Potent glycan inhibitors of myelin-associated glycoprotein enhance axon outgrowth in vitro

Molecular approaches for spinal cord injury treatment

Injuries to the spinal cord result in permanent disabilities that limit daily life activities. The main reasons for these poor outcomes are the limited regenerative capacity of central neurons and the inhibitory milieu that is established upon traumatic injuries. Despite decades of research, there is still no efficient treatment for spinal cord injury. Many strategies are tested in preclinical studies that focus on ameliorating the functional outcomes after spinal cord injury. Among these, molecular compounds are currently being used for neurological recovery, with promising results. These molecules target the axon collapsed growth cone, the inhibitory microenvironment, the survival of neurons and glial cells, and the re-establishment of lost connections. In this review we focused on molecules that are being used, either in preclinical or clinical studies, to treat spinal cord injuries, such as drugs, growth and neurotrophic factors, enzymes, and purines. The mechanisms of action of these molecules are discussed, considering traumatic spinal cord injury in rodents and humans.

Genome-wide CRISPR screens reveal a specific ligand for the glycan-binding immune checkpoint receptor Siglec-7

Glyco-immune checkpoint receptors, molecules that inhibit immune cell activity following binding to glycosylated cell-surface antigens, are emerging as attractive targets for cancer immunotherapy. Defining biologically relevant ligands that bind and activate such receptors, however, has historically been a significant challenge. Here, we present a CRISPRi genomic screening strategy that allowed unbiased identification of the key genes required for cell-surface presentation of glycan ligands on leukemia cells that bind the glyco-immune checkpoint receptors Siglec-7 and Siglec-9. This approach revealed a selective interaction between Siglec-7 and the mucin-type glycoprotein CD43. Further work identified a specific N-terminal glycopeptide region of CD43 containing clusters of disialylated O-glycan tetrasaccharides that form specific Siglec-7 binding motifs. Knockout or blockade of CD43 in leukemia cells relieves Siglec-7-mediated inhibition of immune killing activity. This work identifies a potential target for immune checkpoint blockade therapy and represents a generalizable approach to dissection of glycan-receptor interactions in living cells.

Detection and Characterization of Vesicular Gangliosides Binding to Myelin-Associated Glycoprotein on Supported Lipid Bilayers

In the nervous system, a myelin sheath that originates from oligodendrocytes or Schwann cells wraps around axons to facilitate electrical signal transduction. The interface between an axon and myelin is maintained by a number of biomolecular interactions. Among the interactions are those between GD1a and GT1b gangliosides on the axon and myelin-associated glycoprotein (MAG) on myelin. Interestingly, these interactions can also inhibit neuronal outgrowth. Ganglioside-MAG interactions are often studied in cellular or animal models where their relative concentrations are not easily controlled or in assays where the gangliosides and MAG are not presented as part of fluid lipid bilayers. Here, we present an approach to characterize MAG-ganglioside interactions in real time, where MAG, GD1a, and GT1b contents are controlled and they are in their in vivo orientation within fluid lipid bilayers. Using a quartz crystal microbalance with dissipation monitoring (QCM-D) biosensor functionalized with a supported lipid bilayer (SLB) and MAG, we detect vesicular GD1a and GT1b binding and determine the interaction kinetics as a function of vesicular ganglioside content. MAG-bound vesicles are deformed similarly, regardless of the ganglioside or its mole fraction. We further demonstrate how MAG-ganglioside interactions can be disrupted by antiganglioside antibodies that override MAG-based neuron growth inhibition.

Oligodendroglial glycolipids in (Re)myelination: implications for multiple sclerosis research

Covering: up to 2020 This short review surveys aspects of glycolipid-based natural products and their biological relevance in multiple sclerosis (MS). The role of isolated gangliosides in disease models is discussed together with an overview of ganglioside-inspired small molecule drugs and imaging probes. The discussion is extended to neurodegeneration in a more general context and addresses the need for more efficient synthetic methods to generate (glyco)structures that are of therapeutic relevance.

Structural advances of Siglecs: insight into synthetic glycan ligands for immunomodulation

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are transmembrane proteins of the immunoglobulin (Ig) superfamily predominantly expressed on the cells of our immune system. Siglecs recognize sialic acid via their terminal V-set domain. In mammals, sialic acid-terminated glycolipids and glycoproteins are the ligands of Siglecs, and the monomeric affinity of Siglecs for their sialic acid-containing ligands is weak. Significant efforts have been devoted toward the development of chemically modified sialoside ligands to target Siglecs with higher affinity and selectivity. In this review we discuss natural and synthetic sialoside ligands for each human Siglec, emphasizing the ligand binding determinants uncovered from recent advances in protein structural information. Potential therapeutic applications of these ligands are also discussed.

Sialic acid and biology of life: An introduction

Sialic acids are important molecule with high structural diversity. They are known to occur in higher animals such as Echinoderms, Hemichordata, Cephalochorda, and Vertebrata and also in other animals such as Platyhelminthes, Cephalopoda, and Crustaceae. Plants are known to lack sialic acid. But they are reported to occur in viruses, bacteria, protozoa, and fungi. Deaminated neuraminic acid although occurs in vertebrates and bacteria, is reported to occur in abundance in the lower vertebrates. Sialic acids are mostly located in terminal ends of glycoproteins and glycolipids, capsular and tissue polysialic acids, bacterial lipooligosaccharides/polysaccharides, and in different forms that dictate their role in biology. Sialic acid play important roles in human physiology of cell-cell interaction, communication, cell-cell signaling, carbohydrate-protein interactions, cellular aggregation, development processes, immune reactions, reproduction, and in neurobiology and human diseases in enabling the infection process by bacteria and virus, tumor growth and metastasis, microbiome biology, and pathology. It enables molecular mimicry in pathogens that allows them to escape host immune responses. Recently sialic acid has found role in therapeutics. In this chapter we have highlighted the (i) diversity of sialic acid, (ii) their occurrence in the diverse life forms, (iii) sialylation and disease, and (iv) sialic acid and therapeutics.

The sugar code: letters and vocabulary, writers, editors and readers and biosignificance of functional glycan-lectin pairing

Ubiquitous occurrence in Nature, abundant presence at strategically important places such as the cell surface and dynamic shifts in their profile by diverse molecular switches qualifies the glycans to serve as versatile biochemical signals. However, their exceptional structural complexity often prevents one noting how simple the rules of objective-driven assembly of glycan-encoded messages are. This review is intended to provide a tutorial for a broad readership. The principles of why carbohydrates meet all demands to be the coding section of an information transfer system, and this at unsurpassed high density, are explained. Despite appearing to be a random assortment of sugars and their substitutions, seemingly subtle structural variations in glycan chains by a sophisticated enzymatic machinery have emerged to account for their specific biological meaning. Acting as 'readers' of glycan-encoded information, carbohydrate-specific receptors (lectins) are a means to turn the glycans' potential to serve as signals into a multitude of (patho)physiologically relevant responses. Once the far-reaching significance of this type of functional pairing has become clear, the various modes of spatial presentation of glycans and of carbohydrate recognition domains in lectins can be explored and rationalized. These discoveries are continuously revealing the intricacies of mutually adaptable routes to achieve essential selectivity and specificity. Equipped with these insights, readers will gain a fundamental understanding why carbohydrates form the third alphabet of life, joining the ranks of nucleotides and amino acids, and will also become aware of the importance of cellular communication via glycan-lectin recognition.

GM1 ganglioside prevents axonal regeneration inhibition and cognitive deficits in a mouse model of traumatic brain injury

Traumatic Brain Injury (TBI) is one of the most common causes of neurological damage in young populations. It has been previously suggested that one of the mechanisms that underlie brain injury is Axonal Outgrowth Inhibition (AOI) that is caused by altered composition of the gangliosides on the axon surface. In the present study, we have found a significant reduction of GM1 ganglioside levels in the cortex in a closed head traumatic brain injury model of a mouse, induced by a weight drop device. In addition, axonal regeneration in the brains of the injured mice was affected as seen by the expression of the axonal marker pNF-H and the growth cones (visualized by F-actin and β-III-tubulin). NeuN immunostaining revealed mTBI-induced damage to neuronal survival. Finally, as expected, spatial and visual memories (measured by the Y-maze and the Novel Object Recognition tests, respectively) were also damaged 7 and 30 days post injury. A single low dose of GM1 shortly after the injury (2 mg/kg; IP) prevented all of the deficits mentioned above. These results reveal additional insights into the neuroprotective characteristics of GM1 in prevention of biochemical, cellular and cognitive changes caused by trauma, and may suggest a potential intervention for mTBI.

Glycan Chains of Gangliosides: Functional Ligands for Tissue Lectins (Siglecs/Galectins)

Molecular signals on the cell surface are responsible for adhesion and communication. Of relevance in this respect, their chemical properties endow carbohydrates with the capacity to store a maximum of information in a minimum of space. One way to present glycans on the cell surface is their covalent conjugation to a ceramide anchor. Among the resulting glycosphingolipids, gangliosides are special due to the presence of at least one sialic acid in the glycan chains. Their spatial accessibility and the dynamic regulation of their profile are factors that argue in favor of a role of glycans of gangliosides as ligands (counterreceptors) for carbohydrate-binding proteins (lectins). Indeed, as discovered first for a bacterial toxin, tissue lectins bind gangliosides and mediate contact formation (trans) and signaling (cis). While siglecs have a preference for higher sialylated glycans, certain galectins also target the monosialylated pentasaccharide of ganglioside GM1. Enzymatic interconversion of ganglioside glycans by sialidase action, relevant for neuroblastoma cell differentiation and growth control in vitro, for axonogenesis and axon regeneration, as well as for proper communication between effector and regulatory T cells, changes lectin-binding affinity profoundly. The GD1a-to-GM1 "editing" is recognized by such lectins, for example, myelin-associated glycoprotein (siglec-4) losing affinity and galectin-1 gaining reactivity, and then translated into postbinding signaling. Orchestrations of loss/gain of affinity, of ganglioside/lectin expression, and of lectin presence in a network offer ample opportunities for fine-tuning. Thus glycans of gangliosides such as GD1a and GM1 are functional counterreceptors by a pairing with tissue lectins, an emerging aspect of ganglioside and lectin functionality.

Restoring GM1 ganglioside expression ameliorates axonal outgrowth inhibition and cognitive impairments induced by blast traumatic brain injury

Blast induced traumatic brain injury (B-TBI) may cause various degrees of cognitive and behavioral disturbances but the exact brain pathophysiology involved is poorly understood. It was previously suggested that ganglioside alteration on the axon surface as well as axonal regenerating inhibitors (ARIs) such as myelin associated glycoprotein (MAG) were involved in axonal outgrowth inhibition (AOI), leading to brain damage. GM1 ganglioside content in the brain was significantly reduced while GD1 ganglioside was not affected. The axonal regeneration was also reduced as seen by the phosphorylated NF-H expression. Moreover, B-TBI induced a significant elevation in MAG expression in the brains of the injured mice. The blast injured mice exhibited a significant decline in spatial memory as seen by the Y-maze test. In addition, the injured mice showed pronounced damage to the visual memory (as evaluated by the Novel object recognition test). A single low dose of GM1 (2 mg/kg; IP), shortly after the injury, prevented both the cognitive and the cellular changes in the brains of the injured mice. These results enlighten part of the complicated mechanism that underlies the damage induced by B-TBI and may also suggest a potential new treatment strategy for brain injuries.

Sweet complementarity: the functional pairing of glycans with lectins

Carbohydrates establish the third alphabet of life. As part of cellular glycoconjugates, the glycans generate a multitude of signals in a minimum of space. The presence of distinct glycotopes and the glycome diversity are mapped by sugar receptors (antibodies and lectins). Endogenous (tissue) lectins can read the sugar-encoded information and translate it into functional aspects of cell sociology. Illustrated by instructive examples, each glycan has its own ligand properties. Lectins with different folds can converge to target the same epitope, while intrafamily diversification enables functional cooperation and antagonism. The emerging evidence for the concept of a network calls for a detailed fingerprinting. Due to the high degree of plasticity and dynamics of the display of genes for lectins the validity of extrapolations between different organisms of the phylogenetic tree yet is inevitably limited.

Functional interplay between ganglioside GM1 and cross-linking galectin-1 induces axon-like neuritogenesis via integrin-based signaling and TRPC5-dependent Ca²⁺ influx

Axon‐like neuritogenesis in neuroblastoma (NG108‐15) cells and primary cerebellar granular neurons is furthered by the presence of ganglioside GM1. We describe here that galectin‐1 (Gal‐1), a homobivalent endogenous lectin, is an effector by cross‐linking the ganglioside and its associated glycoprotein α₅β₁‐integrin. The thereby triggered signaling cascade involves autophosphorylation of focal adhesion kinase and activation of phospholipase Cγ and phosphoinositide‐3 kinase. This leads to a transient increase in the intracellular Ca²⁺ concentration by opening of TRPC5 channels, which belong to the signal transduction‐gated cation channels. Controls with GM1‐defective cells (NG‐CR72 and neurons from ganglio‐series KO mice) were retarded in axonal growth, underscoring the relevance of GM1 as functional counterreceptor for Gal‐1. The lectin's presence was detected in the NG108‐15 cells, suggesting an autocrine mechanism of action, and in astrocytes in situ. Gal‐1, as cross‐linking lectin, can thus translate metabolic conversion of ganglioside GD1a to GM1 by neuraminidase action into axon growth.

Galectin‐1 (Gal‐1) was shown an effector of axonogenesis in cerebellar granule neurons (CGNs) and NG108‐15 cells by cross‐linking GM1 ganglioside and its associated glycoprotein α₅β₁‐integrin. The resulting signaling led to a transient increase in intracellular Ca²⁺ by opening TRPC5 channels. CGNs deficient in GM1 showed retarded axonogenesis, underscoring the relevance of GM1 as functional counterreceptor for Gal‐1 in this process. This Gal‐1/GM1‐induced signaling was manifest only at the earliest, initiating stage of axon development.

Determination of major sialylated N-glycans and identification of branched sialylated N-glycans that dynamically change their content during development in the mouse cerebral cortex

Oligosaccharides of glycoproteins expressed on the cell surface play important roles in cell-cell interactions, particularly sialylated N-glycans having a negative charge, which interact with sialic acid-binding immunoglobulin-like lectins (siglecs). The entire structure of sialylated N-glycans expressed in the mouse brain, particularly the linkage type of sialic acid residues attached to the backbone N-glycans, has not yet been elucidated. An improved method to analyze pyridylaminated sugar chains using high performance liquid chromatography (HPLC) was developed to determine the entire structure of sialylated N-linked sugar chains expressed in the adult and developing mouse cerebral cortices. Three classes of sialylated sugar chains were prevalent: 1) N-glycans containing α(2-3)-sialyl linkages on a type 2 antennary (Galβ(1-4)GlcNAc), 2) sialylated N-glycans with α(2-6)-sialyl linkages on a type 2 antennary, and 3) a branched sialylated N-glycan with a [Galβ(1-3){NeuAcα(2-6)}GlcNAc-] structure, which was absent at embryonic day 12 but then increased during development. This branched type sialylated N-glycan structure comprised approximately 2 % of the total N-glycans in the adult brain. Some N-glycans (containing type 2 antennary) were found to change their type of sialic acid linkage from α(2-6)-Gal to α(2-3)-Gal. Thus, the linkages and expression levels of sialylated N-glycans change dramatically during brain development.

Sialic acids in the brain: gangliosides and polysialic acid in nervous system development, stability, disease, and regeneration

Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

Glycosphingolipid functions

The combination of carbohydrate and lipid generates unusual molecules in which the two distinctive halves of the glycoconjugate influence the function of each other. Membrane glycolipids can act as primary receptors for carbohydrate binding proteins to mediate transmembrane signaling despite restriction to the outer bilayer leaflet. The extensive heterogeneity of the lipid moiety plays a significant, but still largely unknown, role in glycosphingolipid function. Potential interplay between glycolipids and their fatty acid isoforms, together with their preferential interaction with cholesterol, generates a complex mechanism for the regulation of their function in cellular physiology.

High affinity sialoside ligands of myelin associated glycoprotein

Myelin associated glycoprotein (Siglec-4) is a myelin adhesion receptor, that is, well established for its role as an inhibitor of axonal outgrowth in nerve injury, mediated by binding to sialic acid containing ligands on the axonal membrane. Because disruption of myelin-ligand interactions promotes axon outgrowth, we have sought to develop potent ligand based inhibitors using natural ligands as scaffolds. Although natural ligands of MAG are glycolipids terminating in the sequence NeuAcα2-3Galβ1-3(±NeuAcα2-6)GalNAcβ-R, we previously established that synthetic O-linked glycoprotein glycans with the same sequence α-linked to Thr exhibited ∼1000-fold increased affinity (∼1μM). Attempts to increase potency by introducing a benzoylamide substituent at C-9 of the α2-3 sialic acid afforded only a two-fold increase, instead of increases of >100-fold observed for other sialoside ligands of MAG. Surprisingly, however, introduction of a 9-N-fluoro-benzoyl substituent on the α2-6 sialic acid increased affinity 80-fold, resulting in a potent inhibitor with a K(d) of 15nM. Docking this ligand to a model of MAG based on known crystal structures of other siglecs suggests that the Thr positions the glycan such that aryl substitution of the α2-3 sialic acid produces a steric clash with the GalNAc, while attaching an aryl substituent to the other sialic acid positions the substituent near a hydrophobic pocket that accounts to the increase in affinity.

Functionalization of electrospun fibers of poly(epsilon-caprolactone) with star shaped NCO-poly(ethylene glycol)-stat-poly(propylene glycol) for neuronal cell guidance

Microfibers produced with electrospinning have recently been used in tissue engineering. In the development of artificial implants for nerve regeneration they are of particular interest as guidance structures for cell migration and axonal growth. Using electrospinning we produced parallel-orientated biocompatible fibers in the submicron range consisting of poly(epsilon-caprolactone) (PCL) and star shaped NCO-poly(ethylene glycol)-stat-poly(propylene glycol) (sPEG). Addition of the bioactive peptide sequence glycine-arginine-glycine-aspartate-serine (GRGDS) or the extracellular matrix protein fibronectin to the electrospinning solution resulted in functionalized fibers. Surface characteristics and biological properties of functionalized and non-functionalised fibers were investigated. Polymer solutions and electrospinning process parameters were varied to obtain high quality orientated fibers. A polymer mixture containing high molecular weight PCL, PCL-diol, and sPEG permitted a chemical reaction between hydroxyl groups of the diol and isocyanante groups of the sPEG. Surface analysis demonstrated that sPEG at the fiber surface minimized protein adhesion. In vitro experiments using dorsal root ganglia explants showed that the cell repellent property of pure PCL/sPEG fibers was overcome by functionalization either with GRGDS peptide or fibronectin. In this way cell migration and axonal outgrowth along fibers were significantly increased. Thus, functionalized electrospun PCL/sPEG fibers, while preventing non-specific protein adsorption, are a suitable substrate for biological and medical applications.

Sialidase enhances recovery from spinal cord contusion injury

Axons fail to regenerate in the injured spinal cord, limiting motor and autonomic recovery and contributing to long-term morbidity. Endogenous inhibitors, including those on residual myelin, contribute to regeneration failure. One inhibitor, myelin-associated glycoprotein (MAG), binds to sialoglycans and other receptors on axons. MAG inhibition of axon outgrowth in some neurons is reversed by treatment with sialidase, an enzyme that hydrolyzes sialic acids and eliminates MAG-sialoglycan binding. We delivered recombinant sialidase intrathecally to rats following a spinal cord contusive injury. Sialidase (or saline solution) was infused to the injury site continuously for 2 wk and then motor behavior, autonomic physiology, and anatomic outcomes were determined 3 wk later. Sialidase treatment significantly enhanced hindlimb motor function, improved bulbospinally mediated autonomic reflexes, and increased axon sprouting. These findings validate sialoglycans as therapeutic targets and sialidase as a candidate therapy for spinal cord injury.

Brain gangliosides in axon-myelin stability and axon regeneration

Gangliosides, sialic acid-bearing glycosphingolipids, are expressed at high abundance and complexity in the brain. Altered ganglioside expression results in neural disorders, including seizures and axon degeneration. Brain gangliosides function, in part, by interacting with a ganglioside-binding lectin, myelin-associated glycoprotein (MAG). MAG, on the innermost wrap of the myelin sheath, binds to gangliosides GD1a and GT1b on axons. MAG-ganglioside binding ensures optimal axon-myelin cell-cell interactions, enhances long-term axon-myelin stability and inhibits axon outgrowth after injury. Knowledge of the molecular interactions of brain gangliosides may improve understanding of axon-myelin stability and provide opportunities to enhance recovery after nerve injury.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

From carbohydrate leads to glycomimetic drugs

Carbohydrates are the most abundant natural products. Besides their role in metabolism and as structural building blocks, they are fundamental constituents of every cell surface, where they are involved in vital cellular recognition processes. Carbohydrates are a relatively untapped source of new drugs and therefore offer exciting new therapeutic opportunities. Advances in the functional understanding of carbohydrate-protein interactions have enabled the development of a new class of small-molecule drugs, known as glycomimetics. These compounds mimic the bioactive function of carbohydrates and address the drawbacks of carbohydrate leads, namely their low activity and insufficient drug-like properties. Here, we examine examples of approved carbohydrate-derived drugs, discuss the potential of carbohydrate-binding proteins as new drug targets (focusing on the lectin families) and consider ways to overcome the challenges of developing this unique class of novel therapeutics.

A review of published reports on neuroprotection in spinal cord injury

STUDY DESIGN

Literature review.

OBJECTIVES

To review the main published current neuroprotection research trends and results in spinal cord injury (SCI).

SETTING

This paper is the result of a collaboration between a group of European scientists.

METHODS

Recent studies, especially in genetic, immune, histochemical and bio (nano)-technological fields, have provided new insight into the cellular and molecular mechanisms occurring within the central nervous system (NS), including SCIs. As a consequence, a new spectrum of therapies aiming to antagonize the 'secondary injury' pathways (that is, to provide neuroprotection) and also to repair such classically irreparable structures is emerging. We reviewed the most significant published works related to such novel, but not yet entirely validated, clinical practice therapies.

RESULTS

There have been identified many molecules, primarily expressed by heterogenous glial and neural subpopulations of cells, which are directly or indirectly critical for tissue damaging/sparing/re-growth inhibiting, angiogenesis and neural plasticity, and also various substances/energy vectors with regenerative properties, such as MAG (myelin-associated glycoprotein), Omgp (oligodendrocyte myelin glycoprotein), KDI (synthetic: Lysine-Asparagine-Isoleucine 'gamma-1 of Laminin Kainat Domain'), Nogo (Neurite outgrowth inhibitor), NgR (Nogo protein Receptor), the Rho signaling pathway (superfamily of 'Rho-dopsin gene-including neurotransmitter-receptors'), EphA4 (Ephrine), GFAP (Glial Fibrillary Acidic Protein), different subtypes of serotonergic and glutamatergic receptors, antigens, antibodies, immune modulators, adhesion molecules, scavengers, neurotrophic factors, enzymes, hormones, collagen scar inhibitors, remyelinating agents and neurogenetic/plasticity inducers, all aiming to preserve/re-establish the morphology and functional connections across the lesion site. Accordingly, modern research and experimental SCI therapies focus on several intricate, rather overlapping, therapeutic objectives and means, such as neuroprotective, neurotrophic, neurorestorative, neuroreparative, neuroregenerative, neuro(re)constructive and neurogenetic interventions.

CONCLUSION

The first three of these therapeutical directions are generically assimilated as neuroprotective, and are synthetically presented and commented in this paper in an attempt to conceptually systematize them; thus, the aim of this article is, by emphasizing the state-of-the art in the domain, to optimize theoretical support in selecting the most effective pharmacological and physical interventions for preventing, as much as possible, paralysis, and for maximizing recovery chances after SCI.

Gangliosides in cell recognition and membrane protein regulation

Gangliosides, sialic acid-bearing glycosphingolipids, are expressed on all vertebrate cells, and are the major glycans on nerve cells. They are anchored to the plasma membrane through their ceramide lipids with their varied glycans extending into the extracellular space. Through sugar-specific interactions with glycan-binding proteins on apposing cells, gangliosides function as receptors in cell-cell recognition, regulating natural killer cell cytotoxicity via Siglec-7, myelin-axon interactions via Siglec-4 (myelin-associated glycoprotein), and inflammation via E-selectin. Gangliosides also interact laterally in their own membranes, regulating the responsiveness of signaling proteins including the insulin, epidermal growth factor, and vascular endothelial growth factor receptors. In these ways, gangliosides act as regulatory elements in the immune system, in the nervous system, in metabolic regulation, and in cancer progression.

Inhibitory activity of myelin-associated glycoprotein on sensory neurons is largely independent of NgR1 and NgR2 and resides within Ig-Like domains 4 and 5

Myelin-associated glycoprotein (MAG) is a sialic acid binding Ig-like lectin (Siglec) which has been characterized as potent myelin-derived inhibitor of neurite outgrowth. Two members of the Nogo-receptor (NgR) family, NgR1 and NgR2, have been identified as neuronal binding proteins of MAG. In addition, gangliosides have been proposed to bind to and confer the inhibitory activity of MAG on neurons. In this study, we investigated the individual contribution of NgRs and gangliosides to MAG-mediated inhibition of sensory neurons derived from dorsal root ganglia (DRG) of ngr1, ngr2 or ngr1/ngr2 deletion mutants. We found no disinhibition of neurite growth in the absence of either NgR1 or NgR2. Sensory neurons deficient for both NgR proteins displayed only a moderate reduction of MAG-mediated inhibition of neurite growth. If treated with Vibrio cholerae neuraminidase (VCN), inhibition by MAG is further attenuated but still not annulled. Thus, disrupting all known protein and ganglioside receptors for MAG in sensory neurons does not fully abolish its inhibitory activity pointing to the existence of as yet unidentified receptors for MAG. Moreover, by employing a variety of protein mutants, we identified the Ig-like domains 4 or 5 of MAG as necessary and sufficient for growth arrest, whereas abolishing MAG's ability to bind to sialic acid did not interfere with its inhibitory activity. These findings provide new insights into the inhibitory function of MAG and suggest similarities but also major differences in MAG inhibition between sensory and central nervous system (CNS) neurons.

Cell surface O-glycans limit Staphylococcus aureus adherence to corneal epithelial cells

The mucin-rich environment of the intact corneal epithelium is thought to contribute to the prevention of Staphylococcus aureus infection. This study examined whether O-glycans, which constitute the majority of the mucin mass of epithelial cell glycocalyces, prevented bacterial adhesion and growth. Abrogation of mucin O glycosylation using the chemical primer benzyl-alpha-GalNAc resulted in increased adherence of parental strain RN6390 to apical human corneal-limbal epithelial (HCLE) cells and to biotinylated cell surface protein in static and liquid phase adhesion assays, consistent with a role of mucin O-glycans in preventing bacterial adhesion. Comparable results were found with ALC135, an isogenic mutant strain defective in the accessory gene regulators agr and sar, indicating that the agr- and/or sar-regulated virulence factors did not play a major role in mediating adhesion to the corneal cell surface after mucin O-glycan truncation. In exoglycosidase digestion studies, treatment with sialidase from Arthrobacter ureafaciens--which hydrolyzed mucin-associated O-acetyl sialic acid--but not from Clostridium perfringens resulted in an increase in RN6390 and ALC135 adhesion. Abrogation of mucin O glycosylation in HCLE cell cultures did not affect bacterial growth. Overall, these data indicate that mucin O-glycans contribute to the prevention of bacterial adherence to the apical surface of corneal epithelial cells and suggest that alteration of cell surface glycosylation from disease or trauma, including that stemming from contact lens wear, could contribute to a higher risk of infection.

The inhibition site on myelin-associated glycoprotein is within Ig-domain 5 and is distinct from the sialic acid binding site

Myelin-associated glycoprotein (MAG) is a potent inhibitor of axonal regeneration. It contains five Ig-like domains and is a sialic binding protein. Previously, we showed that the sialic acid binding site on MAG maps to arginine 118 in Ig domain 1 (Kelm et al., 1994). However, sialic acid binding was neither necessary nor sufficient for MAG to bring about inhibition of neurite outgrowth. Consistent with this, we now map a distinct inhibition site on MAG to Ig domain 5 (Ig-5). We show that when a truncated form of MAG missing Ig domains 1 and 2 is expressed by Chinese hamster ovary (CHO) cells, it does not bind sialic acid, but still inhibits neurite outgrowth almost as effectively as full-length MAG. To determine whether the inhibition site mapped to Ig-3, Ig-4, or Ig-5, we made chimeric molecules of various combinations of these three MAG Ig domains fused to Ig domains from another Ig family member, sialoadhesin (Sn), which also binds to sialic acid in the same linkage as MAG. The MAG-Sn molecules were expressed in CHO cells and all contained five Ig domains and were able to bind sialic acid. However, only the chimeric molecules containing MAG Ig-5 inhibited neurite outgrowth. Furthermore, peptides corresponding to sequences in MAG Ig-5, but not Ig-4 or Sn Ig-5, are able to block inhibition of neurite outgrowth by both wild-type MAG and CNS myelin. We conclude that the inhibition site on MAG is carried by Ig domain 5 and that this site is distinct from the sialic-acid binding site.

Synthesis of sialic acid derivatives as ligands for the myelin-associated glycoprotein (MAG)

The trisaccharide substructure 13 of the ganglioside GQ1balpha shows a remarkable affinity for the myelin-associated glycoprotein (MAG). In the search for structurally simplified and pharmacokinetically improved mimics of 13, sialosides with modifications at the reducing and non-reducing end were synthesized. The biological evaluation of mimics 12a-o was performed in a competitive target-based assay. It was found that the relative inhibitory potency (rIP) of antagonist 12h was enhanced by more than 1000-fold in comparison to the reference trisaccharide 13, despite the former having a much simpler structure. In addition, the sialic acid derivatives, for example, 12h, have clearly improved pharmacokinetic properties due to the presence of aromatic moieties, a lower molecular weight, and a reduced number of polar hydroxy functions compared to the reference compound 13.

Glycan-based interactions involving vertebrate sialic-acid-recognizing proteins

All cells in nature are covered by a dense and complex array of carbohydrates. Given their prominence on cell surfaces, it is not surprising that these glycans mediate and/or modulate many cellular interactions. Proteins that bind sialic acid, a sugar that is found on the surface of the cell and on secreted proteins in vertebrates, are involved in a broad range of biological processes, including intercellular adhesion, signalling and microbial attachment. Studying the roles of such proteins in vertebrates has improved our understanding of normal physiology, disease and human evolution.

Glial inhibition of CNS axon regeneration

Damage to the adult CNS often leads to persistent deficits due to the inability of mature axons to regenerate after injury. Mounting evidence suggests that the glial environment of the adult CNS, which includes inhibitory molecules in CNS myelin as well as proteoglycans associated with astroglial scarring, might present a major hurdle for successful axon regeneration. Here, we evaluate the molecular basis of these inhibitory influences and their contributions to the limitation of long-distance axon repair and other types of structural plasticity. Greater insight into glial inhibition is crucial for developing therapies to promote functional recovery after neural injury.

Sialidase enhances spinal axon outgrowth in vivo

The adult CNS is an inhibitory environment for axon outgrowth, severely limiting recovery from traumatic injury. This limitation is due, in part, to endogenous axon regeneration inhibitors (ARIs) that accumulate at CNS injury sites. ARIs include myelin-associated glycoprotein, Nogo, oligodendrocyte-myelin glycoprotein, and chondroitin sulfate proteoglycans (CSPGs). Some ARIs bind to specific receptors on the axon growth cone to halt outgrowth. Reversing or blocking the actions of ARIs may promote recovery after CNS injury. We report that treatment with sialidase, an enzyme that cleaves one class of axonal receptors for myelin-associated glycoprotein, enhances spinal axon outgrowth into implanted peripheral nerve grafts in a rat model of brachial plexus avulsion, a traumatic injury in which nerve roots are torn from the spinal cord. Repair using peripheral nerve grafts is a promising restorative surgical treatment in humans, although functional improvement remains limited. To model brachial plexus avulsion in the rat, C8 nerve roots were cut flush to the spinal cord and a peroneal nerve graft was inserted into the lateral spinal cord at the lesion site. Infusion of Clostridium perfringens sialidase to the injury site markedly increased the number of spinal axons that grew into the graft (2.6-fold). Chondroitinase ABC, an enzyme that cleaves a different ARI (CSPGs), also enhanced axon outgrowth in this model. In contrast, phosphatidylinositol-specific phospholipase C, which cleaves oligodendrocyte-myelin glycoprotein and Nogo receptors, was without benefit. Molecular therapies targeting sialoglycoconjugates and CSPGs may aid functional recovery after brachial plexus avulsion or other nervous system injuries and diseases.

Zwitterionic capsular polysaccharides: the new MHCII-dependent antigens

The immune system has evolved the ability for T cells to recognize nearly any biological polymer, including peptides, protein superantigens, and glycolipids through presentation by the major histocompatibility complex (MHC) proteins such as MHC class I (MHCI), MHC class II (MHCII), and CD1. A recent and unexpected addition to this list is the zwitterionic capsular polysaccharide (ZPS). These bacterial molecules utilize MHCII presentation to activate T cells via recognition by alphabeta T cell receptor (alphabetaTCR) proteins. In this review, we explore what is currently known about ZPS processing and presentation within antigen-presenting cells (APCs) and the immune response that follows.

Siglecs--the major subfamily of I-type lectins

Animal glycan-recognizing proteins can be broadly classified into two groups-lectins (which typically contain an evolutionarily conserved carbohydrate-recognition domain [CRD]) and sulfated glycosaminoglycan (SGAG)-binding proteins (which appear to have evolved by convergent evolution). Proteins other than antibodies and T-cell receptors that mediate glycan recognition via immunoglobulin (Ig)-like domains are called "I-type lectins." The major homologous subfamily of I-type lectins with sialic acid (Sia)-binding properties and characteristic amino-terminal structural features are called the "Siglecs" (Sia-recognizing Ig-superfamily lectins). The Siglecs can be divided into two groups: an evolutionarily conserved subgroup (Siglecs-1, -2, and -4) and a CD33/Siglec-3-related subgroup (Siglecs-3 and -5-13 in primates), which appear to be rapidly evolving. This article provides an overview of historical and current information about the Siglecs.