Use of nuclear magnetic resonance spectroscopy in evaluation of ganglioside structure, conformation, and dynamics

East-Asian Helicobacter pylori strains synthesize heptan-deficient lipopolysaccharide

The lipopolysaccharide O-antigen structure expressed by the European Helicobacter pylori model strain G27 encompasses a trisaccharide, an intervening glucan-heptan and distal Lewis antigens that promote immune escape. However, several gaps still remain in the corresponding biosynthetic pathway. Here, systematic mutagenesis of glycosyltransferase genes in G27 combined with lipopolysaccharide structural analysis, uncovered HP0102 as the trisaccharide fucosyltransferase, HP1283 as the heptan transferase, and HP1578 as the GlcNAc transferase that initiates the synthesis of Lewis antigens onto the heptan motif. Comparative genomic analysis of G27 lipopolysaccharide biosynthetic genes in strains of different ethnic origin revealed that East-Asian strains lack the HP1283/HP1578 genes but contain an additional copy of HP1105 and JHP0562. Further correlation of different lipopolysaccharide structures with corresponding gene contents led us to propose that the second copy of HP1105 and the JHP0562 may function as the GlcNAc and Gal transferase, respectively, to initiate synthesis of the Lewis antigen onto the Glc-Trio-Core in East-Asian strains lacking the HP1283/HP1578 genes. In view of the high gastric cancer rate in East Asia, the absence of the HP1283/HP1578 genes in East-Asian H. pylori strains warrants future studies addressing the role of the lipopolysaccharide heptan in pathogenesis.

Enhanced Ordering in Monolayers Containing Glycosphingolipids: Impact of Carbohydrate Structure

The influence of carbohydrate structure on the ordering of glycosphingolipids (GSLs) and surrounding phospholipids was investigated in monolayers at the air-water interface. Binary mixtures composed of GSLs, chosen to span a range of carbohydrate complexity, and zwitterionic dipalmitoylphosphatidylcholine phospholipid, were studied. X-ray reflectivity was used to measure the out-of-plane structure of the monolayers and characterize the extension and conformation of the GSL carbohydrates. Using synchrotron grazing incidence x-ray diffraction, the in-plane packing of the lipid acyl chains and the area per molecule within ordered domains were characterized at different mole ratios of the two components. Our findings indicate that GSL-containing mixtures, regardless of the carbohydrate size, enhance the ordering of the surrounding lipids, resulting in a larger fraction of ordered phase of the monolayer and greater dimensions of the ordered domains. Reduction of the averaged area per molecule within the ordered domains was also observed but only in the cases where there was a size mismatch between the phospholipid headgroups and GSL components, suggesting that the condensation mechanism involves the relief of steric interactions between headgroups in mixtures.

The redefinition of Helicobacter pylori lipopolysaccharide O-antigen and core-oligosaccharide domains

Helicobacter pylori lipopolysaccharide promotes chronic gastric colonisation through O-antigen host mimicry and resistance to mucosal antimicrobial peptides mediated primarily by modifications of the lipid A. The structural organisation of the core and O-antigen domains of H. pylori lipopolysaccharide remains unclear, as the O-antigen attachment site has still to be identified experimentally. Here, structural investigations of lipopolysaccharides purified from two wild-type strains and the O-antigen ligase mutant revealed that the H. pylori core-oligosaccharide domain is a short conserved hexasaccharide (Glc-Gal-DD-Hep-LD-Hep-LD-Hep-KDO) decorated with the O-antigen domain encompassing a conserved trisaccharide (-DD-Hep-Fuc-GlcNAc-) and variable glucan, heptan and Lewis antigens. Furthermore, the putative heptosyltransferase HP1284 was found to be required for the transfer of the third heptose residue to the core-oligosaccharide. Interestingly, mutation of HP1284 did not affect the ligation of the O-antigen and resulted in the attachment of the O-antigen onto an incomplete core-oligosaccharide missing the third heptose and the adjoining Glc-Gal residues. Mutants deficient in either HP1284 or O-antigen ligase displayed a moderate increase in susceptibility to polymyxin B but were unable to colonise the mouse gastric mucosa. Finally, mapping mutagenesis and colonisation data of previous studies onto the redefined organisation of H. pylori lipopolysaccharide revealed that only the conserved motifs were essential for colonisation. In conclusion, H. pylori lipopolysaccharide is missing the canonical inner and outer core organisation. Instead it displays a short core and a longer O-antigen encompassing residues previously assigned as the outer core domain. The redefinition of H. pylori lipopolysaccharide domains warrants future studies to dissect the role of each domain in host-pathogen interactions. Also enzymes involved in the assembly of the conserved core structure, such as HP1284, could be attractive targets for the design of new therapeutic agents for managing persistent H. pylori infection causing peptic ulcers and gastric cancer.

Gangliosides of the Vertebrate Nervous System

Gangliosides, sialylated glycosphingolipids, found on all vertebrate cells and tissues, are major molecular determinants on the surfaces of vertebrate nerve cells. Composed of a sialylated glycan attached to a ceramide lipid, the same four structures-GM1, GD1a, GD1b, and GT1b-represent the vast majority (>90%) of gangliosides in the brains of all mammals and birds. Primarily found on the outer surface of the plasma membrane with their glycans facing outward, gangliosides associate laterally with each other, sphingomyelin, cholesterol, and select proteins in lipid rafts-the dynamic functional subdomains of the plasma membrane. The functions of gangliosides in the human nervous system are revealed by congenital mutations in ganglioside biosynthetic genes. Mutations in ST3GAL5, which codes for an enzyme early in brain ganglioside biosynthesis, result in an early-onset seizure disorder with profound motor and cognitive decay, whereas mutations in B4GALNT1, a gene encoding a later step, result in hereditary spastic paraplegia accompanied by intellectual deficits. The molecular functions of brain gangliosides include regulation of receptors in the same membrane via lateral (cis) associations and regulation of cell-cell recognition by trans interaction with ganglioside binding proteins on apposing cells. Gangliosides also affect the aggregation of Aβ (Alzheimer's disease) and α-synuclein (Parkinson's Disease). As analytical, biochemical, and genetic tools advance, research on gangliosides promises to reveal mechanisms of molecular control related to nerve and glial cell differentiation, neuronal excitability, axon outgrowth after nervous system injury, and protein folding in neurodegenerative diseases.

Ganglioside regulation of AMPA receptor trafficking

Gangliosides are major cell-surface determinants on all vertebrate neurons. Human congenital disorders of ganglioside biosynthesis invariably result in intellectual disability and are often associated with intractable seizures. To probe the mechanisms of ganglioside functions, affinity-captured ganglioside-binding proteins from rat cerebellar granule neurons were identified by quantitative proteomic mass spectrometry. Of the six proteins that bound selectively to the major brain ganglioside GT1b (GT1b:GM1 > 4; p < 10(-4)), three regulate neurotransmitter receptor trafficking: Thorase (ATPase family AAA domain-containing protein 1), soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (γ-SNAP), and the transmembrane protein Nicalin. Thorase facilitates endocytosis of GluR2 subunit-containing AMPA-type glutamate receptors (AMPARs) in an ATPase-dependent manner; its deletion in mice results in learning and memory deficits (J. Zhang et al., 2011b). GluR2-containing AMPARs did not bind GT1b, but bound specifically to another ganglioside, GM1. Addition of noncleavable ATP (ATPγS) significantly disrupted ganglioside binding, whereas it enhanced AMPAR association with Thorase, NSF, and Nicalin. Mutant mice lacking GT1b expressed markedly higher brain Thorase, whereas Thorase-null mice expressed higher GT1b. Treatment of cultured hippocampal neurons with sialidase, which cleaves GT1b (and other sialoglycans), resulted in a significant reduction in the size of surface GluR2 puncta. These data support a model in which GM1-bound GluR2-containing AMPARs are functionally segregated from GT1b-bound AMPAR-trafficking complexes. Release of ganglioside binding may enhance GluR2-containing AMPAR association with its trafficking complexes, increasing endocytosis. Disrupting ganglioside biosynthesis may result in reduced synaptic expression of GluR2-contianing AMPARs resulting in intellectual deficits and seizure susceptibility in mice and humans.

Self-assembled micelles of monosialogangliosides as nanodelivery vehicles for taxanes

We demonstrate herein that taxanes (paclitaxel (Ptx) and docetaxel (Dtx)) can be spontaneously loaded into ganglioside nanomicelles. The efficiency of gangliosides to solubilize taxanes was highly dependent on their self-aggregating structure. Thus, GM3 that forms unilamellar vesicles was less efficient to solubilize taxanes than gangliosides that form micelles (i.e. GM1 and GM2). Sialic acid cyclization of GM1 by acid treatment led to an important reduction in its capacity to solubilize taxanes, as also did the replacement of the fatty acid of ceramide by a dicholoracetyl group. Water solubility of paclitaxel (Ptx) is less than 1 μg mL⁻¹ and increased up to 6.3mg.mL⁻¹ upon its association with GM1 micelles. The incorporation of Ptx in GM1 reached an optimum at GM1/Ptx 20/1 molar ratio when performed at room temperature. An increase in the solubilization capacity of GM1 micelles was observed upon dehydration of their polar head group by pre-treatment at 55 °C. Loading of Ptx into the micelle induced a structural reorganization that led to an important protection of Ptx reducing its hydrolysis at alkaline pH. Diffusion of either GM1 or Ptx was restricted upon mixed-micelle formation indicating that they are kinetically more stable than pure ganglioside micelles. X-ray powder diffraction of lyophilized GM1 micelles with Ptx showed a change in their internal structure from a crystalline state to completely amorphous. Taxane-ganglioside mixed micelles were stable in solution for at least 4months and also upon freeze-thawing or lyophilization-solubilization cycles. Upon mixing with human blood constituents, GM1/Ptx micelles did not induce hemolysis or platelet aggregation and were spontaneously covered with human serum albumin (HSA), which could aid in the delivery of micellar content to tumors. In vitro antimitotic activity of GM1/Ptx mixed micelles was qualitatively equivalent to that of free drug in DMSO solution.

Formation of GM1 ganglioside clusters on the lipid membrane containing sphingomyeline and cholesterol

GM1 gangliosides form a microdomain with sphingomyeline (SM) and cholesterol (Chol) and are deeply involved in the aggregation of amyloid beta (Aβ) peptides on neural membranes. We performed molecular dynamics simulations on two kinds of lipid bilayers containing GM1 ganglioside: GM1/SM/Chol and GM1/POPC. Both 10 and 100 ns simulations and another set of 10 ns simulations with different initial lipid arrangement essentially showed the same computational results. GM1 molecules in the GM1/SM/Chol membrane were condensed, whereas those in GM1/POPC membrane scattered. That is, the formation of GM1 cluster was observed only on the GM1/SM/Chol mixed membrane. There appeared numerous hydrogen bonds among glycan portions of the GM1 clusters due to the condensation. A comparison in distribution of lipid molecules between the two kinds of membranes suggested that cholesterol had important roles to prevent the membrane from interdigitation and to stabilize other lipids for interacting with each other. This property of cholesterol promotes the formation of GM1 clusters.

Condensing and fluidizing effects of ganglioside GM1 on phospholipid films

Mixed monolayers of the ganglioside G(M1) and the lipid dipalmitoylphosphatidlycholine (DPPC) at air-water and solid-air interfaces were investigated using various biophysical techniques to ascertain the location and phase behavior of the ganglioside molecules in a mixed membrane. The effects induced by G(M1) on the mean molecular area of the binary mixtures and the phase behavior of DPPC were followed for G(M1) concentrations ranging from 5 to 70 mol %. Surface pressure isotherms and fluorescence microscopy imaging of domain formation indicate that at low concentrations of G(M1) (<25 mol %), the monolayer becomes continually more condensed than DPPC upon further addition of ganglioside. At higher G(M1) concentrations (>25 mol %), the mixed monolayer becomes more expanded or fluid-like. After deposition onto a solid substrate, atomic force microscopy imaging of these lipid monolayers showed that G(M1) and DPPC pack cooperatively in the condensed phase domain to form geometrically packed complexes that are more ordered than either individual component as evidenced by a more extended total height of the complex arising from a well-packed hydrocarbon tail region. Grazing incidence x-ray diffraction on the DPPC/G(M1) binary mixture provides evidence that ordering can emerge when two otherwise fluid components are mixed together. The addition of G(M1) to DPPC gives rise to a unit cell that differs from that of a pure DPPC monolayer. To determine the region of the G(M1) molecule that interacts with the DPPC molecule and causes condensation and subsequent expansion of the monolayer, surface pressure isotherms were obtained with molecules modeling the backbone or headgroup portions of the G(M1) molecule. The observed concentration-dependent condensing and fluidizing effects are specific to the rigid, sugar headgroup portion of the G(M1) molecule.

Involvement of very long fatty acid-containing lactosylceramide in lactosylceramide-mediated superoxide generation and migration in neutrophils

The neutral glycosphingolipid lactosylceramide (LacCer) forms lipid rafts (membrane microdomains) coupled with the Src family kinase Lyn on the plasma membranes of human neutrophils; ligand binding to LacCer activates Lyn, resulting in neutrophil functions, such as superoxide generation and migration (Iwabuchi and Nagaoka, Lactosylceramide-enriched glycosphingolipid signaling domain mediates superoxide generation from human neutrophils, Blood 100, 1454-1464, 2002 and Sato et al. Induction of human neutrophil chemotaxis by Candida albicans-derived beta-1,6-long glycoside side-chain-branched beta glycan, J. Leukoc. Biol. 84, 204-211, 2006). Neutrophilic differentiated HL-60 cells (D-HL-60 cells) express almost the same amount of LacCer as neutrophils. However, D-HL-60 cells do not have Lyn-associated LacCer-enriched lipid rafts and lack LacCer-mediated superoxide-generating and migrating abilities. Here, we examined the roles of LacCer molecular species of different fatty acid compositions in these processes. Liquid chromatography-mass spectrometry analyses revealed that the very long fatty acid C24:0 and C24:1 chains were the main components of LacCer (31.6% on the total fatty acid content) in the detergent-resistant membrane fraction (DRM) from neutrophil plasma membranes. In contrast, plasma membrane DRM of D-HL-60 cells included over 70% C16:0-LacCer, but only 13.6% C24-LacCer species. D-HL-60 cells loaded with C24:0 or C24:1-LacCer acquired LacCer-mediated migrating and superoxide-generating abilities, and allowed Lyn coimmunoprecipitation by anti-LacCer antibody. Lyn knockdown by siRNA completely abolished the effect of C24:1-LacCer loading on LacCer-mediated migration of D-HL-60 cells. Immunoelectron microscopy revealed that LacCer clusters were closely associated with Lyn molecules in neutrophils and C24:1-LacCer-loaded D-HL-60 cells, but not in D-HL-60 cells or C16:0-LacCer-loaded cells. Taken together, these observations suggest that LacCer species with very long fatty acids are specifically necessary for Lyn-coupled LacCer-enriched lipid raft-mediated neutrophil superoxide generation and migration.

Gangliosides as components of lipid membrane domains

Cell membrane components are organized as specialized domains involved in membrane-associated events such as cell signaling, cell adhesion, and protein sorting. These membrane domains are enriched in sphingolipids and cholesterol but display a low protein content. Theoretical considerations and experimental data suggest that some properties of gangliosides play an important role in the formation and stabilization of specific cell lipid membrane domains. Gangliosides are glycolipids with strong amphiphilic character and are particularly abundant in the plasma membranes, where they are inserted into the external leaflet with the hydrophobic ceramide moiety and with the oligosaccharide chain protruding into the extracellular medium. The geometry of the monomer inserted into the membrane, largely determined by the very large surface area occupied by the oligosaccharide chain, the ability of the ceramide amide linkage to form a network of hydrogen bonds at the water-lipid interface of cell membranes, the Delta(4) double bond of sphingosine proximal to the water-lipid interface, the capability of the oligosaccharide chain to interact with water, and the absence of double bonds into the double-tailed hydrophobic moiety are the ganglioside features that will be discussed in this review, to show how gangliosides are responsible for the formation of cell lipid membrane domains characterized by a strong positive curvature.

Structure and dynamics of the conserved protein GPI anchor core inserted into detergent micelles

A suitable approach which combines nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations have been used to study the structure and the dynamics of the glycosylphosphatidylinositol (GPI) anchor Manalphal-2Manalpha1-6Manalphal -4GlcNalpha1-6myo-inositol-1-OPO(3)-sn-1,2-dimyristoylglycerol (1) incorporated into dodecylphosphatidylcholine (DPC) micelles. The results have been compared to those previously obtained for the products obtainable from (1) after phospholipase cleavage, in aqueous solution. Relaxation and diffusion NMR experiments were used to establish the formation of stable aggregates and the insertion of (1) into the micelles. MD calculations were performed including explicit water, sodium and chloride ions and using the Particle Mesh Ewald approach for the evaluation of the electrostatic energy term. The MD predicted three dimensional structure and dynamics were substantiated by nuclear overhauser effect (NOE) measurements and relaxation data. The pseudopentasaccharide structure, which was not affected by incorporation of (1) into the micelle, showed a complex dynamic behaviour with a faster relative motion at the terminal mannopyranose unit and decreased mobility close to the micelle. This motion may be better described as an oscillation relative to the membrane rather than a folding event.

Conformational alteration of bradykinin in presence of GM1 micelle

We report here the interaction of bradykinin with ganglioside GM1 by circular dichroism, steady-state fluorescence, and one-dimensional 1H NMR spectroscopy. Circular dichroism spectroscopy is indicative of a turn formation of bradykinin backbone in the presence of GM1 micelle. The fluorescence quenching efficiencies of iodide and acrylamide are substantially reduced, indicating a shielding of phenylalanine residue of bradykinin from aqueous environment. Significant line broadening of NMR resonances of bradykinin, suggestive of motional restriction, is observed.

Structural alterations of enkephalins in the presence of GM1 ganglioside micelles

The enkephalins are endogenous neurotransmitters and bind with high affinity at the delta-receptor. Gangliosides, the major glycans of nerve cells, known to interact both with receptors and ligands on the cell surface, have been implicated to modulate the actions of opioid receptors by allosteric regulation (Wu, G.; Lu, Z. H.; Wei, T. J.; Howells, R. D.; Christoffers, K.; Leeden R. W. Ann NY Acad Sci 1998, 845, 126-138). We have studied the interactions between enkephalins and monosialylated ganglioside GM1 using NMR spectroscopy and fluorescence. The structural models of enkephalins in the presence of GM1 micelles were generated using two-dimensional (1)H-ROESY experiments along with restrained molecular dynamics simulations. We report a conformational alteration of enkephalins in the presence of GM1 micelles.

Melittin-GM1 interaction: a model for a side-by-side complex

We report here the interaction of melittin with ganglioside GM1 by steady-state fluorescence, one-dimensional (1)H NMR spectroscopy and molecular modeling. In the presence of GM1 the emission maximum of melittin is blue shifted and fluorescence quenching efficiencies of iodide and acrylamide are substantially reduced, indicating a shielding of tryptophan of melittin from aqueous environment. Significant line broadening of NMR resonances of melittin, suggestive of motional restriction, is observed. Molecular modeling indicates a melittin-GM1 complex with N-terminal hydrophobic stretch of melittin associating with the ceramide tail and C-terminal hydrophilic end of melittin having favorable electrostatic interaction with the carbohydrate head group of GM1.

Structure of the main ganglioside from the brain of Xenopus laevis

The main component of the ganglioside(1) mixture from the brain of the adult amphibian Xenopus laevis accounts for 35% of the total, as lipid bound sialic acid. This ganglioside has been purified and characterized by thin layer chromatography, partial and exhaustive enzymatic hydrolysis with sialidase, TLC-overlay procedures with anti-Gg(4)Cer and anti-Neu5Acalpha6GalNAcbeta specific monoclonal antibodies and mass spectrometry. All together the results suggest the following structure: Neu5Acalpha8Neu5Acalpha3Galbeta3(Neu5Acalpha8Neu5Acalpha6)GalNAcbeta4Galbeta4Glcbeta1Cer, or, IV(3)-alpha-Neu5Ac(2),III(6)-alpha-Neu5Ac(2)-Gg(4)Cer.