Dally regulates Dpp morphogen gradient formation by stabilizing Dpp on the cell surface

Hedgehog on the Move: Glypican-Regulated Transport and Gradient Formation in Drosophila

Glypicans (Glps) are a family of heparan sulphate proteoglycans that are attached to the outer plasma membrane leaflet of the producing cell by a glycosylphosphatidylinositol anchor. Glps are involved in the regulation of many signalling pathways, including those that regulate the activities of Wnts, Hedgehog (Hh), Fibroblast Growth Factors (FGFs), and Bone Morphogenetic Proteins (BMPs), among others. In the Hh-signalling pathway, Glps have been shown to be essential for ligand transport and the formation of Hh gradients over long distances, for the maintenance of Hh levels in the extracellular matrix, and for unimpaired ligand reception in distant recipient cells. Recently, two mechanistic models have been proposed to explain how Hh can form the signalling gradient and how Glps may contribute to it. In this review, we describe the structure, biochemistry, and metabolism of Glps and their interactions with different components of the Hh-signalling pathway that are important for the release, transport, and reception of Hh.

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.

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.

SMOC-1 interacts with both BMP and glypican to regulate BMP signaling in C. elegans

Secreted modular calcium-binding proteins (SMOCs) are conserved matricellular proteins found in organisms from Caenorhabditis elegans to humans. SMOC homologs characteristically contain 1 or 2 extracellular calcium-binding (EC) domain(s) and 1 or 2 thyroglobulin type-1 (TY) domain(s). SMOC proteins in Drosophila and Xenopus have been found to interact with cell surface heparan sulfate proteoglycans (HSPGs) to exert both positive and negative influences on the conserved bone morphogenetic protein (BMP) signaling pathway. In this study, we used a combination of biochemical, structural modeling, and molecular genetic approaches to dissect the functions of the sole SMOC protein in C. elegans. We showed that CeSMOC-1 binds to the heparin sulfate proteoglycan GPC3 homolog LON-2/glypican, as well as the mature domain of the BMP2/4 homolog DBL-1. Moreover, CeSMOC-1 can simultaneously bind LON-2/glypican and DBL-1/BMP. The interaction between CeSMOC-1 and LON-2/glypican is mediated specifically by the EC domain of CeSMOC-1, while the full interaction between CeSMOC-1 and DBL-1/BMP requires full-length CeSMOC-1. We provide both in vitro biochemical and in vivo functional evidence demonstrating that CeSMOC-1 functions both negatively in a LON-2/glypican-dependent manner and positively in a DBL-1/BMP-dependent manner to regulate BMP signaling. We further showed that in silico, Drosophila and vertebrate SMOC proteins can also bind to mature BMP dimers. Our work provides a mechanistic basis for how the evolutionarily conserved SMOC proteins regulate BMP signaling.

RNAi screen in the Drosophila wing of genes encoding proteins related to cytoskeleton organization and cell division

Cell division and cytoskeleton organization are fundamental processes participating in the development of Drosophila imaginal discs. In this manuscript we describe the phenotypes in the adult fly wing generated by knockdowns of 85% of Drosophila genes encoding proteins likely related to the regulation of cell division and cytoskeleton organization. We also compile a molecular classification of these proteins into classes that describe their expected or known main biochemical characteristics, as well as mRNA expression in the wing disc and likely protein subcellular localization for a subset of these genes. Finally, we analyze in more detail one protein family of cytoskeleton genes (Arp2/3 complex), and define the consequences of interfering with cell division for wing growth and patterning.

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.

Stemming Tumoral Growth: A Matter of Grotesque Organogenesis

The earliest metazoans probably evolved from single-celled organisms which found the colonial system to be a beneficial organization. Over the course of their evolution, these primary colonial organisms increased in size, and division of labour among the cells became a remarkable feature, leading to a higher level of organization: the biological organs. Primitive metazoans were the first organisms in evolution to show organ-type structures, which set the grounds for complex organs to evolve. Throughout evolution, and concomitant with organogenesis, is the appearance of tissue-specific stem cells. Tissue-specific stem cells gave rise to multicellular living systems with distinct organs which perform specific physiological functions. This setting is a constructive role of evolution; however, rebel cells can take over the molecular mechanisms for other purposes: nowadays we know that cancer stem cells, which generate aberrant organ-like structures, are at the top of a hierarchy. Furthermore, cancer stem cells are the root of metastasis, therapy resistance, and relapse. At present, most therapeutic drugs are unable to target cancer stem cells and therefore, treatment becomes a challenging issue. We expect that future research will uncover the mechanistic "forces" driving organ growth, paving the way to the implementation of new strategies to impair human tumorigenesis.

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.

C. elegans SMOC-1 interacts with both BMP and glypican to regulate BMP signaling

Secreted modular calcium binding (SMOC) proteins are conserved matricellular proteins found in organisms from C. elegans to humans. SMOC homologs characteristically contain one or two extracellular calcium (EC) binding domain(s) and one or two thyroglobulin type-1 (TY) domain(s). SMOC proteins in Drosophila and Xenopus have been found to interact with cell surface heparan sulfate protein glycans (HSPGs) to exert both positive and negative influences on the conserved bone morphogenetic protein (BMP) signaling pathway. In this study, we used a combination of biochemical, structural modeling, and molecular genetic approaches to dissect the functions of the sole SMOC protein in C. elegans . We showed that SMOC-1 binds LON-2/glypican, as well as the mature domain of DBL-1/BMP. Moreover, SMOC-1 can simultaneously bind LON-2/glypican and DBL-1/BMP. The interaction between SMOC-1 and LON-2/glypican is mediated by the EC domain of SMOC-1, while the interaction between SMOC-1 and DBL-1/BMP involves full-length SMOC-1. We further showed that while SMOC-1(EC) is sufficient to promote BMP signaling when overexpressed, both the EC and TY domains are required for SMOC-1 function at the endogenous locus. Finally, when overexpressed, SMOC-1 can promote BMP signaling in the absence of LON-2/glypican. Taken together, our findings led to a model where SMOC-1 functions both negatively in a LON-2-dependent manner and positively in a LON-2-independent manner to regulate BMP signaling. Our work provides a mechanistic basis for how the evolutionarily conserved SMOC proteins regulate BMP signaling.

Improving bone morphogenetic protein (BMP) production in CHO cells through understanding of BMP synthesis, signaling and endocytosis

Bone morphogenetic proteins (BMPs) are a group of growth factors with the clinical potential to regulate cartilage and bone formation. Functionally active mature recombinant human BMPs (rhBMPs), produced primarily in Chinese hamster ovary (CHO) cells for clinical applications, are considered difficult to express because they undergo maturation processes, signaling pathways, or endocytosis. Although BMPs are a family of proteins with similar mature domain sequence identities, their individual properties are diverse. Thus, understanding the properties of individual rhBMPs is essential to improve rhBMP production in CHO cells. In this review, we discuss various approaches to improve rhBMP production in CHO cells by understanding the overall maturation process, signaling pathways and endocytosis of individual rhBMPs.

Azot expression in the Drosophila gut modulates organismal lifespan

Cell Competition emerged in Drosophila as an unexpected phenomenon, when confronted clones of fit vs unfit cells genetically induced. During the last decade, it has been shown that this mechanism is physiologically active in Drosophila and higher organisms. In Drosophila, Flower (Fwe) eliminates unfit cells during development, regeneration and disease states. Furthermore, studies suggest that Fwe signaling is required to eliminate accumulated unfit cells during adulthood extending Drosophila lifespan. Indeed, ahuizotl (azot) mutants accumulate unfit cells during adulthood and after physical insults in the brain and other epithelial tissues, showing a decrease in organismal lifespan. On the contrary, flies carrying three functional copies of the gene, unfit cell culling seems to be more efficient and show an increase in lifespan. During aging, Azot is required for the elimination of unfit cells, however, the specific organs modulating organismal lifespan by Azot remain unknown. Here we found a potential connection between gut-specific Azot expression and lifespan which may uncover a more widespread organ-specific mechanism modulating organismal survival.

Regulation of stem cell fate by HSPGs: implication in hair follicle cycling

Heparan sulfate proteoglycans (HSPGs) are part of proteoglycan family. They are composed of heparan sulfate (HS)-type glycosaminoglycan (GAG) chains covalently linked to a core protein. By interacting with growth factors and/or receptors, they regulate numerous pathways including Wnt, hedgehog (Hh), bone morphogenic protein (BMP) and fibroblast growth factor (FGF) pathways. They act as inhibitor or activator of these pathways to modulate embryonic and adult stem cell fate during organ morphogenesis, regeneration and homeostasis. This review summarizes the knowledge on HSPG structure and classification and explores several signaling pathways regulated by HSPGs in stem cell fate. A specific focus on hair follicle stem cell fate and the possibility to target HSPGs in order to tackle hair loss are discussed in more dermatological and cosmeceutical perspectives.

Functional characterization of chitin synthesis pathway genes, HaAGM and HaUAP, reveal their crucial roles in ecdysis and survival of Helicoverpa armigera (Hübner)

The chitin metabolic pathway is one of the most lucrative targets for designing pest management regimes. Inhibition of the chitin synthesis pathway causes detrimental effects on the normal growth and development of insects. Phospho-N-acetylglucosamine mutase (AGM) and UDP-N-acetylglucosamine pyrophosphorylase (UAP) are two key chitin biosynthesis enzymes in insects including Helicoverpa armigera, a pest of global significance. In the present study, we have identified, cloned and recombinantly expressed AGM and UAP from H. armigera (HaAGM and HaUAP). Biochemical characterization of recombinant HaAGM and HaUAP exhibited high affinities for their natural substrates N-acetyl glucosamine-6-phosphate (K_m 38.72 ± 2.41) and N-acetyl glucosamine-1-phosphate (K_m 3.66 ± 0.13), respectively. In the coupled enzyme-catalytic assay, HaAGM and HaUAP yielded the end-products, inorganic pyrophosphate and UDP-GlcNAc, confirming their active participation in the chitin synthesis pathway of H. armigera. Gene expression profiling revealed that HaAGM and HaUAP genes were expressed in all developmental stages and key tissues. These genes also showed substantial responses towards the moulting hormone 20-hydroxyecdysone and chitin biosynthesis inhibitor, novaluron. Remarkably, the RNAi-mediated knockdown of either HaAGM or HaUAP led to severe developmental deformities and significant mortality ranging from 65.61 to 72.54%. Overall findings suggest that HaAGM and HaUAP play crucial roles in the ecdysis and survival of H. armigera. Further, these genes could serve as potential targets for designing pest management strategies for H. armigera.

Silencing uridine diphosphate N-acetylglucosamine pyrophosphorylase gene impairs larval development in Henosepilachna vigintioctopunctata

BACKGROUND

Uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) diphosphorylase (UAP) catalyzes the formation of UDP-GlcNAc, the precursor for the production of chitin in ectodermally derived epidermal cells and midgut, for GlcNAcylation of proteins and for generation of glycosyl-phosphatidyl-inositol anchors in all tissues in Drosophila melanogaster.

RESULTS

Here, we identified a putative HvUAP gene in Henosepilachna vigintioctopunctata. Knockdown of HvUAP at the second-, third- and fourth-instar stages impaired larval development. Most resultant HvUAP hypomorphs showed arrested development at the third-, fourth-instar larval or prepupal stages, and became paralyzed, depending on the age when treated. Some HvUAP-silenced larvae had weak and soft scoli. A portion of HvUAP-depleted beetles formed misshapen pupae. No HvUAP RNA interference pupae successfully emerged as adults. Dissection and microscopic observation revealed that knockdown of HvUAP affected gut growth and food ingestion, reduced cuticle thickness, and negatively affected the formation of newly generated cuticle layers during ecdysis. Furthermore, HvUAP deficiency inhibited development of the tracheal respiratory system and thinned tracheal taenidia.

CONCLUSION

The phenotypical defects in HvUAP hypomorphs suggest that HvUAP is involved in the production of chitin. Moreover, our findings will enable the development of a double-stranded RNA-based pesticide to control H. vigintioctopunctata. © 2021 Society of Chemical Industry.

Metabolic control of progenitor cell propagation during Drosophila tracheal remodeling

Adult progenitor cells in the trachea of Drosophila larvae are activated and migrate out of niches when metamorphosis induces tracheal remodeling. Here we show that in response to metabolic deficiency in decaying tracheal branches, signaling by the insulin pathway controls the progenitor cells by regulating Yorkie (Yki)-dependent proliferation and migration. Yki, a transcription coactivator that is regulated by Hippo signaling, promotes transcriptional activation of cell cycle regulators and components of the extracellular matrix in tracheal progenitor cells. These findings reveal that regulation of Yki signaling by the insulin pathway governs proliferation and migration of tracheal progenitor cells, thereby identifying the regulatory mechanism by which metabolic depression drives progenitor cell activation and cell division that underlies tracheal remodeling.

Tracheal remodeling is a key step during Drosophila metamorphosis. Here they report that tracheal progenitor cells are activated through Yorkie-dependent proliferation and migration, which is induced by metabolic deficit and insulin signaling.

Two Modulators of Skeletal Development: BMPs and Proteoglycans

During embryogenesis, skeletal development is tightly regulated by locally secreted growth factors that interact with proteoglycans (PGs) in the extracellular matrix (ECM). Bone morphogenetic proteins (BMPs) are multifunctional growth factors that play critical roles in cartilage maturation and bone formation. BMP signals are transduced from plasma membrane receptors to the nucleus through both canonical Smad and noncanonical p38 mitogen-activated protein kinase (MAPK) pathways. BMP signalling is modulated by a variety of endogenous and exogenous molecular mechanisms at different spatiotemporal levels and in both positive and negative manners. As an endogenous example, BMPs undergo extracellular regulation by PGs, which generally regulate the efficiency of ligand-receptor binding. BMP signalling can also be exogenously perturbed by a group of small molecule antagonists, such as dorsomorphin and its derivatives, that selectively bind to and inhibit the intracellular kinase domain of BMP type I receptors. In this review, we present a current understanding of BMPs and PGs functions in cartilage maturation and osteoblast differentiation, highlighting BMP–PG interactions. We also discuss the identification of highly selective small-molecule BMP receptor type I inhibitors. This review aims to shed light on the importance of BMP signalling and PGs in cartilage maturation and bone formation.

A role for Flower and cell death in controlling morphogen gradient scaling

During development, morphogen gradients encode positional information to pattern morphological structures during organogenesis¹. Some gradients, like that of Dpp in the fly wing, remain proportional to the size of growing organs-that is, they scale. Gradient scaling keeps morphological patterns proportioned in organs of different sizes^2,3. Here we show a mechanism of scaling that ensures that, when the gradient is smaller than the organ, cell death trims the developing tissue to match the size of the gradient. Scaling is controlled by molecular associations between Dally and Pentagone, known factors involved in scaling, and a key factor that mediates cell death, Flower^4-6. We show that Flower activity in gradient expansion is not dominated by cell death, but by the activity of Dally/Pentagone on scaling. Here we show a potential connection between scaling and cell death that may uncover a molecular toolbox hijacked by tumours.

The function of Glypicans in the mammalian embryo

Glypicans are proteoglycans that are bound to the outer surface of the plasma membrane by a glycosylphosphatidylinositol anchor. The mammalian genome contains six members of the glypican family (GPC1 to GPC6). Although the degree of sequence homology within the family is rather low, the three-dimensional structure of these proteoglycans is highly conserved. Glypicans are predominantly expressed during embryonic development. Genetic and biochemical studies have shown that glypicans can stimulate or inhibit the signaling pathways triggered by Wnts, Hedgehogs, Fibroblast Growth Factors, and Bone Morphogenetic Proteins. The study of mutant mouse strains demonstrated that glypicans have important functions in the developmental morphogenesis of various organs. In addition, a role of glypicans in synapsis formation has been established. Notably, glypican loss-of-function mutations are the cause of three human inherited syndromes. Recent analysis of glypican compound mutant mice have demonstrated that members of this protein family display redundant functions during embryonic development.

The wing imaginal disc

The Drosophila wing imaginal disc is a tissue of undifferentiated cells that are precursors of the wing and most of the notum of the adult fly. The wing disc first forms during embryogenesis from a cluster of ∼30 cells located in the second thoracic segment, which invaginate to form a sac-like structure. They undergo extensive proliferation during larval stages to form a mature larval wing disc of ∼35,000 cells. During this time, distinct cell fates are assigned to different regions, and the wing disc develops a complex morphology. Finally, during pupal stages the wing disc undergoes morphogenetic processes and then differentiates to form the adult wing and notum. While the bulk of the wing disc comprises epithelial cells, it also includes neurons and glia, and is associated with tracheal cells and muscle precursor cells. The relative simplicity and accessibility of the wing disc, combined with the wealth of genetic tools available in Drosophila, have combined to make it a premier system for identifying genes and deciphering systems that play crucial roles in animal development. Studies in wing imaginal discs have made key contributions to many areas of biology, including tissue patterning, signal transduction, growth control, regeneration, planar cell polarity, morphogenesis, and tissue mechanics.

The NDNF-like factor Nord is a Hedgehog-induced extracellular BMP modulator that regulates Drosophila wing patterning and growth

Hedgehog (Hh) and Bone Morphogenetic Proteins (BMPs) pattern the developing Drosophila wing by functioning as short- and long-range morphogens, respectively. Here, we show that a previously unknown Hh-dependent mechanism fine-tunes the activity of BMPs. Through genome-wide expression profiling of the Drosophila wing imaginal discs, we identify nord as a novel target gene of the Hh signaling pathway. Nord is related to the vertebrate Neuron-Derived Neurotrophic Factor (NDNF) involved in congenital hypogonadotropic hypogonadism and several types of cancer. Loss- and gain-of-function analyses implicate Nord in the regulation of wing growth and proper crossvein patterning. At the molecular level, we present biochemical evidence that Nord is a secreted BMP-binding protein and localizes to the extracellular matrix. Nord binds to Decapentaplegic (Dpp) or the heterodimer Dpp-Glass-bottom boat (Gbb) to modulate their release and activity. Furthermore, we demonstrate that Nord is a dosage-dependent BMP modulator, where low levels of Nord promote and high levels inhibit BMP signaling. Taken together, we propose that Hh-induced Nord expression fine-tunes both the range and strength of BMP signaling in the developing Drosophila wing.

Morphogen gradient scaling by recycling of intracellular Dpp

Morphogen gradients are fundamental to establish morphological patterns in developing tissues¹. During development, gradients scale to remain proportional to the size of growing organs^2,3. Scaling is a universal gear adjusting patterns to size in living organisms^3-8, yet its mechanisms remain unclear. Here, focusing on the Dpp gradient in the Drosophila wing disc, we unravel a cell biological basis behind scaling. From small to large discs, scaling of the Dpp gradient is achieved by increasing the contribution of the internalized Dpp molecules to Dpp transport: to expand the gradient, endocytosed molecules are re-exocytosed to spread extracellularly. To regulate the contribution of endocytosed Dpp to the spreading extracellular pool during tissue growth, it is the Dpp binding rates that are progressively modulated by the extracellular factor Pentagone, driving scaling. Thus, for some morphogens, evolution may act on endocytic trafficking to regulate the range of the gradient and its scaling, which could allow adaptation of shape and pattern to different sizes of organs in different species.

Generation of Drosophila Heparan Sulfate Mutant Cell Lines from Existing Fly Strains

Genetic studies using a model organism, Drosophila melanogaster, have been contributing to elucidating the in vivo functions of heparan sulfate proteoglycans (HSPGs). On the other hand, biochemical analysis of Drosophila glycosaminoglycans (GAGs) has been limited, mainly due to the insufficient amount of the material obtained from the animal. Recently, a novel in vitro system has been developed by establishing mutant cell lines for heparan sulfate (HS)-modifying enzyme genes. Metabolic radiolabeling of GAGs allows us to assess uncharacterized features of Drosophila GAGs and the effects of the mutations on HS structures and function. The novel in vitro system will provide us with a direct link between detailed structural information of Drosophila HS and a wealth of knowledge on biological phenotypic data obtained over the last two decades using this animal model.

Syndecan-2 expression enriches for hematopoietic stem cells and regulates stem cell repopulating capacity

The discovery of novel hematopoietic stem cell (HSC) surface markers can enhance understanding of HSC identity and function. We have discovered a population of primitive bone marrow (BM) HSCs distinguished by their expression of the heparan sulfate proteoglycan, Syndecan-2, which serves as both a marker and regulator of HSC function. Syndecan-2 expression was increased 10-fold in CD150+CD48-CD34-c-Kit+Sca-1+Lineage- cells (long-term - HSCs, LT-HSCs) compared to differentiated hematopoietic cells. Isolation of BM cells based solely on Syndecan-2 surface expression produced a 24-fold enrichment for LT-HSCs, 6-fold enrichment for alpha-catulin+c-kit+ HSCs, and yielded HSCs with superior in vivo repopulating capacity compared to CD150+ cells. Competitive repopulation assays revealed the HSC frequency to be 17-fold higher in Syndecan-2+CD34-KSL cells compared to Syndecan-2-CD34-KSL cells and indistinguishable from CD150+CD34-KSL cells. Syndecan-2 expression also identified nearly all repopulating HSCs within the CD150+CD34-KSL population. Mechanistically, Syndecan-2 regulates HSC repopulating capacity through control of expression of Cdkn1c (p57) and HSC quiescence. Loss of Syndecan-2 expression caused increased HSC cell cycle entry, downregulation of Cdkn1c and loss of HSC long-term - repopulating capacity. Syndecan-2 is a novel marker of HSCs which regulates HSC repopulating capacity via control of HSC quiescence.

Heparan sulfate proteoglycan molecules, syndecan and perlecan, have distinct roles in the maintenance of Drosophila germline stem cells

The Drosophila female germline stem cell (GSC) niche provides an excellent model for understanding the stem cell niche in vivo. The GSC niche is composed of stromal cells that provide growth factors for the maintenance of GSCs and the associated extracellular matrix (ECM). Although the function of stromal cells/growth factors has been well studied, the function of the ECM in the GSC niche is largely unknown. In this study, we investigated the function of syndecan and perlecan, molecules of the heparan sulfate proteoglycan (HSPG) family, as the main constituents of the ECM. We found that both of these genes were expressed in niche stromal cells, and knockdown of them in stromal cells decreased GSC number, indicating that these genes are important niche components. Interestingly, our genetic analysis revealed that the effects of syndecan and perlecan on the maintenance of GSC were distinct. While the knockdown of perlecan in the GSC niche increased the number of cystoblasts, a phenotype suggestive of delayed differentiation of GSCs, the same was not true in the context of syndecan. Notably, the overexpression of syndecan and perlecan did not cause an expansion of the GSC niche, opposing the results reported in the context of glypican, another HSPG gene. Altogether, our data suggest that HSPG genes contribute to the maintenance of GSCs through multiple mechanisms, such as the control of signal transduction, and ligand distribution/stabilization. Therefore, our study paves the way for a deeper understanding of the ECM functions in the stem cell niche.

Blockage of undesirable endocytosis of recombinant human growth/differentiation factor-5 in Chinese hamster ovary cell cultures requires heparin analogs with specific chain lengths

Cell surface heparan sulfate proteoglycan (HSPG)-mediated endocytosis lowers the yield of recombinant human bone morphogenetic proteins (rhBMPs), such as rhBMP-2 and rhBMP-4, from Chinese hamster ovary (CHO) cell cultures. Exogenous recombinant human growth/differentiation factor-5 (rhGDF-5), a member of the BMP family, bound to cell surface HSPGs and was actively internalized into CHO cells. Knockdown of heparan sulfate (HS) synthesis enzymes in CHO cells revealed that the chain length and N-sulfation of HS affected the binding of rhGDF-5 to HSPGs and subsequent rhGDF-5 internalization. To increase product yield by minimizing rhGDF-5 internalization in recombinant CHO (rCHO) cell cultures, heparin, and dextran sulfate (DS) of various polysaccharide chain lengths, which are structural analogs of HS, were examined for blockage of rhGDF-5 internalization. Heparin fragments of four monosaccharides (MW of 1.2 kDa) and DS (MW of 15 kDa) did not inhibit rhGDF-5 internalization whereas unfractionated heparin and DS of 200 kDa could significantly inhibit it. Compared to the control cultures, supplementation with unfractionated heparin or DS of 200 kDa at 1 g L^-1 resulted in more than a 10-fold increase in the maximum rhGDF-5 concentration. Taken together, the supplementation of structural HS analogs improved rhGDF-5 production in rCHO cell cultures by inhibiting rhGDF-5 internalization. GRAPHICAL ABSTRACT AND LAY SUMMARY: Cell surface heparan sulfate proteoglycan (HSPG)-mediated endocytosis lowers the yield of rhGDF-5 from CHO cell cultures. In this study, the authors found that the length and N-sulfation of HS chain determine the binding of rhGDF-5 to HSPGs and subsequent rhGDF-5 internalization. Based on this finding, the authors successfully used heparin analogs with specific chain lengths to enhance the rhGDF-5 yield by blocking rhGDF-5 internalization.

A Random Walk Approach to Transport in Tissues and Complex Media: From Microscale Descriptions to Macroscale Models

The biological processes necessary for the development and continued survival of any organism are often strongly influenced by the transport properties of various biologically active species. The transport phenomena involved vary over multiple temporal and spatial scales, from organism-level behaviors such as the search for food, to systemic processes such as the transport of oxygen from the lungs to distant organs, down to microscopic phenomena such as the stochastic movement of proteins in a cell. Each of these processes is influenced by many interrelated factors. Identifying which factors are the most important, and how they interact to determine the overall result is a problem of great importance and interest. Experimental observations are often fit to relatively simple models, but in reality the observations are the output of complicated functions of the physicochemical, topological, and geometrical properties of a given system. Herein we use multistate continuous-time random walks and generalized master equations to model transport processes involving spatial jumps, immobilization at defined sites, and stochastic internal state changes. The underlying spatial models, which are framed as graphs, may have different classes of nodes, and walkers may have internal states that are governed by a Markov process. A general form of the solutions, using Fourier-Laplace transforms and asymptotic analysis, is developed for several spatially infinite regular lattices in one and two spatial dimensions, and the theory is developed for the analysis of transport and internal state changes on general graphs. The goal in each case is to shed light on how experimentally observable macroscale transport coefficients can be explained in terms of microscale properties of the underlying processes. This work is motivated by problems arising in transport in biological tissues, but the results are applicable to a broad class of problems that arise in other applications.

Mad dephosphorylation at the nuclear pore is essential for asymmetric stem cell division

Stem cells divide asymmetrically to generate a stem cell and a differentiating daughter cell. Yet, it remains poorly understood how a stem cell and a differentiating daughter cell can receive distinct levels of niche signal and thus acquire different cell fates (self-renewal versus differentiation), despite being adjacent to each other and thus seemingly exposed to similar levels of niche signaling. In the Drosophila ovary, germline stem cells (GSCs) are maintained by short range bone morphogenetic protein (BMP) signaling; the BMP ligands activate a receptor that phosphorylates the downstream molecule mothers against decapentaplegic (Mad). Phosphorylated Mad (pMad) accumulates in the GSC nucleus and activates the stem cell transcription program. Here, we demonstrate that pMad is highly concentrated in the nucleus of the GSC, while it quickly decreases in the nucleus of the differentiating daughter cell, the precystoblast (preCB), before the completion of cytokinesis. We show that a known Mad phosphatase, Dullard (Dd), is required for the asymmetric partitioning of pMad. Our mathematical modeling recapitulates the high sensitivity of the ratio of pMad levels to the Mad phosphatase activity and explains how the asymmetry arises in a shared cytoplasm. Together, these studies reveal a mechanism for breaking the symmetry of daughter cells during asymmetric stem cell division.

Generation of extracellular morphogen gradients: the case for diffusion

Cells within developing tissues rely on morphogens to assess positional information. Passive diffusion is the most parsimonious transport model for long-range morphogen gradient formation but does not, on its own, readily explain scaling, robustness and planar transport. Here, we argue that diffusion is sufficient to ensure robust morphogen gradient formation in a variety of tissues if the interactions between morphogens and their extracellular binders are considered. A current challenge is to assess how the affinity for extracellular binders, as well as other biophysical and cell biological parameters, determines gradient dynamics and shape in a diffusion-based transport system. Technological advances in genome editing, tissue engineering, live imaging and in vivo biophysics are now facilitating measurement of these parameters, paving the way for mathematical modelling and a quantitative understanding of morphogen gradient formation and modulation.

Hedgehog signaling activation required for glypican-6-mediated regulation of invasion, migration, and epithelial-mesenchymal transition of gastric cancer cells

Gastric cancer (GC) is the fifth most common cancer worldwide and one of the most aggressive cancers in China. Glypican 6 is highly expressed in gastric adenocarcinoma and may act as a diagnostic and prognostic marker; however, the functional importance and molecular mechanism of glypican 6 in GC remains unclear. In the current study, we aimed to reveal the function and mechanism of glypican 6 in two GC cell lines: MKN-45 and SGC-7901. We found higher expression of glypican 6 in MKN-45 and SGC-7901 cells than in cells from the normal gastric mucosa epithelial cell line GES-1. Glypican 6 knockdown suppressed MKN-45 and SGC-7901 cell proliferation. A Transwell assay confirmed that glypican 6 silencing inhibited the migration and invasiveness of MKN-45 and SGC-7901 cells. Epithelial-to-mesenchymal transition (EMT) markers were determined by western blotting, and the results showed reduced Vimentin expression and elevated E-cadherin expression in glypican 6 short interfering RNA (siRNA) transfected MKN-45 and SGC-7901 cells. However, glypican 6 overexpression in GES-1 cells showed no significant promotion on GES-1 cells proliferation and migration. Further studies confirmed that glypican 6 siRNA regulated Hedgehog and Gli1 signaling and participated in the function of glypican 6 on MKN-45 and SGC-7901 cell migration and invasion. Our findings suggest that decreased glypican 6 expression inhibits the migration and invasion ability of GC cells.

A unified mechanism for the control of Drosophila wing growth by the morphogens Decapentaplegic and Wingless

Development of the Drosophila wing-a paradigm of organ development-is governed by 2 morphogens, Decapentaplegic (Dpp, a BMP) and Wingless (Wg, a Wnt). Both proteins are produced by defined subpopulations of cells and spread outwards, forming gradients that control gene expression and cell pattern as a function of concentration. They also control growth, but how is unknown. Most studies have focused on Dpp and yielded disparate models in which cells throughout the wing grow at similar rates in response to the grade or temporal change in Dpp concentration or to the different amounts of Dpp "equalized" by molecular or mechanical feedbacks. In contrast, a model for Wg posits that growth is governed by a progressive expansion in morphogen range, via a mechanism in which a minimum threshold of Wg sustains the growth of cells within the wing and recruits surrounding "pre-wing" cells to grow and enter the wing. This mechanism depends on the capacity of Wg to fuel the autoregulation of vestigial (vg)-the selector gene that specifies the wing state-both to sustain vg expression in wing cells and by a feed-forward (FF) circuit of Fat (Ft)/Dachsous (Ds) protocadherin signaling to induce vg expression in neighboring pre-wing cells. Here, we have subjected Dpp to the same experimental tests used to elucidate the Wg model and find that it behaves indistinguishably. Hence, we posit that both morphogens act together, via a common mechanism, to control wing growth as a function of morphogen range.

Epigenetic regulation of drosophila germline stem cell maintenance and differentiation

Gametogenesis is one of the most extreme cellular differentiation processes that takes place in Drosophila male and female germlines. This process begins at the germline stem cell, which undergoes asymmetric cell division (ACD) to produce a self-renewed daughter that preserves its stemness and a differentiating daughter cell that undergoes epigenetic and genomic changes to eventually produce haploid gametes. Research in molecular genetics and cellular biology are beginning to take advantage of the continually advancing genomic tools to understand: (1) how germ cells are able to maintain their identity throughout the adult reproductive lifetime, and (2) undergo differentiation in a balanced manner. In this review, we focus on the epigenetic mechanisms that address these two questions through their regulation of germline-soma communication to ensure germline stem cell identity and activity.

Selective endocytosis of recombinant human BMPs through cell surface heparan sulfate proteoglycans in CHO cells: BMP-2 and BMP-7

Cell surface heparan sulfate proteoglycan (HSPG)-mediated endocytosis results in poor yields of recombinant human bone morphogenetic proteins (rhBMPs) from CHO cell cultures. Upon incubation of rhBMP-2 and rhBMP-7 with CHO cells at 37 °C, both rhBMP-2 and rhBMP-7 bound to the cell surface HSPGs in CHO cells, but only rhBMP-2 was actively internalized into CHO cells. Cell surface HSPGs were found to serve as the main receptor for rhBMP-2 internalization. It was also found that the cell surface HSPG-mediated endocytosis of rhBMP-2 occurred through both the clathrin- and caveolin-dependent pathways. Blockage of rhBMP-2 internalization by the addition of structural analogs of HSPGs such as dextran sulfate (DS) and heparin dramatically increased rhBMP-2 production in recombinant CHO (rCHO) cell cultures. Compared to the control cultures, addition of DS (1.0 g/L) and heparin (0.2 g/L) resulted in a 22.0- and 19.0-fold increase in the maximum rhBMP-2 concentration, respectively. In contrast, the production of rhBMP-7, which was not internalized into the rCHO cells, did not dramatically increase upon addition of DS and heparin. Taken together, rhBMPs have a different fate in terms of HSPG-mediated internalization in CHO cells. HSPG-mediated endocytosis of each rhBMP should be understood individually to increase the rhBMP yield in rCHO cell cultures.

Scaling a Dpp Morphogen Gradient through Feedback Control of Receptors and Co-receptors

Gradients of decapentaplegic (Dpp) pattern Drosophila wing imaginal discs, establishing gene expression boundaries at specific locations. As discs grow, Dpp gradients expand, keeping relative boundary positions approximately stationary. Such scaling fails in mutants for Pentagone (pent), a gene repressed by Dpp that encodes a diffusible protein that expands Dpp gradients. Although these properties fit a recent mathematical model of automatic gradient scaling, that model requires an expander that spreads with minimal loss throughout a morphogen field. Here, we show that Pent's actions are confined to within just a few cell diameters of its site of synthesis and can be phenocopied by manipulating non-diffusible Pent targets strictly within the Pent expression domain. Using genetics and mathematical modeling, we develop an alternative model of scaling driven by feedback downregulation of Dpp receptors and co-receptors. Among the model's predictions is a size beyond which scaling fails-something we observe directly in wing discs.

Integrins Modulate Extracellular Matrix Organization to Control Cell Signaling during Hematopoiesis

During hematopoiesis, progenitor cells receive and interpret a diverse array of regulatory signals from their environment. These signals control the maintenance of the progenitors and regulate the production of mature blood cells. Integrins are well known in vertebrates for their roles in hematopoiesis, particularly in assisting in the migration to, as well as the physical attachment of, progenitors to the niche. However, whether and how integrins are also involved in the signaling mechanisms that control hematopoiesis remains to be resolved. Here, we show that integrins play a key role during fly hematopoiesis in regulating cell signals that control the behavior of hematopoietic progenitors. Integrins can regulate hematopoiesis directly, via focal adhesion kinase (FAK) signaling, and indirectly, by directing extracellular matrix (ECM) assembly and/or maintenance. ECM organization and density controls blood progenitor behavior by modulating multiple signaling pathways, including bone morphogenetic protein (BMP) and Hedgehog (Hh). Furthermore, we show that integrins and the ECM are reduced following infection, which may assist in activating the immune response. Our results provide mechanistic insight into how integrins can shape the signaling environment around hematopoietic progenitors.

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.

Drosophila jumu modulates apoptosis via a JNK-dependent pathway and is required for other processes in wing development

Previous studies in several model organisms have revealed that members of the Forkhead (Fkh) transcription factor family have multiple functions. Drosophila Jumeau (Jumu), a member of this family, participates in cardiogenesis, hematopoiesis and immune system homeostasis. Here, we show that loss of jumu function positively regulates or triggers apoptosis via a JNK-dependent pathway in wing development. jumu mutants showed reduced wing size and increased apoptosis. Moreover, we observed a loss of the anterior cross vein (ACV) phenotype that was similar to that observed in wings in which JNK signaling has been ectopically activated. The JNK signaling markers puckered (puc) and p-JNK were also significantly increased in the wing discs of jumu mutants. In addition, apoptosis induced by the loss of jumu was rescued by knocking down JNK, indicating a role for JNK in reducing jumu-induced apoptosis. Jumu could also control wing margin development via the positive regulation of cut expression, and the observed wing margin defect did not result from a loss of jumu-induced apoptosis. Further, jumu deficiency in the pupal wing could induce multiple wing hairs via a Rho1-mediated planar cell polarity pathway, but abnormal Rho1 expression was not why jumu loss induced apoptosis via a JNK-dependent pathway in wing discs.

Dlp-mediated Hh and Wnt signaling interdependence is critical in the niche for germline stem cell progeny differentiation

Although multiple signaling pathways work synergistically in various niches to control stem cell self-renewal and differentiation, it remains poorly understood how they cooperate with one another molecularly. In the Drosophila ovary, Hh and Wnt pathways function in the niche to promote germline stem cell (GSC) progeny differentiation. Here, we show that glypican Dlp-mediated Hh and Wnt signaling interdependence operates in the niche to promote GSC progeny differentiation by preventing BMP signaling. Hh/Wnt-mediated dlp repression is essential for their signaling interdependence in niche cells and for GSC progeny differentiation by preventing BMP signaling. Mechanistically, Hh and Wnt downstream transcription factors directly bind to the same dlp regulatory region and recruit corepressors composed of transcription factor Croc and Egg/H3K9 trimethylase to repress Dlp expression. Therefore, our study reveals a novel mechanism for Hh/Wnt signaling-mediated direct dlp repression and a novel regulatory mechanism for Dlp-mediated Hh/Wnt signaling interdependence in the GSC differentiation niche.

Regulatory feedback on receptor and non-receptor synthesis for robust signaling

Elaborate regulatory feedback processes are thought to make biological development robust, that is, resistant to changes induced by genetic or environmental perturbations. How this might be done is still not completely understood. Previous numerical simulations on reaction-diffusion models of Dpp gradients in Drosophila wing imaginal disc have showed that feedback (of the Hill function type) on (signaling) receptors and/or non-(signaling) receptors are of limited effectiveness in promoting robustness. Spatial nonuniformity of the feedback processes has also been shown theoretically to lead to serious shape distortion and a principal cause for ineffectiveness. Through mathematical modeling and analysis, the present article shows that spatially uniform nonlocal feedback mechanisms typically modify gradient shape through a shape parameter (that does not change with location). This in turn enables us to uncover new multi-feedback instrument for effective promotion of robust signaling gradients.

Chondroitin sulfate proteoglycan Windpipe modulates Hedgehog signaling in Drosophila

Proteoglycans, a class of carbohydrate-modified proteins, often modulate growth factor signaling on the cell surface. However, the molecular mechanism by which proteoglycans regulate signal transduction is largely unknown. In this study, using a recently developed glycoproteomic method, we found that Windpipe (Wdp) is a novel chondroitin sulfate proteoglycan (CSPG) in Drosophila. Wdp is a single-pass transmembrane protein with leucin-rich repeat (LRR) motifs and bears three CS sugar chain attachment sites in the extracellular domain. Here we show that Wdp modulates the Hedgehog (Hh) pathway. In the wing disc, overexpression of wdp inhibits Hh signaling, which is dependent on its CS chains and the LRR motifs. The wdp null mutant flies show a specific defect (supernumerary scutellar bristles) known to be caused by Hh overexpression. RNA interference knockdown and mutant clone analyses showed that loss of wdp leads to the up-regulation of Hh signaling. Altogether, our study demonstrates a novel role of CSPGs in regulating Hh signaling.

The regulation of Drosophila ovarian stem cell niches by signaling crosstalk

The Drosophila female ovary is an excellent model for investigating how multiple stem cell types are coordinately regulated in vivo. The ovary contains at least two stem cell types, germline stem cells (GSCs) and somatic follicular stem cells (FSCs). Although GSCs and FSCs are maintained within a distinct extra-cellular microenvironment, known as a niche, they share some common signaling molecules to generate their own niche. To properly maintain these stem cell types, understanding how signaling molecules are regulated is essential. In this review, we summarize the recent understanding of the mechanisms maintaining GSCs and FSCs from the perspective of growth factor regulation and discuss how these regulatory mechanisms contribute to stem cell maintenance, competition, and survival.

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.

Growth control in the Drosophila wing disk

The regulation of size and shape is a fundamental requirement of biological development and has been a subject of scientific study for centuries, but we still lack an understanding of how organisms know when to stop growing. Imaginal wing disks of the fruit fly Drosophila melanogaster, which are precursors of the adult wings, are an archetypal tissue for studying growth control. The growth of the disks is dependent on many inter- and intra-organ factors such as morphogens, mechanical forces, nutrient levels, and hormones that influence gene expression and cell growth. Extracellular signals are transduced into gene-control signals via complex signal transduction networks, and since cells typically receive many different signals, a mechanism for integrating the signals is needed. Our understanding of the effect of morphogens on tissue-level growth regulation via individual pathways has increased significantly in the last half century, but our understanding of how multiple biochemical and mechanical signals are integrated to determine whether or not a cell decides to divide is still rudimentary. Numerous fundamental questions are involved in understanding the decision-making process, and here we review the major biochemical and mechanical pathways involved in disk development with a view toward providing a basis for beginning to understand how multiple signals can be integrated at the cell level, and how this translates into growth control at the level of the imaginal disk. This article is categorized under: Analytical and Computational Methods > Computational Methods Biological Mechanisms > Cell Signaling Models of Systems Properties and Processes > Cellular Models.

Control of size, fate and time by the Hh morphogen in the eyes of flies

Molecules of the hedgehog (hh) family are involved in the specification and patterning of eyes in vertebrates and invertebrates. These organs, though, are of very different sizes, raising the question of how Hh molecules operate at such different scales. In this paper we discuss the strategies used by Hh to control the development of the two eye types in Drosophila: the large compound eye and the small ocellus. We first describe the distinct ways in which these two eyes develop and the evidence for the key role played by Hh in both; then we consider the potential for variation in the range of action of a "typical" morphogen and measure this range ("characteristic length") for Hh in different organs, including the compound eye and the ocellus. Finally, we describe how different feedback mechanisms are used to extend the Hh range of action to pattern the large and even the small eye. In the ocellus, the basic Hh signaling pathway adds to its dynamics the attenuation of its receptor as cell differentiate. This sole regulatory change can result in the decoding of the Hh gradient by receiving cells as a wave of constant speed. Therefore, in the fly ocellus, the Hh morphogen adds to its spatial patterning role a novel one: patterning along a time axis.

Glypican-6 stimulates intestinal elongation by simultaneously regulating Hedgehog and non-canonical Wnt signaling

We report here that Glypican-6 (GPC6)-null mice display at birth small intestines that are 75% shorter than those of normal littermates. Notably, we demonstrate that the role of GPC6 in intestinal elongation is mediated by both Hedgehog (Hh) and non-canonical Wnt signaling. Based on results from in vitro experiments, we had previously proposed that GPC6 stimulates Hh signaling by interacting with Hh and Patched1 (Ptc1), and facilitating/stabilizing their interaction. Here we provide strong support to this hypothesis by showing that GPC6 binds to Ptc1 in the mesenchymal layer of embryonic intestines. This study also provides experimental evidence that strongly suggests that GPC6 inhibits the activity of Wnt5a on the intestinal epithelium by binding to this growth factor, and reducing its release from the surrounding mesenchymal cells. Finally, we show that whereas the mesenchymal layer of GPC6-null intestines displays reduced cell proliferation and a thinner smooth muscle layer, epithelial cell differentiation is not altered in the mutant gut.

Drosophila Ovarian Germline Stem Cell Cytocensor Projections Dynamically Receive and Attenuate BMP Signaling

In the Drosophila ovarian germline, Bone Morphogenetic Protein (BMP) signals released by niche cells promote germline stem cell (GSC) maintenance. Although BMP signaling is known to repress expression of a key differentiation factor, it remains unclear whether BMP-responsive transcription also contributes positively to GSC identity. Here, we identify the GSC transcriptome using RNA sequencing (RNA-seq), including the BMP-induced transcriptional network. Based on these data, we provide evidence that GSCs form two types of cellular projections. Genetic manipulation and live ex vivo imaging reveal that both classes of projection allow GSCs to access a reservoir of Dpp held away from the GSC-niche interface. Moreover, microtubule-rich projections, termed "cytocensors", form downstream of BMP and have additional functionality, which is to attenuate BMP signaling. In this way, cytocensors allow dynamic modulation of signal transduction to facilitate differentiation following GSC division. This ability of cytocensors to attenuate the signaling response expands the repertoire of functions associated with signaling projections.

Establishment and characterization of Drosophila cell lines mutant for heparan sulfate modifying enzymes

A class of carbohydrate-modified proteins, heparan sulfate proteoglycans (HSPGs), play critical roles both in normal development and during disease. Genetic studies using a model organism, Drosophila, have been contributing to understanding the in vivo functions of HSPGs. Despite the many strengths of the Drosophila model for in vivo studies, biochemical analysis of Drosophila HS is somewhat limited, mainly due to the insufficient amount of the material obtained from the animal. To overcome this obstacle, we generated mutant cell lines for four HS modifying enzymes that are critical for the formation of ligand binding sites on HS, Hsepi, Hs2st, Hs6st and Sulf1, using a recently established method. Morphological and immunological analyses of the established lines suggest that they are spindle-shaped cells of mesodermal origin. The disaccharide profiles of HS from these cell lines showed characteristics of lack of each enzyme as well as compensatory modifications by other enzymes. Metabolic radiolabeling of HS allowed us to assess chain length and net charge of the total population of HS in wild-type and Hsepi mutant cell lines. We found that Drosophila HS chains are significantly shorter than those from mammalian cells. BMP signaling assay using Hs6st cells indicates that molecular phenotypes of these cell lines are consistent with previously known in vivo phenomena. The established cell lines will provide us with a direct link between detailed structural information of Drosophila HS and a wealth of knowledge on biological phenotypic data obtained over the last two decades using this animal model.

Signaling network involved in the GPC3-induced inhibition of breast cancer progression: role of canonical Wnt pathway

PURPOSE

We have shown that GPC3 overexpression in breast cancer cells inhibits in vivo tumor progression, by acting as a metastatic suppressor. GPC3-overexpressing cells are less clonogenic, viable and motile, while their homotypic adhesion is increased. We have presented evidences indicating that GPC3 inhibits canonical Wnt and Akt pathways, while non-canonical Wnt and p38MAPK cascades are activated. In this study, we aimed to investigate whether GPC3-induced Wnt signaling inhibition modulates breast cancer cell properties as well as to describe the interactions among pathways modulated by GPC3.

METHODS

Fluorescence microscopy, qRT-PCR microarray, gene reporter assay and Western blotting were performed to determine gene expression levels, signaling pathway activities and molecule localization. Lithium was employed to activate canonical Wnt pathway and treated LM3-GPC3 cell viability, migration, cytoskeleton organization and homotypic adhesion were assessed using MTS, wound healing, phalloidin staining and suspension growth assays, respectively.

RESULTS

We provide new data demonstrating that GPC3 blocks-also at a transcriptional level-both autocrine and paracrine canonical Wnt activities, and that this inhibition is required for GPC3 to modulate migration and homotypic adhesion. Our results indicate that GPC3 is secreted into the extracellular media, suggesting that secreted GPC3 competes with Wnt factors or interacts with them and thus prevents Wnt binding to Fz receptors. We also describe the complex network of interactions among GPC3-modulated signaling pathways.

CONCLUSION

GPC3 is operating through an intricate molecular signaling network. From the balance of these interactions, the inhibition of breast metastatic spread induced by GPC3 emerges.

Somatic clones heterozygous for recessive disease alleles of BMPR1A exhibit unexpected phenotypes in Drosophila

The majority of mutations studied in animal models are designated as recessive based on the absence of visible phenotypes in germline heterozygotes. Accordingly, genetic studies primarily rely on homozygous loss-of-function to determine gene requirements, and a conceptually-related ‘two-hit model’ remains the central paradigm in cancer genetics. Here we investigate pathogenesis due to somatic mutation in epithelial tissues, a process that predominantly generates heterozygous cell clones. To study somatic mutation in Drosophila, we generated inducible alleles that mimic human Juvenile polyposis-associated BMPR1A mutations. Unexpectedly, four of these mutations had no phenotype in heterozygous carriers but exhibited clear tissue-level effects when present in somatic clones of heterozygous cells. We conclude that these alleles are indeed recessive when present in the germline, but nevertheless deleterious when present in heterozygous clones. This unforeseen effect, deleterious heteromosaicism, suggests a ‘one-hit’ mechanism for disease initiation that may explain some instances of pathogenesis associated with spontaneous mutation.