Characterization of GM1 ganglioside by direct inlet chemical ionization mass spectrometry

Hippocampal lipid differences in Alzheimer's disease: a human brain study using matrix-assisted laser desorption/ionization-imaging mass spectrometry

INTRODUCTION

Alzheimer's disease (AD), the leading cause of dementia, is pathologically characterized by β-amyloid plaques and tau tangles. However, there is also evidence of lipid dyshomeostasis-mediated AD pathology. Given the structural diversity of lipids, mass spectrometry is a useful tool for studying lipid changes in AD. Although there have been a few studies investigating lipid changes in the human hippocampus in particular, there are few reports on how lipids change in each hippocampal subfield (e.g., Cornu Ammonis [CA] 1-4, dentate gyrus [DG] etc.). Since each subfield has its own function, we postulated that there could be lipid changes that are unique to each.

METHODS

We used matrix-assisted laser desorption/ionization-imaging mass spectrometry to investigate specific lipid changes in each subfield in AD. Data from the hippocampus region of six age- and gender-matched normal and AD pairs were analyzed with SCiLS lab 2015b software (SCiLS GmbH, Germany; RRID:SCR_014426), using an analysis workflow developed in-house. Hematoxylin, eosin, and luxol fast blue staining were used to precisely delineate each anatomical hippocampal subfield. Putative lipid identities, which were consistent with published data, were assigned using MS/MS.

RESULTS

Both positively and negatively charged lipid ion species were abundantly detected in normal and AD tissue. While the distribution pattern of lipids did not change in AD, the abundance of some lipids changed, consistent with trends that have been previously reported. However, our results indicated that the majority of these lipid changes specifically occur in the CA1 region. Additionally, there were many lipid changes that were specific to the DG.

CONCLUSIONS

Matrix-assisted laser desorption/ionization-imaging mass spectrometry and our analysis workflow provide a novel method to investigate specific lipid changes in hippocampal subfields. Future work will focus on elucidating the role that specific lipid differences in each subfield play in AD pathogenesis.

High-energy collision-induced dissociation of ceramide ions from permethylated glycosphingolipids

Ceramide fragments from permethylated glycosphingolipids (GSLs) were studied by highenergy collision-induced dissociation (CID). In comparison with ceramlde fragments of underivatized GSLs, many more product ions including charge-remote fragment ions were observed. These ions provided detailed structural information on the ceramides. The relative intensity and the mass interval between the L and M ions were used to assign the position of the double bond. The position of the hydroxyl group was assigned with the Ln and K ions. Because the ceramide fragments and not the pseudomolecular ions were selected as the precursor ions, the size of GSLs had little effect on the quality of the product ion spectra. The sensitivity of this approach was in the range of picomoles.

Desorption mass spectrometry of glycosphingolipids

Desorption mass spectrometry has become an important tool for sequencing and mapping of glycosphingolipids of natural, synthetic, or semi-synthetic origin. The appropriate combination of different desorption mass spectrometric techniques with other spectroscopic, enzymatic, chemical, and/or immunological methods represents the most direct and efficient way to establish frequent, yet unknown, molecular structure-function relationships.

Analysis of native neutral glycosphingolipids by combined high performance liquid chromatography/mass spectrometry

Conditions for normal phase high performance liquid chromatography-chemical ionization mass spectrometry of underivatized glycosphingolipids with the use of a moving belt transport interface were investigated. Chromatography was performed on a 2 mm ID X 3 cm column packed with 5 micron spherical silica. A gradient of increasing methanol and water in methylene chloride was used to resolve mono-, di-, tri-, and tetraglycosylceramides and sulfatides in less than 15 min. A polyimide belt was used to transport the sample residues to the mass spectrometer where ammonia chemical ionization mass spectrometry was used to obtain spectra. One to 5 micrograms of each component was sufficient to obtain full spectral scans. Mono- and dihexosylceramide spectra showed [M + H]+ ions of good abundance, while the higher glycosphingolipids have molecular weights that exceed the range of the mass spectrometer utilized. All glycosphingolipids gave ions characteristic of their ceramide, fatty acid, long chain base and carbohydrate components. Sequence information which reflected the relative position of hexose and N-acetylhexosamine residues was also obtained.

Enhancement of structural information in FAB ionized carbohydrate samples by neutral gas collision

Fast atom bombardment (FAB) ionization and two coupled analyzers (BE-EB) have been combined with neutral gas collision (C) to enhance structural information in the mass spectra of oligosaccharides. (B and E are abbreviations for magnetic and electric sectors respectively.) FAB ionization and the first analysers (BE) have provided parent ions free from biological and liquid matrix contaminants. Structural detail of these products were observed after collision and daughter ion analysis in a second coupled analyser (EB). Starting from complex mixtures, this instrumental approach, BE-C-EB, has provided specific oligomeric sequence information which was not observed in the normal FAB mass spectra. Collision spectra obtained from isomeric linear and branched oligosaccharides show unique fragments that can be directly related to structure.

Structural characterization of glycosphingolipids by direct chemical ionization mass spectrometry

This report describes the use of direct chemical ionization mass spectrometry with ammonia as the reagent gas (NH3-DCI) for structure analysis of underivatized, permethylated and permethylated and reduced glycosphingolipids. In contrast to ionization by electron impact, the NH3-DCI mass spectra exhibit intense molecular and carbohydrate sequence-related ions using microgram amounts of sample. Underivatized glycosphingolipids with up to two sugar residues yield abundant protonated and ammonia-cationized molecular ions and structurally significant fragments. Permethylation in conjunction with NH3-DCI can be used to obtain molecular weight as well as oligosaccharide sequence and branching information on neutral, acidic and complex-type glycosphingolipids with up to five sugar residues. Reduction of the permethylated derivatives gives rise to several new, structurally significant fragments in the corresponding NH3-DCI mass spectra which enable fatty acid and base compositions to be determined. Isotopically labeled reagent gases have been used to confirm the assignment of fragment structures and to demonstrate that the ions observed are unique to the NH3-DCI mass spectra.

Fast atom bombardment mass spectrometry of glycosphingolipids. Glycosphingolipids containing neutral sugars

Natural and synthetic glycosphingolipids containing neutral sugars have been analyzed by positive and negative ion fast atom bombardment mass spectrometry. Basic structural characterization including saccharide size and sequence and ceramide composition is possible on the basis of the fragment ions observed. The degree of fragmentation could be increased by using higher sample concentrations and lower fast atom beam energies. Commercially available synthetic compounds that had been presumed to be pure were shown to contain homologous fatty acids. Mixtures of glycosphingolipids such as those obtained from Gaucher's spleen and from human erythrocytes can be characterized and quantitated.

Isolation and characterization of a novel disialoganglioside from bovine adrenal medulla

A novel GDlb ganglioside which contains only N-glycolylneuraminic acid was isolated from bovine adrenal medulla by DEAE-Sephadex A-25 and Iatrobeads column chromatography. The concentration of this ganglioside was 0.9 nmol lipid-bound sialic acid per gram fresh tissue, which accounted for 2.7% of the disialoganglioside fraction. The structure was elucidated by sugar analysis, neuraminidase digestion, and permethylation studies. The complete structure of this ganglioside was identified as GDlb (NeuGc)2 or II3 (NeuGc)2-GgOse4Cer.

Chemical-ionization mass spectra of the permethylated sialo-oligosaccharides liberated from gangliosides

Permethylated mono- and di-sialo-oligosaccharides liberated from several parent gangliosides have been examined by chemical-ionization mass spectrometry with ammonia as the reagent gas in order to elucidate their structures. Several major fragment-ions, in addition to both the protonated and ammonium adduct molecular-ions, may be readily assigned without interference from the ceramide moiety. Sialic acid-containing di-, tri-, and tetra-saccharide ions can be clearly observed and used to determine the sugar residue to which the sialic acid residue is attached. The neutral-sugar skeletons produced by the loss of sialic acid give rise to both the protonated and the ammonium adduct ions; in the case of tetrasaccharides, these are further degraded to produce di- and tri-saccharide ions. These characteristic ions are useful for the determination of the number of sugar residues and their sequence in an oligosaccharide structure. The chemical-ionization mass spectra of GM3- and GM1-oligosaccharides with isobutane show the ions corresponding to each monosaccharide residue. These results indicate that chemical-ionization mass spectrometry is highly useful in determining the complete sugar-sequence of gangliosides.

Application of field desorption and secondary ion mass spectrometry for glycolipid analysis

Field desorption (FD) and secondary ion mass spectrometry (SI-MS) mass spectra of several glycolipids are presented to demonstrate their potential for the analysis of glycolipids. FD and SI-MS give useful information on molecular weight, ceramide structure and sugar sequence. In general, FD provides clearer fragment ion peaks for the analysis of sugar sequence than SI-MS. For underivatized acidic glycolipids such as gangliosides, sulfatide and seminolipid, SI-MS provides quasimolecular ions which are hardly produced by FD. In contrast to underivatized gangliosides, permethylated samples give molecular ion species of high intensity in both FD and SI-MS, but no fragment ions pertinent to carbohydrate sequence could be observed in FD spectra. SI-MS spectra of permethylated samples provide good information on sugar chain structure. Thus FD and SI-MS mass spectra complement each other, and the combination of these ionization methods will provide powerful tools for glycolipid analysis.