Atmospheric Pressure MALDI Fourier Transform Mass Spectrometry of Labile Oligosaccharides

Recent advances in mass spectrometry imaging of single cells

Mass spectrometry imaging (MSI) is a sensitive, specific, label-free imaging analysis technique that can simultaneously obtain the spatial distribution, relative content, and structural information of hundreds of biomolecules in cells and tissues, such as lipids, small drug molecules, peptides, proteins, and other compounds. The study of molecular mapping of single cells can reveal major scientific issues such as the activity pattern of living organisms, disease pathogenesis, drug-targeted therapy, and cellular heterogeneity. Applying MSI technology to the molecular mapping of single cells can provide new insights and ideas for the study of single-cell metabolomics. This review aims to provide an informative resource for those in the MSI community who are interested in single-cell imaging. Particularly, we discuss advances in imaging schemes and sample preparation, instrumentation improvements, data processing and analysis, and 3D MSI over the past few years that have allowed MSI to emerge as a powerful technique in the molecular imaging of single cells. Also, we highlight some of the most cutting-edge studies in single-cell MSI, demonstrating the future potential of single-cell MSI. Visualizing molecular distribution at the single-cell or even sub-cellular level can provide us with richer cell information, which strongly contributes to advancing research fields such as biomedicine, life sciences, pharmacodynamic testing, and metabolomics. At the end of the review, we summarize the current development of single-cell MSI technology and look into the future of this technology.

Analysis of Synthetic Monodisperse Polysaccharides by Wide Mass Range Ultrahigh-Resolution MALDI Mass Spectrometry

Carbohydrates, such as oligo- and polysaccharides, are highly abundant biopolymers that are involved in numerous processes. The study of their structure and functions is commonly based on a material that is isolated from complex natural sources. However, a more precise analysis requires pure compounds with well-defined structures that can be obtained from chemical or enzymatic syntheses. Novel synthetic strategies have increased the accessibility of larger monodisperse polysaccharides, posing a challenge to the analytical methods used for their molecular characterization. Here, we present wide mass range ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) as a powerful platform for the analysis of synthetic oligo- and polysaccharides. Synthetic carbohydrates 16-, 64-, 100-, and 151-mers were mass analyzed and characterized by MALDI in-source decay FT-ICR MS. Detection of fragment ions generated from glycosidic bond cleavage (or cross-ring cleavage) provided information of the monosaccharide content and the linkage type, allowing for the corroboration of the carbohydrate compositions and structures.

Recent advances in the mass spectrometric analysis of glycosphingolipidome - A review

Aberrant expression of glycosphingolipids has been implicated in a myriad of diseases, but our understanding of the strucural diversity, spatial distribution, and biological function of this class of biomolecules remains limited. These challenges partly stem from a lack of sensitive tools that can detect, identify, and quantify glycosphingolipids at the molecular level. Mass spectrometry has emerged as a powerful tool poised to address most of these challenges. Here, we review the recent developments in analytical glycosphingolipidomics with an emphasis on sample preparation, mass spectrometry and tandem mass spectrometry-based structural characterization, label-free and labeling-based quantification. We also discuss the nomenclature of glycosphingolipids, and emerging technologies like ion mobility spectrometry in differentiation of glycosphingolipid isomers. The intrinsic advantages and shortcomings of each method are carefully critiqued in line with an individual's research goals. Finally, future perspectives on analytical sphingolipidomics are stated, including a need for novel and more sensive methods in isomer separation, low abundance species detection, and profiling the spatial distribution of glycosphingolipid molecular species in cells and tissues using imaging mass spectrometry.

Mass Spectrometry Imaging of N-Glycans from Formalin-Fixed Paraffin-Embedded Tissue Sections Using a Novel Subatmospheric Pressure Ionization Source

N-linked glycosylation, featuring various glycoforms, is one of the most common and complex protein post-translational modifications (PTMs) controlling protein structures and biological functions. It has been revealed that abnormal changes of protein N-glycosylation patterns are associated with many diseases. Hence, unraveling the disease-related alteration of glycosylation, especially the glycoforms, is crucial and beneficial to improving our understanding about the pathogenic mechanisms of various diseases. In past decades, given the capability of in situ mapping of biomolecules and their region-specific localizations, matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) has been widely applied to the discovery of potential biomarkers for many diseases. In this study, we coupled a novel subatmospheric pressure (SubAP)/MALDI source with a Q Exactive HF hybrid quadrupole-orbitrap mass spectrometer for in situ imaging of N-linked glycans from formalin-fixed paraffin-embedded (FFPE) tissue sections. The utility of this new platform for N-glycan imaging analysis was demonstrated with a variety of FFPE tissue sections. A total of 55 N-glycans were successfully characterized and visualized from a FFPE mouse brain section. Furthermore, 29 N-glycans with different spatial distribution patterns could be identified from a FFPE mouse ovarian cancer tissue section. High-mannose N-glycans exhibited elevated expression levels in the tumor region, indicating the potential association of this type of N-glycans with tumor progression.

Mass Spectrometry Approaches to Glycomic and Glycoproteomic Analyses

Glycomic and glycoproteomic analyses involve the characterization of oligosaccharides (glycans) conjugated to proteins. Glycans are produced through a complicated nontemplate driven process involving the competition of enzymes that extend the nascent chain. The large diversity of structures, the variations in polarity of the individual saccharide residues, and the poor ionization efficiencies of glycans all conspire to make the analysis arguably much more difficult than any other biopolymer. Furthermore, the large number of glycoforms associated with a specific protein site makes it more difficult to characterize than any post-translational modification. Nonetheless, there have been significant progress, and advanced separation and mass spectrometry methods have been at its center and the main reason for the progress. While glycomic and glycoproteomic analyses are still typically available only through highly specialized laboratories, new software and workflow is making it more accessible. This review focuses on the role of mass spectrometry and separation methods in advancing glycomic and glycoproteomic analyses. It describes the current state of the field and progress toward making it more available to the larger scientific community.

5-Methoxysalicylic acid matrix for ganglioside analysis with matrix-assisted laser desorption/ionization mass spectrometry

In this note, we report that high quality ganglioside profiles with minimal loss of sialic acid residues can be obtained in the positive ion mode by using a 5-methoxysalicylic acid (MSA) matrix for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). Our results showed that MSA produced much less sialic acid losses from gangliosides than DHB, although MSA and DHB are differ only by their functional groups at their 5-positions (-OH for DHB and -OCH(3) for MSA). Furthermore, our data also demonstrated that addition of an alkali metal additive was effective for simplifying ganglioside profiles, but not necessary for stabilizing glycosidic bonds of gangliosides if MSA was used as a matrix. This suggests that MALDI MS with MSA has a potential to gain additional benefits from the positive-ion mode analyses without losing performance in ganglioside profiling.

Energy-dependent electron activated dissociation of metal-adducted permethylated oligosaccharides

The effects of varying the electron energy and cationizing agents on electron activated dissociation (ExD) of metal-adducted oligosaccharides were explored, using permethylated maltoheptaose as the model system. Across the examined range of electron energy, the metal-adducted oligosaccharide exhibited several fragmentation processes, including electron capture dissociation (ECD) at low energies, hot-ECD at intermediate energies, and electronic excitation dissociation (EED) at high energies. The dissociation threshold depended on the metal charge carrier(s), whereas the types and sequence spans of product ions were influenced by the metal-oligosaccharide binding pattern. Theoretical modeling contributed insight into the metal-dependent behavior of carbohydrates during low-energy ECD. When ExD was applied to a permethylated high mannose N-linked glycan, EED provided more structural information than either collision-induced dissociation (CID) or low-energy ECD, thus demonstrating its potential for oligosaccharide linkage analysis.

Recent advances in sialic acid-focused glycomics

Recent emergences of glycobiology, glycotechnology and glycomics have been clarifying enormous roles of carbohydrates in biological recognition systems. For example, cell surface carbohydrates existing as glycoconjugates (glycolipids, glycoproteins and proteoglycans) play crucial roles in cell-cell communication, cell proliferation and differentiation, tumor metastasis, inflammatory response or viral infection. In particular, sialic acids (SAs) existing as terminal residues in carbohydrate chains on cell surface are involved in signal recognition and adhesion to ligands, antibodies, enzymes and microbes. In addition, plasma free SAs and sialoglycans have shown great potential for disease biomarker discovery. Therefore, the development of efficient analytical methods for structural and functional studies of SAs and sialylglycans are very important and highly demanded. The problems of SAs and sialylglycans analysis are vanishingly small sample amount, complicated and unstable structures, and complex mixtures. Nevertheless, in the past decade, mass spectrometry in combination with chemical derivatization and modern separation methodologies has become a powerful and versatile technique for structural analysis of SAs and sialylglycans. This review summarizes these recent advances in glycomic studies on SAs and sialylglycans. Specially, derivatization and capturing of SAs and sialylglycans combined with mass spectrometry analysis are highlighted.

Harnessing glycomics technologies: integrating structure with function for glycan characterization

Glycans, or complex carbohydrates, are a ubiquitous class of biological molecule which impinge on a variety of physiological processes ranging from signal transduction to tissue development and microbial pathogenesis. In comparison to DNA and proteins, glycans present unique challenges to the study of their structure and function owing to their complex and heterogeneous structures and the dominant role played by multivalency in their sequence-specific biological interactions. Arising from these challenges, there is a need to integrate information from multiple complementary methods to decode structure-function relationships. Focusing on acidic glycans, we describe here key glycomics technologies for characterizing their structural attributes, including linkage, modifications, and topology, as well as for elucidating their role in biological processes. Two cases studies, one involving sialylated branched glycans and the other sulfated glycosaminoglycans, are used to highlight how integration of orthogonal information from diverse datasets enables rapid convergence of glycan characterization for development of robust structure-function relationships.

Localization and imaging of gangliosides in mouse brain tissue sections by laserspray ionization inlet

A new ionization method for the analysis of fragile gangliosides without undesired fragmentation or salt adduction is presented. In laserspray ionization inlet (LSII), the matrix/analyte sample is ablated at atmospheric pressure, and ionization takes place in the ion transfer capillary of the mass spectrometer inlet by a process that is independent of a laser wavelength or voltage. The softness of LSII allows the identification of gangliosides up to GQ1 with negligible sialic acid loss. This is of importance to the field of MS imaging, as undesired fragmentation has made it difficult to accurately map the spatial distribution of fragile ganglioside lipids in tissue. Proof-of-principle structural characterization of endogenous gangliosides using MS(n) fragmentation of multiply charged negative ions on a LTQ Velos and subsequent imaging of the GD1 ganglioside is demonstrated. This is the first report of multiply charged negative ions using inlet ionization. We find that GD1 is detected at higher levels in the mouse cortex and hippocampus compared with the thalamus. In LSII with the laser aligned in transmission geometry relative to the inlet, images were obtained in approximately 60 min using an inexpensive nitrogen laser.

Structural characterization of neutral oligosaccharides by laser-enhanced in-source decay of MALDI-FTICR MS

MALDI in-source decay (ISD) technique described to date has proven to be a convenient and rapid method for sequencing purified peptides and proteins. However, the general ISD still can not produce adequate fragments for the detailed structural elucidation of oligosaccharides. In this study, an efficient and practical method termed the laser-enhanced ISD (LEISD) technique of MALDI-FTICR MS allows highly reliable and abundant fragmentation of the neutral oligosaccharides, which was attributed to the ultrahigh irradiation laser of mJ level. The yield of ISD fragmentation was evaluated under different laser powers for 7 neutral oligosaccharides using DHB as matrix. Better quality ISD spectra including fragment ions in low-mass region were obtained at higher laser power. Results from the LEISD of oligosaccharides demonstrated that a significantly better signal-to-noise ratio (S/N) and more structural information could be obtained in comparison to the conventional CID. It was also suggested that the valuable A ions derived from cross-ring cleavage of the linear oligosaccharides allowed the distinction among α(1→4)-, α(1→6)-, β(1→4)-, and β(1→3)-linked isobaric structures according to fragment types and intensities. In addition, ideal fragmentation ions observed by LEISD method facilitated the determination of the sequences and branched points of complex oligosaccharides from human milk.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Profile of native N-linked glycan structures from human serum using high performance liquid chromatography on a microfluidic chip and time-of-flight mass spectrometry

Protein glycosylation involves the addition of monosaccharides in a stepwise process requiring no glycan template. Therefore, identifying the numerous glycoforms, including isomers, can help elucidate the biological function(s) of particular glycans. A method to assess the diversity of the N-linked oligosaccharides released from human serum without derivatization has been developed using on-line nanoLC and high resolution TOF MS. The N-linked oligosaccharides were analyzed with MALDI FT-ICR MS and microchip LC MS (HPLC-Chip/TOF MS). Two microfluidic chips were employed, the glycan chip (40 nL enrichment column, 43 x 0.075 mm(2) i.d. analytical column) and the high capacity chip (160 nL enrichment column, 140 x 0.075 mm(2) i.d. analytical column), both with graphitized carbon as the stationary phase. Both chips offered good sensitivity and reproducibility in separating a heterogeneous mixture of neutral and anionic oligosaccharides between injections. Increasing the length and volume of the enrichment and the analytical columns improved resolution of the peaks. Complex type N-linked oligosaccharides were the most abundant oligosaccharides in human serum accounting for approximately 96% of the total glycans identified, while hybrid and high mannose type oligosaccharides comprise the remaining approximately 4%.

Glycoproteomic analyses of ovarian cancer cell lines and sera from ovarian cancer patients show distinct glycosylation changes in individual proteins

Ovarian cancer is difficult to diagnose in women because symptoms of the disease are often not noticed until the disease has progressed to an advanced untreatable stage. Although a serum test, CA125, is currently available to assist with monitoring treatment of ovarian cancer, this test lacks the necessary specificity and sensitivity for early detection. Therefore, better biomarkers of ovarian cancer are needed. A glycoprotein analysis approach was undertaken using high resolution Fourier transform ion cyclotron resonance mass spectrometry to analyze glycosylated proteins present in the conditioned media of ovarian cancer cell lines and in sera obtained from ovarian cancer patients and normal controls. In this study, glycosylated proteins were separated by gel electrophoresis, and individual glycoproteins were selected for glycosylation analysis and protein identification. The attached glycans from each protein were released and profiled by mass spectrometry. Glycosylation of a mucin protein and a large glycosylated protein isolated from the ES2 ovarian cancer cell line was determined to consist of mostly O-linked glycans. Four prominent glycoproteins of approximate 517, 370, 250, 163 kDa from serum samples were identified as two forms of apolipoprotein B-100, fibronectin, and immunoglobulin A1, respectively. Mass spectrometric analysis of glycans isolated from apolipoprotein B-100 (517 kD) showed the presence of small, specific O-linked oligosaccharides. In contrast, analysis of fibronectin (250 kD) and immunoglobulin A1 (163 kD) produced N-linked glycan fragments in forms that were sufficiently different from the glycans obtained from the corresponding protein band present in the normal serum samples. This study shows that not only a single protein but several are aberrantly glycosylated, and those abnormal glycosylation changes can be detected and may ultimately serve as glycan biomarkers for ovarian cancer.

Collisionally activated dissociation and electron capture dissociation provide complementary structural information for branched permethylated oligosaccharides

Doubly charged sodiated and permethylated linear malto-oligosaccharides ({Glc}6-{Glc}9), branched N-linked glycans (high-mannose type GlcNAc2Man5-9, and complex asialo- and disialylated-biantennary glycans) were analyzed by tandem mass spectrometry using collisionally-activated dissociation (CAD) and "hot" electron capture dissociation (ECD) available in a custom-built ESI FTICR mass spectrometer. For linear permethylated malto-oligosaccharides, both CAD and "hot" ECD produced glycosidic cleavages (B, Y, C, and Z ions), cross-ring cleavages (A- and X-type), and internal cleavages (B/Y and C/Y type) to provide sequence and linkage information. For the branched N-linked glycans, CAD and "hot" ECD provided complementary structural information. CAD generated abundant B and Y fragment ions by glycosidic cleavages, whereas "hot" ECD produced dominant C and Z ions. A-type cross-ring cleavages were present in CAD spectra. Complementary A- and X-type cross-ring fragmentation pairs were generated by "hot" ECD, and these delineated the branching patterns and linkage positions. For example, 0, 4An and 3, 5An ions defined the linkage position of the major branch as the 6-position of the central core mannose residue. The internal fragments observed in CAD were more numerous and abundant than in "hot" ECD spectra. Since the triply charged (sodiated) molecular ion of the permethylated disialylated-biantennary N-linked glycan has relatively high abundance, it was isolated and fragmented in a "hot" ECD experiment and extensive fragment ions (glycosidic and complementary pairs of cross-ring cleavages) were generated to fully confirm the sequence, branching, and linkage assignments for this glycan.

Peptide analysis: Solid phase extraction–elution on diamond combined with atmospheric pressure matrix-assisted laser desorption/ionization–Fourier transform ion cyclotron resonance mass spectrometry

Electrospray ionization (ESI) has been an indispensable ion generation technique for mass spectrometric analysis of biopolymers such as intact proteins and protein digests operated at atmospheric pressure. Since its advent in 1998, atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) quickly became a popular alternative for the analysis of peptides. Although AP-MALDI sources typically share the same vacuum interface and ion transmission hardware with ESI, it is generally found that ESI is superior in detection sensitivity. Here we present a method based on solid phase extraction and elution with surface-functionalized diamond nanocrystals (which we previously referred to as "SPEED") that not only streamlines AP-MALDI mass spectrometric analyses of peptides and other small biomolecules under typical operational conditions but also outruns ESI in ultimate detectable concentration by at least one order of magnitude.

Matrix-assisted laser desorption/ionization Fourier transform mass spectrometry quantitation via in cell combination

Herein we describe a novel method for quantitation using a Fourier transform mass spectrometer (FTMS) equipped with a MALDI ion source. The unique instrumental configuration of FTMS and its ion trapping and storing capabilities enable ion packets originating from two physically distinct samples to be combined in the ion cyclotron resonance (ICR) cell prior to detection. These features are exploited to combine analyte ions from two differentially labeled samples spotted separately and then combined in the ICR cell to generate a single mass spectrum containing isotopically paired peaks for quantitative comparison of relative ion abundances. The utility of this new quantitation via in cell combination (QUICC) approach is explored using peptide standards, a bovine serum albumin tryptic digest, and a crude neuronal tissue extract. We show that spectra acquired using the QUICC scheme are comparable to those obtained from premixing the isotopically labeled samples in solution. In addition, we show direct tissue in situ isotopic formaldehyde labeling of a crustacean neuroendocrine organ, thus demonstrating the potential application of the QUICC methodology for direct tissue quantitative analysis.