Myelin-associated glycoprotein modulates apoptosis of motoneurons during early postnatal development via NgR/p75(NTR) receptor-mediated activation of RhoA signaling pathways

Myelin-associated glycoprotein (MAG) is a minor constituent of nervous system myelin, selectively expressed on the periaxonal myelin wrap. By engaging multiple axonal receptors, including Nogo-receptors (NgRs), MAG exerts a nurturing and protective effect the axons it ensheaths. Pharmacological activation of NgRs has a modulatory role on p75^NTR-dependent postnatal apoptosis of motoneurons (MNs). However, it is not clear whether this reflects a physiological role of NgRs in MN development. NgRs are part of a multimeric receptor complex, which includes p75^NTR, Lingo-1 and gangliosides. Upon ligand binding, this multimeric complex activates RhoA/ROCK signaling in a p75^NTR-dependent manner. The aim of this study was to analyze a possible modulatory role of MAG on MN apoptosis during postnatal development. A time course study showed that Mag-null mice suffer a loss of MNs during the first postnatal week. Also, these mice exhibited increased susceptibility in an animal model of p75^NTR-dependent MN apoptosis induced by nerve-crush injury, which was prevented by treatment with a soluble form of MAG (MAG-Fc). The protective role of MAG was confirmed in in vitro models of p75^NTR-dependent MN apoptosis using the MN1 cell line and primary cultures. Lentiviral expression of shRNA sequences targeting NgRs on these cells abolished protection by MAG-Fc. Analysis of RhoA activity using a FRET-based RhoA biosensor showed that MAG-Fc activates RhoA. Pharmacological inhibition of p75^NTR/RhoA/ROCK pathway, or overexpression of a p75^NTR mutant unable to activate RhoA, completely blocked MAG-Fc protection against apoptosis. The role of RhoA/ROCK signaling was further confirmed in the nerve-crush model, where pretreatment with ROCK inhibitor Y-27632 blocked the pro-survival effect of MAG-Fc. These findings identify a new protective role of MAG as a modulator of apoptosis of MNs during postnatal development by a mechanism involving the p75^NTR/RhoA/ROCK signaling pathway. Also, our results highlight the relevance of the nurture/protective effects of myelin on neurons.

Anti-ganglioside antibody-mediated neuronal cytotoxicity and its protection by intravenous immunoglobulin: implications for immune neuropathies

Antibodies against GD1a, GM1 and related gangliosides are frequently present in patients with the motor variant of Guillain-Barré syndrome (GBS), and their pathological role in this variant of GBS is now widely accepted. However, two basic issues related to anti-ganglioside antibody-mediated neural injury are not completely resolved: (i) some anti-ganglioside antibodies can cross-react with glycoproteins and therefore the nature of antigens targeted by these antibodies is not well established; and (ii) although pathological studies suggest that complement activation occurs in GBS, experimental data for the role of complement remain inconclusive. To address these issues, we developed and characterized a simple anti-ganglioside antibody-mediated cytotoxicity assay. Our results demonstrate first, that both GBS sera containing anti-ganglioside antibodies and monoclonal anti-ganglioside antibodies cause neuronal cell lysis by targeting specific cell surface gangliosides, and secondly, that this cell lysis is complement dependent. In this assay, the GD1a cell membrane pool appears to be more susceptible to anti-ganglioside antibody-mediated injury than the GM1 pool. Further, human intravenous immunoglobulin (i.v.Ig), now a standard treatment for GBS, significantly decreased cytotoxicity in this assay. Our data indicate that the mechanisms of i.v.Ig-mediated protection in this assay include anti-idiotypic antibodies and downregulation of complement activation. This simple cytotoxicity assay can potentially be used for screening of (i) pathogenic anti-ganglioside antibodies in patients with immune-mediated neuropathies; and (ii) new/experimental therapies to prevent anti-ganglioside antibody-mediated neural injury.

Localization of major gangliosides in the PNS: implications for immune neuropathies

Antibodies targeting major gangliosides that are broadly distributed in the nervous system are sometimes associated with clinical symptoms that imply selective nerve damage. For example, anti-GD1a antibodies are associated with acute motor axonal neuropathy (AMAN), a form of Guillain-Barré syndrome that selectively affects motor nerves, despite reports that GD1a is present in human axons and myelin and is not expressed differentially in motor versus sensory roots. We used a series of high-affinity monoclonal antibodies (mAbs) against the major nervous system gangliosides GM1, GD1a, GD1b and GT1b to test whether any of them bind motor or sensory fibres differentially in rodent and human peripheral nerves. The following observations were made. (i) Some of the anti-GD1a antibodies preferentially stained motor fibres, supporting the association of human anti-GD1a antibodies with predominant motor neuropathies such as AMAN. (ii) A GD1b antibody preferentially stained the large dorsal root ganglion (DRG) neurones, in keeping with the proposed role of human anti-GD1b antibodies in sensory ataxic neuropathies. (iii) Two mAbs with broad structural cross-reactivity bound to both gangliosides and peripheral nerve proteins. (iv) Myelin was poorly stained; all clones stained axons nearly exclusively. Our findings suggest that anti-ganglioside antibody fine specificity as well as differences in ganglioside accessibility in axons and myelin influence the selectivity of injury to different fibre systems and cell types in human autoimmune neuropathies.

Glycolipids support E-selectin-specific strong cell tethering under flow

This study provides functional evidence that glycosphingolipids constitute ligands for E-selectin but not P-selectin. Chinese hamster ovary (CHO) cells expressing E-selectin (CHO-E) or P-selectin (CHO-P) were perfused over alpha2,3-sialyl Lewis X (alpha2,3-sLe(x)) presented as the hexaosylceramide glycosphingolipid adsorbed in a monolayer containing phosphatidylcholine and cholesterol. CHO-E cells tethered extensively and formed slow, stable rolling interactions with alpha2,3-sLe(x) glycosphingolipid but not with the comparable alpha2,6-sLe(x) glycosphingolipid. Tethering/rolling varied with wall shear stress, selectin density, and ligand density. In contrast, alpha2,3-sLe(x) glycosphingolipid supported only limited, fast CHO-P cell rolling. As calculated from a stochastic model of cell rolling, the step size between successive bond releases from the alpha2,3-sLe(x) glycosphingolipid was smaller for CHO-E than CHO-P cells, whereas the opposite effect was observed for the waiting time between these events. Detachment assays revealed stronger adhesive interactions of CHO-E than CHO-P cells with alpha2,3-sLe(x) glycosphingolipid. These findings indicate that glycosphingolipids expressing an appropriate oligosaccharide mediate cell tethering/rolling via E-selectin but not P-selectin.

Brain gangliosides: functional ligands for myelin stability and the control of nerve regeneration

Gangliosides, sialylated glycosphingolipids which are the predominant glycans on vertebrate nerve cell surfaces, are emerging as components of membrane rafts, where they can mediate important physiological functions. Myelin associated glycoprotein (MAG), a minor constituent of myelin, is a sialic acid binding lectin with two established physiological functions: it is involved in myelin-axon stability and cytoarchitecture, and controls nerve regeneration. MAG is found selectively on the myelin membranes directly apposed to the axon surface, where it has been proposed to mediate myelin-axon interactions. Although the nerve cell surface ligands for MAG remain to be established, evidence supports a functional role for sialylated glycoconjugates. Here we review recent studies that reflect on the role of gangliosides, sialylated glycosphingolipids, as functional MAG ligands. MAG binds to gangliosides with the terminal sequence 'NeuAc alpha 3Gal beta 3GalNAc' which is found on the major nerve gangliosides GD1a and GT1b. Gangliosides lacking that terminus (e.g., GM1 or GD1b), or having any biochemical modification of the terminal NeuAc residue fail to support MAG binding. Genetically engineered mice lacking the GalNAc transferase required for biosynthesis of the 'NeuAc alpha 3Gal beta 3GalNAc' terminus have grossly impaired myelination and progressive neurodegeneration. Notably the MAG level in these animals is dysregulated. Furthermore, removal of NeuAc residues from nerve cells reverses MAG-mediated inhibition of neuritogenesis, and neurons from mice lacking the 'NeuAc alpha 3 Gal beta 3GalNAc' terminus have an attenuated response to MAG. Cross-linking nerve cell surface gangliosides can mimic MAG-mediated inhibition of nerve regeneration. Taken together these observations implicate gangliosides as functional MAG ligands.

Fucosyltransferases in Schistosoma mansoni development

Glycoconjugate-bound fucose, abundant in the parasite Schistosoma mansoni, has been found in the form of Fucalpha1,3GlcNAc, Fucalpha1,2Fuc, Fucalpha1,6GlcNAc, and perhaps Fucalpha1,4GlcNAc linkages. Here we quantify fucosyltransferase activities in three developmental stages of S. mansoni. Assays were performed using fluorophore-assisted carbohydrate electrophoresis with detection of radioactive fucose incorporation from GDP-[(14)C]-fucose into structurally defined acceptors. The total fucosyltransferase-specific activity in egg extracts was 50-fold higher than that in the other life stages tested (cercaria and adult worms). A fucosyltransferase was detected that transferred fucose to type-2 oligosaccharides (Galbeta1,4GlcNAc-R), both sialylated (with the sialic acid attached to the terminal Gal by alpha2,3 or 2,6 linkage) and nonsialylated. Another fucosyltransferase was identified that transferred fucose to lactose-based and type-2 fucosylated oligosaccharides, such as LNFIII (Galbeta1,4(Fucalpha1,3)GlcNAcbeta1,3Galbeta1,4Glc). A low level of fucosyltransferase that transfers fucose to no-sialylated type-1 oligosaccharides (Galbeta1,3GlcNAc-R) was also detected. These studies revealed multifucosylated products of the reactions. In addition, the effects of fucose-type iminosugars inhibitors were tested on schistosome fucosyltransferases. A new fucose-type 1-N-iminosugar was four- to sixfold more potent as an inhibitor of schistosome fucosyltransferases in vitro than was deoxyfuconojirimycin. In vivo, this novel 1-iminosugar blocked the expression of a fucosylated epitope (mAb 128C3/3 antigen) that is associated with the pathogenesis of schistosomiasis.

Segregation of gangliosides GM1 and GD3 on cell membranes, isolated membrane rafts, and defined supported lipid monolayers

Lateral assemblies of sphingolipids, glycosphingolipids and cholesterol, termed rafts, are postulated to be present in biological membranes and to function in important cellular phenomena. We probed whether rafts are heterogeneous by determining the relative distribution of two gangliosides, GM1 and GD3, in artificial supported monolayers, in intact rat primary cerebellar granule neurones, and in membrane rafts isolated from rat cerebellum. Fluorescence resonance energy transfer (FRET) using fluorophore-labelled cholera toxin B subunit (which binds GM1) and mAb R24 (which binds GD3) revealed that GM1 spontaneously self-associates but does not co-cluster with GD3 in supported monolayers and on intact neurones. Cholera toxin and immunocytochemical labelling of isolated membrane rafts from rat cerebellum further demonstrated that GM1 does not co-localise with GD3. Furthermore, whereas the membrane raft resident proteins Lyn and caveolin both co-localise with GD3 in isolated membrane rafts, GM1 appears in separate and distinct aggregates. These data support prior reports that membrane rafts are heterogeneous, although the mechanisms for establishing and maintaining such heterogeneity remain to be determined.

A functional role for complex gangliosides: motor deficits in GM2/GD2 synthase knockout mice

Although gangliosides are abundant molecular determinants on all vertebrate nerve cells (comprising approximately 1.5% of brain dry weight) their functions have remained obscure. We report that mice engineered to lack a key enzyme in complex ganglioside biosynthesis (GM2/GD2 synthase), and which express only the simple ganglioside molecular species GM3 and GD3, develop significant and progressive behavioral neuropathies, including deficits in reflexes, strength, coordination, and balance. Quantitative indices of motor abilities, applied at 8 and 12 months of age, also revealed progressive gait disorders in complex ganglioside knockout mice compared to controls, including reduced stride length, stride width, and increased hindpaw print length as well as a marked reduction in rearing. Compared to controls, null mutant mice tended to walk in small labored movements. Twelve-month-old complex ganglioside knockout mice also displayed significant incidence of tremor and catalepsy. These comprehensive neurobehavioral studies establish an essential role for complex gangliosides in the maintenance of normal neural physiology in mice, consistent with a role in maintaining axons and myelin (Sheikh, K. A. , J. Sun, Y. Liu, H. Kawai, T. O. Crawford, R. L. Proia, J. W. Griffin, and R. L. Schnaar. 1999. Mice lacking complex gangliosides develop Wallerian degeneration and myelination defects. Proc. Natl. Acad. Sci. USA 96: 7532-7537), and may provide insights into the mechanisms underlying certain neural degenerative diseases.

High-affinity anti-ganglioside IgG antibodies raised in complex ganglioside knockout mice: reexamination of GD1a immunolocalization

Gangliosides, sialic acid-bearing glycosphingolipids, are highly enriched in the vertebrate nervous system. Anti-ganglioside antibodies are associated with various human neuropathies, although the pathogenicity of these antibodies remains unproven. Testing the pathogenic role of anti-ganglioside antibodies will be facilitated by developing high-affinity IgG-class complement-fixing monoclonal anti-bodies against major brain gangliosides, a goal that has been difficult to achieve. In this study, mice lacking complex gangliosides were used as immune-naive hosts to raise anti-ganglioside antibodies. Wild-type mice and knockout mice with a disrupted gene for GM2/GD2 synthase (UDP-N-acetyl-D-galactosamine : GM3/GD3 N-acetyl-D-glactosaminyltransferase) were immunized with GD1a conjugated to keyhole limpet hemocyanin. The knockout mice produced a vigorous anti-GD1a IgG response, whereas wildtype littermates failed to do so. Fusion of spleen cells from an immunized knockout mouse with myeloma cells yielded numerous IgG anti-GD1a antibody-producing colonies. Ganglioside binding studies revealed two specificity classes; one colony representing each class was cloned and characterized. High-affinity monoclonal antibody was produced by each hybridoma : an IgG1 that bound nearly exclusively to GD1a and an IgG2b that bound GD1a, GT1b, and GT1aalpha. Both antibodies readily readily detected gangliosides via ELISA, TLC immune overlay, immunohistochemistry, and immunocytochemistry. In contrast to prior reports using anti-GD1a and anti-GT1b IgM class monoclonal antibodies, the new antibodies bound avidly to granule neurons in brain tissue sections and cell cultures. Mice lacking complex gangliosides are improved hosts for raising high-affinity, high-titer anti-ganglioside IgG antibodies for probing for the distribution and physiology of gangliosides and the pathophysiology of anti-ganglioside antibodies.

Conversion of cellular sialic acid expression from N-acetyl- to N-glycolylneuraminic acid using a synthetic precursor, N-glycolylmannosamine pentaacetate: inhibition of myelin-associated glycoprotein binding to neural cells

Sialic acids are prominent termini of mammalian glycoconjugates and are key binding determinants for cell-cell recog-nition lectins. Binding of the sialic acid-dependent lectin, myelin-associated glycoprotein (MAG), to nerve cells is implicated in the inhibition of nerve regeneration after injury. Therefore, blocking MAG binding to nerve cell sialoglycoconjugates might enhance nerve regeneration. Previously, we reported that certain sialoglycoconjugates bearing N-acetylneuraminic acid (NeuAc) but not N-glycolylneuraminic acid (NeuGc) support MAG binding (Collins et al., 1997a). We now report highly efficient conversion of sialic acids on living neural cells from exclusively NeuAc to predominantly NeuGc using a novel synthetic metabolic precursor, N-glycolylmannosamine pentaacetate (Man-NGc-PA). When NG108-15 neuroblastoma-glioma hybrid cells, which normally express only NeuAc (and bind to MAG), were cultured in the presence of 1 mM ManNGcPA, they expressed 80-90% of their sialic acid precursor pool as NeuGc within 24 h. Within 5 days, 80% of their ganglioside-associated sialic acids and 70% of their glycoprotein-associated sialic acids were converted to NeuGc. Consistent with this result, treatment of NG108-15 cells with ManNGcPA resulted in nearly complete abrogation of MAG binding. These results demonstrate that ManNGcPA treatment efficiently alters the sialic acid structures on living cells, with a commensurate change in recognition by a physiologically important lectin.

Enhanced binding of the neural siglecs, myelin-associated glycoprotein and Schwann cell myelin protein, to Chol-1 (alpha-series) gangliosides and novel sulfated Chol-1 analogs

Extended glycoconjugate binding specificities of three sialic acid-dependent immunoglobulin-like family member lectins (siglecs), myelin-associated glycoprotein (MAG), Schwann cell myelin protein (SMP), and sialoadhesin, were compared by measuring siglec-mediated cell adhesion to immobilized gangliosides. Synthetic gangliosides bearing the alpha-series determinant (NeuAc alpha2,6-linked to GalNAc on a gangliotetraose core) were tested, including GD1alpha (IV(3)NeuAc, III(6)NeuAc-Gg(4)OseCer), GD1alpha with modified sialic acid residues at the III(6)-position, and the "Chol-1" gangliosides GT1aalpha (IV(3)NeuAc, III(6)NeuAc, II(3)NeuAc-Gg(4)OseCer) and GQ1balpha (IV(3)NeuAc, III(6)NeuAc, II(3)(NeuAc)(2)-Gg(4)OseCer). The alpha-series gangliosides displayed enhanced potency for MAG- and SMP-mediated cell adhesion (GQ1balpha > GT1aalpha, GD1alpha > GT1b, GD1a >> GM1 (nonbinding)), whereas sialoadhesin-mediated adhesion was comparable with alpha-series and non-alpha-series gangliosides. GD1alpha derivatives with modified sialic acids (7-, 8-, or 9-deoxy) or sulfate (instead of sialic acid) at the III(6)-position supported adhesion comparable with that of GD1alpha. Notably, a novel GT1aalpha analog with sulfates at two internal sites of sialylation (NeuAcalpha2,3Galbeta1,4GalNAc-6-sulfatebeta1, 4Gal3-sulfatebeta1,4Glcbeta1,1'ceramide) was the most potent siglec-binding structure tested to date (10-fold more potent than GT1aalpha in supporting MAG and SMP binding). Together with prior studies, these data indicate that MAG and SMP display an extended structural specificity with a requirement for a terminal alpha2, 3-linked NeuAc and great enhancement by nearby precisely spaced anionic charges.

Mice lacking complex gangliosides develop Wallerian degeneration and myelination defects

Gangliosides are a family of sialic acid-containing glycosphingolipids highly enriched in the mammalian nervous system. Although they are the major sialoglycoconjugates in the brain, their neurobiological functions remain poorly defined. By disrupting the gene for a key enzyme in complex ganglioside biosynthesis (GM2/GD2 synthase; EC 2.4.1.92) we generated mice that express only simple gangliosides (GM3/GD3) and examined their central and peripheral nervous systems. The complex ganglioside knockout mice display decreased central myelination, axonal degeneration in both the central and peripheral nervous systems, and demyelination in peripheral nerves. The pathological features of their nervous system closely resemble those reported in mice with a disrupted gene for myelin-associated glycoprotein (MAG), a myelin receptor that binds to complex brain gangliosides in vitro. Furthermore, GM2/GD2 synthase knockout mice have reduced MAG expression in the central nervous system. These results indicate that complex gangliosides function in central myelination and maintaining the integrity of axons and myelin. They also support the theory that complex gangliosides are endogenous ligands for MAG. The data extend and clarify prior observations on a similar mouse model, which reported only subtle conduction defects in their nervous system [Takamiya, K., Yamamoto, A., Furukawa, K., Yamashiro, S., Shin, M., Okada, M., Fukumoto, S., Haraguchi, M., Takeda, N., Fujimura, K., et al. (1996) Proc. Natl. Acad. Sci. USA 93, 10662-10667].

The proteoglycan lectin domain binds sulfated cell surface glycolipids and promotes cell adhesion

The lecticans are a group of chondroitin sulfate proteoglycans characterized by the presence of C-type lectin domains. Despite the suggestion that their lectin domains interact with carbohydrate ligands, the identity of such ligands has not been elucidated. We previously showed that brevican, a nervous system-specific lectican, binds the surface of B28 glial cells (Yamada, H., Fredette, B., Shitara, K., Hagihara, K., Miura, R., Ranscht, B., Stallcup, W. B., and Yamaguchi, Y. (1997) J. Neurosci. 17, 7784-7795). In this paper, we demonstrate that two classes of sulfated glycolipids, sulfatides and HNK-1-reactive sulfoglucuronylglycolipids (SGGLs), act as cell surface receptors for brevican. The lectin domain of brevican binds sulfatides and SGGLs in a calcium-dependent manner as expected of a C-type lectin domain. Intact, full-length brevican also binds both sulfatides and SGGLs. The lectin domain immobilized as a substrate supports adhesion of cells expressing SGGLs or sulfatides, which was inhibited by monoclonal antibodies against these glycolipids or by treatment of the substrate with SGGLs or sulfatides. Our findings demonstrate that the interaction between the lectin domains of lecticans and sulfated glycolipids comprises a novel cell substrate recognition system, and suggest that lecticans in extracellular matrices serve as substrate for adhesion and migration of cells expressing these glycolipids in vivo.

Ganglioside GD1 alpha analogues as high-affinity ligands for myelin-associated glycoprotein (MAG)

The first systematic synthesis of ganglioside GD1 alpha analogues carrying N-acetyldeoxyneuraminic acids linked to C-6 of the GalNAc residue was accomplished. The suitably protected GM1b pentasaccharide derivative was regioselectively glycosylated with the phenyl 2-thioglycosides of 7-deoxy, 8-deoxy, and 9-deoxy-N-acetylneuraminic acid promoted by N-iodosuccinimide (NIS)-trifluoromethanesulfonic acid (TfOH) in acetonitrile, and the resulting hexasaccharides were converted to the target GD1 alpha analogues. All of the analogues retained excellent efficiency in supporting the adhesion to myelin-associated glycoprotein (MAG), raising the possibility that the internal sialic acid linked to the GalNAc residue may be replaced by other anionic substituents, in contrast to the terminal sialic acid, which is essential for MAG binding.

Substructural specificity and polyvalent carbohydrate recognition by the Entamoeba histolytica and rat hepatic N-acetylgalactosamine/galactose lectins

Both the Entamoeba histolytica lectin, a virulence factor for the causative agent of amebiasis, and the mammalian hepatic lectin bind to N-acetylgalactosamine (GalNAc) and galactose (Gal) nonreducing termini on oligosaccharides, with preference for GalNAc. Polyvalent GalNAc-derivatized neoglycoproteins have >1000-fold enhanced binding affinity for both lectins (Adler,P., Wood,S.J., Lee,Y.C., Lee,R.T., Petri,W.A.,Jr. and Schnaar,R.L.,1995, J. Biol. Chem ., 270, 5164-5171). Substructural specificity studies revealed that the 3-OH and 4-OH groups of GalNAc were required for binding to both lectins, whereas only the E.histolytica lectin required the 6-OH group. Whereas GalNAc binds with 4-fold lower affinity to the E.histolytica lectin than to the mammalian hepatic lectin, galactosamine and N-benzoyl galactosamine bind with higher affinity to the E. histolytica lectin. Therefore, a synthetic scheme for converting polyamine carriers to poly-N-acyl galactosamine derivatives (linked through the galactosamine primary amino group) was developed to test whether such ligands would bind the E.histolytica lectin with high specificity and high affinity. Contrary to expectations, polyvalent derivatives including GalN6lys5, GalN4desmosine, GalN4StarburstTMdendrimer, and GalN8StarburstTMdendrimer demonstrated highly enhanced binding to the mammalian hepatic lectin but little or no enhancement of binding to the E.histolytica lectin. We propose that the mammalian hepatic lectin binds with greatest affinity to GalNAc "miniclusters," which mimic branched termini of N-linked oligosaccharides, whereas the E.histolytica lectin binds most effectively to "maxiclusters," which may mimic more widely spaced GalNAc residues on intestinal mucins.

Myelin-associated glycoprotein binding to gangliosides. Structural specificity and functional implications

Myelin-associated glycoprotein (MAG), which mediates certain myelin-neuron cell-cell interactions, is a lectin that binds to sialylated glycoconjugates. Gangliosides, the most abundant sialylated glycoconjugates in the brain, may be the functional neuronal ligands for MAG. Cells engineered to express MAG on their surface adhered specifically to gangliosides bearing an alpha 2,3-linked N-acetylneuraminic acid on a terminal galactose, with the following relative potency: GQ1b alpha >> GD1a, GT1b >> GM3, GM4 (GM1, GD1b, GD3, and GQ1b did not support adhesion). MAG binding was abrogated by modification of the carboxylic acid, any hydroxyl, or the N-acetyl group of the ganglioside's N-acetylneuraminic acid moiety. Related immunoglobulin (Ig) superfamily members either failed to bind gangliosides (CD22) or bound with less stringent specificity (sialoadhesin), whereas a modified form of MAG (bearing three of its five extra-cellular Ig-like domains) bound only GQ1b alpha. Enzymatic removal of sialic acids from the surface of intact nerve cells altered their functional interaction with myelin. These data are consistent with a role for gangliosides in MAG-neuron interactions.

Molecularly cloned mammalian glucosamine-6-phosphate deaminase localizes to transporting epithelium and lacks oscillin activity

Glucosamine-6-phosphate deaminase (GNPDA) catalyzes the conversion of glucosamine-6-phosphate to fructose-6-phosphate, a reaction that under physiological conditions proceeds to the formation of fructose-6-phosphate. Though first identified in mammalian tissues in 1956, the enzyme has not previously been molecularly characterized in mammalian tissues, although a bacterial GNPDA has been cloned. Recently, a protein displaying similarity to bacterial GNPDA was purified and cloned from sperm extract. It was proposed that this protein was the factor, found in sperm extracts, that causes calcium oscillations in cells; thus, the protein was named 'oscillin.' We demonstrate that oscillin is the mammalian form of glucosamine 6-phosphate deaminase by showing that cloned oscillin has a robust GNPDA activity and can account for all such activity in mammalian tissues extracts. In situ hybridization and immunohistochemistry localize GNPDA selectively to tissues with high energy requirements such as the apical zone of transporting epithelia in the proximal convoluted tubules of the kidney and the small intestine; to neurons (but not glia) and especially to nerve terminals in the brain; and to motile sperm. Recombinant GNPDA and GNPDA purified to homogeneity from hamster sperm fail to elevate intracellular calcium when injected into mouse eggs over a wide range of concentrations under conditions in which sperm extracts elicit pronounced calcium oscillations. Thus, the calcium-releasing or oscillin activity of sperm extracts is due to a substance other than GNPDA. Since GNPDA is the sole enzyme linking hexosamine systems with glycolytic pathways, we propose that it provides a source of energy in the form of phosphosugar derived from the catabolism of hexosamines found in glycoproteins, glycolipids, and sialic acid-containing macromolecules. Evidence that GNPDA can regulate hexosamine stores comes from our observation that transfection of GNPDA into HEK-293 cells reduces cellular levels of sialic acid.

The chicken genome contains two functional nonallelic beta1,4-galactosyltransferase genes. Chromosomal assignment to syntenic regions tracks fate of the two gene lineages in the human genome

Two distinct but related groups of cDNA clones, CKbeta4GT-I and CKbeta4GT-II, have been isolated by screening a chicken hepatoma cDNA library with a bovine beta1,4-galactosyltransferase (beta4GT) cDNA clone. CKbeta4GT-I is predicted to encode a type II transmembrane glycoprotein of 41 kDa with one consensus site for N-linked glycosylation. CKbeta4GT-II is predicted to encode a type II transmembrane glycoprotein of 43 kDa with five potential N-linked glycosylation sites. At the amino acid level, the coding regions of CKbeta4GT-I and CKbeta4GT-II are 52% identical to each other and 62 and 49% identical, respectively, to bovine beta4GT. Despite this divergence in amino acid sequence, high levels of expression of each cDNA in Trichoplusia ni insect cells demonstrate that both CKbeta4GT-I and CKbeta4GT-II encode an alpha-lactalbumin-responsive, UDP-galactose:N-acetylglucosamine beta4-galactosyltransferase. An analysis of CKbeta4GT-I and CKbeta4GT-II genomic clones established that the intron positions within the coding region are conserved when compared with each other, and these positions are identical to the mouse and human beta4GT genes. Thus CKbeta4GT-I and CKbeta4GT-II are the result of the duplication of an ancestral gene and subsequent divergence. CKbeta4GT-I maps to chicken chromosome Z in a region of conserved synteny with the centromeric region of mouse chromosome 4 and human chromosome 9p, where beta4-galactosyltransferase (EC 2.4.1.38) had previously been mapped. Consequently, during the evolution of mammals, it is the CKbeta4GT-I gene lineage that has been recruited for the biosynthesis of lactose. CKbeta4GT-II maps to a region of chicken chromosome 8 that exhibits conserved synteny with human chromosome 1p. An inspection of the current human gene map of expressed sequence tags reveals that there is a gene noted to be highly similar to beta4GT located in this syntenic region on human chromosome 1p. Because both the CKbeta4GT-I and CKbeta4GT-II gene lineages are detectable in mammals, duplication of the ancestral beta4-galactosyltransferase gene occurred over 250 million years ago in an ancestral species common to both mammals and birds.

Identification of a ganglioside recognition domain of tetanus toxin using a novel ganglioside photoaffinity ligand

Tetanus toxin entry into vertebrate motorneurons may involve binding of neuronal surface gangliosides containing the "1b" substructure (a NeuAcalpha2,8NeuAc group on an internal galactose residue). The domains of tetanus toxin involved in ganglioside binding are known to reside within the carboxyl-terminal half of the toxin's heavy chain ("C fragment"). We developed a novel photoaffinity reagent based upon the structure of the 1b ganglioside GD1b (125I-azido-GD1b) to define the ganglioside-binding domains of tetanus toxin. Using this ligand, we observed radiolabeling of tetanus toxin C fragment which could be specifically inhibited by a ganglioside of the 1b series (GT1b), but not by a non-1b series ganglioside (GM3). When tetanus toxin C fragment was proteolyzed with clostripain, whether before or after reaction with 125I-azido-GD1b, a radiolabeled band was observed by SDS-polyacrylamide gel electrophoresis autoradiography, which was selectively inhibited by GT1b. Protein sequencing of proteolyzed tetanus toxin C fragment co-migrating with that band revealed the carboxyl-terminal 34 amino acid residues of tetanus toxin. Matrix-assisted laser desorption/ionization mass spectrometry of a photoaffinity labeled synthetic polypeptide representing the 34-amino acid domain revealed modification at a single residue (His1293). We propose that this domain of tetanus toxin is sufficient for ganglioside binding.

Distribution of cells bearing the HNK-1 epitope in the human placenta

HNK-1 (Leu-7 antigen or CD57) is a unique carbohydrate moiety found in certain glycosphingolipids and several cell adhesion glycoproteins on the cell membrane. Previous studies have suggested that HNK-1 carbohydrates act as adhesive ligands in cell-cell interactions. Using a monoclonal antibody reactive to the HNK-1 moiety and an immunoperoxidase method on formalin-fixed paraffin-embedded tissue, the expression of the HNK-1 epitope in human placentae was confined to the intermediate trophoblast (IT) in trophoblastic columns. The number of HNK-1 immunoreactive IT cells increased from the proximal to the midportion of the trophoblastic column, and then disappeared at the junction of the column with the basal plate where IT infiltrates the endomyometrium and becomes extravillous IT. Neither cytotrophoblast nor syncytiotrophoblast reacted with the HNK-1 antibody. In hydatidiform moles, HNK-1 immunoreactivity was localized to areas that structurally resembled trophoblastic columns. In contrast, placental site trophoblastic tumours which do not contain structures analogous to trophoblastic columns did not express HNK-1 epitope. Expression of HNK-1 was only rarely observed in choriocarcinomas, being present in less than 5 per cent of trophoblastic cells in two of 13 cases. The murine placenta, which lacks trophoblastic columns, was negative for HNK-1. Thin-layer chromatography immunostaining demonstrated the HNK-1 reactive glycosphingolipids in placental lipid extracts, whereas Western blot analysis from placental protein extract failed to reveal detectable glycoproteins that demonstrated HNK-1 immunoreactivity. In conclusion, the specific localization of HNK-1 reactive glycosphingolipids in trophoblastic columns of the human placenta suggests that the HNK-1 moiety may play an important role in maintaining cohesion between intermediate trophoblastic cells in the trophoblastic columns thereby contributing to the structural integrity of the villi that anchor the placenta to the basal plate.

Multivalent ganglioside and sphingosine conjugates modulate myelin protein kinases

Gangliosides, added exogenously at concentrations of 10-100 microM, inhibit intrinsic protein kinase activities in purified rat brain myelin. Multivalent neoganglioproteins--gangliosides covalently attached, via their lipid moieties, to bovine serum albumin--were much more potent, inhibiting myelin protein phosphorylation half-maximally at a concentration of 100 nM. Different ganglioside conjugates varied 10-fold in inhibitory potency; GT1b-conjugates being the most potent and GM3-conjugates being the least. Conjugates of ganglioside oligosaccharides, lacking the lipid moiety, did not inhibit myelin protein phosphorylation, whereas conjugates of sphingosine inhibited nearly as potently as GT1b conjugates. Conjugate-mediated inhibition of myelin protein phosphorylation was due to inhibition of a protein serine kinase activity rather than activation of a phosphatase activity. We conclude that (i) clustered gangliosides or sphingosine are potent myelin protein kinase inhibitors, and (ii) sphingolipid metabolism is not required for myelin protein kinase inhibition. In contrast to their effects on myelin protein phosphorylation, ganglioside conjugates stimulated phosphorylation of a presumptive axon membrane protein. The data support the conclusion that gangliosides and other sphingolipids, when appropriately clustered, are potent modulators of central nervous system protein phosphorylation.

Binding specificities of the sialoadhesin family of I-type lectins. Sialic acid linkage and substructure requirements for binding of myelin-associated glycoprotein, Schwann cell myelin protein, and sialoadhesin

The carbohydrate binding specificities of three sialoadhesins, a subgroup of I-type lectins (immunoglobulin superfamily lectins), were compared by measuring lectin-transfected COS cell adhesion to natural and synthetic gangliosides. The neural sialoadhesins, myelin-associated glycoprotein (MAG) and Schwann cell myelin protein (SMP), had similar and stringent binding specificities. Each required an alpha2,3-linked sialic acid on the terminal galactose of a neutral saccharide core, and they shared the following rank-order potency of binding: GQ1balpha >> GD1a = GT1b >> GM3 = GM4 >> GM1, GD1b, GD3, GQ1b (nonbinders). In contrast, sialoadhesin had less exacting specificity, binding to gangliosides that bear either terminal alpha2,3- or alpha2,8-linked sialic acids with the following rank-order potency of binding: GQ1balpha > GD1a = GD1b = GT1b = GM3 = GM4 > GD3 = GQ1b >> GM1 (nonbinder). CD22 did not bind to any ganglioside tested. Binding of MAG, SMP, and sialoadhesin was abrogated by chemical modification of either the sialic acid carboxylic acid group or glycerol side chain on a target ganglioside. Synthetic ganglioside GM3 derivatives further distinguished lectin binding specificities. Deoxy and/or methoxy derivatives of the 4-, 7-, 8-, or 9-position of sialic acid attenuated or eliminated binding of MAG, as did replacement of the sialic acid acetamido group with a hydroxyl. In contrast, the 4- and 7-deoxysialic acid derivatives supported sialoadhesin binding at near control levels (the other derivatives did not support binding). These data are consistent with sialoadhesin binding to one face of the sialic acid moiety, whereas MAG (and SMP) may have more complex binding sites or may bind sialic acids only in the context of more restricted oligosaccharide conformations.

Sialic acid specificity of myelin-associated glycoprotein binding

Myelin-associated glycoprotein (MAG), a nervous system cell adhesion molecule, is an I-type lectin that binds to sialylated glycoconjugates, including gangliosides bearing characteristic structural determinants (Yang, L. J.-S., Zeller, C. B., Shaper, N. L., Kiso, M., Hasegawa, A., Shapiro, R. E., and Schnaar, R. L. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 814-818). Two cell adhesion systems, COS-1 monkey kidney fibroblasts transiently transfected to express MAG and Chinese hamster ovary (CHO) cells stably transfected to express MAG, were used to probe the structural specificity of MAG-ganglioside binding. Both cell types bound to the same gangliosides: GQ1b alpha (IV3NeuAc,III6NeuAc,II3(NeuAc)2Gg4Cer) > GT1b = GD1a > GM3 > GM1, GD1b, and GQ1b (the latter do not support adhesion). Binding was enhanced by pretreatment of MAG-expressing cells with neuraminidase. MAG-expressing Chinese hamster ovary cells bound directly to gangliosides resolved on thin layer chromatograms, allowing detection of MAG binding species in a mixture. The simplest ganglioside ligand for MAG was GM3 bearing N-acetylneuraminic acid, whereas GM3 bearing N-glycolylneuraminic acid did not support adhesion. Chemical modifications of N-acetylneuraminic acid residues (on GD1a) abrogated MAG binding. Mild periodate oxidation of sialic acids to their corresponding seven-carbon (or eight-carbon) sialic acid aldehydes abolished MAG binding, as did further conversion to the corresponding primary alcohols. Eliminating the anionic charge by ethyl esterification, amidation, or reduction also abolished MAG-mediated cell adhesion. These data demonstrate that MAG-ganglioside binding is highly specific and defines key carbohydrate structural determinants for MAG-mediated cell adhesion to gangliosides.

Detection and isolation of lectin-transfected COS cells based on cell adhesion to immobilized glycosphingolipids

Two methods are described, one for detection and one for isolation of COS cells transiently expressing vertebrate lectins. The methods are based on specific cell adhesion to polystyrene microwells or magnetic beads adsorbed with glycosphingolipids. In the first method, glycolipids were adsorbed to wells of 96-well polystyrene plates. A suspension of lectin-transfected COS cells was added and the plate was incubated to allow cell adhesion to occur. The plate was then immersed in buffer, inverted (while immersed), and placed in a fluid-filled Plexiglas centrifugation chamber which was sealed to avoid introducing an air-liquid interface. The chamber, with the inverted plate enclosed, was centrifuged to remove nonadherent cells. The plate was then removed from the carrier (while immersed) and righted, and adherent cells were quantitated enzymatically or immunochemically using a 96-well plate reader. COS cells transfected with an expression plasmid carrying the gene for the rat Kupffer cell lectin (fucose and N-acetylgalactosamine specific) adhered specifically to globotetraosylceramide. Glycolipid- and lectin-specific cell adhesion was readily detected even when COS cells were transfected with a plasmid mixture containing 0.5% lectin-carrying plasmid and 99.5% irrelevant plasmid. This sensitivity will facilitate screening of plasmid pools to detect and isolate plasmids expressing mammalian lectin genes. To isolate COS cells transiently expressing lectin, glycosphingolipids were adsorbed to carboxylated magnetic polystyrene microspheres, which were mixed with the lectin-transfected COS cells. Adherent cells were collected on a fixed magnet and plasmid recovered for subsequent amplification.

Distinctive ganglioside patterns revealed by anti-ganglioside antibody binding to differentiating CG-4 oligodendrocytes

Oligodendrocytes are central nervous system glial cells responsible for myelination of neuronal axons. During brain development oligodendrocyte progenitor cells progress through a series of morphologically and immunohistochemically distinct differentiation steps leading to mature myelin-producing oligodendrocytes. Much of this same differentiation sequence is expressed in vitro by primary oligodendrocyte progenitor cells, and by the clonal progenitor cell line CG-4. We report the use of highly specific monoclonal antibodies against GM1, GD1a, GD1b, GT1b, and GQ1b to determine major brain ganglioside expression and morphological distribution during CG-4 differentiation in vitro. Prominent anti-GD1b antibody stained defined a highly arborized intermediate stage of oligodendrocyte differentiation. In contrast, anti-GT1b antibody bound to discrete patches on the cell bodies of early progenitor cells and more mature oligodendrocytes, and to sites of progenitor arborization. The other anti-ganglioside antibodies tested did not bind above background levels. Cells with anti-GD1b antibody binding and morphology similar to those in differentiating CG-4 cells were detected in rat brain primary cell cultures enriched in oligodendrocyte precursors. The remarkably distinctive ganglioside immunoreactivity on differentiating oligodendrocytes suggests the possibility of a functional role for their surface expression.

Gangliosides are neuronal ligands for myelin-associated glycoprotein

Nerve cells depend on specific interactions with glial cells for proper function. Myelinating glial cells are thought to associate with neuronal axons, in part, via the cell-surface adhesion protein, myelin-associated glycoprotein (MAG). MAG is also thought to be a major inhibitor of neurite outgrowth (axon regeneration) in the adult central nervous system. Primary structure and in vitro function place MAG in an immunoglobulin-related family of sialic acid-binding lactins. We report that a limited set of structurally related gangliosides, known to be expressed on myelinated neurons in vivo, are ligands for MAG. When major brain gangliosides were adsorbed as artificial membranes on plastic microwells, only GT1b and GD1a supported cell adhesion of MAG-transfected COS-1 cells. Furthermore, a quantitatively minor ganglioside expressed on cholinergic neurons, GQ1b alpha (also known as Chol-1 alpha-b), was much more potent than GT1b or GD1a in supporting MAG-mediated cell adhesion. Adhesion to either GT1b or GQ1b alpha was abolished by pretreatment of the adsorbed gangliosides with neuraminidase. On the basis of structure-function studies of 19 test glycosphingolipids, an alpha 2,3-N-acetylneuraminic acid residue on the terminal galactose of a gangliotetraose core is necessary for MAG binding, and additional sialic acid residues linked to the other neutral core saccharides [Gal(II) and GalNAc(III)] contribute significantly to binding affinity. MAG-mediated adhesion to gangliosides was blocked by pretreatment of the MAG-transfected COS-1 cells with anti-MAG monoclonal antibody 513, which is known to inhibit oligodendrocyte-neuron binding. These data are consistent with the conclusion that MAG-mediated cell-cell interactions involve MAG-ganglioside recognition and binding.

High affinity binding of the Entamoeba histolytica lectin to polyvalent N-acetylgalactosaminides

Entamoeba histolytica trophozoites initiate pathogenic colonization by adherence to host glycoconjugates via an amebic surface lectin which binds to galactose (Gal) and N-acetylgalactosamine (GalNAc) residues. Monovalent and multivalent carbohydrate ligands were screened for inhibition of E. histolytica lectin-mediated human red cell hemagglutination, revealing that: (i) the synthetic multivalent neoglycoprotein GalNAc39BSA (having an average of 39 GalNAc residues linked to bovine serum albumin) was 140,000-fold more potent an inhibitor than monovalent GalNAc and 500,000-fold more potent than monovalent Gal; and (ii) small synthetic multivalent ligands which bind with high affinity to the mammalian hepatic Gal/GalNAc lectin do not bind with high affinity to the E. histolytica lectin. Radioligand binding studies revealed saturable binding of 125I-GalNAc39BSA to E. histolytica membranes (KD = 10 +/- 3 nM, Bmax = 0.9 +/- 0.08 pmol/mg membrane protein). Maximal binding required the presence of calcium chloride (300 microM) or sodium chloride (50 mM), and had a broad pH maximum (pH 6-9). GalNAc39BSA was 200,000-fold more potent than monovalent GalNAc in blocking radio-ligand binding. Among synthetic saccharide-derivatized linear polymers, the GalNAc beta and GalNAc alpha 3Gal beta derivatives were the most potent, with GalNAc alpha and GalNAc alpha 3(Fuc alpha 2)Gal beta derivatives much weaker. The data support a model in which a unique pattern of spaced multiple GalNAc residues are the highest affinity targets for the E. histolytica lectin.

Solubilization of a membrane-associated protein from rat nervous system tissues which binds anionic glycolipids and phospholipids

An anionic glycolipid and phospholipid binding protein was characterized in detergent-solubilized rat brain synaptosomes using a synthetic, polyvalent radiolabeled ganglioside-protein conjugate as radioligand. Gangliosides are prominent cell surface glycoconjugates in vertebrate brain, where they may function in membrane protein regulation or in cell-cell recognition. The neoganglioprotein (GT1b)13BSA was radioiodinated and used to probe solubilized synaptosomal proteins for ganglioside binding activity using a receptor-ligand precipitation assay. Binding data revealed a high affinity (KD = 1 nM), saturable (Bmax = 173 pmol/mg protein) binding activity that was proteinase sensitive, calcium independent and maximal at neutral pH. Size exclusion chromatography of the synaptosomal (GT1b)13BSA binding activity indicated a M(r) of approximately 28 kDa. Binding activity with similar characteristics was solubilized from other rat tissues, with activity from sciatic nerve = muscle > synaptosomes > central nervous system myelin = liver. Gangliosides added as mixed detergent-lipid micelles inhibited (GT1b)13BSA binding: GT1b, GD1a and GD1b were the most effective inhibitors (IC50 approximately 200 nM), while GM1 and GM3 were 5-fold less effective. In addition, the sphingolipids sulfatide and sialylneolactotetraosylceramide were effective inhibitors, with IC50 values of 300 nM and 200 nM, respectively. The neutral sphingolipid GA1 did not block (GT1b)13BSA binding. Phosphatidylcholine and phosphatidylethanolamine were non-inhibitory, however phosphatidylglycerol, phosphatidylserine, and phosphatidylinositol inhibited half-maximally at 200-300 nM. Inhibition by both gangliosides and anionic phospholipids was competitive with (GT1b)13BSA. We conclude that a binding protein for anionic glycolipids and phospholipids is distributed on nerve and muscle membranes.

Sialoadhesin, myelin-associated glycoprotein and CD22 define a new family of sialic acid-dependent adhesion molecules of the immunoglobulin superfamily

BACKGROUND

Protein-carbohydrate interactions are believed to be important in many biological processes that involve cell-cell communication. Apart from the selectins, the only well-characterized vertebrate sialic acid-dependent adhesion molecules are CD22 and sialoadhesin; CD22 is a member of the immunoglobulin superfamily that is expressed by B lymphocytes and sialoadhesin is a macrophage receptor. The recent cloning of the gene encoding sialoadhesin has shown that it is also immunoglobulin-like. Both proteins share sequence similarity with the myelin-associated glycoprotein, an adhesion molecule of oligodendrocytes and Schwann cells that has been implicated in the process of myelination, raising the important question of whether myelin-associated glycoprotein is also a sialic acid-binding protein.

RESULTS

We have investigated the binding properties of these three receptors when expressed either in monkey COS cells or as chimaeric proteins containing the Fc portion of human immunoglobulin G. We demonstrate that, like sialoadhesin and CD22, myelin-associated glycoprotein mediates cell adhesion by binding to cell-surface glycans that contain sialic acid. We have dissected the specificities of these three adhesins further: whereas sialoadhesin binds equally to the sugar moieties NeuAc alpha 2-->3Gal beta 1-->3(4)GlcNAc or NeuAc alpha 2-->3Gal beta 1-->3GalNAc, myelin-associated glycoprotein recognizes only NeuAc alpha 2-->3Gal beta 1-->3GalNAc and CD22 binds specifically to NeuAc alpha 2-->6Gal beta 1-->4GlcNAc. Furthermore, we show that the recognition of sialylated glycans on the surfaces of particular cell types leads to the selective binding of sialoadhesin to neutrophils, myelin-associated glycoprotein to neurons and CD22 to lymphocytes.

CONCLUSIONS

Our findings demonstrate that a subgroup of the immunoglobulin superfamily can mediate diverse biological processes through recognition of specific sialylated glycans on cell surfaces. We propose that this subgroup of proteins be called the sialoadhesin family.

Carbohydrate recognition in the peripheral nervous system: a calcium-dependent membrane binding site for HNK-1 reactive glycolipids potentially involved in Schwann cell adhesion

The carbohydrate determinants recognized by the HNK-1 antibody are potential cell-cell recognition ligands in the peripheral nervous system (PNS). The HNK-1 reactive sulfoglucuronylneolacto (SGNL) glycolipids specifically support Schwann cell adhesion, suggesting the presence of a cell surface receptor specific for SGNL-oligosaccharides. We directly probed PNS membranes for receptors complementary to SGNL determinants using a synthetic radioligand consisting of radioiodinated serum albumin derivatized with multiple SGNL-oligosaccharides. A high-affinity, saturable, calcium-dependent binding site for this ligand was found in PNS myelin membranes. Binding activity was carbohydrate-specific (most potently inhibited by SGNL-lipids compared to other glycolipids) and PNS-specific (absent from comparable central nervous system membranes). The SGNL-specific binding activity on PNS membranes reported here may be involved in peripheral myelination or myelin stabilization.

The HNK-1 reactive sulfoglucuronyl glycolipids are ligands for L-selectin and P-selectin but not E-selectin

E-selectin, L-selectin, and P-selectin are related cell adhesion molecules that bind via their lectin domains to sialyl Lewis x and related carbohydrate determinants. Reports have indicated that sulfated glycolipids and polysaccharides also bind selectins. To extend these findings, we compared binding of selectin-IgG chimeras to immobilized sulfated and sialylated glycosphingolipids. E-, L-, and P-selectin chimeras all bound to surfaces absorbed with 2,3-sialyl Lewis x glycolipid or sulfatide (galactosylceramide I3-sulfate) but not to surfaces adsorbed with control sulfated lipids (octadecyl sulfate, sphingosine sulfate). Notably, the L- and P-selectin chimeras but not E-selectin chimera bound to surfaces adsorbed with sulfoglucuronyl glycosphingolipids (SGNL lipids; e.g., IV3 glucuronylneolactotetraosylceramide V3-sulfate). These unusual lipids have been reported as antigenic determinants for monoclonal IgM antibodies produced in patients with neuropathy associated with paraproteinemia and react with the mouse monoclonal antibody HNK-1. Binding of L- and P-selectin chimeras to SGNL lipids was specifically inhibited by appropriate anti-selectin antibodies. While binding of all three selectin chimeras to sialyl Lewis x was blocked by removal of calcium, binding to SGNL lipid was only modestly reduced by EDTA. Chemically desulfated SGNL lipid retained binding activity for L- and P-selectin chimeras, while methyl esterification of the glucuronic acid eliminated binding. We conclude that SGNL lipids, unlike sialyl Lewis x and sulfatides, selectively support L- and P-selectin but not E-selectin chimera binding. The presence of SGNL lipids on brain microvascular endothelium (and other endothelia) may implicate these molecules in leukocyte trafficking to the nervous system and elsewhere.

Complex carbohydrates in drug development

Recent advances in carbohydrate chemistry and biochemistry afford the opportunity to develop bioactive complex carbohydrates, per se, as drugs or as lead compounds in drug development. Complex carbohydrates are unique among biopolymers in their inherent potential to generate diverse molecular structures. While proteins vary only in the linear sequence of their monomer constituents, individual monosaccharides can combine at any of several sites on each carbohydrate ring, in linear or branched arrays, and with varied stereochemistry at each linkage bond. This chapter addresses some salient features of mammalian glycoconjugate structure and biosynthesis, and presents examples of the biological activities of complex carbohydrates. The chapter presents selected examples that will provide an accurate introduction to their pharmacological potential. In addition to their independent functions, oligosaccharides can modify the activities of proteins to which they are covalently attached. Many glycoprotein enzymes and hormones require glycosylation for expression and function. The chapter discusses the ancillary role of carbohydrates that is of great importance to the use of engineered glycoproteins as pharmaceuticals.

Hepatocyte adhesion to carbohydrate-derivatized surfaces. II. Regulation of cytoskeletal organization and cell morphology

Rat hepatic lectins mediate adhesion of isolated rat hepatocytes to synthetic surfaces derivatized with galactosides. Initial weak adhesion is followed by rapid adhesion strengthening. After hepatocytes contact galactose-derivatized gels, the hepatic lectins move rapidly into an inaccessible patch at the adhesive surface (Weisz, O. A., and R. L. Schnaar. 1991. J. Cell Biol. 115:485-493). Hepatic lectin patching, which occurs both at 37 degrees C and 4 degrees C, is not responsible for adhesion strengthening, which does not occur at 4 degrees C. Of various cytoskeletal and metabolic perturbants tested, only a combination of hyperosmotic medium, colchicine, and cytochalasin caused a marked (72%) reduction of adhesion strengthening (without reducing weak cell adhesion). Clathrin and actin were readily detected in the adhesive patch by immunofluorescence microscopy. Rat hepatocytes also adhered avidly to surfaces derivatized with asialofetuin, a high-affinity ligand for the rat hepatic lectins. However, hepatic lectin molecules did not migrate into a patch on the asialofetuin-derivatized surface, suggesting that hepatic lectin-asialofetuin binding may have resulted in the rapid formation of a ring of essentially irreversibly adherent receptors that prevented diffusion of additional lectin molecules into the contact site. The cells were unable to increase their adhesive contact area by flattening onto the derivatized surface. Treatment of cells with cytochalasin, however, did result in an increase in the size of the contact area. Cells adhering to surfaces derivatized with an adhesion-promoting peptide (containing an arg-gly-asp sequence) had larger contact areas than those adhering to galactoside-derivatized surfaces. A model is proposed in which carbohydrate-mediated adhesion causes specific reorganization of cytoskeletal components, leading to strengthened adhesion and a characteristic spherical cell morphology.

Hepatocyte adhesion to carbohydrate-derivatized surfaces. I. Surface topography of the rat hepatic lectin

The rat hepatic lectins, galactose- and N-acetylgalactosamine-binding proteins found on the hepatocyte cell surface, mediate adhesion of isolated primary rat hepatocytes to artificial galactose-derivatized polyacrylamide gels. Biochemical and immunohistochemical techniques were used to examine the topographical redistribution of the rat hepatic lectins in response to galactose-mediated cell adhesion. Hepatocytes isolated from rat liver by collagenase perfusion had an average of 7 x 10(5) cell surface lectin molecules per cell, representing 30-50% of the total lectin molecules per cell, the remainder residing in intracellular pools. Hepatocytes incubated on galactose-derivatized surfaces, whether at 0-4 degrees C or 37 degrees C, rapidly lost greater than 80% of their accessible cell surface lectin binding sites into an adhesive patch of characteristic morphology. The kinetics of rat hepatic lectin disappearance were used to estimate a lateral diffusion coefficient greater than 9 x 10(-9) cm2/s at 37 degrees C, suggesting rapid and unimpeded lectin diffusion in the plane of the membrane. Indirect immunofluorescence labeling of adherent cells using antihepatic lectin antibody revealed a structured ring of receptors surrounding an area of exclusion (patch) of reproducible size and shape which represented approximately 8% of the hepatocyte cell surface. Notably, adherent cells, which had lost greater than 80% of their accessible surface binding sites, still endocytosed soluble galactose-terminated radioligand at greater than 50% of the rate of nonadherent control cells. No net movement of rat hepatic lectin from intracellular pools to the cell surface was found on cells recovered after adhesion to galactose-derivatized surfaces at 37 degrees C, suggesting that the physical size and/or lectin density of the patch was restricted by kinetic or topological constraints.

Rotaviruses specifically bind to the neutral glycosphingolipid asialo-GM1

Rotaviruses are the major etiologic agents of severe diarrhea in children. Many rotaviruses encode a hemagglutinin which binds to sialic acids. We report that rotaviruses specifically recognize the neutral glycosphingolipid gangliotetraosylceramide (asialo-GM1 or GA1). GA1 resolved by thin-layer chromatography is bound by rotavirus, and binding is blocked by neutralizing rotavirus antiserum. Similar glycosphingolipid structures, such as globoside, gangliotriaosylceramide, and GA1 oxidized with galactose oxidase are ineffective in binding rotavirus. Other enteric viruses also specifically bind GA1. GA1 adsorbed to polystyrene beads inhibits rotavirus replication in vitro (as do anti-GA1 antibodies). The use of orally administered immobilized GA1 or anti-GA1 antibodies may prove useful in preventing or attenuating rotaviral and other enteric viral infections.

A membrane receptor for gangliosides is associated with central nervous system myelin

A binding protein specific for major neuronal gangliosides was detected on rat brain membranes using a synthetic ganglioside-protein conjugate, 125I-(GT1b)4BSA (bovine serum albumin derivatized with 4 mol of ganglioside GT1b/mol of protein), as a radioligand. Specific binding of the ligand displayed marked regional variation within the brain, with white matter-enriched regions demonstrating the highest binding activity. Autoradiographic localization of 125I-(GT1b)4BSA binding to tissue sections revealed selective association with myelinated pathways throughout the brain. The ligand also bound preferentially to brain subcellular fractions enriched in myelin, even after removal of axolemma. In contrast, peripheral nerve myelin had little binding activity. The myelin-associated ganglioside receptor detected by 125I-(GT1b)4BSA binding appears to be a novel oligodendroglial membrane protein which preferentially recognizes neuronal gangliosides.

Tumor cell haptotaxis on immobilized N-acetylglucosamine gradients

Polyacrylamide surfaces covalently derivatized with quantifiable gradients of glycosides superimposed on a uniform adhesive background of coimmobilized Arg-Gly-Asp-containing adhesion peptide were synthesized. Substrate-directed cell redistribution (haptotaxis) was measured by seeding derivatized surfaces uniformly with B16F10 murine melanoma cells. After 4-32 hr, cells on gradients of N-acetylglucosamine (GlcNAc) redistributed markedly; higher cell densities were found at gel positions having a higher immobilized GlcNAc density. In contrast, cells seeded on otherwise identical gels having a uniform concentration of immobilized GlcNAc, or on gels having gradients of glucose or galactose, did not redistribute. Soluble inhibitors containing nonreducing terminal GlcNAc (but not those with terminal GalNAc or Gal) blocked redistribution on immobilized GlcNAc gradients. Redistribution was not affected by the presence or absence of serum in the medium. An affinity-purified antibody against beta-1,4-galactosyltransferase, a GlcNAc-binding protein reported to be expressed on B16F10 cell surfaces, attenuated GlcNAc-directed redistribution. When cells were seeded on surfaces derivatized with various uniform densities of immobilized GlcNAc coimmobilized with an invariant density of immobilized Arg-Gly-Asp-peptide, neither cell attachment nor proliferation rate were enhanced on the gels having a higher GlcNAc density. These data indicate that the redistribution on immobilized GlcNAc gradients was due to cell motility. Although gels derivatized with Arg-Gly-Asp-peptide alone supported strong B16F10 cell adhesion, surfaces derivatized with uniform high concentrations of GlcNAc did not. We conclude that cell recognition of substratum gradients that support, at best, weak adhesion (GlcNAc) on an otherwise uniform strongly adhesive background (Arg-Gly-Asp-peptide) may be sufficient to direct cell migration.

Immature cortical neurons are uniquely sensitive to glutamate toxicity by inhibition of cystine uptake

Using the N18-RE-105 neuroblastoma X retina cell line, we previously described Ca2(+)-dependent quisqualate-type glutamate toxicity caused by the inhibition of high-affinity cystine uptake, leading to glutathione depletion and accumulation of cellular oxidants. We now demonstrate that primary cultures of rat cortical neurons (E17; 24-72 h in culture), but not glia, also degenerate when exposed to culture medium with reduced cystine or containing competitive inhibitors of cystine uptake, including glutamate. At this developmental stage, neurotoxicity did not occur as a consequence of continuous exposure to glutamate receptor subtype agonists, N-methyl-D-aspartate, kainate, or 2(RS)-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. However, those that inhibited neuronal cystine uptake--quisqualate, glutamate, homocysteate, beta-N-oxalyl-L-alpha,beta-diaminopropionic acid, and ibotenate--were neurotoxic. Toxicity related to quisqualate did not correlate with the development of quisqualate-stimulated phosphatidylinositol turnover. The toxic potencies of glutamate, quisqualate, and homocysteate were inversely proportional to the concentration of cystine in the medium, suggesting that they competitively inhibit cystine uptake. Autoradiographic analysis of the cellular localization of L-[35S]cystine uptake indicated that embryonic neurons have a high-affinity transport system that is sensitive to quisqualate, whereas non-neuronal cells in the same cultures have a low-affinity system that is insensitive to quisqualate but potently blocked by D-aspartate and glutamate. Exposure to glutamate or homocysteate resulted in a time-dependent depletion of the cellular antioxidant glutathione. The centrally acting antioxidant idebenone and alpha-tocopherol completely blocked the neurotoxicity resulting from glutamate exposure. We propose that competitive inhibition of cystine transport and reduction of extracellular cystine levels result in neuronal cell death due to accumulation of cellular oxidants.