Heparan sulfate proteoglycan syndecan promotes axonal and myotube guidance by slit/robo signaling

Drosophila noktochor regulates night sleep via a local mushroom body circuit

We show that a sleep-regulating, Ig-domain protein (NKT) is secreted from Drosophila mushroom body (MB) α'/β' neurons to act locally on other MB cell types. Pan-neuronal or broad MB expression of membrane-tethered NKT (tNkt) protein reduced sleep, like that of an NKT null mutant, suggesting blockade of a receptor mediating endogenous NKT action. In contrast, expression in neurons requiring NKT (the MB α'/β' cells), or non-MB sleep-regulating centers, did not reduce night sleep, indicating the presence of a local MB sleep-regulating circuit consisting of communicating neural subtypes. We suggest that the leucocyte-antigen-related like (Lar) transmembrane receptor may mediate NKT action. Knockdown or overexpression of Lar in the MB increased or decreased sleep, respectively, indicating the receptor promotes wakefulness. Surprisingly, selective expression of tNkt or knockdown of Lar in MB wake-promoting cells increased rather than decreased sleep, suggesting that NKT acts on wake- as well as sleep-promoting cell types to regulate sleep.

Hippo cell signaling and HS-proteoglycans regulate tissue form and function, age-dependent maturation, extracellular matrix remodeling, and repair

This review examined how Hippo cell signaling and heparan sulfate (HS)-proteoglycans (HSPGs) regulate tissue form and function. Despite being a nonweight-bearing tissue, the brain is regulated by Hippo mechanoresponsive cell signaling pathways during embryonic development. HS-proteoglycans interact with growth factors, morphogens, and extracellular matrix components to regulate development and pathology. Pikachurin and Eyes shut (Eys) interact with dystroglycan to stabilize the photoreceptor axoneme primary cilium and ribbon synapse facilitating phototransduction and neurotransduction with bipolar retinal neuronal networks in ocular vision, the primary human sense. Another HSPG, Neurexin interacts with structural and adaptor proteins to stabilize synapses and ensure specificity of neural interactions, and aids in synaptic potentiation and plasticity in neurotransduction. HSPGs also stabilize the blood-brain barrier and motor neuron basal structures in the neuromuscular junction. Agrin and perlecan localize acetylcholinesterase and its receptors in the neuromuscular junction essential for neuromuscular control. The primary cilium is a mechanosensory hub on neurons, utilized by YES associated protein (YAP)-transcriptional coactivator with PDZ-binding motif (TAZ) Hippo, Hh, Wnt, transforming growth factor (TGF)-β/bone matrix protein (BMP) receptor tyrosine kinase cell signaling. Members of the glypican HSPG proteoglycan family interact with Smoothened and Patched G-protein coupled receptors on the cilium to regulate Hh and Wnt signaling during neuronal development. Control of glycosyl sulfotransferases and endogenous protease expression by Hippo TAZ YAP represents a mechanism whereby the fine structure of HS-proteoglycans can be potentially modulated spatiotemporally to regulate tissue morphogenesis in a similar manner to how Hippo signaling controls sialyltransferase expression and mediation of cell-cell recognition, dysfunctional sialic acid expression is a feature of many tumors.

Role of the SLIT-ROBO signaling pathway in renal pathophysiology and various renal diseases

SLIT ligand and its receptor ROBO were initially recognized for their role in axon guidance in central nervous system development. In recent years, as research has advanced, the role of the SLIT-ROBO signaling pathway has gradually expanded from axonal repulsion to cell migration, tumor development, angiogenesis, and bone metabolism. As a secreted protein, SLIT regulates various pathophysiological processes in the kidney, such as proinflammatory responses and fibrosis progression. Many studies have shown that SLIT-ROBO is extensively involved in various aspects of kidney development and maintenance of structure and function. The SLIT-ROBO signaling pathway also plays an important role in different types of kidney disease. This article reviews the advances in the study of the SLIT-ROBO pathway in various renal pathophysiological and kidney disorders and proposes new directions for further research in this field.

Structure and evolution of neuronal wiring receptors and ligands

One of the fundamental properties of a neuronal circuit is the map of its connections. The cellular and developmental processes that allow for the growth of axons and dendrites, selection of synaptic targets, and formation of functional synapses use neuronal surface receptors and their interactions with other surface receptors, secreted ligands, and matrix molecules. Spatiotemporal regulation of the expression of these receptors and cues allows for specificity in the developmental pathways that wire stereotyped circuits. The families of molecules controlling axon guidance and synapse formation are generally conserved across animals, with some important exceptions, which have consequences for neuronal connectivity. Here, we summarize the distribution of such molecules across multiple taxa, with a focus on model organisms, evolutionary processes that led to the multitude of such molecules, and functional consequences for the diversification or loss of these receptors.

Zebrafish Slit2 and Slit3 Act Together to Regulate Retinal Axon Crossing at the Midline

Slit-Robo signaling regulates midline crossing of commissural axons in different systems. In zebrafish, all retinofugal axons cross at the optic chiasm to innervate the contralateral tectum. Here, the mutant for the Robo2 receptor presents severe axon guidance defects, which were not completely reproduced in a Slit2 ligand null mutant. Since slit3 is also expressed around this area at the stage of axon crossing, we decided to analyze the possibility that it collaborates with Slit2 in this process. We found that the disruption of slit3 expression by sgRNA-Cas9 injection caused similar, albeit slightly milder, defects than those of the slit2 mutant, while the same treatment in the slit2−/−^mz background caused much more severe defects, comparable to those observed in robo2 mutants. Tracking analysis of in vivo time-lapse experiments indicated differential but complementary functions of these secreted factors in the correction of axon turn errors around the optic chiasm. Interestingly, RT-qPCR analysis showed a mild increase in slit2 expression in slit3-deficient embryos, but not the opposite. Our observations support the previously proposed “repulsive channel” model for Slit-Robo action at the optic chiasm, with both Slits acting in different manners, most probably relating to their different spatial expression patterns.

Neural Tissue Homeostasis and Repair Is Regulated via CS and DS Proteoglycan Motifs

Chondroitin sulfate (CS) is the most abundant and widely distributed glycosaminoglycan (GAG) in the human body. As a component of proteoglycans (PGs) it has numerous roles in matrix stabilization and cellular regulation. This chapter highlights the roles of CS and CS-PGs in the central and peripheral nervous systems (CNS/PNS). CS has specific cell regulatory roles that control tissue function and homeostasis. The CNS/PNS contains a diverse range of CS-PGs which direct the development of embryonic neural axonal networks, and the responses of neural cell populations in mature tissues to traumatic injury. Following brain trauma and spinal cord injury, a stabilizing CS-PG-rich scar tissue is laid down at the defect site to protect neural tissues, which are amongst the softest tissues of the human body. Unfortunately, the CS concentrated in gliotic scars also inhibits neural outgrowth and functional recovery. CS has well known inhibitory properties over neural behavior, and animal models of CNS/PNS injury have demonstrated that selective degradation of CS using chondroitinase improves neuronal functional recovery. CS-PGs are present diffusely in the CNS but also form denser regions of extracellular matrix termed perineuronal nets which surround neurons. Hyaluronan is immobilized in hyalectan CS-PG aggregates in these perineural structures, which provide neural protection, synapse, and neural plasticity, and have roles in memory and cognitive learning. Despite the generally inhibitory cues delivered by CS-A and CS-C, some CS-PGs containing highly charged CS disaccharides (CS-D, CS-E) or dermatan sulfate (DS) disaccharides that promote neural outgrowth and functional recovery. CS/DS thus has varied cell regulatory properties and structural ECM supportive roles in the CNS/PNS depending on the glycoform present and its location in tissue niches and specific cellular contexts. Studies on the fruit fly, Drosophila melanogaster and the nematode Caenorhabditis elegans have provided insightful information on neural interconnectivity and the role of the ECM and its PGs in neural development and in tissue morphogenesis in a whole organism environment.

Functions of the extracellular matrix in development: Lessons from Caenorhabditis elegans

Cell-extracellular matrix interactions are crucial for the development of an organism from the earliest stages of embryogenesis. The main constituents of the extracellular matrix are collagens, laminins, proteoglycans and glycosaminoglycans that form a network of interactions. The extracellular matrix and its associated molecules provide developmental cues and structural support from the outside of cells during development. The complex nature of the extracellular matrix and its ability for continuous remodeling poses challenges when investigating extracellular matrix-based signaling during development. One way to address these challenges is to employ invertebrate models such as Caenorhabditis elegans, which are easy to genetically manipulate and have an invariant developmental program. C. elegans also expresses fewer extracellular matrix protein isoforms and exhibits reduced redundancy compared to mammalian models, thus providing a simpler platform for exploring development. This review summarizes our current understanding of how the extracellular matrix controls the development of neurons, muscles and the germline in C. elegans.

Syndecan receptors: pericellular regulators in development and inflammatory disease

The syndecans are the major family of transmembrane proteoglycans, usually bearing multiple heparan sulfate chains. They are present on virtually all nucleated cells of vertebrates and are also present in invertebrates, indicative of a long evolutionary history. Genetic models in both vertebrates and invertebrates have shown that syndecans link to the actin cytoskeleton and can fine-tune cell adhesion, migration, junction formation, polarity and differentiation. Although often associated as co-receptors with other classes of receptors (e.g. integrins, growth factor and morphogen receptors), syndecans can nonetheless signal to the cytoplasm in discrete ways. Syndecan expression levels are upregulated in development, tissue repair and an array of human diseases, which has led to the increased appreciation that they may be important in pathogenesis not only as diagnostic or prognostic agents, but also as potential targets. Here, their functions in development and inflammatory diseases are summarized, including their potential roles as conduits for viral pathogen entry into cells.

New SDC function prediction based on protein-protein interaction using bioinformatics tools

The precise roles for SDC have been complex to specify. Assigning and reanalyzing protein and peptide identification to novel protein functions is one of the most important challenges in postgenomic era. Here, we provide SDC molecular description to support, contextualize and reanalyze the corresponding protein-protein interaction (PPI). From SDC-1 data mining, we discuss the potential of bioinformatics tools to predict new biological rules of SDC. Using these methods, we have assembled new possibilities for SDC biology from PPI data, once, the understanding of biology complexity cannot be capture from one simple question.

The Role of the Slit/Robo Signaling Pathway

The Slit family is a family of secreted proteins that play important roles in various physiologic and pathologic activities via interacting with Robo receptors. Slit/Robo signaling was first identified in the nervous system, where it functions in neuronal axon guidance; nevertheless, an increasing number of studies have shown that Slit/Robo signaling even regulates other activities, such as angiogenesis, inflammatory cell chemotaxis, tumor cell migration and metastasis. Although the precise role of the ligand-receptor in organisms has been obscure and the conclusions drawn are sometimes paradoxical, tremendous advances in understanding the Slit/Robo signaling pathway have been made. As such, our review summarizes the characteristics of the Slit/Robo signaling pathway and its role in various cell types.

Structure and Function of Roundabout Receptors

The creation of complex neuronal networks relies on ligand-receptor interactions that mediate attraction or repulsion towards specific targets. Roundabouts comprise a family of single-pass transmembrane receptors facilitating this process upon interaction with the soluble extracellular ligand Slit protein family emanating from the midline. Due to the complexity and flexible nature of Robo receptors , their overall structure has remained elusive until now. Recent structural studies of the Robo 1 and Robo 2 ectodomains have provided the basis for a better understanding of their signalling mechanism. These structures reveal how Robo receptors adopt an auto-inhibited conformation on the cell surface that can be further stabilised by cis and/or trans oligmerisation arrays. Upon Slit -N binding Robo receptors must undergo a conformational change for Ig4 mediated dimerisation and signaling, probably via endocytosis. Furthermore, it's become clear that Robo receptors do not only act alone, but as large and more complex cell surface receptor assemblies to manifest directional and growth effects in a concerted fashion. These context dependent assemblies provide a mechanism to fine tune attractive and repulsive signals in a combinatorial manner required during neuronal development. While a mechanistic understanding of Slit mediated Robo signaling has advanced significantly further structural studies on larger assemblies are required for the design of new experiments to elucidate their role in cell surface receptor complexes. These will be necessary to understand the role of Slit -Robo signaling in neurogenesis, angiogenesis, organ development and cancer progression. In this chapter, we provide a review of the current knowledge in the field with a particular focus on the Roundabout receptor family.

Different Isoforms of the Neuronal Guidance Molecule Slit2 Directly Cause Chemoattraction or Chemorepulsion of Human Neutrophils

The movement of neutrophils between blood and tissues appears to be regulated by chemoattractants and chemorepellents. Compared with neutrophil chemoattractants, relatively little is known about neutrophil chemorepellents. Slit proteins are endogenously cleaved into a variety of N- and C-terminal fragments, and these fragments are neuronal chemorepellents and inhibit chemoattraction of many cell types, including neutrophils. In this report, we show that the ∼140-kDa N-terminal Slit2 fragment (Slit2-N) is a chemoattractant and the ∼110-kDa N-terminal Slit2 fragment (Slit2-S) is a chemorepellent for human neutrophils. The effects of both Slit2 fragments were blocked by Abs to the Slit2 receptor Roundabout homolog 1 or the Slit2 coreceptor Syndecan-4. Slit2-N did not appear to activate Ras but increased phosphatidylinositol 3,4,5-triphosphate levels. Slit2-N-induced chemoattraction was unaffected by Ras inhibitors, reversed by PI3K inhibitors, and blocked by Cdc42 and Rac inhibitors. In contrast, Slit2-S activated Ras but did not increase phosphatidylinositol 3,4,5-triphosphate levels. Slit2-S-induced chemorepulsion was blocked by Ras and Rac inhibitors, not affected by PI3K inhibitors, and reversed by Cdc42 inhibitors. Slit2-N, but not Slit2-S, increased neutrophil adhesion, myosin L chain 2 phosphorylation, and polarized actin formation and single pseudopods at the leading edge of cells. Slit2-S induced multiple pseudopods. These data suggest that Slit2 isoforms use similar receptors but different intracellular signaling pathways and have different effects on the cytoskeleton and pseudopods to induce neutrophil chemoattraction or chemorepulsion.

Making Connections: Guidance Cues and Receptors at Nonneural Cell-Cell Junctions

The field of axon guidance was revolutionized over the past three decades by the identification of highly conserved families of guidance cues and receptors. These proteins are essential for normal neural development and function, directing cell and axon migration, neuron-glial interactions, and synapse formation and plasticity. Many of these genes are also expressed outside the nervous system in which they influence cell migration, adhesion and proliferation. Because the nervous system develops from neural epithelium, it is perhaps not surprising that these guidance cues have significant nonneural roles in governing the specialized junctional connections between cells in polarized epithelia. The following review addresses roles for ephrins, semaphorins, netrins, slits and their receptors in regulating adherens, tight, and gap junctions in nonneural epithelia and endothelia.

Syndecan-4 influences mammalian myoblast proliferation by modulating myostatin signalling and G1/S transition

Myostatin, a TGF‐β superfamily member, is a negative regulator of muscle growth. Here we describe how myostatin activity is regulated by syndecan‐4, a ubiquitous transmembrane heparan sulfate proteoglycan. During muscle regeneration the levels of both syndecan‐4 and promyostatin decline gradually after a sharp increase, concurrently with the release of mature myostatin. Promyostatin and syndecan‐4 co‐immunoprecipitate, and the interaction is heparinase‐sensitive. ShRNA‐mediated silencing of syndecan‐4 reduces C2C12 myoblast proliferation via blocking the progression from G1‐ to S‐phase of the cell cycle, which is accompanied by elevated levels of myostatin and p21(Waf1/Cip1), and decreases in cyclin E and cyclin D1 expression. Our results suggest that syndecan‐4 functions as a reservoir for promyostatin regulating the local bioavailability of mature myostatin.

miR-92 Suppresses Robo1 Translation to Modulate Slit Sensitivity in Commissural Axon Guidance

Temporospatial regulation of guidance signaling is essential for axon outgrowth and pathfinding in the developing nervous system. Regulation of Robo1 levels in commissural neurons modulates Slit sensitivity facilitating proper axon guidance. The mechanisms underlying this regulation in the vertebrate nervous system are not well understood. Here, we report that miR-92, a highly conserved microRNA (miRNA), regulates chicken Robo1 expression in commissural neurons by binding to the 3' untranslated region (3' UTR) of Robo1 mRNA. miR-92 and Robo1 are differentially expressed in the developing spinal cord. miR-92 interacts with the Robo1 3'UTR to cause translational repression, but not mRNA degradation. Disruption of the miR-92/Robo1 3' UTR interaction induces premature responsiveness to Slit2 repulsion of precrossing commissural axons (CAs) in vitro and causes CA projection defects in vivo. These results indicate that miR-92 represses Robo1 expression thereby regulating Slit sensitivity to control CA projection and midline crossing.

A Traveling Wave Ion Mobility Spectrometry (TWIMS) Study of the Robo1-Heparan Sulfate Interaction

Roundabout 1 (Robo1) interacts with its receptor Slit to regulate axon guidance, axon branching, and dendritic development in the nervous system and to regulate morphogenesis and many cell functions in the nonneuronal tissues. This interaction is known to be critically regulated by heparan sulfate (HS). Previous studies suggest that HS is required to promote the binding of Robo1 to Slit to form the minimal signaling complex, but the molecular details and the structural requirements of HS for this interaction are still unclear. Here, we describe the application of traveling wave ion mobility spectrometry (TWIMS) to study the conformational details of the Robo1-HS interaction. The results suggest that Robo1 exists in two conformations that differ by their compactness and capability to interact with HS. The results also suggest that the highly flexible interdomain hinge region connecting the Ig1 and Ig2 domains of Robo1 plays an important functional role in promoting the Robo1-Slit interaction. Moreover, variations in the sulfation pattern and size of HS were found to affect its binding affinity and selectivity to interact with different conformations of Robo1. Both MS measurements and CIU experiments show that the Robo1-HS interaction requires the presence of a specific size and pattern of modification of HS. Furthermore, the effect of N-glycosylation on the conformation of Robo1 and its binding modes with HS is reported. Graphical Abstract ᅟ.

Slit-Robo signalling in heart development

The Slit ligands and their Robo receptors are well-known for their roles during axon guidance in the central nervous system but are still relatively unknown in the cardiac field. However, data from different animal models suggest a broad involvement of the pathway in many aspects of heart development, from cardiac cell migration and alignment, lumen formation, chamber formation, to the formation of the ventricular septum, semilunar and atrioventricular valves, caval veins, and pericardium. Absence of one or more of the genes in the pathway results in defects ranging from bicuspid aortic valves to ventricular septal defects and abnormal venous connections to the heart. Congenital heart defects are the most common congenital malformations found in life new-born babies and progress in methods for large scale human genetic testing has significantly enhanced the identification of new causative genes involved in human congenital heart disease. Recently, loss of function variants in ROBO1 have also been linked to ventricular septal defects and tetralogy of Fallot in patients. Here, we will give an overview of the role of the Slit-Robo signalling pathway in Drosophila, zebrafish, and mouse heart development. The extent of these data warrant further attention on the SLIT-ROBO signalling pathway as a candidate for an array of human congenital heart defects.

Heparan Sulfate Proteoglycans as Emerging Players in Synaptic Specificity

Neural circuits consist of distinct neuronal cell types connected in specific patterns. The specificity of these connections is achieved in a series of sequential developmental steps that involve the targeting of neurites, the identification of synaptic partners, and the formation of specific types of synapses. Cell-surface proteins play a critical role in each of these steps. The heparan sulfate proteoglycan (HSPG) family of cell-surface proteins is emerging as a key regulator of connectivity. HSPGs are expressed throughout brain development and play important roles in axon guidance, synapse development and synapse function. New insights indicate that neuronal cell types express unique combinations of HSPGs and HS-modifying enzymes. Furthermore, HSPGs interact with cell type-specific binding partners to mediate synapse development. This suggests that cell type-specific repertoires of HSPGs and specific patterns of HS modifications on the cell surface are required for the development of specific synaptic connections. Genome-wide association studies have linked these proteins to neurodevelopmental and neuropsychiatric diseases. Thus, HSPGs play an important role in the development of specific synaptic connectivity patterns important for neural circuit function, and their dysfunction may be involved in the development of brain disorders.

Diverse roles for glycosaminoglycans in neural patterning

The nervous system coordinates the functions of most multicellular organisms and their response to the surrounding environment. Its development involves concerted cellular interactions, including migration, axon guidance, and synapse formation. These processes depend on the molecular constituents and structure of the extracellular matrices (ECM). An essential component of ECMs are proteoglycans, i.e., proteins containing unbranched glycan chains known as glycosaminoglycans (GAGs). A defining characteristic of GAGs is their enormous molecular diversity, created by extensive modifications of the glycans during their biosynthesis. GAGs are widely expressed, and their loss can lead to catastrophic neuronal defects. Despite their importance, we are just beginning to understand the function and mechanisms of GAGs in neuronal development. In this review, we discuss recent evidence suggesting GAGs have specific roles in neuronal patterning and synaptogenesis. We examine the function played by the complex modifications present on GAG glycans and their roles in regulating different aspects of neuronal patterning. Moreover, the review considers the function of proteoglycan core proteins in these processes, stressing their likely role as co-receptors of different signaling pathways in a redundant and context-dependent manner. We conclude by discussing challenges and future directions toward a better understanding of these fascinating molecules during neuronal development. Developmental Dynamics 247:54-74, 2018. © 2017 Wiley Periodicals, Inc.

Control of nerve cord formation by Engrailed and Gooseberry-Neuro: A multi-step, coordinated process

One way to better understand the molecular mechanisms involved in the construction of a nervous system is to identify the downstream effectors of major regulatory proteins. We previously showed that Engrailed (EN) and Gooseberry-Neuro (GsbN) transcription factors act in partnership to drive the formation of posterior commissures in the central nervous system of Drosophila. In this report, we identified genes regulated by both EN and GsbN through chromatin immunoprecipitation ("ChIP on chip") and transcriptome experiments, combined to a genetic screen relied to the gene dose titration method. The genomic-scale approaches allowed us to define 175 potential targets of EN-GsbN regulation. We chose a subset of these genes to examine ventral nerve cord (VNC) defects and found that half of the mutated targets show clear VNC phenotypes when doubly heterozygous with en or gsbn mutations, or when homozygous. This strategy revealed new groups of genes never described for their implication in the construction of the nerve cord. Their identification suggests that, to construct the nerve cord, EN-GsbN may act at three levels, in: (i) sequential control of the attractive-repulsive signaling that ensures contralateral projection of the commissural axons, (ii) temporal control of the translation of some mRNAs, (iii) regulation of the capability of glial cells to act as commissural guideposts for developing axons. These results illustrate how an early, coordinated transcriptional control may orchestrate the various mechanisms involved in the formation of stereotyped neuronal networks. They also validate the overall strategy to identify genes that play crucial role in axonal pathfinding.

Midline axon guidance in the Drosophila embryonic central nervous system

Studies in the fruit fly Drosophila melanogaster have provided many fundamental insights into the genetic regulation of neural development, including the identification and characterization of evolutionarily conserved axon guidance pathways and their roles in important guidance decisions. Due to its highly organized and fast-developing embryonic nervous system, relatively small number of neurons, and molecular and genetic tools for identifying, labeling, and manipulating individual neurons or small neuronal subsets, studies of axon guidance in the Drosophila embryonic CNS have allowed researchers to dissect these genetic mechanisms with a high degree of precision. In this review, we discuss the major axon guidance pathways that regulate midline crossing of axons and the formation and guidance of longitudinal axon tracts, two processes that contribute to the development of the precise three-dimensional structure of the insect nerve cord. We focus particularly on recent insights into the roles and regulation of canonical midline axon guidance pathways, and on additional factors and pathways that have recently been shown to contribute to axon guidance decisions at and near the midline.

Inter-adipocyte Adhesion and Signaling by Collagen IV Intercellular Concentrations in Drosophila

Sheet-forming Collagen IV is the main component of basement membranes, which are planar polymers of extracellular matrix underlying epithelia and surrounding organs in all animals. Adipocytes in both insects and mammals are mesodermal in origin and often classified as mesenchymal. However, they form true tissues where cells remain compactly associated. Neither the mechanisms providing this tissue-level organization nor its functional significance are known. Here we show that discrete Collagen IV intercellular concentrations (CIVICs), distinct from basement membranes and thicker in section, mediate inter-adipocyte adhesion in Drosophila. Loss of these Collagen-IV-containing structures in the larval fat body caused intercellular gaps and disrupted continuity of the adipose tissue layer. We also found that Integrin and Syndecan matrix receptors attach adipocytes to CIVICs and direct their formation. Finally, we show that Integrin-mediated adhesion to CIVICs promotes normal adipocyte growth and prevents autophagy through Src-Pi3K-Akt signaling. Our results evidence a surprising non-basement membrane role of Collagen IV in non-epithelial tissue morphogenesis while demonstrating adhesion and signaling functions for these structures.

The glycosylation pathway is required for the secretion of Slit and for the maintenance of the Slit receptor Robo on axons

Slit proteins act as repulsive axon guidance cues by activating receptors of the Roundabout (Robo) family. During early neurogenesis in Drosophila melanogaster, Slit prevents the growth cones of longitudinal tract neurons from inappropriately crossing the midline, thus restricting these cells to trajectories parallel to the midline. Slit is expressed in midline glial cells, and Robo is present in longitudinal axon tracts and growth cones. We showed that the enzyme Mummy (Mmy) controlled Slit-Robo signaling through mechanisms that affected both the ligand and the receptor. Mmy was required for the glycosylation of Slit, which was essential for Slit secretion. Mmy was also required for maintaining the abundance and spatial distribution of Robo through an indirect mechanism that was independent of Slit secretion. Moreover, secretion of Slit was required to maintain the fasciculation and position of longitudinal axon tracts, thus maintaining the hardwiring of the nervous system. Thus, Mmy is required for Slit secretion and for maintaining Robo abundance and distribution in the developing nervous system in Drosophila.

The role of Slit-Robo signaling in the regulation of tissue barriers

The role of Slit/Robo signaling has extended from initial axon repulsion in the developing nervous system to organ morphogenesis, cancer development and angiogenesis. Slit/Robo signaling regulates similar pathways within these processes. Slit/Robo ensures the homeostasis of the dynamic interaction between cell-cell and cell-matrix interactions. The dysregulation of Slit/Robo signaling damages the tissue barrier, resulting in developmental abnormalities or disease. Here, we summarize how Slit/Robo controls kidney morphogenesis and describe the dual roles of Slit/Robo signaling in the regulation of tumorigenesis and angiogenesis.

Functional Requirements for Heparan Sulfate Biosynthesis in Morphogenesis and Nervous System Development in C. elegans

The regulation of cell migration is essential to animal development and physiology. Heparan sulfate proteoglycans shape the interactions of morphogens and guidance cues with their respective receptors to elicit appropriate cellular responses. Heparan sulfate proteoglycans consist of a protein core with attached heparan sulfate glycosaminoglycan chains, which are synthesized by glycosyltransferases of the exostosin (EXT) family. Abnormal HS chain synthesis results in pleiotropic consequences, including abnormal development and tumor formation. In humans, mutations in either of the exostosin genes EXT1 and EXT2 lead to osteosarcomas or multiple exostoses. Complete loss of any of the exostosin glycosyltransferases in mouse, fish, flies and worms leads to drastic morphogenetic defects and embryonic lethality. Here we identify and study previously unavailable viable hypomorphic mutations in the two C. elegans exostosin glycosyltransferases genes, rib-1 and rib-2. These partial loss-of-function mutations lead to a severe reduction of HS levels and result in profound but specific developmental defects, including abnormal cell and axonal migrations. We find that the expression pattern of the HS copolymerase is dynamic during embryonic and larval morphogenesis, and is sustained throughout life in specific cell types, consistent with HSPGs playing both developmental and post-developmental roles. Cell-type specific expression of the HS copolymerase shows that HS elongation is required in both the migrating neuron and neighboring cells to coordinate migration guidance. Our findings provide insights into general principles underlying HSPG function in development.

During animal development, cells and neurons navigate long distances to reach their final target destinations. Migrating cells are guided by extracellular molecular cues, and cellular responses to these cues are regulated by heparan sulfate proteoglycans. Heparan sulfate proteoglycans are proteins with long heparan sulfate polysaccharide chains attached. Here we identify and study previously unavailable viable mutants that disrupt the elongation of the heparan sulfate chains in the nematode C. elegans. Our analysis shows that these HS-chain-elongation mutations affect the development of the nervous system as they result in misguided migrations of neurons and axons. Furthermore, we find that heparan sulfate chain elongation occurs in numerous cell types during development and that the coordinated production of heparan sulfate proteoglycans, in both the migrating cell and neighboring tissues, ensures proper migration. Our findings highlight the critical roles of heparan sulfate proteoglycans in nervous system development and the evolutionary conservation of the molecular mechanisms driving guided migrations.

Panels of chemically-modified heparin polysaccharides and natural heparan sulfate saccharides both exhibit differences in binding to Slit and Robo, as well as variation between protein binding and cellular activity

Heparin/heparan sulfate (HS) glycosaminoglycans are required for Slit-Robo cellular responses. Evidence exists for interactions between each combination of Slit, Robo and heparin/HS and for formation of a ternary complex. Heparin/HS are complex mixtures displaying extensive structural diversity. The relevance of this diversity has been studied to a limited extent using a few select chemically-modified heparins as models of HS diversity. Here we extend these studies by parallel screening of structurally diverse panels of eight chemically-modified heparin polysaccharides and numerous natural HS oligosaccharide chromatographic fractions for binding to both Drosophila Slit and Robo N-terminal domains and for activation of a chick retina axon response to the Slit fragment. Both the polysaccharides and oligosaccharide fractions displayed variability in binding and cellular activity that could not be attributed solely to increasing sulfation, extending evidence for the importance of structural diversity to natural HS as well as model modified heparins. They also displayed differences in their interactions with Slit compared to Robo, with Robo preferring compounds with higher sulfation. Furthermore, the patterns of cellular activity across compounds were different to those for binding to each protein, suggesting that biological outcomes are selectively determined in a subtle manner that does not simply reflect the sum of the separate interactions of heparin/HS with Slit and Robo.

Slit-Robo signaling

Slits are secreted proteins that bind to Roundabout (Robo) receptors. Slit-Robo signaling is best known for mediating axon repulsion in the developing nervous system. However, in recent years the functional repertoire of Slits and Robo has expanded tremendously and Slit-Robo signaling has been linked to roles in neurogenesis, angiogenesis and cancer progression among other processes. Likewise, our mechanistic understanding of Slit-Robo signaling has progressed enormously. Here, we summarize new insights into Slit-Robo evolutionary and system-dependent diversity, receptor-ligand interactions, signaling crosstalk and receptor activation.

In vivo functional analysis of Drosophila Robo1 immunoglobulin-like domains

BACKGROUND

In animals with bilateral symmetry, midline crossing of axons in the developing central nervous system is regulated by Slit ligands and their neuronal Roundabout (Robo) receptors. Multiple structural domains are present in an evolutionarily conserved arrangement in Robo family proteins, but our understanding of the functional importance of individual domains for midline repulsive signaling is limited.

METHODS

We have examined the functional importance of each of the five conserved immunoglobulin-like (Ig) domains within the Drosophila Robo1 receptor. We generated a series of Robo1 variants, each lacking one of the five Ig domains (Ig1-5), and tested each for their ability to bind Slit when expressed in cultured Drosophila cells. We used a transgenic approach to express each variant in robo1's normal expression pattern in wild-type and robo1 mutant embryos, and examined the effects of deleting each domain on receptor expression, axonal localization, regulation, and midline repulsive signaling in vivo.

RESULTS

We show that individual deletion of Ig domains 2-5 does not interfere with Robo1's ability to bind Slit, while deletion of Ig1 strongly disrupts Slit binding. None of the five Ig domains (Ig1-5) are individually required for proper expression of Robo1 in embryonic neurons, for exclusion from commissural axon segments in wild-type embryos, or for downregulation by Commissureless (Comm), a negative regulator of Slit-Robo repulsion in Drosophila. Each of the Robo1 Ig deletion variants (with the exception of Robo1∆Ig1) were able to restore midline crossing in robo1 mutant embryos to nearly the same extent as full-length Robo1, indicating that Ig domains 2-5 are individually dispensable for midline repulsive signaling in vivo.

CONCLUSIONS

Our findings indicate that four of the five Ig domains within Drosophila Robo1 are dispensable for its role in midline repulsion, despite their strong evolutionary conservation, and highlight a unique requirement for the Slit-binding Ig1 domain in the regulation of midline crossing.

The role of heparan sulphate in development: the ectodermal story

Heparan sulphate (HS) is ubiquitously expressed and is formed of repeating glucosamine and glucuronic/iduronic acid units which are generally highly sulphated. HS is found in tissues bound to proteins forming HS proteoglycans (HSPGs) which are present on the cell membrane or in the extracellular matrix. HSPGs influence a variety of biological processes by interacting with physiologically important proteins, such as morphogens, creating storage pools, generating morphogen gradients and directly mediating signalling pathways, thereby playing vital roles during development. This review discusses the vital role HS plays in the development of tissues from the ectodermal lineage. The ectodermal layer differentiates to form the nervous system (including the spine, peripheral nerves and brain), eye, epidermis, skin appendages and tooth enamel.

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.

Proteoglycans and axon guidance: a new relationship between old partners

Neural circuits are formed with great precision during development. Accumulated evidence over the past three decades has demonstrated that growing axons are navigated toward their targets by the combined actions of attractants and repellents together with their receptors. It has long been known that proteoglycans, glycosylated proteins possessing covalently attached glycosaminoglycans, play a critical role in axon guidance; however, the molecular mechanisms by which proteoglycans regulate axon behaviors remain largely unknown. Glycosaminoglycans such as heparan sulfate and chondroitin sulfate are large linear polysaccharides composed of repeating disaccharide units that are highly modified by specific sulfation and epimerization. Recent biochemical and molecular biological studies have identified the enzymes that are involved in the biosynthesis of glycosaminoglycans. Interestingly, many mutants lacking glycosaminoglycan-synthesizing enzymes or proteoglycans in several model organisms show defects in specific nerve tract formation. In parallel, detailed biochemical studies have identified the molecular interactions between axon guidance molecules and glycosaminoglycans that have specific modification in their sugar chains. This review summarizes the structure and function of axon guidance molecules and glycosaminoglycans, and then tries to combine the knowledge from these studies to understand the role of proteoglycans from a new vantage point. Deciphering the sugar code is important for understanding the complicated nature of proteoglycans in axon guidance. Neural circuits are formed by the combined actions of axon guidance molecules. Proteoglycans play critical roles in regulating axon guidance through the interaction between signaling molecules and glycosaminoglycan chains attached to the core protein. This paper summarizes the structure and functions of axon guidance molecules and glycosaminoglycans and reviews the molecular mechanisms by which proteoglycans regulate axon guidance from a new vantage point. This article is part of the 60th Anniversary special issue.

Heparan sulfate proteoglycans: a sugar code for vertebrate development?

Heparan sulfate proteoglycans (HSPGs) have long been implicated in a wide range of cell-cell signaling and cell-matrix interactions, both in vitro and in vivo in invertebrate models. Although many of the genes that encode HSPG core proteins and the biosynthetic enzymes that generate and modify HSPG sugar chains have not yet been analyzed by genetics in vertebrates, recent studies have shown that HSPGs do indeed mediate a wide range of functions in early vertebrate development, for example during left-right patterning and in cardiovascular and neural development. Here, we provide a comprehensive overview of the various roles of HSPGs in these systems and explore the concept of an instructive heparan sulfate sugar code for modulating vertebrate development.

Summary: This Review article examines the role of heparan sulfate proteoglycans in vertebrate development and explores the concept of an instructive 'sugar code' for modulating development.

BmRobo2/3 is required for axon guidance in the silkworm Bombyx mori

Axon guidance is critical for proper wiring of the nervous system. During the neural development, the axon guidance molecules play a key role and direct axons to choose the correct way to reach the target. Robo, as the receptor of axon guidance molecule Slit, is evolutionarily conserved from planarians to humans. However, the function of Robo in the silkworm, Bombyx mori, remained unknown. In this study, we cloned robo2/3 from B. mori (Bmrobo2/3), a homologue of robo2/3 in Tribolium castaneum. Moreover, BmRobo2/3 was localized in the neuropil, and RNAi-mediated knockdown of Bmrobo2/3 resulted in the longitudinal connectives forming closer to the midline. These data demonstrate that BmRobo2/3 is required for axon guidance in the silkworm.

Functional Conservation of the Glide/Gcm Regulatory Network Controlling Glia, Hemocyte, and Tendon Cell Differentiation in Drosophila

High-throughput screens allow us to understand how transcription factors trigger developmental processes, including cell specification. A major challenge is identification of their binding sites because feedback loops and homeostatic interactions may mask the direct impact of those factors in transcriptome analyses. Moreover, this approach dissects the downstream signaling cascades and facilitates identification of conserved transcriptional programs. Here we show the results and the validation of a DNA adenine methyltransferase identification (DamID) genome-wide screen that identifies the direct targets of Glide/Gcm, a potent transcription factor that controls glia, hemocyte, and tendon cell differentiation in Drosophila. The screen identifies many genes that had not been previously associated with Glide/Gcm and highlights three major signaling pathways interacting with Glide/Gcm: Notch, Hedgehog, and JAK/STAT, which all involve feedback loops. Furthermore, the screen identifies effector molecules that are necessary for cell-cell interactions during late developmental processes and/or in ontogeny. Typically, immunoglobulin (Ig) domain-containing proteins control cell adhesion and axonal navigation. This shows that early and transiently expressed fate determinants not only control other transcription factors that, in turn, implement a specific developmental program but also directly affect late developmental events and cell function. Finally, while the mammalian genome contains two orthologous Gcm genes, their function has been demonstrated in vertebrate-specific tissues, placenta, and parathyroid glands, begging questions on the evolutionary conservation of the Gcm cascade in higher organisms. Here we provide the first evidence for the conservation of Gcm direct targets in humans. In sum, this work uncovers novel aspects of cell specification and sets the basis for further understanding of the role of conserved Gcm gene regulatory cascades.

Cleaved Slit directs embryonic muscles

The formation of functional musculoskeletal system relies on proper connectivity between muscles and their corresponding tendon cells. In Drosophila, larval muscles are born during early embryonic stages, and elongate toward tendons that are embedded within the ectoderm in later. The Slit/Robo signaling pathway had been implicated in the process of muscle elongation toward tendons. Here we discuss our recent findings regarding the critical contribution of Slit cleavage for immobilization and stabilization of the Slit signal on the tendon cells. Slit cleavage produces 2 polypeptides, the N-terminal Slit-N, which is extremely stable, undergoes oligomerization, and associates with the tendon cell surfaces, and the C-terminal Slit-C, which rapidly degrades. Slit cleavage leads to immobilization of Slit signaling on tendons, leading to a short-range repulsion, which eventually arrest further muscle elongation. Robo2, which is co-expressed with Slit by the tendon cells facilitates Slit cleavage. This activity does not require the cytoplasmic signaling domain of Robo2. We suggest that Robo2-dependent Slit cleavage, and the formation of Slit-N oligomers on the tendon cell surfaces direct muscle elongation, and provide a stop signal for the approaching muscle, through binding to Robo and Robo3 receptors expressed by the muscles.

"GAG-ing with the neuron": The role of glycosaminoglycan patterning in the central nervous system

Proteoglycans (PGs) are a diverse family of proteins that consist of one or more glycosaminoglycan (GAG) chains, covalently linked to a core protein. PGs are major components of the extracellular matrix (ECM) and play critical roles in development, normal function and damage-response of the central nervous system (CNS). GAGs are classified based on their disaccharide subunits, into the following major groups: chondroitin sulfate (CS), heparan sulfate (HS), heparin (HEP), dermatan sulfate (DS), keratan sulfate (KS) and hyaluronic acid (HA). All except HA are modified by sulfation, giving GAG chains specific charged structures and binding properties. While significant neuroscience research has focused on the role of one PG family member, chondroitin sulfate proteoglycan (CSPG), there is ample evidence in support of a role for the other PGs in regulating CNS function in normal and pathological conditions. This review discusses the role of all the identified PG family members (CS, HS, HEP, DS, KS and HA) in normal CNS function and in the context of pathology. Understanding the pleiotropic roles of these molecules in the CNS may open the door to novel therapeutic strategies for a number of neurological conditions.

Building from the Ground up: Basement Membranes in Drosophila Development

Basement membranes (BMs) are sheetlike extracellular matrices found at the basal surfaces of epithelial tissues. The structural and functional diversity of these matrices within the body endows them with the ability to affect multiple aspects of cell behavior and communication; for this reason, BMs are integral to many developmental processes. The power of Drosophila genetics, as applied to the BM, has yielded substantial insight into how these matrices influence development. Here, we explore three facets of BM biology to which Drosophila research has made particularly important contributions. First, we discuss how newly synthesized BM proteins are secreted to and assembled exclusively on basal epithelial surfaces. Next, we examine how regulation of the structural properties of the BM mechanically supports and guides tissue morphogenesis. Finally, we explore how BMs influence development through the modulation of several major signaling pathways.

Glypican Is a Modulator of Netrin-Mediated Axon Guidance

Netrin is a key axon guidance cue that orients axon growth during neural circuit formation. However, the mechanisms regulating netrin and its receptors in the extracellular milieu are largely unknown. Here we demonstrate that in Caenorhabditis elegans, LON-2/glypican, a heparan sulfate proteoglycan, modulates UNC-6/netrin signaling and may do this through interactions with the UNC-40/DCC receptor. We show that developing axons misorient in the absence of LON-2/glypican when the SLT-1/slit guidance pathway is compromised and that LON-2/glypican functions in both the attractive and repulsive UNC-6/netrin pathways. We find that the core LON-2/glypican protein, lacking its heparan sulfate chains, and secreted forms of LON-2/glypican are functional in axon guidance. We also find that LON-2/glypican functions from the epidermal substrate cells to guide axons, and we provide evidence that LON-2/glypican associates with UNC-40/DCC receptor-expressing cells. We propose that LON-2/glypican acts as a modulator of UNC-40/DCC-mediated guidance to fine-tune axonal responses to UNC-6/netrin signals during migration.

The differential roles of Slit2-exon 15 splicing variants in angiogenesis and HUVEC permeability

Slit2, a secreted glycoprotein, is down-regulated in many cancers. Slit2/Robo signaling pathway plays an important, but controversial, role in angiogenesis. We identified splicing variants of Slit2 at exon 15, Slit2-WT and Slit2-ΔE15, with differential effects on proliferation and invasive capability of lung cancer cells. The aim of this study was to elucidate the differential roles of these exon 15 splicing variants in angiogenesis. Our results revealed that both Slit2-WT and Slit2-ΔE15 inhibit motility of human umbilical vein endothelial cells (HUVECs). The conditioned medium (CM) collected from CL1-5/VC or CL1-5/Slit2-WT lung adenocarcinoma cells blocked HUVEC tube formation and angiogenesis on chorioallantoic membrane (CAM) assay when compared with untreated HUVECs and CAM, respectively. However, CM of CL1-5/Slit2-ΔE15 restored the quality of tubes and the size of vessels. Although both Slit2-WT and Slit2-ΔE15 inhibited permeability induced by CM of cancer cells, Slit2-ΔE15 exhibited stronger effect. These results suggested that Slit2-ΔE15 plays important roles in normalization of blood vessels by enhancing tube quality and tightening endothelial cells, while Slit2-WT only enhances tightening of endothelial cells. It appears that Robo4 is responsible for Slit2 isoform-mediated inhibition of permeability, while neither Robo1 nor Robo4 is required for Slit2-ΔE15-enhanced tube quality. The results of this study suggest that Slit2-ΔE15 splicing form is a promising molecule for normalizing blood vessels around a tumor, which, in turn, may increase efficacy of chemotherapy and radiotherapy.

Nerve injury induces the expression of syndecan-1 heparan sulfate proteoglycan in primary sensory neurons

Heparan sulfate proteoglycans (HSPGs) have important functions in development of the central nervous system; however, their functions in nerve injury are not yet fully understood. We previously reported the expression of syndecan-1, a type of HSPG, in cranial motor neurons after nerve injury, suggesting the importance of syndecan-1 in the pathology of motor nerve injury. In this study, we examined the expression of syndecan-1, a type of HSPG, in primary sensory neurons after nerve injury in mice. Sciatic nerve axotomy strongly induced the expression of syndecan-1 in a subpopulation of injured dorsal root ganglion (DRG) neurons, which were small in size and had CGRP- or isolectin B4-positive fibers. Syndecan-1 was also distributed in the dorsal horn of the spinal cord ipsilateral to the axotomy, and located on the membrane of axons in lamina II of the dorsal horn. Not only sciatic nerve axotomy, infraorbital nerve axotomy also induced the expression of syndecan-1 in trigeminal ganglion neurons. Moreover, syndecan-1 knockdown in cultured DRG neurons induced a shorter neurite extension. These results suggest that syndecan-1 expression in injured primary sensory neurons may have functional roles in nerve regeneration and synaptic plasticity, resulting in the development of neuropathic pain.

Analyzing the role of heparan sulfate proteoglycans in axon guidance in vivo in zebrafish

One of the most fascinating questions in the field of neurobiology is to understand how neuronal connections are properly formed. During development, neurons extend axons that are guided along defined paths by attractive and repulsive cues to reach their brain target. Most of these guidance factors are regulated by heparan sulfate proteoglycans (HSPGs), a family of cell-surface and extracellular core proteins with attached heparan sulfate (HS) glycosaminoglycans. The unique diversity and structural complexity of HS sugar chains, as well as the variety of core proteins, have been proposed to generate a complex "sugar code" essential for brain wiring. While the functions of HSPGs have been well characterized in C. elegans or Drosophila, relatively little is known about their roles in nervous system development in vertebrates. In this chapter, we describe the advantages and the different methods available to study the roles of HSPGs in axon guidance directly in vivo in zebrafish. We provide protocols for visualizing axons in vivo, including precise dye labeling and time-lapse imaging, and for disturbing the functions of HS-modifying enzymes and core proteins, including morpholino, DNA, or RNA injections.

Fell-Muir Lecture: Syndecans: from peripheral coreceptors to mainstream regulators of cell behaviour

In the 25 years, as the first of the syndecan family was cloned, interest in these transmembrane proteoglycans has steadily increased. While four distinct members are present in mammals, one is present in invertebrates, including C. elegans that is such a powerful genetic model. The syndecans, therefore, have a long evolutionary history, indicative of important roles. However, these roles have been elusive. The knockout in the worm has a developmental neuronal phenotype, while knockouts of the syndecans in the mouse are mild and mostly limited to post-natal rather than developmental effects. Moreover, their association with high-affinity receptors, such as integrins, growth factor receptors, frizzled and slit/robo, have led to the notion that syndecans are coreceptors, with minor roles. Given that their heparan sulphate chains can gather many different protein ligands, this gave credence to views that the importance of syndecans lay with their ability to concentrate ligands and that only the extracellular polysaccharide was of significance. Syndecans are increasingly identified with roles in the pathogenesis of many diseases, including tumour progression, vascular disease, arthritis and inflammation. This has provided impetus to understanding syndecan roles in more detail. It emerges that while the cytoplasmic domains of syndecans are small, they have clear interactive capabilities, most notably with the actin cytoskeleton. Moreover, through the binding and activation of signalling molecules, it is likely that syndecans are important receptors in their own right. Here, an overview of syndecan structure and function is provided, with some prospects for the future.

Neuron-glia interactions through the Heartless FGF receptor signaling pathway mediate morphogenesis of Drosophila astrocytes

Astrocytes are critically important for neuronal circuit assembly and function. Mammalian protoplasmic astrocytes develop a dense ramified meshwork of cellular processes to form intimate contacts with neuronal cell bodies, neurites, and synapses. This close neuron-glia morphological relationship is essential for astrocyte function, but it remains unclear how astrocytes establish their intricate morphology, organize spatial domains, and associate with neurons and synapses in vivo. Here we characterize a Drosophila glial subtype that shows striking morphological and functional similarities to mammalian astrocytes. We demonstrate that the Fibroblast growth factor (FGF) receptor Heartless autonomously controls astrocyte membrane growth, and the FGFs Pyramus and Thisbe direct astrocyte processes to ramify specifically in CNS synaptic regions. We further show that the shape and size of individual astrocytes are dynamically sculpted through inhibitory or competitive astrocyte-astrocyte interactions and Heartless FGF signaling. Our data identify FGF signaling through Heartless as a key regulator of astrocyte morphological elaboration in vivo.

Influence of the extracellular matrix on endogenous and transplanted stem cells after brain damage

The limited regeneration capacity of the adult central nervous system (CNS) requires strategies to improve recovery of patients. In this context, the interaction of endogenous as well as transplanted stem cells with their environment is crucial. An understanding of the molecular mechanisms could help to improve regeneration by targeted manipulation. In the course of reactive gliosis, astrocytes upregulate Glial fibrillary acidic protein (GFAP) and start, in many cases, to proliferate. Beside GFAP, subpopulations of these astroglial cells coexpress neural progenitor markers like Nestin. Although cells express these markers, the proportion of cells that eventually give rise to neurons is limited in many cases in vivo compared to the situation in vitro. In the first section, we present the characteristics of endogenous progenitor-like cells and discuss the differences in their neurogenic potential in vitro and in vivo. As the environment plays an important role for survival, proliferation, migration, and other processes, the second section of the review describes changes in the extracellular matrix (ECM), a complex network that contains numerous signaling molecules. It appears that signals in the damaged CNS lead to an activation and de-differentiation of astrocytes, but do not effectively promote neuronal differentiation of these cells. Factors that influence stem cells during development are upregulated in the damaged brain as part of an environment resembling a stem cell niche. We give a general description of the ECM composition, with focus on stem cell-associated factors like the glycoprotein Tenascin-C (TN-C). Stem cell transplantation is considered as potential treatment strategy. Interaction of transplanted stem cells with the host environment is critical for the outcome of stem cell-based therapies. Possible mechanisms involving the ECM by which transplanted stem cells might improve recovery are discussed in the last section.

Syndecan promotes axon regeneration by stabilizing growth cone migration

Growth cones facilitate the repair of nervous system damage by providing the driving force for axon regeneration. Using single-neuron laser axotomy and in vivo time-lapse imaging, we show that syndecan, a heparan sulfate (HS) proteoglycan, is required for growth cone function during axon regeneration in C. elegans. In the absence of syndecan, regenerating growth cones form but are unstable and collapse, decreasing the effective growth rate and impeding regrowth to target cells. We provide evidence that syndecan has two distinct functions during axon regeneration: (1) a canonical function in axon guidance that requires expression outside the nervous system and depends on HS chains and (2) an intrinsic function in growth cone stabilization that is mediated by the syndecan core protein, independently of HS. Thus, syndecan is a regulator of a critical choke point in nervous system repair.

Characterization of the interaction between Robo1 and heparin and other glycosaminoglycans

Roundabout 1 (Robo1) is the cognate receptor for secreted axon guidance molecule, Slits, which function to direct cellular migration during neuronal development and angiogenesis. The Slit2-Robo1 signaling is modulated by heparan sulfate, a sulfated linear polysaccharide that is abundantly expressed on the cell surface and in the extracellular matrix. Biochemical studies have further shown that heparan sulfate binds to both Slit2 and Robo1 facilitating the ligand-receptor interaction. The structural requirements for heparan sulfate interaction with Robo1 remain unknown. In this report, surface plasmon resonance (SPR) spectroscopy was used to examine the interaction between Robo1 and heparin and other GAGs and determined that heparin binds to Robo1 with an affinity of ~650 nM. SPR solution competition studies with chemically modified heparins further determined that although all sulfo groups on heparin are important for the Robo1-heparin interaction, the N-sulfo and 6-O-sulfo groups are essential for the Robo1-heparin binding. Examination of differently sized heparin oligosaccharides and different GAGs also demonstrated that Robo1 prefers to bind full-length heparin chains and that GAGs with higher sulfation levels show increased Robo1 binding affinities.

Post-transcriptional regulation of myotube elongation and myogenesis by Hoi Polloi

Striated muscle development requires the coordinated expression of genes involved in sarcomere formation and contractility, as well as genes that determine muscle morphology. However, relatively little is known about the molecular mechanisms that control the early stages of muscle morphogenesis. To explore this facet of myogenesis, we performed a genetic screen for regulators of somatic muscle morphology in Drosophila, and identified the putative RNA-binding protein (RBP) Hoi Polloi (Hoip). Hoip is expressed in striated muscle precursors within the muscle lineage and controls two genetically separable events: myotube elongation and sarcomeric protein expression. Myotubes fail to elongate in hoip mutant embryos, even though the known regulators of somatic muscle elongation, target recognition and muscle attachment are expressed normally. In addition, a majority of sarcomeric proteins, including Myosin Heavy Chain (MHC) and Tropomyosin, require Hoip for their expression. A transgenic MHC construct that contains the endogenous MHC promoter and a spliced open reading frame rescues MHC protein expression in hoip embryos, demonstrating the involvement of Hoip in pre-mRNA splicing, but not in transcription, of muscle structural genes. In addition, the human Hoip ortholog NHP2L1 rescues muscle defects in hoip embryos, and knockdown of endogenous nhp2l1 in zebrafish disrupts skeletal muscle development. We conclude that Hoip is a conserved, post-transcriptional regulator of muscle morphogenesis and structural gene expression.