guiding the migratory path of primordial germ cells (PGCs) from the outside surface of the Drosophila embryo through the midgut and mesoderm to the SGPs (somatic gonadal precursors) has been controversial. Here we present new experiments demonstrating (1) that Hh produced by mesodermal cells guides PGC migration, (2) that HMG CoenzymeA reductase (Hmgcr) potentiates guidance signals emanating from the SGPs, functioning upstream of hh and of 2 Hh pathway genes important for Hh-containing cytonemes, and (3) that factors required in Hh receiving cells in other contexts function in PGCs to help direct migration toward the SGPs. We also compare the data reported by 4 different laboratories that have studied the role of the Hh pathway in guiding PGC migration.

A deficiency screen of the 3rd chromosome for dominant modifiers of the Drosophila ER integral membrane protein, Jagunal

The mechanism surrounding chromosome inheritance during cell division has been well documented, however, organelle inheritance during mitosis is less understood. Recently, the endoplasmic reticulum (ER) has been shown to reorganize during mitosis, dividing asymmetrically in proneuronal cells prior to cell fate selection, indicating a programmed mechanism of inheritance. ER asymmetric partitioning in proneural cells relies on the highly conserved ER integral membrane protein, Jagunal (Jagn). Knockdown of Jagn in the compound Drosophila eye displays a pleotropic rough eye phenotype in 48% of the progeny. To identify genes involved in Jagn dependent ER partitioning pathway, we performed a dominant modifier screen of the 3rd chromosome for enhancers and suppressors of this Jagn-RNAi-induced rough eye phenotype. We screened through 181 deficiency lines covering the 3L and 3R chromosomes and identified 12 suppressors and 10 enhancers of the Jagn-RNAi phenotype. Based on the functions of the genes covered by the deficiencies, we identified genes that displayed a suppression or enhancement of the Jagn-RNAi phenotype. These include Division Abnormally Delayed (Dally), a heparan sulfate proteoglycan, the γ-secretase subunit Presenilin, and the ER resident protein Sec63. Based on our understanding of the function of these targets, there is a connection between Jagn and the Notch signaling pathway. Further studies will elucidate the role of Jagn and identified interactors within the mechanisms of ER partitioning during mitosis.

Regulation of morphogen pathways by a Drosophila chondroitin sulfate proteoglycan Windpipe

Morphogens provide quantitative and robust signaling systems to achieve stereotypic patterning and morphogenesis. Heparan sulfate (HS) proteoglycans (HSPGs) are key components of such regulatory feedback networks. In Drosophila, HSPGs serve as co-receptors for a number of morphogens, including Hedgehog (Hh), Wingless (Wg), Decapentaplegic (Dpp) and Unpaired (Upd, or Upd1). Recently, Windpipe (Wdp), a chondroitin sulfate (CS) proteoglycan (CSPG), was found to negatively regulate Upd and Hh signaling. However, the roles of Wdp, and CSPGs in general, in morphogen signaling networks are poorly understood. We found that Wdp is a major CSPG with 4-O-sulfated CS in Drosophila. Overexpression of wdp modulates Dpp and Wg signaling, showing that it is a general regulator of HS-dependent pathways. Although wdp mutant phenotypes are mild in the presence of morphogen signaling buffering systems, this mutant in the absence of Sulf1 or Dally, molecular hubs of the feedback networks, produces high levels of synthetic lethality and various severe morphological phenotypes. Our study indicates a close functional relationship between HS and CS, and identifies the CSPG Wdp as a novel component in morphogen feedback pathways.

Heparan sulfate proteoglycans (HSPGs) of the ocular lens

Heparan sulfate proteoglycans (HSPGs) reside in most cells; on their surface, in the pericellular milieu and/or extracellular matrix. In the eye, HSPGs can orchestrate the activity of key signalling molecules found in the ocular environment that promote its development and homeostasis. To date, our understanding of the specific roles played by individual HSPG family members, and the heterogeneity of their associated sulfated HS chains, is in its infancy. The crystalline lens is a relatively simple and well characterised ocular tissue that provides an ideal stage to showcase and model the expression and unique roles of individual HSPGs. Individual HSPG core proteins are differentially localised to eye tissues in a temporal and spatial developmental- and cell-type specific manner, and their loss or functional disruption results in unique phenotypic outcomes for the lens, and other ocular tissues. More recent work has found that different HS sulfation enzymes are also presented in a cell- and tissue-specific manner, and that disruption of these different sulfation patterns affects specific HS-protein interactions. Not surprisingly, these sulfated HS chains have also been reported to be required for lens and eye development, with dysregulation of HS chain structure and function leading to pathogenesis and eye-related phenotypes. In the lens, HSPGs undergo significant and specific changes in expression and function that can drive pathology, or in some cases, promote tissue repair. As master signalling regulators, HSPGs may one day serve as valuable biomarkers, and even as putative targets for the development of novel therapeutics, not only for the eye but for many other systemic pathologies.

Regulation of autophagy, lipid metabolism, and neurodegenerative pathology by heparan sulfate proteoglycans

Heparan sulfate modified proteins or proteoglycans (HSPGs) are an abundant class of cell surface and extracellular matrix molecules. They serve important co-receptor functions in the regulation of signaling as well as membrane trafficking. Many of these activities directly affect processes associated with neurodegeneration including uptake and export of Tau protein, disposition of Amyloid Precursor Protein-derived peptides, and regulation of autophagy. In this review we focus on the impact of HSPGs on autophagy, membrane trafficking, mitochondrial quality control and biogenesis, and lipid metabolism. Disruption of these processes are a hallmark of Alzheimer's disease (AD) and there is evidence that altering heparan sulfate structure and function could counter AD-associated pathological processes. Compromising presenilin function in several systems has provided instructive models for understanding the molecular and cellular underpinnings of AD. Disrupting presenilin function produces a constellation of cellular deficits including accumulation of lipid, disruption of autophagosome to lysosome traffic and reduction in mitochondrial size and number. Inhibition of heparan sulfate biosynthesis has opposing effects on all these cellular phenotypes, increasing mitochondrial size, stimulating autophagy flux to lysosomes, and reducing the level of intracellular lipid. These findings suggest a potential mechanism for countering pathology found in AD and related disorders by altering heparan sulfate structure and influencing cellular processes disrupted broadly in neurodegenerative disease. Vertebrate and invertebrate model systems, where the cellular machinery of autophagy and lipid metabolism are conserved, continue to provide important translational guideposts for designing interventions that address the root cause of neurodegenerative pathology.

Complex protein interactions mediate Drosophila Lar function in muscle tissue

The type IIa family of receptor protein tyrosine phosphatases (RPTPs), including Lar, RPTPσ and RPTPδ, are well-studied in coordinating actin cytoskeletal rearrangements during axon guidance and synaptogenesis. To determine whether this regulation is conserved in other tissues, interdisciplinary approaches were utilized to study Lar-RPTPs in the Drosophila musculature. Here we find that the single fly ortholog, Drosophila Lar (Dlar), is localized to the muscle costamere and that a decrease in Dlar causes aberrant sarcomeric patterning, deficits in larval locomotion, and integrin mislocalization. Sequence analysis uncovered an evolutionarily conserved Lys-Gly-Asp (KGD) signature in the extracellular region of Dlar. Since this tripeptide sequence is similar to the integrin-binding Arg-Gly-Asp (RGD) motif, we tested the hypothesis that Dlar directly interacts with integrin proteins. However, structural analyses of the fibronectin type III domains of Dlar and two vertebrate orthologs that include this conserved motif indicate that this KGD tripeptide is not accessible and thus unlikely to mediate physical interactions with integrins. These results, together with the proteomics identification of basement membrane (BM) proteins as potential ligands for type IIa RPTPs, suggest a complex network of protein interactions in the extracellular space that may mediate Lar function and/or signaling in muscle tissue.

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.

Hedgehog pathway modulation by glypican 3-conjugated heparan sulfate

Glypicans are a family of cell surface heparan sulfate proteoglycans that play critical roles in multiple cell signaling pathways. Glypicans consist of a globular core, an unstructured stalk modified with sulfated glycosaminoglycan chains, and a glycosylphosphatidylinositol anchor. Though these structural features are conserved, their individual contribution to glypican function remains obscure. Here, we investigate how glypican 3 (GPC3), which is mutated in Simpson-Golabi-Behmel tissue overgrowth syndrome, regulates Hedgehog signaling. We find that GPC3 is necessary for the Hedgehog response, surprisingly controlling a downstream signal transduction step. Purified GPC3 ectodomain rescues signaling when artificially recruited to the surface of GPC3-deficient cells but has dominant-negative activity when unattached. Strikingly, the purified stalk, modified with heparan sulfate but not chondroitin sulfate, is necessary and sufficient for activity. Our results demonstrate a novel function for GPC3-associated heparan sulfate and provide a framework for the functional dissection of glycosaminoglycans by in vivo biochemical complementation. This article has an associated First Person interview with the first author of the paper.

Molecular Genetic Techniques for the Proteoglycan Functions in Drosophila

Several classes of heparan sulfate proteoglycan (HSPG) core proteins and all HS biosynthetic/modifying enzymes are evolutionarily conserved from human to Drosophila melanogaster. This genetically tractable model offers highly sophisticated techniques to manipulate gene function in a spatially and temporally controlled manner. Thus, Drosophila genetics has been a powerful system to explore functions of HSPGs in vivo. In this chapter, we will introduce three genetic techniques available in Drosophila: TARGET (temporal and regional gene expression targeting), MARCM (mosaic analysis with a repressible cell marker), and FLP-Out.

Glypicans define unique roles for the Hedgehog co-receptors boi and ihog in cytoneme-mediated gradient formation

The conserved family of Hedgehog (Hh) signaling proteins plays a key role in cell–cell communication in development, tissue repair, and cancer progression, inducing distinct concentration-dependent responses in target cells located at short and long distances. One simple mechanism for long distance dispersal of the lipid modified Hh is the direct contact between cell membranes through filopodia-like structures known as cytonemes. Here we have analyzed in Drosophila the interaction between the glypicans Dally and Dally-like protein, necessary for Hh signaling, and the adhesion molecules and Hh coreceptors Ihog and Boi. We describe that glypicans are required to maintain the levels of Ihog, but not of Boi. We also show that the overexpression of Ihog, but not of Boi, regulates cytoneme dynamics through their interaction with glypicans, the Ihog fibronectin III domains being essential for this interaction. Our data suggest that the regulation of glypicans over Hh signaling is specifically given by their interaction with Ihog in cytonemes. Contrary to previous data, we also show that there is no redundancy of Ihog and Boi functions in Hh gradient formation, being Ihog, but not of Boi, essential for the long-range gradient.

Glypicans Dally and Dally-like control injury-induced allodynia in Drosophila

Over 100 million people are challenged by the effects of chronic pain in the United States alone. This burden also impacts the U.S. economy; 600 billion dollars annually is spent on medical care, medications, and lost productivity in the workplace. Current opioid treatments cause adverse effects including nausea, constipation, tolerance, and addiction liability. Nociceptive sensitization is thought to perpetuate chronic pain, but too little is known about its mechanisms. Components of the pathways that sensitize the nociceptors after injury are likely to be valuable targets for novel medications for the relief or prevention of chronic pain. Utilizing the Drosophila melanogaster cell targeting and RNA interference toolkit, we are investigating the bone morphogenetic protein pathway and its role in ultraviolet light injury-induced nociceptive sensitization. Bone morphogenetic proteins are well known as secreted developmental morphogens that control development, but other functions are known. We have previously identified bone morphogenetic protein signaling components used in nociceptors to modulate injury-induced allodynia, including Decapentaplegic (Dpp, orthologous to mammalian bone morphogenetic protein 2/4), and its downstream signaling components. The morphogen Hedgehog has also been shown to be necessary for allodynia following injury. Here, we show that two membrane-embedded regulators of the Dpp and Hedgehog pathways, Dally and Dally-like, are necessary for injury-induced thermal allodynia, as the formation of sensitization was reduced when either component was suppressed. These bone morphogenetic protein components are highly conserved and, because dysregulation of nociceptor sensitization underlies chronic pain, the homologs of Dally and Dally-like may represent novel therapeutic targets in humans challenged by chronic pain. Furthermore, because of their extracellular location, Dally and Dally-like represent attractive therapeutic drug targets because such drugs would not need to cross the plasma membrane.

Notch signaling governs the expression of glypican Dally to define the stem cell niche

Extracellular glypicans play pivotal roles in organogenesis, stem cell maintenance and cancer development. However, the growth phenotypes associated with different levels of glypican are not consistent in development or tumorigenesis. This requires clarification on how the spatial patterns of glypican relate to the distribution of signaling molecules in different cellular contexts, and how glypican expression is regulated. We have previously reported that Dally, one of the glypican members in Drosophila, is required in the niche for the maintenance of germline stem cells (GSCs) via short-range BMP signaling in ovary. However, the regulatory mechanism of glypican pattern in the ovarian stem cell niche remains elusive. Our current data demonstrate that the Notch pathway is genetically upstream of Dally and its function to maintain GSCs relies on Dally expression. Combining yeast and fruit fly genetics, we illustrate that Dally is under the transcriptional control of Notch signaling via the transcription factor Su(H). Further, we assayed human glypicans and disease-associated variants in Drosophila ovary, which can serve as an effective system to evaluate the structure–function relationship of human homologs.

Summary: Spatial regulation of a cell surface glycoprotein defines the territory of germline stem cells.

Characterization of C. elegans Chondroitin Proteoglycans and Their Large Functional and Structural Heterogeneity; Evolutionary Aspects on Structural Differences Between Humans and the Nematode

Proteoglycans regulate important cellular pathways in essentially all metazoan organisms. While considerable effort has been devoted to study structural and functional aspects of proteoglycans in vertebrates, the knowledge of the core proteins and proteoglycan-related functions in invertebrates is relatively scarce, even for C.elegans. This nematode produces a large amount of non-sulfated chondroitin in addition to small amount of low-sulfated chondroitin chains (Chn and CS chains, respectively). Until recently, 9 chondroitin core proteins (CPGs) had been identified in C.elegans, none of which showed any homology to vertebrate counterparts or to other invertebrate core proteins. By using a glycoproteomic approach, we recently characterized the chondroitin glycoproteome of C.elegans, resulting in the identification of 15 novel CPG core proteins in addition to the 9 previously established. Three of the novel core proteins displayed homology to human proteins, indicating that CPG and CSPG core proteins may be more conserved throughout evolution than previously perceived. Bioinformatic analysis of the primary amino acid sequences revealed that the core proteins contained a broad range of functional domains, indicating that specialization of proteoglycan-mediated functions may have evolved early in metazoan evolution. This review specifically discusses our recent data in relation to previous knowledge of core proteins and GAG-attachment sites in Chn and CS proteoglycans of C.elegans and humans, and point out both converging and diverging aspects of proteoglycan evolution.

Molecular control of the female germline stem cell niche size in Drosophila

Adult stem cells have a unique capacity to renew themselves and generate differentiated cells that are needed in the body. These cells are recruited and maintained by the surrounding microenvironment, known as the stem cell niche, during organ development. Thus, the stem cell niche is required for proper tissue homeostasis, and its dysregulation is associated with tumorigenesis and tissue degeneration. The identification of niche components and the mechanisms that regulate niche establishment and maintenance, however, are just beginning to be uncovered. Germline stem cells (GSCs) of the Drosophila ovary provide an excellent model for studying the stem cell niche in vivo because of their well-characterized cell biology and the availability of genetic tools. In this review, we introduce the ovarian GSC niche, and the key signaling pathways for niche precursor segregation, niche specification, and niche extracellular environment establishment and niche maintenance that are involved in regulating niche size during development and adulthood.

Heparan sulfate negatively regulates intestinal stem cell proliferation in Drosophila adult midgut

Tissue homeostasis is maintained by differentiated progeny of residential stem cells. Both extrinsic signals and intrinsic factors play critical roles in the proliferation and differentiation of adult intestinal stem cells (ISCs). However, how extrinsic signals are transduced into ISCs still remains unclear. Here, we find that heparan sulfate (HS), a class of glycosaminoglycan (GAG) chains, negatively regulates progenitor proliferation and differentiation to maintain midgut homeostasis under physiological conditions. Interestingly, HS depletion in progenitors results in inactivation of Decapentaplegic (Dpp) signaling. Dpp signal inactivation in progenitors resembles HS-deficient intestines. Ectopic Dpp signaling completely rescued the defects caused by HS depletion. Taken together, these data demonstrate that HS is required for Dpp signaling to maintain midgut homeostasis. Our results provide insight into the regulatory mechanisms of how extrinsic signals are transduced into stem cells to regulate their proliferation and differentiation.

A Genetic Screen Using the Drosophila melanogaster TRiP RNAi Collection To Identify Metabolic Enzymes Required for Eye Development

The metabolic enzymes that compose glycolysis, the citric acid cycle, and other pathways within central carbon metabolism have emerged as key regulators of animal development. These enzymes not only generate the energy and biosynthetic precursors required to support cell proliferation and differentiation, but also moonlight as regulators of transcription, translation, and signal transduction. Many of the genes associated with animal metabolism, however, have never been analyzed in a developmental context, thus highlighting how little is known about the intersection of metabolism and development. Here we address this deficiency by using the Drosophila TRiP RNAi collection to disrupt the expression of over 1,100 metabolism-associated genes within cells of the eye imaginal disc. Our screen not only confirmed previous observations that oxidative phosphorylation serves a critical role in the developing eye, but also implicated a host of other metabolic enzymes in the growth and differentiation of this organ. Notably, our analysis revealed a requirement for glutamine and glutamate metabolic processes in eye development, thereby revealing a role of these amino acids in promoting Drosophila tissue growth. Overall, our analysis highlights how the Drosophila eye can serve as a powerful tool for dissecting the relationship between development and metabolism.

Bringing Heparan Sulfate Glycomics Together with Proteomics for the Design of Novel Therapeutics: A Historical Perspective

Increasing knowledge of how peptides bind saccharides, and of how saccharides bind peptides, is starting to revolutionize understanding of cell-extracellular matrix relationships. Here, a historical perspective is taken of the relationship between heparan sulfate glycosaminoglycans and how they interact with peptide growth factors in order to both drive and modulate signaling through the appropriate cognate receptors. Such knowledge is guiding the preparation of targeted sugar mimetics that will impact the treatment of many different kinds of diseases, including cancer.

Wingless Signaling: A Genetic Journey from Morphogenesis to Metastasis

This FlyBook chapter summarizes the history and the current state of our understanding of the Wingless signaling pathway. Wingless, the fly homolog of the mammalian Wnt oncoproteins, plays a central role in pattern generation during development. Much of what we know about the pathway was learned from genetic and molecular experiments in Drosophila melanogaster, and the core pathway works the same way in vertebrates. Like most growth factor pathways, extracellular Wingless/Wnt binds to a cell surface complex to transduce signal across the plasma membrane, triggering a series of intracellular events that lead to transcriptional changes in the nucleus. Unlike most growth factor pathways, the intracellular events regulate the protein stability of a key effector molecule, in this case Armadillo/β-catenin. A number of mysteries remain about how the "destruction complex" destabilizes β-catenin and how this process is inactivated by the ligand-bound receptor complex, so this review of the field can only serve as a snapshot of the work in progress.

Mutation update for the GPC3 gene involved in Simpson-Golabi-Behmel syndrome and review of the literature

Simpson-Golabi-Behmel syndrome (SGBS) is an X-linked multiple congenital anomalies and overgrowth syndrome caused by a defect in the glypican-3 gene (GPC3). Until now, GPC3 mutations have been reported in isolated cases or small series and the global genotypic spectrum of these mutations has never been delineated. In this study, we review the 57 previously described GPC3 mutations and significantly expand this mutational spectrum with the description of 29 novel mutations. Compiling our data and those of the literature, we provide an overview of 86 distinct GPC3 mutations identified in 120 unrelated families, ranging from single nucleotide variations to complex genomic rearrangements and dispersed throughout the entire coding region of GPC3. The vast majority of them are deletions or truncating mutations (frameshift, nonsense mutations) predicted to result in a loss-of-function. Missense mutations are rare and the two which were functionally characterized, impaired GPC3 function by preventing GPC3 cleavage and cell surface addressing respectively. This report by describing for the first time the wide mutational spectrum of GPC3 could help clinicians and geneticists in interpreting GPC3 variants identified incidentally by high-throughput sequencing technologies and also reinforces the need for functional validation of non-truncating mutations (missense, in frame mutations, duplications).

Heparan sulfate proteoglycans in Drosophila neuromuscular development

Heparan sulfate proteoglycans (HSPGs) are glycoconjugates bearing heparan sulfate (HS) chains covalently attached to core proteins, which are ubiquitously distributed on the cell surface and in the extracellular matrix. HSPGs interact with a number of molecules mainly through HS chains, which play critical roles in diverse physiological and disease processes. Among these, recent vertebrate studies showed that HSPGs are closely involved in synapse development and function. However, the detailed molecular mechanisms remain elusive. Genetic studies from fruit flies, Drosophila melanogaster, have begun to reveal the molecular mechanisms by which HSPGs regulate synapse formation at neuromuscular junctions (NMJs). In this review, we introduce Drosophila studies showing how HSPGs regulate various signaling pathways in developing NMJs. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Glypican-5 is a tumor suppressor in non-small cell lung cancer cells

Glypican-5 (GPC5) belongs to the glypican family of proteoglycans that have been implicated in a variety of physiological processes, ranging from cell proliferation to morphogenesis. However, the role of GPC5 in human cancer remains poorly understood. We report that knockdown of GPC5 in bronchial epithelial cells promoted, and forced expression of GPC5 in non-small lung cancer (NSCLC) cells suppressed, the anchorage-independent cell growth. In vivo, expression of GPC5 inhibited xenograft tumor growth of NSCLC cells. Furthermore, we found that GPC5 was expressed predominantly as a membrane protein, and its expression led to diminished phosphorylation of several oncogenic receptor tyrosine kinases, including the ERBB family members ERBB2 and ERBB3, which play critical roles in lung tumorigenesis. Collectively, our results suggest that GPC5 may act as a tumor suppressor, and reagents that activate GPC5 may be useful for treating NSCLC.

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GPC5 suppresses anchorage-independent growth of lung cancer cells.

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GPC5 suppresses xenograft growth of lung cancer cells.

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GPC5 is localized to the membrane and suppresses oncogenic RTKs.

Functions of Heparan Sulfate Proteoglycans in Development: Insights From Drosophila Models

Heparan sulfate proteoglycans (HSPGs) are a class of carbohydrate-modified proteins involved in key biological processes, including growth factor signaling, cell adhesion, and enzymatic catalysis. HSPGs serve as coreceptors for a number of ligand molecules to regulate their signaling and distribution. These HS-dependent factors include fibroblast growth factors, bone morphogenetic proteins, Wnt-related factors, hedgehog, and cytokines. Several classes of HSPGs are evolutionarily conserved from humans to the genetically tractable model organism Drosophila. Sophisticated molecular genetic tools available in Drosophila provide for a powerful system to address unanswered questions regarding in vivo functions of HSPGs. These studies have highlighted the functions of HSPGs in the regulation of significant developmental events, such as morphogen gradient formation, nervous system formation, and the stem cell niche. Drosophila genetics has also established HSPGs as key factors in feedback controls that ensure robustness in developmental systems.

GPI-AP release in cellular, developmental, and reproductive biology

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) contain a covalently linked GPI anchor located on outer cell membranes. GPI-APs are ubiquitously conserved from protozoa to vertebrates and are critical for physiological events such as development, immunity, and neurogenesis in vertebrates. Both membrane-anchored and soluble GPI-APs play a role in regulating their protein conformation and functional properties. Several pathways mediate the release of GPI-APs from the plasma membrane by vesiculation or cleavage. Phospholipases and putative substrate-specific GPI-AP-releasing enzymes, such as NOTUM, glycerophosphodiesterase 2, and angiotensin-converting enzyme, have been characterized in mammals. Here, the protein modifications resulting from the cleavage of the GPI anchor are discussed in the context of its physiological functions.

Dally Proteoglycan Mediates the Autonomous and Nonautonomous Effects on Tissue Growth Caused by Activation of the PI3K and TOR Pathways

How cells acquiring mutations in tumor suppressor genes outcompete neighboring wild-type cells is poorly understood. The phosphatidylinositol 3-kinase (PI3K)–phosphatase with tensin homology (PTEN) and tuberous sclerosis complex (TSC)-target of rapamycin (TOR) pathways are frequently activated in human cancer, and this activation is often causative of tumorigenesis. We utilized the Gal4-UAS system in Drosophila imaginal primordia, highly proliferative and growing tissues, to analyze the impact of restricted activation of these pathways on neighboring wild-type cell populations. Activation of these pathways leads to an autonomous induction of tissue overgrowth and to a remarkable nonautonomous reduction in growth and proliferation rates of adjacent cell populations. This nonautonomous response occurs independently of where these pathways are activated, is functional all throughout development, takes place across compartments, and is distinct from cell competition. The observed autonomous and nonautonomous effects on tissue growth rely on the up-regulation of the proteoglycan Dally, a major element involved in modulating the spreading, stability, and activity of the growth promoting Decapentaplegic (Dpp)/transforming growth factor β(TGF-β) signaling molecule. Our findings indicate that a reduction in the amount of available growth factors contributes to the outcompetition of wild-type cells by overgrowing cell populations. During normal development, the PI3K/PTEN and TSC/TOR pathways play a major role in sensing nutrient availability and modulating the final size of any developing organ. We present evidence that Dally also contributes to integrating nutrient sensing and organ scaling, the fitting of pattern to size.

The loss of tumor suppressor genes induces a nonautonomous reduction of growth and proliferation rates in adjacent cell populations by competing for available growth factors; the proteoglycan Dally helps to mediate this effect.

The final size of a developing organ is finely modulated by nutrient conditions through the activity of nutrient sensing pathways, and deregulation of these pathways is often causative of tumorigenesis. Besides the well-known roles of these pathways in inducing tissue and cell growth, here we identify a nonautonomous effect of activation of these pathways on growth and proliferation rates and on the final size of neighboring cell populations. We reveal that the observed autonomous and nonautonomous effects on tissue growth and proliferation rates rely on the up-regulation of the proteoglycan Dally, a major factor involved in modulating the spreading, stability, and activity of the growth promoting Decapentaplegic (Dpp)/transforming growth factor β(TGF-β) signaling molecule. Our data indicate that a reduction in the amount of available growth factors contributes to the outcompetition of wild-type cells by overgrowing cell populations. Whereas nutrient-sensing pathways modulate the final size of the adult structure according to nutrient availability to the feeding animal, Dpp plays an organ-intrinsic role in the coordination of growth and patterning. We identify the proteoglycan Dally as the rate-limiting factor that contributes to the tissue-autonomous and nonautonomous effects on growth caused by targeted activation of the nutrient-sensing pathways. Thus, our results unravel a role of Dally as a molecular bridge between the organ-intrinsic and organ-extrinsic mechanisms that regulate organ size.

Glypican4 promotes cardiac specification and differentiation by attenuating canonical Wnt and Bmp signaling

Glypicans are heparan sulphate proteoglycans (HSPGs) attached to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor, and interact with various extracellular growth factors and receptors. The Drosophila division abnormal delayed (dally) was the first glypican loss-of-function mutant described that displays disrupted cell divisions in the eye and morphological defects in the wing. In human, as in most vertebrates, six glypican-encoding genes have been identified (GPC1-6), and mutations in several glypican genes cause multiple malformations including congenital heart defects. To understand better the role of glypicans during heart development, we studied the zebrafish knypek mutant, which is deficient for Gpc4. Our results demonstrate that knypek/gpc4 mutant embryos display severe cardiac defects, most apparent by a strong reduction in cardiomyocyte numbers. Cell-tracing experiments, using photoconvertable fluorescent proteins and genetic labeling, demonstrate that Gpc4 'Knypek' is required for specification of cardiac progenitor cells and their differentiation into cardiomyocytes. Mechanistically, we show that Bmp signaling is enhanced in the anterior lateral plate mesoderm of knypek/gpc4 mutants and that genetic inhibition of Bmp signaling rescues the cardiomyocyte differentiation defect observed in knypek/gpc4 embryos. In addition, canonical Wnt signaling is upregulated in knypek/gpc4 embryos, and inhibiting canonical Wnt signaling in knypek/gpc4 embryos by overexpression of the Wnt inhibitor Dkk1 restores normal cardiomyocyte numbers. Therefore, we conclude that Gpc4 is required to attenuate both canonical Wnt and Bmp signaling in the anterior lateral plate mesoderm to allow cardiac progenitor cells to specify and differentiate into cardiomyocytes. This provides a possible explanation for how congenital heart defects arise in glypican-deficient patients.

Genetic approaches in the study of heparan sulfate functions in Drosophila

Several classes of heparan sulfate proteoglycan (HSPG) core proteins and all HS biosynthetic/modifying enzymes are evolutionarily conserved from human to Drosophila melanogaster. This genetically tractable model offers highly sophisticated techniques to manipulate gene function in a spatially and temporally controlled manner. Thus, Drosophila has been a powerful system to explore the functions of HSPGs in vivo. In this chapter, we will introduce two genetic techniques available in Drosophila: TARGET (temporal and regional gene expression targeting) and MARCM (mosaic analysis with a repressible cell marker).

Sonic hedgehog processing and release are regulated by glypican heparan sulfate proteoglycans

All Hedgehog morphogens are released from producing cells, despite being synthesized as N- and C-terminally lipidated molecules, a modification that firmly tethers them to the cell membrane. We have previously shown that proteolytic removal of both lipidated peptides, called shedding, releases bioactive Sonic hedgehog (Shh) morphogens from the surface of transfected Bosc23 cells. Using in vivo knockdown together with in vitro cell culture studies, we now show that glypican heparan sulfate proteoglycans regulate this process, through their heparan sulfate chains, in a cell autonomous manner. Heparan sulfate specifically modifies Shh processing at the cell surface, and purified glycosaminoglycans enhance the proteolytic removal of N- and C-terminal Shh peptides under cell-free conditions. The most likely explanation for these observations is direct Shh processing in the extracellular compartment, suggesting that heparan sulfate acts as a scaffold or activator for Shh ligands and the factors required for their turnover. We also show that purified heparan sulfate isolated from specific cell types and tissues mediates the release of bioactive Shh from pancreatic cancer cells, revealing a previously unknown regulatory role for these versatile molecules in a pathological context.

The transcription factor optomotor-blind antagonizes Drosophila haltere growth by repressing decapentaplegic and hedgehog targets

In Drosophila, decapentaplegic, which codes for a secreted signaling molecule, is activated by the Hedgehog signaling pathway at the anteroposterior compartment border of the two dorsal primordia; the wing and the haltere imaginal discs. In the wing disc, Decapentaplegic and Hedgehog signaling targets are implicated in cell proliferation and cell survival. However, most of their known targets in the wing disc are not expressed in the haltere disc due to their repression by the Hox gene Ultrabithorax. The T-box gene optomotor-blind escapes this repression in the haltere disc, and therefore is expressed in both the haltere and wing discs. Optomotor-blind is a major player during wing development and its function has been intensely investigated in this tissue, however, its role in haltere development has not been reported so far. Here we show that Optomotor-blind function in the haltere disc differs from that in the wing disc. Unlike its role in the wing, Optomotor-blind does not prevent apoptosis in the haltere but rather limits growth by repressing several Decapentaplegic and Hedgehog targets involved both in wing proliferation and in modulating the spread of morphogens similar to Ultrabithorax function but without disturbing Ultrabithorax expression.

Effect of genetic variation in a Drosophila model of diabetes-associated misfolded human proinsulin

The identification and validation of gene–gene interactions is a major challenge in human studies. Here, we explore an approach for studying epistasis in humans using a Drosophila melanogaster model of neonatal diabetes mellitus. Expression of the mutant preproinsulin (hINS^C96Y) in the eye imaginal disc mimics the human disease: it activates conserved stress-response pathways and leads to cell death (reduction in eye area). Dominant-acting variants in wild-derived inbred lines from the Drosophila Genetics Reference Panel produce a continuous, highly heritable distribution of eye-degeneration phenotypes in a hINS^C96Y background. A genome-wide association study (GWAS) in 154 sequenced lines identified a sharp peak on chromosome 3L, which mapped to a 400-bp linkage block within an intron of the gene sulfateless (sfl). RNAi knockdown of sfl enhanced the eye-degeneration phenotype in a mutant-hINS-dependent manner. RNAi against two additional genes in the heparan sulfate (HS) biosynthetic pathway (ttv and botv), in which sfl acts, also modified the eye phenotype in a hINS^C96Y-dependent manner, strongly suggesting a novel link between HS-modified proteins and cellular responses to misfolded proteins. Finally, we evaluated allele-specific expression difference between the two major sfl-intronic haplotypes in heterozygtes. The results showed significant heterogeneity in marker-associated gene expression, thereby leaving the causal mutation(s) and its mechanism unidentified. In conclusion, the ability to create a model of human genetic disease, map a QTL by GWAS to a specific gene, and validate its contribution to disease with available genetic resources and the potential to experimentally link the variant to a molecular mechanism demonstrate the many advantages Drosophila holds in determining the genetic underpinnings of human disease.

Identification and expression analysis of zebrafish glypicans during embryonic development

Heparan sulfate Proteoglycans (HSPG) are ubiquitous molecules with indispensable functions in various biological processes. Glypicans are a family of HSPG’s, characterized by a Gpi-anchor which directs them to the cell surface and/or extracellular matrix where they regulate growth factor signaling during development and disease. We report the identification and expression pattern of glypican genes from zebrafish. The zebrafish genome contains 10 glypican homologs, as opposed to six in mammals, which are highly conserved and are phylogenetically related to the mammalian genes. Some of the fish glypicans like Gpc1a, Gpc3, Gpc4, Gpc6a and Gpc6b show conserved synteny with their mammalian cognate genes. Many glypicans are expressed during the gastrulation stage, but their expression becomes more tissue specific and defined during somitogenesis stages, particularly in the developing central nervous system. Existence of multiple glypican orthologs in fish with diverse expression pattern suggests highly specialized and/or redundant function of these genes during embryonic development.

Analysis of Drosophila glucuronyl C5-epimerase: implications for developmental roles of heparan sulfate sulfation compensation and 2-O-sulfated glucuronic acid

During the biosynthesis of heparan sulfate (HS), glucuronyl C5-epimerase (Hsepi) catalyzes C5-epimerization of glucuronic acid (GlcA), converting it to iduronic acid (IdoA). Because HS 2-O-sulfotransferase (Hs2st) shows a strong substrate preference for IdoA over GlcA, C5-epimerization is required for normal HS sulfation. However, the physiological significance of C5-epimerization remains elusive. To understand the role of Hsepi in development, we isolated Drosophila Hsepi mutants. Homozygous mutants are viable and fertile with only minor morphological defects, including the formation of an ectopic crossvein in the wing, but they have a short lifespan. We propose that two mechanisms contribute to the mild phenotypes of Hsepi mutants: HS sulfation compensation and possible developmental roles of 2-O-sulfated GlcA (GlcA2S). HS disaccharide analysis showed that loss of Hsepi resulted in a significant impairment of 2-O-sulfation and induced compensatory increases in N- and 6-O-sulfation. Simultaneous block of Hsepi and HS 6-O-sulfotransferase (Hs6st) activity disrupted tracheoblast formation, a well established FGF-dependent process. This result suggests that the increase in 6-O-sulfation in Hsepi mutants is critical for the rescue of FGF signaling. We also found that the ectopic crossvein phenotype can be induced by expression of a mutant form of Hs2st with a strong substrate preference for GlcA-containing units, suggesting that this phenotype is associated with abnormal GlcA 2-O-sulfation. Finally, we show that Hsepi formed a complex with Hs2st and Hs6st in S2 cells, raising the possibility that this complex formation contributes to the close functional relationships between these enzymes.

dBre1/dSet1-dependent pathway for histone H3K4 trimethylation has essential roles in controlling germline stem cell maintenance and germ cell differentiation in the Drosophila ovary

The Drosophila ovarian germline stem cells (GSCs) constantly experience self-renewal and differentiation, ensuring the female fertility throughout life. The balance between GSC self-renewal and differentiation is exquisitely regulated by the stem cell niche, the stem cells themselves and systemic factors. Increasing evidence has shown that the GSC regulation also involves epigenetic mechanisms including chromatin remodeling and histone modification. Here, we find that dBre1, an E3 ubiquitin ligase, functions in controlling GSC self-renewal and germ cell differentiation via distinct mechanisms. Removal or knock down of dBre1 function in the germline or somatic niche cell lineage leads to a gradual GSC loss and disruption of H3K4 trimethylation in the Drosophila ovary. Further studies suggest that the defective GSC maintenance is attributable to compromised BMP signaling emitted from the stem cell niche and impaired adhesion of GSCs to their niche. On the other hand, dBre1-RNAi expression in escort cells causes a loss of H3K4 trimethylation and accumulation of spectrosome-containing single germ cells in the germarium. Reducing dpp or dally levels suppresses the germ cell differentiation defects, indicating that dBre1 limits BMP signaling activities for the differentiation control. Strikingly, all phenotypes observed in dBre1 mutant ovaries can be mimicked by RNAi-based reduced expression of dSet1, a Drosophila H3K4 trimethylase. Moreover, genetic studies favor that dBre1 interacts with dSet1 in controlling GSC maintenance and germ cell differentiation. Taken together, we identify a dBre1/dSet1-dependent pathway for the H3K4 methylation involved in the cell fate regulation in the Drosophila ovary.

Egfr signaling controls the size of the stem cell precursor pool in the Drosophila ovary

In many animals, germline progenitors are kept undifferentiated to give rise to germline stem cells (GSCs), enabling continuous production of gametes throughout animal life. In the Drosophila ovary, GSCs arise from a subset of primordial germ cells (PGCs) that stay undifferentiated even after gametogenesis has started. How a certain population of PGCs is protected against differentiation, and the significance of its regulatory mechanisms on GSC establishment remain elusive. Here we show that epidermal growth factor receptor (Egfr) signaling in somatic stromal intermingled cells (ICs), activated by its ligand produced in germ cells, controls the size of the PGC pool at the onset of gametogenesis. Egfr signaling in ICs limits the number of cells that express the heparan sulfate proteoglycan Dally, which is required for the movement and stability of the locally-produced stromal morphogen, Decapentaplegic (Dpp, a BMP2/4 homologue). Dpp is received by PGCs and maintains them in an undifferentiated state. Altering Egfr signaling levels changes the size of the PGC pool and affects the number of GSCs established during development. While excess GSC formation is compensated by the adult stage, insufficient GSC formation can lead to adult ovarioles that completely lack GSCs, suggesting that ensuring an absolute size of the PGC pool is crucial for the GSC system.

The role of Drosophila heparan sulfate 6-O-endosulfatase in sulfation compensation

The biosynthesis of heparan sulfate proteoglycans is tightly regulated by multiple feedback mechanisms, which support robust developmental systems. One of the regulatory network systems controlling heparan sulfate (HS) biosynthesis is sulfation compensation. A previous study using Drosophila HS 2-O- and 6-O-sulfotransferase (Hs2st and Hs6st) mutants showed that loss of sulfation at one position is compensated by increased sulfation at other positions, supporting normal FGF signaling. Here, we show that HS sulfation compensation rescues both Decapentaplegic and Wingless signaling, suggesting a universal role of this regulatory system in multiple pathways in Drosophila. Furthermore, we identified Sulf1, extracellular HS 6-O-endosulfatase, as a novel component of HS sulfation compensation. Simultaneous loss of Hs2st and Sulf1 led to 6-O-oversulfation, leading to patterning defects, overgrowth, and lethality. These phenotypes are caused at least partly by abnormal up-regulation of Hedgehog signaling. Thus, sulfation compensation depends on the coordinated activities of Hs2st, Hs6st, and Sulf1.

Mammalian Notum induces the release of glypicans and other GPI-anchored proteins from the cell surface

Glypicans are heparan sulfate proteoglycans that are attached to the cell surface by a GPI (glycosylphosphatidylinositol)anchor. Glypicans regulate the activity of Wnts, Hedgehogs,bone morphogenetic proteins and fibroblast growth factors. In the particular case of Wnts, it has been proposed that GPI-anchored glypicans stimulate Wnt signalling by facilitating and/or stabilizing the interaction between Wnts and their cell surface receptors. On the other hand, when glypicans are secreted to the extracellular environment, they can act as competitive inhibitors of Wnt. Genetic screens in Drosophila have recently identified a novel inhibitor of Wnt signalling named Notum. The Wnt inhibiting activity of Notum was associated with its ability to release Dlp [Dally (Division abnormally delayed)-like protein; a Drosophila glypican] from the cell surface by cleaving the GPI anchor. Because these studies showed that the other Drosophila glypican Dally was not released from the cell surface by Notum,it remains unclear whether this enzyme is able to cleave glypicans from mammalian cells. Furthermore, it is also not known whether Notum cleaves GPI-anchored proteins that are not members of the glypican family. Here, we show that mammalian Notum can cleave several mammalian glypicans. Moreover, we demonstrate that Notum is able to release GPI-anchored proteins other than glypicans. Another important finding of the present study is that,unlike GPI-phospholipase D, the other mammalian enzyme that cleaves GPI-anchored proteins, Notum is active in the extracellular environment. Finally, by using a cellular system in which GPC3 (glypican-3) stimulates Wnt signalling, we show that Notum can act as a negative regulator of this growth factor.

Finding a niche: studies from the Drosophila ovary

Specialized microenvironments called niches help maintain stem cells in an undifferentiated and self-renewing state. The existence of niches has long been predicted from mammalian studies, but identifying stem cells in their native environments in vivo has remained a challenge in most vertebrates. Many of the mechanistic insights into how niches regulate stem cell maintenance have been obtained using invertebrate models such as Drosophila. Here, we focus on the Drosophila ovarian germline stem cell niche and review recent studies that have begun to reveal how intricate crosstalk between various signaling pathways regulates stem cell maintenance, how the extracellular matrix modulates the signaling output of the niche and how epigenetic programming influences cell development and function both inside and outside the niche to ensure proper tissue homeostasis. These insights will probably inform the study of mammalian niches and how their malfunction contributes to human disease.

Novel contact-dependent bone morphogenetic protein (BMP) signaling mediated by heparan sulfate proteoglycans

We previously proposed a model that DALLY, a Drosophila glypican, acts as a trans co-receptor to regulate BMP signaling in the germ line stem cell niche. To investigate the molecular mechanisms of contact-dependent BMP signaling, we developed novel in vitro assay systems to monitor trans signaling using Drosophila S2 cells. Using immunoblot-based as well as single-cell assay systems, we present evidence that Drosophila glypicans indeed enhance BMP signaling in trans in a contact-dependent manner in vitro. Our analysis showed that heparan sulfate modification is required for the trans co-receptor activity of DALLY. Two BMP-like molecules, Decapentaplegic (DPP) and Glass bottom boat, can mediate trans signaling through a heparan sulfate proteoglycan co-receptor in S2 cells. The in vitro systems reflect the molecular characteristics of heparan sulfate proteoglycan functions observed previously in vivo, such as ligand specificity and biphasic activity dependent on the ligand dosage. In addition, experiments using a DALLY-coated surface suggested that DALLY regulates DPP signaling in trans by its effect on the stability of DPP protein on the surface of the contacting cells. Our findings provide the molecular foundation for novel contact-dependent signaling, which defines the physical space of the stem cell niche in vivo.

Fine-tuning of cell signaling by glypicans

Signaling peptides of the extracellular environment regulate cell biological processes underlying embryonic development, tissue homeostasis, and pathophysiology. The heparan sulphate proteoglycans, glypicans, have evolved as essential modulators of key regulatory proteins such as Wnt, Bmp, Fgf, and Shh. By acting on signal spreading and receptor activation, glypicans can control signal read-out and fate in targeted cells. Genetic and embryological studies have highlighted that glypicans act in a temporal and spatially regulated manner to modulate distinct cellular events. However, alterations of glypican function underlie human congenital malformations and cancer. Recent reports are starting to reveal their mechanism of action and how they can ensure tight modulation of cell signaling.

In vivo manipulation of heparan sulfate structure and its effect on Drosophila development

Heparan sulfate proteoglycans (HSPGs) participate in a wide range of biological processes through interactions with a number of ligand proteins. The nature of these interactions largely depends on the heparan sulfate (HS) moiety of HSPGs, which undergoes a series of modifications by various HS-modifying enzymes (HSMEs). Although the effects of alterations in a single HSME on physiological processes have started to be studied, it remains elusive how a combination of these molecules control the structure and function of HS. Here we systematically manipulated the HS structures and analyzed their effect on morphogenesis and signaling, using the genetically tractable model organism, Drosophila. We generated transgenic fly strains overexpressing HSMEs alone or in combination. Unsaturated disaccharide analyses of HS showed that expression of various HSMEs generates distinct HS structures, and the enzymatic activities of HSMEs are influenced by coexpression of other HSMEs. Furthermore, these transgenic HSME animals showed a different extent of lethality, and a subset of HSMEs caused specific morphological defects due to defective activities of Wnt and bone morphogenetic protein signaling. There is no obvious relationship between HS unsaturated disaccharide composition and developmental defects in HSME animals, suggesting that other structural factors, such as domain organization or sulfation sequence, might regulate the function of HS.

Altering Glypican-1 levels modulates canonical Wnt signaling during trigeminal placode development

Glypicans are conserved cell surface heparan sulfate proteoglycans expressed in a spatiotemporally regulated manner in many developing tissues including the nervous system. Here, we show that Glypican-1 (GPC1) is expressed by trigeminal placode cells as they ingress and contribute to trigeminal sensory neurons in the chick embryo. Either expression of full-length or truncated GPC1 in vivo causes defects in trigeminal gangliogenesis in a manner that requires heparan sulfate side chains. This leads to either abnormal placodal differentiation or organization, respectively, with near complete loss of the ophthalmic (OpV) trigeminal ganglion in the most severe cases after overexpression of full-length GPC1. Interestingly, modulating GPC1 alters levels of endogenous Wnt signaling activity in the forming trigeminal ganglion, as indicated by Wnt reporter expression. Accordingly, GPC1 overexpression phenocopies Wnt inhibition in causing loss of OpV placodal neurons. Furthermore, increased Wnt activity rescues the effects of GPC1 overexpression. Taken together, these results suggest that appropriate levels of GPC1 are essential for proper regulation of canonical Wnt signaling during differentiation and organization of trigeminal placodal cells into ganglia.

Glypican-3 reexpression regulates apoptosis in murine adenocarcinoma mammary cells modulating PI3K/Akt and p38MAPK signaling pathways

Glypican-3 (GPC3) is a proteoglycan involved in proliferation and cell survival. Several reports demonstrated that GPC3 is downregulated in some tumors, such as breast cancer. Previously, we determined that GPC3 reexpression in the murine mammary adenocarcinoma LM3 cells induced an impairment of their invasive and metastatic capacities, associated with a decrease of their motility and an increase of their cell death. We demonstrated that GPC3 inhibits canonical Wnt signaling, as well as it activates non canonical pathway. Now, we identified signaling pathways responsible for the pro-apoptotic role of GPC3 in LM3 cells. We found for the first time that GPC3 inhibits the PI3K/Akt anti-apoptotic pathway while it stimulates the p38MAPK stress-activated one. We report a concomitant modulation of CDK inhibitors as well as of pro- and anti-apoptotic molecules. Our results provide new clues regarding the mechanism involved in the modulation induced by GPC3 of mammary tumor cell growth and survival.

Craniofacial skeletal defects of adult zebrafish Glypican 4 (knypek) mutants

The heparan sulfate proteoglycan Glypican 4 (Gpc4) is part of the Wnt/planar cell polarity pathway, which is required for convergence and extension during zebrafish gastrulation. To observe Glypican 4-deficient phenotypes at later stages, we rescued gpc4(-/-) (knypek) homozygotes and raised them for more than one year. Adult mutants showed diverse cranial malformations of both dermal and endochondral bones, ranging from shortening of the rostral-most skull to loss of the symplectic. Additionally, the adult palatoquadrate cartilage was disorganized, with abnormal chondrocyte orientation. To understand how the palatoquadrate cartilage normally develops, we examined a juvenile series of wild type and mutant specimens. This identified two novel domains of elongated chondrocytes in the larval palatoquadrate, which normally form prior to endochondral ossification. In contrast, gpc4(-/-) larvae never form these domains, suggesting a failure of chondrocyte orientation, though not differentiation. Our findings implicate Gpc4 in the regulation of zebrafish cartilage and bone morphogenesis.

Dual roles of Drosophila glypican Dally-like in Wingless/Wnt signaling and distribution

Heparan sulfate proteoglycans (HSPGs) are cell-surface and extracellular matrix (ECM) macromolecules that comprise a core protein to which heparan sulfate (HS) glycosaminoglycan (GAG) chains are attached. Glypican is a major family of HSPGs that is linked to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. Over the past decade, fruit fly Drosophila has been used as a powerful model system to examine the functions of HSPGs in cell signaling and development. There are two members of Drosophila glypicans named division abnormally delayed (Dally) and Dally-like (Dlp). To study the functions of these two glypicans in development, we have generated the null mutants of dally and dlp. Here, we describe the methods employed to analyze their functions in development with a focus on Dlp in the context of Wingless signaling. Our data suggest that Dlp shows biphasic activity in Wingless/Wnt signaling and distribution.