Proteoglycan-specific molecular switch for RPTPσ clustering and neuronal extension

The roles of protein tyrosine phosphatases in bone-resorbing osteoclasts

Maintaining the proper balance between osteoblast-mediated production of bone and its degradation by osteoclasts is essential for health. Osteoclasts are giant phagocytic cells that are formed by fusion of monocyte-macrophage precursor cells; mature osteoclasts adhere to bone tightly and secrete protons and proteases that degrade its matrix. Phosphorylation of tyrosine residues in proteins, which is regulated by the biochemically-antagonistic activities of protein tyrosine kinases and protein tyrosine phosphatases (PTPs), is central in regulating the production of osteoclasts and their bone-resorbing activity. Here we review the roles of individual PTPs of the classical and dual-specificity sub-families that are known to support these processes (SHP2, cyt-PTPe, PTPRO, PTP-PEST, CD45) or to inhibit them (SHP1, PTEN, MKP1). Characterizing the functions of PTPs in osteoclasts is essential for complete molecular level understanding of bone resorption and for designing novel therapeutic approaches for treating bone disease.

Interaction of receptor type of protein tyrosine phosphatase sigma (RPTPσ) with a glycosaminoglycan library

Receptor type of protein tyrosine phosphatase sigma (RPTPσ) functions as a glycosaminoglycan (GAG) receptor of neuronal cells in both the central and peripheral nervous systems. Both chondroitin sulphate (CS) and heparan sulphate (HS) are important constituents of GAG ligands for RPTPσ, although they have opposite effects on neuronal cells. CS inhibits neurite outgrowth and neural regeneration through RPTPσ, whereas HS enhances them. We prepared recombinant RPTPσ N-terminal fragment containing the GAG binding site and various types of biotin-conjugated GAG (CS and HS) with chemical modification and chemo-enzymatic synthesis. Then interaction of the RPTPσ N-terminal fragment was analysed using GAG-biotin immobilized on streptavidin sensor chips by surface plasmon resonance. Interaction of RPTPσ with the CS library was highly correlated to the degree of disulphated disaccharide E unit, which had two sulphate groups at C-4 and C-6 positions of the N-acetylgalactosamine residue (CSE). The optimum molecular mass of CSE was suggested to be approximately 10 kDa. Heparin showed higher affinity to RPTPσ than the CS library. Our GAG library will not only contribute to the fields of carbohydrate science and cell biology, but also provide medical application to regulate neural regeneration.

PTPσ Drives Excitatory Presynaptic Assembly via Various Extracellular and Intracellular Mechanisms

Leukocyte common antigen-receptor protein tyrosine phosphatases (LAR-RPTPs) are hub proteins that organize excitatory and inhibitory synapse development through binding to various extracellular ligands. Here, we report that knockdown (KD) of the LAR-RPTP family member PTPσ reduced excitatory synapse number and transmission in cultured rat hippocampal neurons, whereas KD of PTPδ produced comparable decreases at inhibitory synapses, in both cases without altering expression levels of interacting proteins. An extensive series of rescue experiments revealed that extracellular interactions of PTPσ with Slitrks are important for excitatory synapse development. These experiments further showed that the intracellular D2 domain of PTPσ is required for induction of heterologous synapse formation by Slitrk1 or TrkC, suggesting that interaction of LAR-RPTPs with distinct intracellular presynaptic proteins, drives presynaptic machinery assembly. Consistent with this, double-KD of liprin-α2 and -α3 or KD of PTPσ substrates (N-cadherin and p250RhoGAP) in neurons inhibited Slitrk6-induced, PTPσ-mediated heterologous synapse formation activity. We propose a synaptogenesis model in presynaptic neurons involving LAR-RPTP-organized retrograde signaling cascades, in which both extracellular and intracellular mechanisms are critical in orchestrating distinct synapse types.SIGNIFICANCE STATEMENT In this study, we sought to test the unproven hypothesis that PTPσ and PTPδ are required for excitatory and inhibitory synapse formation/transmission, respectively, in cultured hippocampal neurons, using knockdown-based loss-of-function analyses. We further performed extensive structure-function analyses, focusing on PTPσ-mediated actions, to address the mechanisms of presynaptic assembly at excitatory synaptic sites. Using interdisciplinary approaches, we systematically applied a varied set of PTPσ deletion variants, point mutants, and splice variants to demonstrate that both extracellular and intracellular mechanisms are involved in organizing presynaptic assembly. Strikingly, extracellular interactions of PTPσ with heparan sulfates and Slitrks, intracellular interactions of PTPσ with liprin-α and its associated proteins through the D2 domain, as well as distinct substrates are all critical.

Identification of novel binding sites for heparin in receptor protein-tyrosine phosphatase (RPTPσ): Implications for proteoglycan signaling

Receptor protein-tyrosine phosphatase RPTPσ has important functions in modulating neural development and regeneration. Compelling evidence suggests that both heparan sulfate (HS) and chondroitin sulfate (CS) glycosaminoglycans (GAGs) bind to a series of Lys residues located in the first Ig domain of RPTPσ. However, HS promotes and CS inhibits axonal growth. Mutation of these Lys residues abolished binding and signal transduction of RPTPσ to CS, whereas HS binding was reduced, and signaling persisted. This activity was mediated through novel heparin-binding sites identified in the juxtamembrane region. Although different functional outcomes of HS and CS have been previously attributed to the differential oligomeric state of RPTPσ upon GAG binding, we found that RPTPσ was clustered by both heparin and CS GAG rich in 4,6-O-disulfated disaccharide units. We propose an additional mechanism by which RPTPσ distinguishes between HS and CS through these novel binding sites.

Deciphering functional glycosaminoglycan motifs in development

Glycosaminoglycans (GAGs) such as heparan sulfate, chondroitin/dermatan sulfate, and keratan sulfate are linear glycans, which when attached to protein backbones form proteoglycans. GAGs are essential components of the extracellular space in metazoans. Extensive modifications of the glycans such as sulfation, deacetylation and epimerization create structural GAG motifs. These motifs regulate protein-protein interactions and are thereby repsonsible for many of the essential functions of GAGs. This review focusses on recent genetic approaches to characterize GAG motifs and their function in defined signaling pathways during development. We discuss a coding approach for GAGs that would enable computational analyses of GAG sequences such as alignments and the computation of position weight matrices to describe GAG motifs.

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.

Increased Excitability and Reduced Excitatory Synaptic Input Into Fast-Spiking CA2 Interneurons After Enzymatic Attenuation of Extracellular Matrix

The neural extracellular matrix (ECM) is enriched with hyaluronic acid, chondroitin sulfate proteoglycans (CSPGs) and the glycoprotein tenascin-R, which play important roles in synaptic plasticity, as shown by studies of the CA1 region of the hippocampus. However, ECM molecules are strongly expressed in the CA2 region, which harbors a high number of fast-spiking interneurons (FSIs) surrounded by a particularly condensed form of ECM, perineuronal nets. Despite this intriguing peculiarity, the functional role of ECM in the CA2 region is mostly unknown. Here, we investigate the acute and delayed effects of chondroitinase ABC (ChABC), an enzyme that digests chondroitin sulfate side chains of CSPGs and greatly attenuates neural ECM, on neuronal excitability and excitatory transmission in the CA2 region. Whole-cell patch clamp recordings of CA2 pyramidal cells (PCs) and FSIs in hippocampal slices revealed that 7 days after injection of ChABC into the CA2 region in vivo, there are alterations in excitability of FSIs and PCs. FSIs generated action potentials with larger amplitudes and longer durations in response to less depolarizing currents compared to controls. PCs were excited at less depolarized membrane potentials, resulted in lower latency of spike generation. The frequency of excitatory postsynaptic currents in FSIs was selectively reduced, while the frequency of inhibitory postsynaptic currents was selectively increased. Acute treatment of hippocampal slices with ChABC did not result in any of these effects. This increase in excitability and changes in synaptic inputs to FSIs after attenuation of ECM suggests a crucial role for perineuronal nets associated with FSIs in regulation of synaptic and electrical properties of these cells.

Structural Characterization of a Heparan Sulfate Pentamer Interacting with LAR-Ig1-2

Leukocyte common antigen-related (LAR) protein is one of the type IIa receptor protein tyrosine phosphatases (RPTPs) that are important for signal transduction in biological processes, including axon growth and regeneration. Glycosaminoglycan chains, including heparan sulfate (HS) and chondroitin sulfate (CS), act as ligands that regulate LAR signaling. Here, we report the structural characterization of the first two immunoglobulin domains (Ig1-2) of LAR interacting with an HS pentasaccharide (GlcNS6S-GlcA-GlcNS3,6S-IdoA2S-GlcNS6S-OME, fondaparinux) using multiple solution-based NMR methods. In the course of the study, we extended an assignment strategy useful for sparsely labeled proteins expressed in mammalian cell culture supplemented with a single type of isotopically enriched amino acid ([¹⁵N]-Lys in this case) by including paramagnetic perturbations to NMR resonances. The folded two-domain structure for LAR-Ig1-2 seen in previous crystal structures has been validated in solution using residual dipolar coupling data, and a combination of chemical shift perturbation on titration of LAR-Ig1-2 with fondaparinux, saturation transfer difference (STD) spectra, and transferred nuclear Overhauser effects (trNOEs) have been employed in the docking program HADDOCK to generate models for the LAR-fondaparinux complex. These models are further analyzed by postprocessing energetic analysis to identify key binding interactions. In addition to providing insight into the ligand interaction mechanisms of type IIa RPTPs and the origin of opposing effects of CS and HS ligands, these results may assist in future design of therapeutic compounds for nervous system repair.

Diverse functions of protein tyrosine phosphatase σ in the nervous and immune systems

Tyrosine phosphorylation is a common means of regulating protein functions and signal transduction in multiple cells. Protein tyrosine phosphatases (PTPs) are a large family of signaling enzymes that remove phosphate groups from tyrosine residues of target proteins and change their functions. Among them, receptor-type PTPs (RPTPs) exhibit a distinct spatial pattern of expression and play essential roles in regulating neurite outgrowth, axon guidance, and synaptic organization in developmental nervous system. Some RPTPs function as essential receptors for chondroitin sulfate proteoglycans that inhibit axon regeneration following CNS injury. Interestingly, certain RPTPs are also important to regulate functions of immune cells and development of autoimmune diseases. PTPσ, a RPTP in the LAR subfamily, is expressed in various immune cells and regulates their differentiation, production of various cytokines and immune responses. In this review, we highlight the physiological and pathological significance of PTPσ and related molecules in both nervous and immune systems.

Targeting Receptor-Type Protein Tyrosine Phosphatases with Biotherapeutics: Is Outside-in Better than Inside-Out?

Protein tyrosine phosphatases (PTPs), of the receptor and non-receptor classes, are key signaling molecules that play critical roles in cellular regulation underlying diverse physiological events. Aberrant signaling as a result of genetic mutation or altered expression levels has been associated with several diseases and treatment via pharmacological intervention at the level of PTPs has been widely explored; however, the challenges associated with development of small molecule phosphatase inhibitors targeting the intracellular phosphatase domain (the “inside-out” approach) have been well documented and as yet there are no clinically approved drugs targeting these enzymes. The alternative approach of targeting receptor PTPs with biotherapeutic agents (such as monoclonal antibodies or engineered fusion proteins; the “outside-in” approach) that interact with the extracellular ectodomain offers many advantages, and there have been a number of exciting recent developments in this field. Here we provide a brief overview of the receptor PTP family and an update on the emerging area of receptor PTP-targeted biotherapeutics for CD148, vascular endothelial-protein tyrosine phosphatase (VE-PTP), receptor-type PTPs σ, γ, ζ (RPTPσ, RPTPγ, RPTPζ) and CD45, and discussion of future potential in this area.

iPSC-derived neural precursor cells: potential for cell transplantation therapy in spinal cord injury

A number of studies have demonstrated that transplantation of neural precursor cells (NPCs) promotes functional recovery after spinal cord injury (SCI). However, the NPCs had been mostly harvested from embryonic stem cells or fetal tissue, raising the ethical concern. Yamanaka and his colleagues established induced pluripotent stem cells (iPSCs) which could be generated from somatic cells, and this innovative development has made rapid progression in the field of SCI regeneration. We and other groups succeeded in producing NPCs from iPSCs, and demonstrated beneficial effects after transplantation for animal models of SCI. In particular, efficacy of human iPSC-NPCs in non-human primate SCI models fostered momentum of clinical application for SCI patients. At the same time, however, artificial induction methods in iPSC technology created alternative issues including genetic and epigenetic abnormalities, and tumorigenicity after transplantation. To overcome these problems, it is critically important to select origins of somatic cells, use integration-free system during transfection of reprogramming factors, and thoroughly investigate the characteristics of iPSC-NPCs with respect to quality management. Moreover, since most of the previous studies have focused on subacute phase of SCI, establishment of effective NPC transplantation should be evaluated for chronic phase hereafter. Our group is currently preparing clinical-grade human iPSC-NPCs, and will move forward toward clinical study for subacute SCI patients soon in the near future.

Proteinaceous Regulators and Inhibitors of Protein Tyrosine Phosphatases

Proper control of the phosphotyrosine content in signal transduction proteins is essential for normal cell behavior and is lost in many pathologies. Attempts to normalize aberrant tyrosine phosphorylation levels in disease states currently involve either the application of small compounds that inhibit tyrosine kinases (TKs) or the addition of growth factors or their mimetics to boost receptor-type TK activity. Therapies that target the TK enzymatic counterparts, the multi-enzyme family of protein tyrosine phosphatases (PTPs), are still lacking despite their undisputed involvement in human diseases. Efforts to pharmacologically modulate PTP activity have been frustrated by the conserved structure of the PTP catalytic core, providing a daunting problem with respect to target specificity. Over the years, however, many different protein interaction-based regulatory mechanisms that control PTP activity have been uncovered, providing alternative possibilities to control PTPs individually. Here, we review these regulatory principles, discuss existing biologics and proteinaceous compounds that affect PTP activity, and mention future opportunities to drug PTPs via these regulatory concepts.

Role of proteoglycans in neuro-inflammation and central nervous system fibrosis

Fibrosis is defined as the thickening and scarring of connective tissue, usually as a consequence of tissue damage. The central nervous system (CNS) is special in the sense that fibrogenic cells are restricted to vascular and meningeal areas. Inflammation and the disruption of the blood-brain barrier can lead to the infiltration of fibroblasts and trigger fibrotic response. While the initial function of the fibrotic tissue is to restore the blood-brain barrier and to limit the site of injury, it also demolishes the structure of extracellular matrix and impedes the healing process by producing inhibitory molecules and forming a physical and biochemical barrier that prevents axon regeneration. As a major constituent in the extracellular matrix, proteoglycans participate in the neuro-inflammation, modulating the fibrotic process. In this review, we will discuss the pathophysiology of fibrosis during acute injuries of the CNS, as well as during chronic neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, multiple sclerosis and age-related neurodegeneration with focus on the functional roles of proteoglycans.

Structural Characterization and Function Prediction of Immunoglobulin-like Fold in Cell Adhesion and Cell Signaling

Domains that belong to an immunoglobulin (Ig) fold are extremely abundant in cell surface receptors, which play significant roles in cell-cell adhesion and signaling. Although the structures of domains in an Ig fold share common topology of β-barrels, functions of receptors in adhesion and signaling are regulated by the very heterogeneous binding between these domains. Additionally, only a small number of domains are directly involved in the binding between two multidomain receptors. It is challenging and time consuming to experimentally detect the binding partners of a given receptor and further determine which specific domains in this receptor are responsible for binding. Therefore, current knowledge in the binding mechanism of Ig-fold domains and their impacts on cell adhesion and signaling is very limited. A bioinformatics study can shed light on this topic from a systematic point of view. However, there is so far no computational analysis on the structural and functional characteristics of the entire Ig fold. We constructed nonredundant structural data sets for all domains in Ig fold, depending on their functions in cell adhesion and signaling. We found that data sets of domains in adhesion receptors show different binding preference from domains in signaling receptors. Using structural alignment, we further built a common structural template for each group of a domain data set. By mapping the protein-protein binding interface of each domain in a group onto the surface of its structural template, we found binding interfaces are highly overlapped within each specific group. These overlapped interfaces, we called consensus binding interfaces, are distinguishable among different data sets of domains. Finally, the residue compositions on the consensus interfaces were used as indicators for multiple machine learning algorithms to predict if they can form homotypic interactions with each other. The overall performance of the cross-validation tests shows that our prediction accuracies ranged between 0.6 and 0.8.

Structural basis of trans-synaptic interactions between PTPδ and SALMs for inducing synapse formation

Synapse formation is triggered by trans-synaptic interactions of cell adhesion molecules, termed synaptic organizers. Three members of type-II receptor protein tyrosine phosphatases (classified as type-IIa RPTPs; PTPδ, PTPσ and LAR) are known as presynaptic organizers. Synaptic adhesion-like molecules (SALMs) have recently emerged as a family of postsynaptic organizers. Although all five SALM isoforms can bind to the type-IIa RPTPs, only SALM3 and SALM5 reportedly have synaptogenic activities depending on their binding. Here, we report the crystal structures of apo-SALM5, and PTPδ–SALM2 and PTPδ–SALM5 complexes. The leucine-rich repeat (LRR) domains of SALMs interact with the second immunoglobulin-like (Ig) domain of PTPδ, whereas the Ig domains of SALMs interact with both the second and third Ig domains of PTPδ. Unexpectedly, the structures exhibit the LRR-mediated 2:2 complex. Our synaptogenic co-culture assay using site-directed SALM5 mutants demonstrates that presynaptic differentiation induced by PTPδ–SALM5 requires the dimeric property of SALM5.

Synaptic organizers are cell adhesion molecules that facilitate synapse formation through trans-synaptic interactions. Here the authors give molecular insights into synaptic differentiation by determining the structures of the synaptic adhesion-like molecules SALM2 and SALM5 bound to the presynaptic organizer PTPδ.

Structural basis of SALM5-induced PTPδ dimerization for synaptic differentiation

SALM5, a synaptic adhesion molecule implicated in autism, induces presynaptic differentiation through binding to the LAR family receptor protein tyrosine phosphatases (LAR-RPTPs) that have been highlighted as presynaptic hubs for synapse formation. The mechanisms underlying SALM5/LAR-RPTP interaction remain unsolved. Here we report crystal structures of human SALM5 LRR-Ig alone and in complex with human PTPδ Ig1–3 (MeA⁻). Distinct from other LAR-RPTP ligands, SALM5 mainly exists as a dimer with LRR domains from two protomers packed in an antiparallel fashion. In the 2:2 heterotetrameric SALM5/PTPδ complex, a SALM5 dimer bridges two separate PTPδ molecules. Structure-guided mutations and heterologous synapse formation assays demonstrate that dimerization of SALM5 is prerequisite for its functionality in inducing synaptic differentiation. This study presents a structural template for the SALM family and reveals a mechanism for how a synaptic adhesion molecule directly induces cis-dimerization of LAR-RPTPs into higher-order signaling assembly.

Synaptic adhesion molecules mediate synaptic differentiation and formation. Here the authors present the structures of the synaptic adhesion molecule SALM5 alone and in complex with the LAR family receptor protein tyrosine phosphatase (LAR-RPTP) PTPδ, which reveals how SALM5 dimerization facilitates higher-order signaling assembly of LAR-RPTPs.

Flexible Roles for Proteoglycan Sulfation and Receptor Signaling

Proteoglycans (PGs) in the extracellular matrix (ECM) play vital roles in axon growth and navigation, plasticity, and regeneration of injured neurons. Different classes of PGs may support or inhibit cell growth, and their functions are determined in part by highly specific structural features. Among these, the pattern of sulfation on the PG sugar chains is a paramount determinant of a diverse and flexible set of outcomes. Recent studies of PG sulfation illustrate the challenges of attributing biological actions to specific sulfation patterns, and suggest ways in which highly similar molecules may exert opposing effects on neurons. The receptors for PGs, which have yet to be fully characterized, display a similarly nuanced spectrum of effects. Different classes of PG function via overlapping families of receptors and signaling pathways. This enables them to control axon growth and guidance with remarkable specificity, but it poses challenges for determining the precise binding interactions and downstream effects of different PGs and their assorted sulfated epitopes. This review examines existing and emerging evidence for the roles of PG sulfation and receptor interactions in determining how these complex molecules influence neuronal development, growth, and function.

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.

Postnatal lethality and chondrodysplasia in mice lacking both chondroitin sulfate N-acetylgalactosaminyltransferase-1 and -2

Chondroitin sulfate (CS) is a sulfated glycosaminoglycan (GAG) chain. In cartilage, CS plays important roles as the main component of the extracellular matrix (ECM), existing as side chains of the major cartilage proteoglycan, aggrecan. Six glycosyltransferases are known to coordinately synthesize the backbone structure of CS; however, their in vivo synthetic mechanism remains unknown. Previous studies have suggested that two glycosyltransferases, Csgalnact1 (t1) and Csgalnact2 (t2), are critical for initiation of CS synthesis in vitro. Indeed, t1 single knockout mice (t1 KO) exhibit slight dwarfism and a reduction in CS content in cartilage compared with wild-type (WT) mice. To reveal the synergetic roles of t1 and t2 in CS synthesis in vivo, we generated systemic single and double knockout (DKO) mice and cartilage-specific t1 and t2 double knockout (Col2-DKO) mice. DKO mice exhibited postnatal lethality, whereas t2 KO mice showed normal size and skeletal development. Col2-DKO mice survived to adulthood and showed severe dwarfism compared with t1 KO mice. Histological analysis of epiphyseal cartilage from Col2-DKO mice revealed disrupted endochondral ossification, characterized by drastic GAG reduction in the ECM. Moreover, DKO cartilage had reduced chondrocyte proliferation and an increased number of apoptotic chondrocytes compared with WT cartilage. Conversely, primary chondrocyte cultures from Col2-DKO knee cartilage had the same proliferation rate as WT chondrocytes and low GAG expression levels, indicating that the chondrocytes themselves had an intact proliferative ability. Quantitative RT-PCR analysis of E18.5 cartilage showed that the expression levels of Col2a1 and Ptch1 transcripts tended to decrease in DKO compared with those in WT mice. The CS content in DKO cartilage was decreased compared with that in t1 KO cartilage but was not completely absent. These results suggest that aberrant ECM caused by CS reduction disrupted endochondral ossification. Overall, we propose that both t1 and t2 are necessary for CS synthesis and normal chondrocyte differentiation but are not sufficient for all CS synthesis in cartilage.

Predicting glycosaminoglycan surface protein interactions and implications for studying axonal growth

Cell-surface carbohydrates play important roles in numerous biological processes through their interactions with various protein-binding partners. These interactions are made possible by the vast structural diversity of carbohydrates and the diverse array of carbohydrate presentations on the cell surface. Among the most complex and important carbohydrates are glycosaminoglycans (GAGs), which display varied stereochemistry, chain lengths, and patterns of sulfation. GAG-protein interactions participate in neuronal development, angiogenesis, spinal cord injury, viral invasion, and immune response. Unfortunately, little structural information is available for these complexes; indeed, for the highly sulfated chondroitin sulfate motifs, CS-E and CS-D, there are no structural data. We describe here the development and validation of the GAG-Dock computational method to predict accurately the binding poses of protein-bound GAGs. We validate that GAG-Dock reproduces accurately (<1-Å rmsd) the crystal structure poses for four known heparin-protein structures. Further, we predict the pose of heparin and chondroitin sulfate derivatives bound to the axon guidance proteins, protein tyrosine phosphatase σ (RPTPσ), and Nogo receptors 1-3 (NgR1-3). Such predictions should be useful in understanding and interpreting the role of GAGs in neural development and axonal regeneration after CNS injury.

Neurocan, an extracellular chondroitin sulfate proteoglycan, stimulates neuroblastoma cells to promote malignant phenotypes

Neurocan (NCAN), a secreted chondroitin sulfate proteoglycan, is one of the major inhibitory molecules for axon regeneration in nervous injury. However, its role in cancer is not clear. Here we observed that high NCAN expression was closely associated with the unfavorable outcome of neuroblastoma (NB). NCAN was also highly and ubiquitously expressed in the early lesions and terminal tumor of TH-MYCN mice, a NB model. Interestingly, exogenous NCAN (i.e., overexpression, recombinant protein and conditioned medium) transformed adherent NB cells into spheres whose malignancies in vitro (anchorage-independent growth and chemoresistance) and in vivo (xenograft tumor growth) were potentiated. Both chondroitin sulfate sugar chains and NCAN's core protein were essential for the sphere formation. The CSG3 domain was essential in the moiety of NCAN. Our comprehensive microarray analysis and RT-qPCR of mRNA expression suggested that NCAN treatment promoted cell division, and urged cells to undifferentiated state. The knockdown of NCAN in tumor sphere cells cultured from TH-MYCN mice resulted in growth suppression in vitro and in vivo. Our findings suggest that NCAN, which stimulates NB cells to promote malignant phenotypes, is an extracellular molecule providing a growth advantage to cancer cells.

Expanding the chondroitin glycoproteome of Caenorhabditis elegans

Chondroitin sulfate proteoglycans (CSPGs) are important structural components of connective tissues in essentially all metazoan organisms. In vertebrates, CSPGs are involved also in more specialized processes such as neurogenesis and growth factor signaling. In invertebrates, however, knowledge of CSPGs core proteins and proteoglycan-related functions is relatively limited, even for Caenorhabditis elegans. This nematode produces large amounts of non-sulfated chondroitin in addition to low-sulfated chondroitin sulfate chains. So far, only nine core proteins (CPGs) have been identified, some of which have been shown to be involved in extracellular matrix formation. We recently introduced a protocol to characterize proteoglycan core proteins by identifying CS-glycopeptides with a combination of biochemical enrichment, enzymatic digestion, and nano-scale liquid chromatography MS/MS analysis. Here, we have used this protocol to map the chondroitin glycoproteome in C. elegans, resulting in the identification of 15 novel CPG proteins in addition to the nine previously established. Three of the newly identified CPGs displayed homology to vertebrate proteins. Bioinformatics analysis of the primary protein sequences revealed that the CPG proteins altogether contained 19 unique functional domains, including Kunitz and endostatin domains, suggesting direct involvement in protease inhibition and axonal migration, respectively. The analysis of the core protein domain organization revealed that all chondroitin attachment sites are located in unstructured regions. Our results suggest that CPGs display a much greater functional and structural heterogeneity than previously appreciated and indicate that specialized proteoglycan-mediated functions evolved early in metazoan evolution.

Astrocyte-Secreted Glypican 4 Regulates Release of Neuronal Pentraxin 1 from Axons to Induce Functional Synapse Formation

The generation of precise synaptic connections between developing neurons is critical to the formation of functional neural circuits. Astrocyte-secreted glypican 4 induces formation of active excitatory synapses by recruiting AMPA glutamate receptors to the postsynaptic cell surface. We now identify the molecular mechanism of how glypican 4 exerts its effect. Glypican 4 induces release of the AMPA receptor clustering factor neuronal pentraxin 1 from presynaptic terminals by signaling through presynaptic protein tyrosine phosphatase receptor δ. Pentraxin then accumulates AMPA receptors on the postsynaptic terminal forming functional synapses. Our findings reveal a signaling pathway that regulates synaptic activity during central nervous system development and demonstrates a role for astrocytes as organizers of active synaptic connections by coordinating both pre and post synaptic neurons. As mutations in glypicans are associated with neurological disorders, such as autism and schizophrenia, this signaling cascade offers new avenues to modulate synaptic function in disease.

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Astrocyte-secreted Gpc4 induces release of NP1 from neurons

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Release of NP1 is mediated through Gpc4 interaction with presynaptic RPTPδ

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Gpc4 or RPTPδ KO causes presynaptic NP1 retention and decreased synapse number

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Astrocytic release of Gpc4 provides spatial and temporal cues for synaptogenesis

LAR-RPTP Clustering Is Modulated by Competitive Binding between Synaptic Adhesion Partners and Heparan Sulfate

The leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs) are cellular receptors of heparan sulfate (HS) and chondroitin sulfate (CS) proteoglycans that direct axonal growth and neuronal regeneration. LAR-RPTPs are also synaptic adhesion molecules that form trans-synaptic adhesion complexes by binding to various postsynaptic adhesion ligands, such as Slit- and Trk-like family of proteins (Slitrks), IL-1 receptor accessory protein-like 1 (IL1RAPL1), interleukin-1 receptor accessory protein (IL-1RAcP) and neurotrophin receptor tyrosine kinase C (TrkC), to regulate synaptogenesis. Here, we determined the crystal structure of the human LAR-RPTP/IL1RAPL1 complex and found that lateral interactions between neighboring LAR-RPTP/IL1RAPL1 complexes in crystal lattices are critical for the higher-order assembly and synaptogenic activity of these complexes. Moreover, we found that LAR-RPTP binding to the postsynaptic adhesion ligands, Slitrk3, IL1RAPL1 and IL-1RAcP, but not TrkC, induces reciprocal higher-order clustering of trans-synaptic adhesion complexes. Although LAR-RPTP clustering was induced by either HS or postsynaptic adhesion ligands, the dominant binding of HS to the LAR-RPTP was capable of dismantling pre-established LAR-RPTP-mediated trans-synaptic adhesion complexes. These findings collectively suggest that LAR-RPTP clustering for synaptogenesis is modulated by a complex synapse-organizing protein network.

Roles of CSGalNAcT1, a key enzyme in regulation of CS synthesis, in neuronal regeneration and plasticity

Chondroitin sulfate (CS) is a sulfated glycosaminoglycan composed of a long chain of repeating disaccharide units that are attached to core proteins, resulting in CS proteoglycans (CSPGs). In the mature brain, CS is concentrated in perineuronal nets (PNNs), which are extracellular structures that surround synapses and regulate synaptic plasticity. In addition, CS is rapidly synthesized after CNS injury to create a physical and chemical barrier that inhibits axon growth. Most previous studies used a bacterial CS-degrading enzyme to investigate the physiological roles of CS. Recent studies have shown that CS is synthesized by more than 15 enzymes, all of which have been characterized in vitro. Here we focus on one of those enzymes, CSGalNAcT1 (T1). We produced T1 knockout mice (KO), which show extensive axon regeneration following spinal cord injury, as well as the loss of onset of ocular dominance plasticity. These results from T1KO mice suggest important roles for extracellular CS in the brain regarding neuronal plasticity and axon regeneration.

Protein tyrosine phosphatase σ regulates autoimmune encephalomyelitis development

Protein tyrosine phosphatases (PTPs) play essential roles in regulating signaling events in multiple cells by tyrosine dephosphorylation. One of them, PTPσ, appears important in regulating function of plasmacytoid dendritic cells (pDC). Here we report that PTPσ deletion in knockout mice and inhibition with a selective antagonist peptide exacerbated symptoms of experimental autoimmune encephalomyelitis (EAE) by enhancing axon and myelin damage in the spinal cord. PTPσ-/- mice displayed pro-inflammatory profiles in the spinal cord and lymphoid organs following MOG peptide immunization. PTPσ deletion promoted a pro-inflammatory phenotype in conventional DCs and directly regulated differentiation of CD4+ T cells. It also facilitated infiltration of T lymphocytes, activation of macrophages in the CNS and development of EAE. Therefore, PTPσ is a key negative regulator in EAE initiation and progression, which acts by regulating functions of DCs, T cells, and other immune cells. PTPσ may become an important molecular target for treating autoimmune disorders.

Defining the molecular basis of interaction between R3 receptor-type protein tyrosine phosphatases and VE-cadherin

Receptor-type protein tyrosine phosphatases (RPTPs) of the R3 subgroup play key roles in the immune, vascular and nervous systems. They are characterised by a large ectodomain comprising multiple FNIII-like repeats, a transmembrane domain, and a single intracellular phosphatase domain. The functional role of the extracellular region has not been clearly defined and potential roles in ligand interaction, dimerization, and regulation of cell-cell contacts have been reported. Here bimolecular fluorescence complementation (BiFC) in live cells was used to examine the molecular basis for the interaction of VE-PTP with VE-cadherin, two proteins involved in endothelial cell contact and maintenance of vascular integrity. The potential of other R3-PTPs to interact with VE-cadherin was also explored using this method. Quantitative BiFC analysis, using a VE-PTP construct expressing only the ectodomain and transmembrane domain, revealed a specific interaction with VE-cadherin, when compared with controls. Controls were sialophorin, an unrelated membrane protein with a large ectodomain, and a membrane anchored C-terminal Venus-YFP fragment, lacking both ectodomain and transmembrane domains. Truncation of the first 16 FNIII-like repeats from the ectodomain of VE-PTP indicated that removal of this region is not sufficient to disrupt the interaction with VE-cadherin, although it occurs predominantly in an intracellular location. A construct with a deletion of only the 17th domain of VE-PTP was, in contrast to previous studies, still able to interact with VE-cadherin, although this also was predominantly intracellular. Other members of the R3-PTP family (DEP-1, GLEPP1 and SAP-1) also exhibited the potential to interact with VE-cadherin. The direct interaction of DEP-1 with VE-cadherin is likely to be of physiological relevance since both proteins are expressed in endothelial cells. Together the data presented in the study suggest a role for both the ectodomain and transmembrane domain of R3-PTPs in interaction with VE-cadherin.

Dendritic space-filling requires a neuronal type-specific extracellular permissive signal in Drosophila

Neurons sometimes completely fill available space in their receptive fields with evenly spaced dendrites to uniformly sample sensory or synaptic information. The mechanisms that enable neurons to sense and innervate all space in their target tissues are poorly understood. Using Drosophila somatosensory neurons as a model, we show that heparan sulfate proteoglycans (HSPGs) Dally and Syndecan on the surface of epidermal cells act as local permissive signals for the dendritic growth and maintenance of space-filling nociceptive C4da neurons, allowing them to innervate the entire skin. Using long-term time-lapse imaging with intact Drosophila larvae, we found that dendrites grow into HSPG-deficient areas but fail to stay there. HSPGs are necessary to stabilize microtubules in newly formed high-order dendrites. In contrast to C4da neurons, non-space-filling sensory neurons that develop in the same microenvironment do not rely on HSPGs for their dendritic growth. Furthermore, HSPGs do not act by transporting extracellular diffusible ligands or require leukocyte antigen-related (Lar), a receptor protein tyrosine phosphatase (RPTP) and the only known Drosophila HSPG receptor, for promoting dendritic growth of space-filling neurons. Interestingly, another RPTP, Ptp69D, promotes dendritic growth of C4da neurons in parallel to HSPGs. Together, our data reveal an HSPG-dependent pathway that specifically allows dendrites of space-filling neurons to innervate all target tissues in Drosophila.

Neural Glycosylphosphatidylinositol-Anchored Proteins in Synaptic Specification

Glycosylphosphatidylinositol (GPI)-anchored proteins are a specialized class of lipid-associated neuronal membrane proteins that perform diverse functions in the dynamic control of axon guidance, synaptic adhesion, cytoskeletal remodeling, and localized signal transduction, particularly at lipid raft domains. Recent studies have demonstrated that a subset of GPI-anchored proteins act as critical regulators of synapse development by modulating specific synaptic adhesion pathways via direct interactions with key synapse-organizing proteins. Additional studies have revealed that alteration of these regulatory mechanisms may underlie various brain disorders. In this review, we highlight the emerging role of GPI-anchored proteins as key synapse organizers that aid in shaping the properties of various types of synapses and circuits in mammals.

Formation and remodeling of the brain extracellular matrix in neural plasticity: Roles of chondroitin sulfate and hyaluronan

BACKGROUND

The extracellular matrix (ECM) of the brain is rich in glycosaminoglycans such as chondroitin sulfate (CS) and hyaluronan. These glycosaminoglycans are organized into either diffuse or condensed ECM. Diffuse ECM is distributed throughout the brain and fills perisynaptic spaces, whereas condensed ECM selectively surrounds parvalbumin-expressing inhibitory neurons (PV cells) in mesh-like structures called perineuronal nets (PNNs). The brain ECM acts as a non-specific physical barrier that modulates neural plasticity and axon regeneration.

SCOPE OF REVIEW

Here, we review recent progress in understanding of the molecular basis of organization and remodeling of the brain ECM, and the involvement of several types of experience-dependent neural plasticity, with a particular focus on the mechanism that regulates PV cell function through specific interactions between CS chains and their binding partners. We also discuss how the barrier function of the brain ECM restricts dendritic spine dynamics and limits axon regeneration after injury.

MAJOR CONCLUSIONS

The brain ECM not only forms physical barriers that modulate neural plasticity and axon regeneration, but also forms molecular brakes that actively controls maturation of PV cells and synapse plasticity in which sulfation patterns of CS chains play a key role. Structural remodeling of the brain ECM modulates neural function during development and pathogenesis.

GENERAL SIGNIFICANCE

Genetic or enzymatic manipulation of the brain ECM may restore neural plasticity and enhance recovery from nerve injury. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Mechanisms of axon regeneration: The significance of proteoglycans

BACKGROUND

Therapeutics specific to neural injury have long been anticipated but remain unavailable. Axons in the central nervous system do not readily regenerate after injury, leading to dysfunction of the nervous system. This failure of regeneration is due to both the low intrinsic capacity of axons for regeneration and the various inhibitors emerging upon injury. After many years of concerted efforts, however, these hurdles to axon regeneration have been partially overcome.

SCOPE OF REVIEW

This review summarizes the mechanisms regulating axon regeneration. We highlight proteoglycans, particularly because it has become increasingly clear that these proteins serve as critical regulators for axon regeneration.

MAJOR CONCLUSIONS

Studies on proteoglycans have revealed that glycans not only assist in the modulation of protein functions but also act as main players-e.g., as functional ligands mediating intracellular signaling through specific receptors on the cell surface. By regulating clustering of the receptors, glycans in the proteoglycan moiety, i.e., glycosaminoglycans, promote or inhibit axon regeneration. In addition, proteoglycans are involved in various types of neural plasticity, ranging from synaptic plasticity to experience-dependent plasticity.

GENERAL SIGNIFICANCE

Although studies on proteins have progressively facilitated our understanding of the nervous system, glycans constitute a new frontier for further research and development in this field. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Identification of function-regulating antibodies targeting the receptor protein tyrosine phosphatase sigma ectodomain

Receptor tyrosine phosphatase sigma (RPTPσ) plays an important role in the regulation of axonal outgrowth and neural regeneration. Recent studies have identified two RPTPσ ligands, chondroitin sulfate proteoglycans (CSPGs) and heparan sulfate proteoglycans (HSPG), which can modulate RPTPσ activity by affecting its dimerization status. Here, we developed a split luciferase assay to monitor RPTPσ dimerization in living cells. Using this system, we demonstrate that heparin, an analog of heparan sulfate, induced the dimerization of RPTPσ, whereas chondroitin sulfate increased RPTPσ activity by inhibiting RPTPσ dimerization. Also, we generated several novel RPTPσ IgG monoclonal antibodies, to identify one that modulates its activity by inducing/stabilizing dimerization in living cells. Lastly, we demonstrate that this antibody promotes neurite outgrowth in SH-SY5Y cells. In summary, we demonstrated that the split luciferase RPTPσ activity assay is a novel high-throughput approach for discovering novel RPTPσ modulators that can promote axonal outgrowth and neural regeneration.

Glypican-2 levels in cerebrospinal fluid predict the status of adult hippocampal neurogenesis

Adult hippocampal neurogenesis is a remarkable form of brain plasticity through which new neurons are generated throughout life. Despite its important roles in cognition and emotion and its modulation in various preclinical disease models, the functional importance of adult hippocampal neurogenesis in human health has not been revealed because of a lack of tools for monitoring adult neurogenesis in vivo. Therefore, we performed an unbiased proteomics screen to identify novel proteins expressed during neuronal differentiation using a human neural stem cell model, and we identified the proteoglycan Glypican-2 (Gpc2) as a putative secreted marker of immature neurons. Exogenous Gpc2 binds to FGF2 and inhibits FGF2-induced neural progenitor cell proliferation. Gpc2 is enriched in neurogenic regions of the adult brain. Its expression is increased by physiological stimuli that increase hippocampal neurogenesis and decreased in transgenic models in which neurogenesis is selectively ablated. Changes in neurogenesis also result in changes in Gpc2 protein level in cerebrospinal fluid (CSF). Gpc2 is detectable in adult human CSF, and first pilot experiments with a longitudinal cohort indicate a decrease over time. Thus, Gpc2 may serve as a potential marker to monitor adult neurogenesis in both animal and human physiology and disease, warranting future studies.

Targeting Tyrosine Phosphatases: Time to End the Stigma

Protein tyrosine phosphatases (PTPs) are a family of enzymes essential for numerous cellular processes, and several PTPs have been validated as therapeutic targets for human diseases. Historically, the development of drugs targeting PTPs has been highly challenging, leading to stigmatization of these enzymes as undruggable targets. Despite these difficulties, efforts to drug PTPs have persisted, and recent years have seen an influx of new probes providing opportunities for biological examination of old and new PTP targets. Here we discuss progress towards drugging PTPs with special emphasis on the development of selective probes with biological activity. We describe the development of new small-molecule orthosteric, allosteric, and oligomerization-inhibiting PTP inhibitors and discuss new studies targeting the receptor PTP (RPTP) subfamily with biologics.

Chondroitin sulfates and their binding molecules in the central nervous system

Chondroitin sulfate (CS) is the most abundant glycosaminoglycan (GAG) in the central nervous system (CNS) matrix. Its sulfation and epimerization patterns give rise to different forms of CS, which enables it to interact specifically and with a significant affinity with various signalling molecules in the matrix including growth factors, receptors and guidance molecules. These interactions control numerous biological and pathological processes, during development and in adulthood. In this review, we describe the specific interactions of different families of proteins involved in various physiological and cognitive mechanisms with CSs in CNS matrix. A better understanding of these interactions could promote a development of inhibitors to treat neurodegenerative diseases.

Contaminants in commercial preparations of 'purified' small leucine-rich proteoglycans may distort mechanistic studies

The present study reports the perplexing results that came about because of seriously impure commercially available reagents. Commercial reagents and chemicals are routinely ordered by scientists and expected to have been rigorously assessed for their purity. Unfortunately, we found this assumption to be risky. Extensive work was carried out within our laboratory using commercially sourced preparations of the small leucine-rich proteoglycans (SLRPs), decorin and biglycan, to investigate their influence on nerve cell growth. Unusual results compelled us to analyse the composition and purity of both preparations of these proteoglycans (PGs) using both mass spectrometry (MS) and Western blotting, with and without various enzymatic deglycosylations. Commercial ‘decorin’ and ‘biglycan’ were found to contain a mixture of PGs including not only both decorin and biglycan but also fibromodulin and aggrecan. The unexpected effects of ‘decorin’ and ‘biglycan’ on nerve cell growth could be explained by these impurities. Decorin and biglycan contain either chondroitin or dermatan sulfate glycosaminoglycan (GAG) chains whereas fibromodulin only contains keratan sulfate and the large (>2500 kDa), highly glycosylated aggrecan contains both keratan and chondroitin sulfate. The different structure, molecular weight and composition of these impurities significantly affected our work and any conclusions that could be made. These findings beg the question as to whether scientists need to verify the purity of each commercially obtained reagent used in their experiments. The implications of these findings are vast, since the effects of these impurities may already have led to inaccurate conclusions and reports in the literature with concomitant loss of researchers’ funds and time.

Inhibition and enhancement of neural regeneration by chondroitin sulfate proteoglycans

The current dogma in neural regeneration research implies that chondroitin sulfate proteoglycans (CSPGs) inhibit plasticity and regeneration in the adult central nervous system (CNS). We argue that the role of the CSPGs can be reversed from inhibition to activation by developmentally expressed CSPG-binding factors. Heparin-binding growth-associated molecule (HB-GAM; also designated as pleiotrophin) has been studied as a candidate molecule that might modulate the role of CSPG matrices in plasticity and regeneration. Studies in vitro show that in the presence of soluble HB-GAM chondroitin sulfate (CS) chains of CSPGs display an enhancing effect on neurite outgrowth. Based on the in vitro studies, we suggest a model according to which the HB-GAM/CS complex binds to the neuron surface receptor glypican-2, which induces neurite growth. Furthermore, HB-GAM masks the CS binding sites of the neurite outgrowth inhibiting receptor protein tyrosine phosphatase sigma (PTPσ), which may contribute to the HB-GAM-induced regenerative effect. In vivo studies using two-photon imaging after local HB-GAM injection into prick-injury of the cerebral cortex reveal regeneration of dendrites that has not been previously demonstrated after injuries of the mammalian nervous system. In the spinal cord, two-photon imaging displays HB-GAM-induced axonal regeneration. Studies on the HB-GAM/CS mechanism in vitro and in vivo are expected to pave the way for drug development for injuries of brain and spinal cord.

Structural basis of myelin-associated glycoprotein adhesion and signalling

Myelin-associated glycoprotein (MAG) is a myelin-expressed cell-adhesion and bi-directional signalling molecule. MAG maintains the myelin–axon spacing by interacting with specific neuronal glycolipids (gangliosides), inhibits axon regeneration and controls myelin formation. The mechanisms underlying MAG adhesion and signalling are unresolved. We present crystal structures of the MAG full ectodomain, which reveal an extended conformation of five Ig domains and a homodimeric arrangement involving membrane-proximal domains Ig4 and Ig5. MAG-oligosaccharide complex structures and biophysical assays show how MAG engages axonal gangliosides at domain Ig1. Two post-translational modifications were identified—N-linked glycosylation at the dimerization interface and tryptophan C-mannosylation proximal to the ganglioside binding site—that appear to have regulatory functions. Structure-guided mutations and neurite outgrowth assays demonstrate MAG dimerization and carbohydrate recognition are essential for its regeneration-inhibiting properties. The combination of trans ganglioside binding and cis homodimerization explains how MAG maintains the myelin–axon spacing and provides a mechanism for MAG-mediated bi-directional signalling.

Myelin-associated glycoprotein (MAG) maintains myelin-axon spacing. Here, the authors report the crystal structures of the MAG full ectodomain in complex with oligosaccharide, and use additional assays to provide insights into the mechanism of MAG-mediated signalling.

Two PTP receptors mediate CSPG inhibition by convergent and divergent signaling pathways in neurons

Receptor protein tyrosine phosphatase σ (PTPσ) and its subfamily member LAR act as transmembrane receptors that mediate growth inhibition of chondroitin sulfate proteoglycans (CSPGs). Inhibition of either receptor increases axon growth into and beyond scar tissues after CNS injury. However, it is unclear why neurons express two similar CSPG receptors, nor whether they use the same or different intracellular pathways. We have now studied the signaling pathways of these two receptors using N2A cells and primary neurons derived from knockout mice. We demonstrate that both receptors share certain signaling pathways (RhoA, Akt and Erk), but also use distinct signals to mediate CSPG actions. Activation of PTPσ by CSPGs selectively inactivated CRMP2, APC, S6 kinase and CREB. By contrast LAR activation inactivated PKCζ, cofilin and LKB1. For the first time, we propose a model of the signaling pathways downstream of these two CSPG receptors. We also demonstrate that deleting both receptors exhibits additive enhancement of axon growth in adult neuronal cultures in vitro. Our findings elucidate the novel downstream pathways of CSPGs and suggest potential synergy of blocking their two PTP receptors.

Glycan Engineering for Cell and Developmental Biology

Cell-surface glycans are a diverse class of macromolecules that participate in many key biological processes, including cell-cell communication, development, and disease progression. Thus, the ability to modulate the structures of glycans on cell surfaces provides a powerful means not only to understand fundamental processes but also to direct activity and elicit desired cellular responses. Here, we describe methods to sculpt glycans on cell surfaces and highlight recent successes in which artificially engineered glycans have been employed to control biological outcomes such as the immune response and stem cell fate.

Glycerophosphodiesterase GDE2 Promotes Neuroblastoma Differentiation through Glypican Release and Is a Marker of Clinical Outcome

Neuroblastoma is a pediatric embryonal malignancy characterized by impaired neuronal differentiation. A better understanding of neuroblastoma differentiation is essential for developing new therapeutic approaches. GDE2 (encoded by GDPD5) is a six-transmembrane-domain glycerophosphodiesterase that promotes embryonic neurogenesis. We find that high GDPD5 expression is strongly associated with favorable outcome in neuroblastoma. GDE2 induces differentiation of neuroblastoma cells, suppresses cell motility, and opposes RhoA-driven neurite retraction. GDE2 alters the Rac-RhoA activity balance and the expression of multiple differentiation-associated genes. Mechanistically, GDE2 acts by cleaving (in cis) and releasing glycosylphosphatidylinositol-anchored glypican-6, a putative co-receptor. A single point mutation in the ectodomain abolishes GDE2 function. Our results reveal GDE2 as a cell-autonomous inducer of neuroblastoma differentiation with prognostic significance and potential therapeutic value.

Site-specific identification of heparan and chondroitin sulfate glycosaminoglycans in hybrid proteoglycans

Heparan sulfate (HS) and chondroitin sulfate (CS) are complex polysaccharides that regulate important biological pathways in virtually all metazoan organisms. The polysaccharides often display opposite effects on cell functions with HS and CS structural motifs presenting unique binding sites for specific ligands. Still, the mechanisms by which glycan biosynthesis generates complex HS and CS polysaccharides required for the regulation of mammalian physiology remain elusive. Here we present a glycoproteomic approach that identifies and differentiates between HS and CS attachment sites and provides identity to the core proteins. Glycopeptides were prepared from perlecan, a complex proteoglycan known to be substituted with both HS and CS chains, further digested with heparinase or chondroitinase ABC to reduce the HS and CS chain lengths respectively, and thereafter analyzed by nLC-MS/MS. This protocol enabled the identification of three consensus HS sites and one hybrid site, carrying either a HS or a CS chain. Inspection of the amino acid sequence at the hybrid attachment locus indicates that certain peptide motifs may encode for the chain type selection process. This analytical approach will become useful when addressing fundamental questions in basic biology specifically in elucidating the functional roles of site-specific glycosylations of proteoglycans.

HB-GAM (pleiotrophin) reverses inhibition of neural regeneration by the CNS extracellular matrix

Chondroitin sulfate (CS) glycosaminoglycans inhibit regeneration in the adult central nervous system (CNS). We report here that HB-GAM (heparin-binding growth-associated molecule; also known as pleiotrophin), a CS-binding protein expressed at high levels in the developing CNS, reverses the role of the CS chains in neurite growth of CNS neurons in vitro from inhibition to activation. The CS-bound HB-GAM promotes neurite growth through binding to the cell surface proteoglycan glypican-2; furthermore, HB-GAM abrogates the CS ligand binding to the inhibitory receptor PTPσ (protein tyrosine phosphatase sigma). Our in vivo studies using two-photon imaging of CNS injuries support the in vitro studies and show that HB-GAM increases dendrite regeneration in the adult cerebral cortex and axonal regeneration in the adult spinal cord. Our findings may enable the development of novel therapies for CNS injuries.

Identification of keratan sulfate disaccharide at C-3 position of glucuronate of chondroitin sulfate from Mactra chinensis

Glycosaminoglycans (GAGs), including chondroitin sulfate (CS), dermatan sulfate, heparin, heparan sulfate and keratan sulfate (KS) are linear sulfated repeating disaccharide sequences containing hexosamine and uronic acid [or galactose (Gal) in the case of KS]. Among the GAGs, CS shows structural variations, such as sulfation patterns and fucosylation, which are responsible for their physiological functions through CS interaction with CS-binding proteins. Here, we solved the structure of KS-branched CS-E derived from a clam, Mactra chinensis KS disaccharide [d-GlcNAc6S-(1→3)-β-d-Gal-(1→] was attached to the C-3 position of GlcA, and consecutive KS-branched disaccharide sequences were found in a CS chain. KS-branched polysaccharides clearly exhibited resistance to degradation by chondroitinase ABC or ACII (at low concentrations) compared with typical CS structures. Furthermore, KS-branched polysaccharides stimulated neurite outgrowth of hippocampal neurons. These results strongly suggest that M. chinensis is a rich source of KS-branched CS, and it has important biological activities.

Extracellular cues influencing oligodendrocyte differentiation and (re)myelination

There is an increasing number of neurologic disorders found to be associated with loss and/or dysfunction of the CNS myelin sheath, ranging from the classic demyelinating disease, multiple sclerosis, through CNS injury, to neuropsychiatric diseases. The disabling burden of these diseases has sparked a growing interest in gaining a better understanding of the molecular mechanisms regulating the differentiation of the myelinating cells of the CNS, oligodendrocytes (OLGs), and the process of (re)myelination. In this context, the importance of the extracellular milieu is becoming increasingly recognized. Under pathological conditions, changes in inhibitory as well as permissive/promotional cues are thought to lead to an overall extracellular environment that is obstructive for the regeneration of the myelin sheath. Given the general view that remyelination is, even though limited in human, a natural response to demyelination, targeting pathologically 'dysregulated' extracellular cues and their downstream pathways is regarded as a promising approach toward the enhancement of remyelination by endogenous (or if necessary transplanted) OLG progenitor cells. In this review, we will introduce the extracellular cues that have been implicated in the modulation of (re)myelination. These cues can be soluble, part of the extracellular matrix (ECM) or mediators of cell-cell interactions. Their inhibitory and permissive/promotional roles with regard to remyelination as well as their potential for therapeutic intervention will be discussed.

Structural Basis for Interactions Between Contactin Family Members and Protein-tyrosine Phosphatase Receptor Type G in Neural Tissues

Protein-tyrosine phosphatase receptor type G (RPTPγ/PTPRG) interacts in vitro with contactin-3-6 (CNTN3-6), a group of glycophosphatidylinositol-anchored cell adhesion molecules involved in the wiring of the nervous system. In addition to PTPRG, CNTNs associate with multiple transmembrane proteins and signal inside the cell via cis-binding partners to alleviate the absence of an intracellular region. Here, we use comprehensive biochemical and structural analyses to demonstrate that PTPRG·CNTN3-6 complexes share similar binding affinities and a conserved arrangement. Furthermore, as a first step to identifying PTPRG·CNTN complexes in vivo, we found that PTPRG and CNTN3 associate in the outer segments of mouse rod photoreceptor cells. In particular, PTPRG and CNTN3 form cis-complexes at the surface of photoreceptors yet interact in trans when expressed on the surfaces of apposing cells. Further structural analyses suggest that all CNTN ectodomains adopt a bent conformation and might lie parallel to the cell surface to accommodate these cis and trans binding modes. Taken together, these studies identify a PTPRG·CNTN complex in vivo and provide novel insights into PTPRG- and CNTN-mediated signaling.

NB-3 signaling mediates the cross-talk between post-traumatic spinal axons and scar-forming cells

Little is known about the molecules mediating the cross-talk between post-traumatic axons and scar-forming cells after spinal cord injury. We found that a sustained NB-3 induction was simultaneously present in the terminations of post-traumatic corticospinal axons and scar-forming cells at the spinal lesion site, where they were in direct contact when axons tried to penetrate the glial scar. The regrowth of corticospinal axons was enhanced in vivo with NB-3 deficiency or interruption of NB-3 trans-homophilic interactions. Biochemical, in vitro and in vivo evidence demonstrated that NB-3 homophilically interacted in trans to initiate a growth inhibitory signal transduction from scar-forming cells to neurons by modulating mTOR activity via CHL1 and PTPσ. NB-3 deficiency promoted BMS scores, electrophysiological transmission, and synapse reformation between regenerative axons and neurons. Our findings demonstrate that NB-3 trans-homophilic interactions mediate the cross-talk between post-traumatic axons and scar-forming cells and impair the intrinsic growth ability of injured axons.