Cell specific-chondroitin sulfate proteoglycan expression during CNS morphogenesis in the chick embryo

Keratan sulfate, an electrosensory neurosentient bioresponsive cell instructive glycosaminoglycan

The roles of keratan sulfate (KS) as a proton detection glycosaminoglycan in neurosensory processes in the central and peripheral nervous systems is reviewed. The functional properties of the KS-proteoglycans aggrecan, phosphacan, podocalyxcin as components of perineuronal nets in neurosensory processes in neuronal plasticity, cognitive learning and memory are also discussed. KS-glycoconjugate neurosensory gels used in electrolocation in elasmobranch fish species and KS substituted mucin like conjugates in some tissue contexts in mammals need to be considered in sensory signalling. Parallels are drawn between KS's roles in elasmobranch fish neurosensory processes and its roles in mammalian electro mechanical transduction of acoustic liquid displacement signals in the cochlea by the tectorial membrane and stereocilia of sensory inner and outer hair cells into neural signals for sound interpretation. The sophisticated structural and functional proteins which maintain the unique high precision physical properties of stereocilia in the detection, transmittance and interpretation of acoustic signals in the hearing process are important. The maintenance of the material properties of stereocilia are essential in sound transmission processes. Specific, emerging roles for low sulfation KS in sensory bioregulation are contrasted with the properties of high charge density KS isoforms. Some speculations are made on how the molecular and electrical properties of KS may be of potential application in futuristic nanoelectronic, memristor technology in advanced ultrafast computing devices with low energy requirements in nanomachines, nanobots or molecular switches which could be potentially useful in artificial synapse development. Application of KS in such innovative areas in bioregulation are eagerly awaited.

Mapping Proteoglycan Function Using Novel Genetic Strategies

Multiple intrinsic and extrinsic factors contribute to stem and neuronal precursor cell maintenance and/or differentiation. Proteoglycans, major residents of the stem cell microenvironment, modulate key signaling cues and are of particular importance. The complexity and diversity of the glycan structure of proteoglycans make their functional characterization a challenging task. In order to test the functional role of glycosaminoglycans (GAGs) in cell self-renewal, maintenance, and differentiation, we have taken a loss-of-function approach by developing a library of both biosynthetic and degradative enzymes to specifically remodel the ECM.

Disease-specific glycosaminoglycan patterns in the extracellular matrix of human lung and brain

A wide variety of diseases throughout the mammalian organism is characterized by abnormal deposition of various components of the extracellular matrix (ECM), including the heterogeneous family of glycosaminoglycans (GAGs), which contribute considerably to the ECM architecture as part of the so-called proteoglycans. The GAG's unique sulfation pattern, derived from highly dynamic and specific modification processes, has a massive impact on critical mediators such as cytokines and growth factors. Due to the strong connection between the specific sulfation pattern and GAG function, slight alterations of this pattern are often associated with enormous changes at the cell as well as at the organ level. This review aims to investigate the connection between modifications of GAG sulfation patterns and the wide range of pathological conditions, mainly focusing on a range of chronic diseases of the central nervous system (CNS) as well as the respiratory tract.

Aggrecan, the Primary Weight-Bearing Cartilage Proteoglycan, Has Context-Dependent, Cell-Directive Properties in Embryonic Development and Neurogenesis: Aggrecan Glycan Side Chain Modifications Convey Interactive Biodiversity

This review examines aggrecan's roles in developmental embryonic tissues, in tissues undergoing morphogenetic transition and in mature weight-bearing tissues. Aggrecan is a remarkably versatile and capable proteoglycan (PG) with diverse tissue context-dependent functional attributes beyond its established role as a weight-bearing PG. The aggrecan core protein provides a template which can be variably decorated with a number of glycosaminoglycan (GAG) side chains including keratan sulphate (KS), human natural killer trisaccharide (HNK-1) and chondroitin sulphate (CS). These convey unique tissue-specific functional properties in water imbibition, space-filling, matrix stabilisation or embryonic cellular regulation. Aggrecan also interacts with morphogens and growth factors directing tissue morphogenesis, remodelling and metaplasia. HNK-1 aggrecan glycoforms direct neural crest cell migration in embryonic development and is neuroprotective in perineuronal nets in the brain. The ability of the aggrecan core protein to assemble CS and KS chains at high density equips cartilage aggrecan with its well-known water-imbibing and weight-bearing properties. The importance of specific arrangements of GAG chains on aggrecan in all its forms is also a primary morphogenetic functional determinant providing aggrecan with unique tissue context dependent regulatory properties. The versatility displayed by aggrecan in biodiverse contexts is a function of its GAG side chains.

Multitasking Human Lectin Galectin-3 Interacts with Sulfated Glycosaminoglycans and Chondroitin Sulfate Proteoglycans

Glycosaminoglycan (GAG) binding proteins (GAGBPs), including growth factors, cytokines, morphogens, and extracellular matrix proteins, interact with both free GAGs and those covalently linked to proteoglycans. Such interactions modulate a variety of cellular and extracellular events, such as cell growth, metastasis, morphogenesis, neural development, and inflammation. GAGBPs are structurally and evolutionarily unrelated proteins that typically recognize internal sequences of sulfated GAGs. GAGBPs are distinct from the other major group of glycan binding proteins, lectins. The multifunctional human galectin-3 (Gal-3) is a β-galactoside binding lectin that preferentially binds to N-acetyllactosamine moieties on glycoconjugates. Here, we demonstrate through microcalorimetric and spectroscopic data that Gal-3 possesses the characteristics of a GAGBP. Gal-3 interacts with unmodified heparin, chondroitin sulfate-A (CSA), -B (CSB), and -C (CSC) as well as chondroitin sulfate proteoglycans (CSPGs). While heparin, CSA, and CSC bind with micromolar affinity, the affinity of CSPGs is nanomolar. Significantly, CSA, CSC, and a bovine CSPG were engaged in multivalent binding with Gal-3 and formed noncovalent cross-linked complexes with the lectin. Binding of sulfated GAGs was completely abolished when Gal-3 was preincubated with β-lactose. Cross-linking of Gal-3 by CSA, CSC, and the bovine CSPG was reversed by β-lactose. Both observations strongly suggest that GAGs primarily occupy the lactose/LacNAc binding site of Gal-3. Hill plot analysis of calorimetric data reveals that the binding of CSA, CSC, and a bovine CSPG to Gal-3 is associated with progressive negative cooperativity effects. Identification of Gal-3 as a GAGBP should help to reveal new functions of Gal-3 mediated by GAGs and proteoglycans.

"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.

Mapping proteoglycan functions with glycosidases

The intrinsic and extrinsic factors that contribute to stem and neuronal precursor cell maintenance and/or differentiation remain poorly understood. Proteoglycans, major residents of the stem cell microenvironment, modulate key signaling cues and are of particular importance. We have taken a loss-of-function approach, by developing a library of bacterial lyases and sulfatases to specifically remodel the ECM and test the functional role of glycosaminoglycans (GAGs) in cell self-renewal, maintenance, and differentiation.

Forward genetics defines Xylt1 as a key, conserved regulator of early chondrocyte maturation and skeletal length

The long bones of the vertebrate body are built by the initial formation of a cartilage template that is later replaced by mineralized bone. The proliferation and maturation of the skeletal precursor cells (chondrocytes) within the cartilage template and their replacement by bone is a highly coordinated process which, if misregulated, can lead to a number of defects including dwarfism and other skeletal deformities. This is exemplified by the fact that abnormal bone development is one of the most common types of human birth defects. Yet, many of the factors that initiate and regulate chondrocyte maturation are not known. We identified a recessive dwarf mouse mutant (pug) from an N-ethyl-N-nitrosourea (ENU) mutagenesis screen. pug mutant skeletal elements are patterned normally during development, but display a ~20% length reduction compared to wild-type embryos. We show that the pug mutation does not lead to changes in chondrocyte proliferation but instead promotes premature maturation and early ossification, which ultimately leads to disproportionate dwarfism. Using sequence capture and high-throughput sequencing, we identified a missense mutation in the Xylosyltransferase 1 (Xylt1) gene in pug mutants. Xylosyltransferases catalyze the initial step in glycosaminoglycan (GAG) chain addition to proteoglycan core proteins, and these modifications are essential for normal proteoglycan function. We show that the pug mutation disrupts Xylt1 activity and subcellular localization, leading to a reduction in GAG chains in pug mutants. The pug mutant serves as a novel model for mammalian dwarfism and identifies a key role for proteoglycan modification in the initiation of chondrocyte maturation.

Exploiting enzyme specificities in digestions of chondroitin sulfates A and C: production of well-defined hexasaccharides

Interactions between proteins and glycosaminoglycans (GAGs) of the extracellular matrix are important to the regulation of cellular processes including growth, differentiation and migration. Understanding these processes can benefit greatly from the study of protein-GAG interactions using GAG oligosaccharides of well-defined structure. Materials for such studies have, however, been difficult to obtain because of challenges in synthetic approaches and the extreme structural heterogeneity in GAG polymers. Here, it is demonstrated that diversity in structures of oligosaccharides derived by limited enzymatic digestion of materials from natural sources can be greatly curtailed by a proper selection of combinations of source materials and digestive enzymes, a process aided by an improved understanding of the specificities of certain commercial preparations of hydrolases and lyases. Separation of well-defined oligosaccharides can then be accomplished by size-exclusion chromatography followed by strong anion-exchange chromatography. We focus here on two types of chondroitin sulfate (CS) as starting material (CS-A, and CS-C) and the use of three digestive enzymes with varying specificities (testicular hyaluronidase and bacterial chondroitinases ABC and C). Analysis using nuclear magnetic resonance and mass spectrometry focuses on isolated CS disaccharides and hexasaccharides. In all, 15 CS hexasaccharides have been isolated and characterized. These serve as useful contributions to growing libraries of well-defined GAG oligosaccharides that can be used in further biophysical assays.

Proteoglycans: gene cloning

Aggrecan is a large proteoglycan that plays roles in numerous tissues during vertebrate development and adult life. The 6,327-nt chick aggrecan coding sequence had been determined from overlapping clones, but a full-length cDNA, needed for use in transgenic expression studies, had not been constructed. The strategy employed to do so was to generate two overlapping cDNA subfragments that shared a unique restriction site in the overlap and then join them at that site. These subfragments were obtained and cloned into the TOPO-TA vector pCR2.1. Digestion of the two constructs with the shared-site enzyme, XbaI, produced vector/5'-cDNA and 3'-cDNA fragments with XbaI-ends; these were ligated to produce the final full-length cDNA.

Neurite outgrowth of neural progenitors in presence of inhibitory proteoglycans

Attempts to promote host regeneration after spinal cord injury (SCI) have often resulted in poor axon extension due to formation of a glial scar, which creates a dense physical barrier around the injury and contains molecules that inhibit regeneration and repair of adult injured axons. Previous studies have shown that, while transplants of multipotent neural stem cells (NSC) integrate poorly in the injury site, the use of neuronal-restricted precursor cells (NRP) together with glial-restricted precursor cells (GRP) allow differentiation and integration of neurons, possibly because NRP are able to overcome chondroitin sulfate proteoglycan (CSPG) inhibition. To investigate this possibility, we grew mixed cultures of NRP/GRP on CSPG at inhibitory concentrations, using embryonic hippocampal cultures as controls. We found that NRP/GRP grown on CSPG survive and differentiate into neurons with no significant changes in neurite length, relative to growth on control polylysine substrate, and in contrast to a significant inhibition of axon growth in hippocampal cultures grown on CSPG-coated substrate. There was, however, a significant decrease in neurite number and branching in both cultures, indicating that CSPG also has important effects on neuronal morphology. These data suggest that embryonic neurons supported by glial cells derived from NRP/GRP transplants are less sensitive to inhibitory effects of CSPG in the glial scar, and are thus an appropriate source for neuronal cell replacement and reconnection of damaged circuits after SCI.

Characterization of glycosaminoglycans by 15N NMR spectroscopy and in vivo isotopic labeling

Characterization of glycosaminoglycans (GAGs), including chondroitin sulfate (CS), dermatan sulfate (DS), and heparan sulfate (HS), is important in developing an understanding of cellular function and in assuring quality of preparations destined for biomedical applications. While use of (1)H and (13)C NMR spectroscopy has become common in characterization of these materials, spectra are complex and difficult to interpret when a more heterogeneous GAG type or a mixture of several types is present. Herein a method based on (1)H-(15)N two-dimensional NMR experiments is described. The (15)N- and (1)H-chemical shifts of amide signals from (15)N-containing acetylgalactosamines in CSs are shown to be quite sensitive to the sites of sulfation (4-, 6-, or 4,6-) and easily distinguishable from those of DS. The amide signals from residual (15)N-containing acetylglucosamines in HS are shown to be diagnostic of the presence of these GAG components as well. Most data were collected at natural abundance of (15)N despite its low percentage. However enrichment of the (15)N-content in GAGs using metabolic incorporation from (15)N-glutamine added to cell culture media is also demonstrated and used to distinguish metabolic states in different cell types.

Differential distribution of aggrecan isoforms in perineuronal nets of the human cerebral cortex

Aggrecan is a component of the CNS extracellular matrix (ECM) and we show here that the three primary alternative spliced transcripts of the aggrecan gene found in cartilage are also present in the adult CNS. Using a unique panel of core protein-directed antibodies against human aggrecan we further show that different aggrecan isoforms are deposited in perineuronal nets (PNNs) and neuropil ECM of Brodmann’s area 6 of the human adult cerebral cortex. According to their distribution pattern, the identified cortical aggrecan isoforms were subdivided into five clusters spanning from cluster 1, comprised isoforms that appeared widespread throughout the cortex, to cluster 5, which was an aggrecan-free subset. Comparison of brain and cartilage tissues showed a different relative abundance of aggrecan isoforms, with cartilage-specific isoforms characterizing cluster 5, and PNN-associated isoforms lacking keratan sulphate chains. In the brain, isoforms of cluster 1 were disclosed in PNNs surrounding small-medium interneurons of layers II–V, small-medium pyramidal neurons of layers III and V and large interneurons of layer VI. Aggrecan PNNs enveloped both neuron bodies and neuronal processes, encompassing pre-terminal nerve fibres, synaptic boutons and terminal processes of glial cells and aggrecan was also observed in continuous ‘coats’ associated with satellite, neuron-associated cells of a putative glial nature. Immunolabelling for calcium-binding proteins and glutamate demonstrated that aggrecan PNNs were linked to defined subsets of cortical interneurons and pyramidal cells. We suggest that in the human cerebral cortex, discrete, layer-specific PNNs are assembled through the participation of selected aggrecan isoforms that characterize defined subsets of cortical neurons.

Collagen-based fibrous scaffold for spatial organization of encapsulated and seeded human mesenchymal stem cells

Living tissues consist of groups of cells organized in a controlled manner to perform a specific function. Spatial distribution of cells within a three-dimensional matrix is critical for the success of any tissue-engineering construct. Fibers endowed with cell-encapsulation capability would facilitate the achievement of this objective. Here we report the synthesis of a cell-encapsulated fibrous scaffold by interfacial polyelectrolyte complexation (IPC) of methylated collagen and a synthetic terpolymer. The collagen component was well distributed in the fiber, which had a mean ultimate tensile strength of 244.6+/-43.0 MPa. Cultured in proliferating medium, human mesenchymal stem cells (hMSCs) encapsulated in the fibers showed higher proliferation rate than those seeded on the scaffold. Gene expression analysis revealed the maintenance of multipotency for both encapsulated and seeded samples up to 7 days as evidenced by Sox 9, CBFA-1, AFP, PPARgamma2, nestin, GFAP, collagen I, osteopontin and osteonectin genes. Beyond that, seeded hMSCs started to express neuronal-specific genes such as aggrecan and MAP2. The study demonstrates the appeal of IPC for scaffold design in general and the promise of collagen-based hybrid fibers for tissue engineering in particular. It lays the foundation for building fibrous scaffold that permits 3D spatial cellular organization and multi-cellular tissue development.

A selective tumor microvasculature thrombogen that targets a novel receptor complex in the tumor angiogenic microenvironment

We have previously shown that part of the heparin-binding domain of the vascular endothelial growth factor (VEGF), designated HBDt, localizes very selectively to surfaces of the endothelial cells of i.t blood vessels. Here, we have coupled the HBDt to the extracellular domain of tissue factor (TFt), to locally initiate the thrombogenic cascade. In tumor-bearing mice, infusion of this HBDt.TFt results in rapid occlusive thrombosis selective only for tumor microvasculature with resultant infarctive destruction of tumors. We now show that infusion of an optimal combination of this HBDt.TFt and its requisite cofactor (factor VIIa) in tumor models results in significant tumor eradication. Binding studies and confocal microscopy indicate that the target for the HBDt.TFt seems to be a trimolecular complex of chondroitin C sulfate proteoglycan, neuropilin-1, and VEGF receptor-2, overexpressed together only in highly angiogenic sites of the tumor microenvironment. The HBDt.TFt was also colocalized with the trimolecular receptor complex in endothelial sprouts from tumor tissues, and its binding inhibited the growth of such sprouts. In vitro, we show that the HBDt structure has its highest affinity for chondroitin 6 sulfate. We show the potential of this HBDt.TFt as a candidate therapeutic and elucidate its target in vivo.

Proteoglycans in brain development

Proteoglycans, as part of the extracellular or cell-surface milieu of most tissues and organ systems, play important roles in morphogenesis by modulating cell-matrix or cell-cell interactions, cell adhesiveness, or by binding and presenting growth and differentiation factors. Chondroitin sulfate proteoglycans which constitute the major population of proteoglycans in the central nervous system may influence formation of neuronal nuclei, establishment of boundaries for axonal growth and act as modulators of neuronal outgrowth during brain development, as well as during regeneration after injury. There is a paucity of information on the role of chondroitin sulfate proteoglycans in central nervous system organogenesis. In the chick embryo, aggrecan has a regionally specific and developmentally regulated expression profile during brain development. By Northern and Western blot analysis, aggrecan expression is first detected in chick brain on embryonic day 7 (E7), increases from E7 to E13, declines markedly after E16, and is not evident in hatchling brains. The time course and pattern of aggrecan expression observed in ventricular zone cells suggested that it might play a role in gliogenesis. We have analyzed the role of aggrecan during brain development using a aggrecan-deficient model, nanomelia. In nanomelic chicks, expression and levels of neurocan and brevican is not affected, indicating a non-redundant role for these members of the aggrecan gene family. Our analysis of the aggrecan-deficient model found a severely altered phenotype which affects cell behavior in a neuronal culture paradigm and expression of astrocytic markers in vivo . Taken together our results suggest a function for aggrecan in the specification of a sub-set of glia precursors that might give rise to astrocytes in vivo.

Aggrecan regulates telencephalic neuronal aggregation in culture

Proteoglycans have been suggested to play roles in pattern formation in the developing central nervous system. In the chick embryo, aggrecan, a chondroitin sulfate proteoglycan, has a regionally-specific and developmentally-regulated expression profile. Telencephalic neuronal cultures, when aggregated, exhibit aggrecan expression patterns comparable to those observed in vivo. The chicken mutation nanomelia produces a truncated aggrecan species that cannot be processed further and is not secreted. Neurons from normal and nanomelic chick embryo telencephalon were scored for aggregate formation and analyzed for distribution of aggrecan protein and expression of aggrecan mRNA. Distinctly different pattern formation, with respect to aggregate size (smaller) and number (fewer) were observed in poly-L-lysine plated neuronal cultures derived from nanomelic embryos when compared to those derived from normal embryos. Significantly, the nanomelic phenotype was subsequently rescued upon addition of the brain-specific form of aggrecan. Modulation of neuronal aggregate formation was mimicked by treatment with chondroitinase ABC but not other glycanases, and was rescued by addition of chondroitin 6-sulfate to the culture media. Lastly, although broad and diffuse distribution of aggrecan among the cell aggregates in the culture paradigm was observed by immunocytochemistry, mRNA in situ hybridization revealed that only a small population of cells in the center of the aggregates was responsible for the production of the secreted aggrecan found associated with neuronal aggregates. These studies suggest a function for aggrecan as a diffusible signal in CNS histomorphogenesis.