Combination of ESI and MALDI mass spectrometry for qualitative, semi-quantitative and in situ analysis of gangliosides in brain

Establishment of a targeted analysis method for gangliosides in mouse tissues by HILIC-ESI-MS/MS

Gangliosides play an imperative role in cell signaling, neuronal recovery, apoptosis, and other physiological processes. For example, GM3 can regulate hypothalamic leptin resistance and control energy homeostasis, GD3 can mediate cell proliferation and differentiation and induce apoptosis, and GQ1b can stimulate neurogenesis. Therefore, the present study sought to establish and optimize the targeted analysis method for ganglioside subclasses and their molecular species using hydrophilic interaction liquid chromatography-triple quadrupole-MS/MS (HILIC-QQQ-MS/MS). Additionally, the fragmentation pattern of different ganglioside subclasses and their retention time patterns were analyzed, providing more accurate qualitative results. The limit of quantitation (LOQ) was as low as 10-4 ng. Moreover, the molecular species of gangliosides in the liver, cortex, and hypothalamus of C57BL/6 mice were analyzed using the established method. A total of 23 ganglioside subclasses with 164 molecular species, including 40 O-acetylated ganglioside molecular species and 28 NeuGc ganglioside molecular species, were identified using the semi-quantitative analysis method of an external standard curve corrected by an internal standard. In addition to NeuGc gangliosides, the contents of ganglioside subclasses were more abundant in the mouse brain than those in the mouse liver; especially, the contents of unsaturated gangliosides in the hypothalamus were much higher than those in the liver. Among them, O-acetylated gangliosides were detected only in the cortex and hypothalamus at a concentration of up to 100 μg/mg protein (40 molecular species). Overall, the proposed method expanded the detectable number of ganglioside subclasses and molecular species in biological samples and provided more opportunities for further study of the biological functions of gangliosides.

MALDI TIMS IMS Reveals Ganglioside Molecular Diversity within Murine S. aureus Kidney Tissue Abscesses

Gangliosides play important roles in innate and adaptive immunity. The high degree of structural heterogeneity results in significant variability in ganglioside expression patterns and greatly complicates linking structure and function. Structural characterization at the site of infection is essential in elucidating host ganglioside function in response to invading pathogens, such as Staphylococcus aureus (S. aureus). Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) enables high-specificity spatial investigation of intact gangliosides. Here, ganglioside structural and spatial heterogeneity within an S. aureus-infected mouse kidney abscess was characterized. Differences in spatial distributions were observed for gangliosides of different classes and those that differ in ceramide chain composition and oligosaccharide-bound sialic acid. Furthermore, integrating trapped ion mobility spectrometry (TIMS) allowed for the gas-phase separation and visualization of monosialylated ganglioside isomers that differ in sialic acid type and position. The isomers differ in spatial distributions within the host-pathogen interface, where molecular patterns revealed new molecular zones in the abscess previously unidentified by traditional histology.

Frontiers in Mass Spectrometry-Based Spatial Metabolomics: Current Applications and Challenges in the Context of Biomedical Research

Metabolites are critical products and mediators of cellular and tissue function, and key signals in cell-to-cell, organ-to-organ and cross-organism communication. Many of these interactions are spatially segregated. Thus, spatial metabolomics can provide valuable insight into healthy tissue function and disease pathogenesis. Here, we review major mass spectrometry-based spatial metabolomics techniques and the biological insights they have enabled, with a focus on brain and microbiota function and on cancer, neurological diseases and infectious diseases. These techniques also present significant translational utility, for example in cancer diagnosis, and for drug development. However, spatial mass spectrometry techniques still encounter significant challenges, including artifactual features, metabolite annotation, open data, and ethical considerations. Addressing these issues represent the future challenges in this field.

Hepatic Topology of Glycosphingolipids in Schistosoma mansoni-Infected Hamsters

Schistosomiasis is a neglected tropical disease caused by worm parasites of the genus Schistosoma. Upon infection, parasite eggs can lodge inside of host organs like the liver. This leads to granuloma formation, which is the main cause of the pathology of schistosomiasis. To better understand the different levels of host-pathogen interaction and pathology, our study focused on the characterization of glycosphingolipids (GSLs). For this purpose, GSLs in livers of infected and noninfected hamsters were studied by combining high-spatial-resolution atmospheric-pressure scanning microprobe matrix-assisted laser desorption/ionization mass spectrometry imaging (AP-SMALDI MSI) with nanoscale hydrophilic interaction liquid chromatography tandem mass spectrometry (nano-HILIC MS/MS). Nano-HILIC MS/MS revealed 60 GSL species with a distinct saccharide and ceramide composition. AP-SMALDI MSI measurements were conducted in positive- and negative-ion mode for the visualization of neutral and acidic GSLs. Based on nano-HILIC MS/MS results, we discovered no downregulated but 50 significantly upregulated GSLs in liver samples of infected hamsters. AP-SMALDI MSI showed that 44 of these GSL species were associated with the granulomas in the liver tissue. Our findings suggest an important role of GSLs during granuloma formation.

Advances in Mass Spectrometry of Gangliosides Expressed in Brain Cancers

Gangliosides are highly abundant in the human brain where they are involved in major biological events. In brain cancers, alterations of ganglioside pattern occur, some of which being correlated with neoplastic transformation, while others with tumor proliferation. Of all techniques, mass spectrometry (MS) has proven to be one of the most effective in gangliosidomics, due to its ability to characterize heterogeneous mixtures and discover species with biomarker value. This review highlights the most significant achievements of MS in the analysis of gangliosides in human brain cancers. The first part presents the latest state of MS development in the discovery of ganglioside markers in primary brain tumors, with a particular emphasis on the ion mobility separation (IMS) MS and its contribution to the elucidation of the gangliosidome associated with aggressive tumors. The second part is focused on MS of gangliosides in brain metastases, highlighting the ability of matrix-assisted laser desorption/ionization (MALDI)-MS, microfluidics-MS and tandem MS to decipher and structurally characterize species involved in the metastatic process. In the end, several conclusions and perspectives are presented, among which the need for development of reliable software and a user-friendly structural database as a search platform in brain tumor diagnostics.

MALDI TIMS IMS of Disialoganglioside Isomers─GD1a and GD1b in Murine Brain Tissue

Gangliosides are acidic glycosphingolipids, containing ceramide moieties and oligosaccharide chains with one or more sialic acid residue(s) and are highly diverse isomeric structures with distinct biological roles. Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) enables the untargeted spatial analysis of gangliosides, among other biomolecules, directly from tissue sections. Integrating trapped ion mobility spectrometry with MALDI IMS allows for the analysis of isomeric lipid structures in situ. Here, we demonstrate the gas-phase separation and identification of disialoganglioside isomers GD1a and GD1b that differ in the position of a sialic acid residue, in multiple samples, including a standard mixture of both isomers, a biological extract, and directly from thin tissue sections. The unique spatial distributions of GD1a/b (d36:1) and GD1a/b (d38:1) isomers were determined in rat hippocampus and spinal cord tissue sections, demonstrating the ability to structurally characterize and spatially map gangliosides based on both the carbohydrate chain and ceramide moieties.

Mass Spectrometry-Based Analysis of Lipid Involvement in Alzheimer’s Disease Pathology—A Review

Irregularities in lipid metabolism have been linked to numerous neurodegenerative diseases. The roles of abnormal brain, plasma, and cerebrospinal fluid (CSF) lipid levels in Alzheimer’s disease (AD) onset and progression specifically have been described to a great extent in the literature. Apparent hallmarks of AD include, but are not limited to, genetic predisposition involving the APOE Ɛ4 allele, oxidative stress, and inflammation. A common culprit tied to many of these hallmarks is disruption in brain lipid homeostasis. Therefore, it is important to understand the roles of lipids, under normal and abnormal conditions, in each process. Lipid influences in processes such as inflammation and blood–brain barrier (BBB) disturbance have been primarily studied via biochemical-based methods. There is a need, however, for studies focused on uncovering the relationship between lipid irregularities and AD by molecular-based quantitative analysis in transgenic animal models and human samples alike. In this review, mass spectrometry as it has been used as an analytical tool to address the convoluted relationships mentioned above is discussed. Additionally, molecular-based mass spectrometry strategies that should be used going forward to further relate structure and function relationships of lipid irregularities and hallmark AD pathology are outlined.

Proteomic Approaches to Unravel Mechanisms of Antibiotic Resistance and Immune Evasion of Bacterial Pathogens

The profound effects of and distress caused by the global COVID-19 pandemic highlighted what has been known in the health sciences a long time ago: that bacteria, fungi, viruses, and parasites continue to present a major threat to human health. Infectious diseases remain the leading cause of death worldwide, with antibiotic resistance increasing exponentially due to a lack of new treatments. In addition to this, many pathogens share the common trait of having the ability to modulate, and escape from, the host immune response. The challenge in medical microbiology is to develop and apply new experimental approaches that allow for the identification of both the microbe and its drug susceptibility profile in a time-sensitive manner, as well as to elucidate their molecular mechanisms of survival and immunomodulation. Over the last three decades, proteomics has contributed to a better understanding of the underlying molecular mechanisms responsible for microbial drug resistance and pathogenicity. Proteomics has gained new momentum as a result of recent advances in mass spectrometry. Indeed, mass spectrometry-based biomedical research has been made possible thanks to technological advances in instrumentation capability and the continuous improvement of sample processing and workflows. For example, high-throughput applications such as SWATH or Trapped ion mobility enable the identification of thousands of proteins in a matter of minutes. This type of rapid, in-depth analysis, combined with other advanced, supportive applications such as data processing and artificial intelligence, presents a unique opportunity to translate knowledge-based findings into measurable impacts like new antimicrobial biomarkers and drug targets. In relation to the Research Topic “Proteomic Approaches to Unravel Mechanisms of Resistance and Immune Evasion of Bacterial Pathogens,” this review specifically seeks to highlight the synergies between the powerful fields of modern proteomics and microbiology, as well as bridging translational opportunities from biomedical research to clinical practice.

Manipulation of Ion Types via Gas-Phase Ion/Ion Chemistry for the Structural Characterization of the Glycan Moiety on Gangliosides

Gangliosides are the most abundant glycolipid among eukaryotic cell membranes and consist of a glycan head moiety containing one or more sialic acids and a ceramide chain. The analysis of the glycan moieties among different subclass gangliosides, including GM, GD, and GT gangliosides, remains a challenge for shotgun lipidomics. Here, we present a novel shotgun lipidomics approach employing gas-phase ion/ion chemistry. The gas-phase derivatization strategy provides a rapid way to manipulate the ion-types of the precursor ions, and, in conjunction with collision induced dissociation (CID), allows for the elucidation of the structures of the glycan moieties from gangliosides. In addition to the enhancement of structural characterization, gas-phase ion chemistry leads to a form of purification of the precursor ions prior to CID by neutralizing isobaric or isomeric ions with different charge states but with similar or identical m/z values. To demonstrate the proposed strategy, both deprotonated GM3 and GM1 gangliosides ([GM-H]^-) were isolated and subjected to reaction with magnesium-Terpy complex cations ([Mg(Terpy)₂]²⁺). The post-reaction product spectra show the elimination of possible contamination, illustrating the ability of charge-switching derivatization to purify the precursor ions. Isomeric differentiation between GD1a and GD1b was achieved by the sequential ion/ion reactions, with the CID of [GD1-H+Mg]⁺ showing diagnostic fragment ions from the isomers. Moreover, isomeric identification among GT1a, GT1b, and GT1c was accomplished while performing a gas-phase magnesium transfer reaction and CID. Lastly, the presented workflow was applied to ganglioside profiling in a porcine brain extract. In total, 34 gangliosides were profiled among only 20 precursor ion m/z values by resolving isomers. Furthermore, the fucosylation site on GM1 and GD1, and N-glycolylneuraminic acid conjugated GT1 isomers was identified. Relative quantification of isomeric two isomeric pairs, GD1a/b C36:1 and GD1a/b C38:1 was also achieved using pure component product ion spectra coupled with a total least-squares method. The results demonstrate the applicability and strength of using shotgun MS coupled with gas-phase ion/ion chemistry to characterize the glycan moiety structures on different subclasses of gangliosides.

Biopharmaceutical quality control with mass spectrometry

Mass spectrometry (MS) is a powerful technique for protein identification, quantification and characterization that is widely applied in biochemical studies, and which can provide data on the quantity, structural integrity and post-translational modifications of proteins. It is therefore a versatile and widely used analytic tool for quality control of biopharmaceuticals, especially in quantifying host-cell protein impurities, identifying post-translation modifications and structural characterization of biopharmaceutical proteins. Here, we summarize recent advances in MS-based analyses of these key quality attributes of the biopharmaceutical development and manufacturing processes.

Mass Spectrometry Imaging for Glycome in the Brain

Glycans are diverse structured biomolecules that play crucial roles in various biological processes. Glycosylation, an enzymatic system through which various glycans are bound to proteins and lipids, is the most common and functionally crucial post-translational modification process. It is known to be associated with brain development, signal transduction, molecular trafficking, neurodegenerative disorders, psychopathologies, and brain cancers. Glycans in glycoproteins and glycolipids expressed in brain cells are involved in neuronal development, biological processes, and central nervous system maintenance. The composition and expression of glycans are known to change during those physiological processes. Therefore, imaging of glycans and the glycoconjugates in the brain regions has become a “hot” topic nowadays. Imaging techniques using lectins, antibodies, and chemical reporters are traditionally used for glycan detection. However, those techniques offer limited glycome detection. Mass spectrometry imaging (MSI) is an evolving field that combines mass spectrometry with histology allowing spatial and label-free visualization of molecules in the brain. In the last decades, several studies have employed MSI for glycome imaging in brain tissues. The current state of MSI uses on-tissue enzymatic digestion or chemical reaction to facilitate successful glycome imaging. Here, we reviewed the available literature that applied MSI techniques for glycome visualization and characterization in the brain. We also described the general methodologies for glycome MSI and discussed its potential use in the three-dimensional MSI in the brain.

Maternal diet with sea urchin gangliosides promotes neurodevelopment of young offspring via enhancing NGF and BDNF expression

Neurodevelopment of fetal and infant brains is an essential process not just during infancy but throughout the whole life. Previous studies have verified the neurotrophic effects of GM1 and milk gangliosides (GLSs) on brain development. However, it remains unclear whether the maternal GLS diet during the perinatal period can program the brain development of young offspring. Sea urchin, as a popular sea food, is a good resource of marine-derived GLSs. This study evaluated the effects of maternal diet with sea urchin gangliosides (SU-GLSs) on the utero and neonatal neurodevelopment and compared their efficacy with common GM1 and sialic acid (SA). Herein, SU-GLSs, as well as GM1 and SA, were orally administered to pregnant mice from pregnancy to lactation. The morphological and functional development of the brain was evaluated in postnatal 15-day (P15) mice. SU-GLSs were superior to GM1 and SA in enhancing neuritogenesis, spinous dendrite growth and synapse function in the hippocampus and cortex of P15 mice. Mechanistic studies found that SU-GLSs upregulated the expressions of NGF and BDNF more effectively than GM1 and SA. Furthermore, different glycosylated SU-GLSs promoted the neural differentiation of Neuro2a cells in a structure-selective manner. Sulfate-type and disialo-type GLSs were more effective than GM1. These findings suggested that maternal SU-GLS diet could promote the neurodevelopment of young offspring and would be a potential nutrition enriching substance for the early developing brain.

ANALYSIS OF CARBOHYDRATES AND GLYCOCONJUGATES BY MATRIX-ASSISTED LASER DESORPTION/IONIZATION MASS SPECTROMETRY: AN UPDATE FOR 2015-2016

This review is the ninth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2016. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented over 30 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show no sign of deminishing. © 2020 Wiley Periodicals, Inc.

Ganglioside isomer analysis using ion polarity switching liquid chromatography-tandem mass spectrometry

Gangliosides are ubiquitously present on cell surface. They are more abundantly expressed in nerve cells and tissues and involved in pathology of various diseases. Diversity of molecular structures in the carbohydrate head group, fatty acyl, and long chain base increases the complexity of analyzing gangliosides. In this study, an ultrahigh-performance liquid chromatography-tandem mass spectrometry method is developed for analysis of the co-eluting ganglioside isomers, which uses ion polarity switching to integrate glycan head isomer identification, ceramide isomer differentiation, and quantification of ganglioside into one analysis. The method is facilitated with an extensive ganglioside target list by combining the various glycan head groups, long chain bases, and the experimentally determined fatty acyls. Correlation between the retention time of ganglioside and its ceramide total carbon number is experimentally validated and used to predict retention time of ganglioside target list for scheduling the final multiple reaction monitoring method. This method was validated according to the FDA guidelines: 96.5% of gangliosides with good accuracy (80-120%), precision (< 15%), and linearity R² > 0.99. The authenticated gangliosides were quantified from mouse brain by isotope dilution. Overall, 165 gangliosides were quantified using 10 mg mouse brain tissue, including 100 isomers of GM1, GM2, GM3, GD1a, GD1b, GD2, GD3, and GT1b.

Streamlined Multimodal DESI and MALDI Mass Spectrometry Imaging on a Singular Dual-Source FT-ICR Mass Spectrometer

The study of biological specimens by mass spectrometry imaging (MSI) has had a profound influence in the various forms of spatial-omics over the past two decades including applications for the identification of clinical biomarker analysis; the metabolic fingerprinting of disease states; treatment with therapeutics; and the profiling of lipids, peptides and proteins. No singular approach is able to globally map all biomolecular classes simultaneously. This led to the development of many complementary multimodal imaging approaches to solve analytical problems: fusing multiple ionization techniques, imaging microscopy or spectroscopy, or local extractions into robust multimodal imaging methods. However, each fusion typically requires the melding of analytical information from multiple commercial platforms, and the tandem utilization of multiple commercial or third-party software platforms—even in some cases requiring computer coding. Herein, we report the use of matrix-assisted laser desorption/ionization (MALDI) in tandem with desorption electrospray ionization (DESI) imaging in the positive ion mode on a singular commercial orthogonal dual-source Fourier transform ion cyclotron resonance (FT-ICR) instrument for the complementary detection of multiple analyte classes by MSI from tissue. The DESI source was 3D printed and the commercial Bruker Daltonics software suite was used to generate mass spectrometry images in tandem with the commercial MALDI source. This approach allows for the generation of multiple modes of mass spectrometry images without the need for third-party software and a customizable platform for ambient ionization imaging. Highlighted is the streamlined workflow needed to obtain phospholipid profiles, as well as increased depth of coverage of both annotated phospholipid, cardiolipin, and ganglioside species from rat brain with both high spatial and mass resolution.

Open-flow microperfusion combined with mass spectrometry for in vivo liver lipidomic analysis

At present, conventional microdialysis (MD) techniques cannot efficiently sample lipids in vivo, possibly due to the high mass transfer resistance and/or the serious adsorption of lipids onto the semi-permeable membrane of a MD probe. The in vivo monitoring of lipids could be of great significance for the study of disease development and mechanisms. In this work, an open-flow microperfusion (OFM) probe was fabricated, and the conditions for sampling lipids via OFM were optimized. Using OFM, the recovery of lipid standards was improved to more than 34.7%. OFM is used for the in vivo sampling of lipids in mouse liver tissue with fibrosis, and it is then combined with mass spectrometry (MS) to perform lipidomic analysis. 156 kinds of lipids were identified in the dialysate collected via OFM, and it was found that the phospholipid levels, including PC, PE, and SM, were significantly higher in a liver suffering from fibrosis. For the first time, OFM combined with MS to sample and analyze lipids has provided a promising platform for in vivo lipidomic studies.

Regional N-glycan and lipid analysis from tissues using MALDI-mass spectrometry imaging

N-glycans and lipids are structural metabolites that play important roles in cellular processes. Both show unique regional distribution in tissues; therefore, spatial analyses of these metabolites are crucial to our understanding of cellular physiology. Matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) is an innovative technique that enables in situ detection of analytes with spatial distribution. This workflow details a MALDI-MSI protocol for the spatial profiling of N-glycans and lipids from tissues following application of enzyme and MALDI matrix.

For complete details on the use and execution of this protocol, please refer to Drake et al. (2018) and Andres et al. (2020).

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MALDI-MSI of N-glycans and lipids from tissues

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Enzyme-assisted release of N-linked glycans from formalin-fixed tissues

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High-velocity deposition of matrix to improve rigor and reproducibility

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Ion mobility separation of analytes from matrix as part of imaging workflow

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.

Combinatory Data-Independent Acquisition and Parallel Reaction Monitoring Method for Deep Profiling of Gangliosides

Ganglioside is an important class of lipid species involved in intercellular signaling and various diseases, especially for neurodegenerative diseases. Systematic ganglioside profiling is challenging because of their naturally low abundance and highly diverse species. Herein, a new data-independent acquisition and parallel reaction monitoring (DIA/PRM) method with superior sensitivity was developed. The untargeted DIA acquisition consecutively records all the precursor ion and fragment ions at the same time, while the targeted PRM analysis with versatile higher collisional dissociation generates full MS/MS spectra for structure elucidation and verification. As compared with traditional data-dependent acquisition (DDA), the DIA/PRM method unbiasedly detected the majority of abundant ganglioside species and as low as 50 pg of ganglioside in an untargeted manner. Gangliosides in four kinds of biological samples including the mouse brain, mouse plasma, HeLa cell, and human colon cancer tissue were systematically identified, and low-abundance ganglioside species were further extended on the basis of linear chromatography retention rules of the most frequently detected ganglioside species. A total of 383 ganglioside features were defined with 329 of them derived from 32 ganglioside species. Taking advantage of the high-resolution MS analysis, rare ganglioside species were further elucidated according to their characteristic fragment ions and neutral losses. In total, 18 gangliosides with a ceramide carbon number from 20 to 25 and modified gangliosides, including 18 acetylated, 8 diacetylated, 1 phosphorylated, 36 N-glycolyneuraminic acid (NeuGc)-containing, and 7 di-NeuGc-containing gangliosides, were newly identified. The developed DIA/PRM method therefore generated a rich ganglioside resource for further functional exploration and is a unique alternative for DDA analysis for global ganglioside profiling in various biological systems.

Lipid rafts and neurodegeneration: structural and functional roles in physiologic aging and neurodegenerative diseases

Lipid rafts are small, dynamic membrane areas characterized by the clustering of selected membrane lipids as the result of the spontaneous separation of glycolipids, sphingolipids, and cholesterol in a liquid-ordered phase. The exact dynamics underlying phase separation of membrane lipids in the complex biological membranes are still not fully understood. Nevertheless, alterations in the membrane lipid composition affect the lateral organization of molecules belonging to lipid rafts. Neural lipid rafts are found in brain cells, including neurons, astrocytes, and microglia, and are characterized by a high enrichment of specific lipids depending on the cell type. These lipid rafts seem to organize and determine the function of multiprotein complexes involved in several aspects of signal transduction, thus regulating the homeostasis of the brain. The progressive decline of brain performance along with physiological aging is at least in part associated with alterations in the composition and structure of neural lipid rafts. In addition, neurodegenerative conditions, such as lysosomal storage disorders, multiple sclerosis, and Parkinson's, Huntington's, and Alzheimer's diseases, are frequently characterized by dysregulated lipid metabolism, which in turn affects the structure of lipid rafts. Several events underlying the pathogenesis of these diseases appear to depend on the altered composition of lipid rafts. Thus, the structure and function of lipid rafts play a central role in the pathogenesis of many common neurodegenerative diseases.jlr;61/5/636/F1F1f1.

Ganglioside Detection from Formalin-Fixed Human Brain Tissue Utilizing MALDI Imaging Mass Spectrometry

Matrix assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS) is used to perform mass spectrometric analysis directly on biological samples providing visual and anatomical spatial information on molecules within tissues. A current obscuration of MALDI-IMS is that it is largely performed on fresh frozen tissue, whereas clinical tissue samples stored long-term are fixed in formalin, and the fixation process is thought to cause signal suppression for lipid molecules. Studies have shown that fresh frozen tissue sections applied with an ammonium formate (AF) wash prior to matrix application in the MALDI-IMS procedure display an increase in observed signal intensity and sensitivity for lipid molecules detected within the brain while maintaining the spatial distribution of molecules throughout the tissue. In this work, we investigate the viability of formalin-fixed tissue imaging in a clinical setting by comparing MALDI data of fresh frozen and postfixed rat brain samples, along with postfixed human brain samples washed with AF to assess the capabilities of ganglioside analysis in MALDI imaging of formalin-fixed tissue. Results herein demonstrate that MALDI-IMS spectra for gangliosides, including GM1, were significantly enhanced in fresh frozen rat brain, formalin-fixed rat brain, and formalin-fixed human brain samples through the use of an AF wash. Improvements in MALDI-IMS image quality were demonstrated, and the spatial distribution of molecules was retained. Results indicate that this method will allow for the analysis of gangliosides from formalin-fixed clinical samples, which can open additional avenues for neurodegenerative disease research.

Developing IR-780 as a Novel Matrix for Enhanced MALDI MS Imaging of Endogenous High-Molecular-Weight Lipids in Brain Tissues

The matrix plays a prominent role in expanding the ability of matrix assisted laser desorption/ionization mass spectrometry (MALDI MS). However, on account of the unclarity of necessary properties of the matrix in MALDI MS, development of a new matrix is still in the exploratory stage and lacks systematic theoretical guidance. Meanwhile, most of the existing matrices are unable to simultaneously detect various high-molecular-weight (high-MW) lipids including (poly-)phosphoinositides, cardiolipins, and gangliosides. In this study, we have successfully screened and optimized the application of commercially available IR-780 as a novel matrix for simultaneously profiling and imaging high-MW lipids in brain tissues by MALDI MS for the first time. The properties of IR-780 related to the matrix of MALDI MS, mainly including the optical properties (UV absorption, fluorescence emission, and photothermal efficiency), proton affinity, collision cross-sections (CCSs), salt-tolerance ability, and homogeneity, were comprehensively characterized, which demonstrated that high photothermal ability and large CCSs might guarantee the superior performance of IR-780 as matrix for the analysis of high-MW lipids in biological samples. This work provided some references for the development of a novel matrix, and especially, the concept of CCS was first introduced as a parameter for the development of a matrix. In addition, the simultaneous identification and imaging of endogenous high-MW lipids in rat brain tissues subjected to traumatic brain injury were successfully performed.

Polydopamine-capped AgNPs as a novel matrix overcoming the ion suppression of phosphatidylcholine for MALDI MS comprehensive imaging of glycerophospholipids and sphingolipids in impact-induced injured brain

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is a powerful tool for the characterization and localization of analytes without the need for extraction, purification, separation or labeling of samples. However, in tissue sections the most abundant lipids, phosphatidylcholines (PCs), could suppress the signals of other classes of coexisting lipids. In this work, polydopamine (PDA)-capped AgNPs (AgNPs@PDA) were synthesized as a matrix of MALDI MSI to analyze lipids in both positive and negative ion modes. By adjusting the thickness of the PDA layer, the signal of silver cluster ions of AgNPs@PDA was effectively controlled, and the ability of AgNPs@PDA serving as a matrix was optimized. More interestingly, using AgNPs@PDA as a matrix, the sensitivity of PCs was dramatically decreased, and the PC signals were greatly suppressed, while for other lipids (including PE, HexCer, PS, PI, PIP, and ST), they were just the opposite. The reason, we believe, is related to the positively charged surface of AgNPs@PDA, and the polyhydroxy and amino groups of PDA. Benefitting from the suppression of the signals of PCs and the improvement of detection sensitivity of other lipids, 58 glycerophospholipids and 25 sphingolipids in brain tissue sections could be imaged in one run with AgNPs@PDA as a matrix by MALDI MSI, much better than when using traditional organic matrices 2,5-dihydroxybenzoic acid and 9-aminoacridine.

Soft Matrix-Assisted Laser Desorption/Ionization for Labile Glycoconjugates

Since its introduction, matrix-assisted laser desorption/ionization (MALDI) has been widely used for the mass analysis of biomolecules. The "soft ionization" of MALDI enables accurate mass determination of intact biomolecules. However, the ionization and desorption processes of MALDI are not adequately soft as many labile biomolecules, such as glycoconjugates containing sialic acid or the sulfate functional group, easily dissociate into fragments and sometimes, no intact molecules are observed. In this study, we compared the conventional matrix of MALDI, namely 2,5-dihydroxybenzoic acid, to various soft matrices of MALDI-specifically, 5-methoxysalicylic acid, diamond nanoparticle trilayers, HgTe nanostructures, ionic liquid, and droplets of frozen solutions-by using three labile glycoconjugates as analytes: gangliosides, heparin, and pullulan. We demonstrated that droplets of frozen solution are the softest matrices for gangliosides and heparin. In particular, droplets of frozen solution do not generate fragments for gangliosides and can be used to determine the relative abundance of various gangliosides, whereas ionic liquid 2,5-dihydroxybenzoic acid butylamine is the most suitable matrix for pullulan mass analysis. Graphical Abstract.

Identification of neutral and acidic glycosphingolipids in the human dermal fibroblasts

Skin fibroblasts are recognized as a valuable model of primary human cells able of mirroring the chronological and biological aging. Here, a lipidomic study of glycosphingolipids (GSL) occurring in the easily accessible human dermal fibroblasts (HDF) is presented. Reversed-phase liquid chromatography with negative electrospray ionization (RPLC-ESI) coupled to either orbitrap or linear ion-trap multiple-stage mass spectrometry was applied to characterize GSL in commercially adult and neonatal primary human fibroblast cells and in skin samples taken from an adult volunteer. Collision-induced dissociation in negative ion mode allowed us to get information on the monosaccharide number and ceramide composition, whereas tandem mass spectra on the ceramide anion was useful to identify the sphingoid base. Nearly sixty endogenous GSL species were successfully recognized, namely 33 hexosyl-ceramides (i.e., HexCer, Hex₂Cer and Hex₃Cer) and 24 gangliosides as monosialic acid GM1, GM2 and GM3, along with 5 globosides Gb4. An average content of GSLs was attained and the most representative GSL in skin fibroblasts were Hex₃Cer, also known as Gb3Cer, followed by Gb4, HexCer and Hex₂Cer , while gangliosides were barely quantifiable. The most abundant GSLs in the examined cell lines share the same ceramide base (i.e. d18:1) and the relative content was d18:1/24:1 > d18:1/24:0 > d18:1/16:0 > d18:1/22:0.

3-Aminophthalhydrazide (Luminol) As a Matrix for Dual-Polarity MALDI MS Imaging

In many aspects of the matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) technique, the discovery of new MALDI matrixes has been a major task for the improvement of ionization efficiency, signal intensity, and molecular coverage. In this work, five analog compounds, including phthalhydrazide, 3-aminophthalhydrazide (3-APH or luminol) and its sodium salt, 4-aminophthalhydrazide (4-APH), and 3-nitrophthalhydrazide (3-NPH) were evaluated as potential matrixes for MALDI Fourier-transform ion cyclotron resonance (FTICR) MSI of metabolites in mouse brain tissue. The five candidate MALDI matrixes were mainly evaluated according to the solid-state ultraviolet absorption, the ion yields and species, and the dual-polarity detection. Among the five candidate matrixes, 3-APH and its sodium salt enabled the detection of endogenous metabolites better than the three other candidates in dual polarities. The best results were observed with 3-APH. Compared with commonly used MALDI matrixes such as 2,5-dihydroxybenzoic acid, α-cyano-4-hydroxycinnamic acid, and 9-aminoacridine, 3-APH exhibited superior performance in dual polarity MALDI MSI, higher sensitivity, broader molecular coverage, and lower background noise. The use of 3-APH led to on-tissue MALDI FTICR MSI of 159 and 207 mouse brain metabolites in the positive and negative ion modes, respectively. Among these metabolites, nucleotides, fatty acids, glycerolipids, glycerophospholipids, sphingolipids, and saccharolipids are included. 3-APH was further used for MALDI FTICR MSI of metabolic responses to ischemia-induced disturbances in mouse brain subjected to middle cerebral artery occlusion (MCAO), thus revealing the alteration of 105 metabolites in the ipsilateral hemispheres. This further emphasizes the great potential of 3-APH as a matrix for the localization of biomarkers in brain diseases.

Mass spectrometry is a multifaceted weapon to be used in the battle against Alzheimer's disease: Amyloid beta peptides and beyond

Amyloid-β peptide (Aβ) accumulation and aggregation have been considered for many years the main cause of Alzheimer's disease (AD), and therefore have been the principal target of investigation as well as of the proposed therapeutic approaches (Grasso [2011] Mass Spectrom Rev. 30: 347-365). However, the amyloid cascade hypothesis, which considers Aβ accumulation the only causative agent of the disease, has proven to be incomplete if not wrong. In recent years, actors such as metal ions, oxidative stress, and other cofactors have been proposed as possible co-agents or, in some cases, main causative factors of AD. In this scenario, MS investigation has proven to be fundamental to design possible diagnostic strategies of this elusive disease, as well as to understand the biomolecular mechanisms involved, in the attempt to find a possible therapeutic solution. We review the current applications of MS in the search for possible Aβ biomarkers of AD to help the diagnosis of the disease. Recent examples of the important contributions that MS has given to prove or build theories on the molecular pathways involved with such terrible disease are also reviewed.

Mass spectrometry for protein sialoglycosylation

Sialic acids are a family of structurally unique and negatively charged nine-carbon sugars, normally found at the terminal positions of glycan chains on glycoproteins and glycolipids. The glycosylation of proteins is a universal post-translational modification in eukaryotic species and regulates essential biological functions, in which the most common sialic acid is N-acetyl-neuraminic acid (2-keto-5-acetamido-3,5-dideoxy-D-glycero-D-galactononulopyranos-1-onic acid) (Neu5NAc). Because of the properties of sialic acids under general mass spectrometry (MS) conditions, such as instability, ionization discrimination, and mixed adducts, the use of MS in the analysis of protein sialoglycosylation is still challenging. The present review is focused on the application of MS related methodologies to the study of both N- and O-linked sialoglycans. We reviewed MS-based strategies for characterizing sialylation by analyzing intact glycoproteins, proteolytic digested glycopeptides, and released glycans. The review concludes with future perspectives in the field.

Improved spectra for MALDI MSI of peptides using ammonium phosphate monobasic in MALDI matrix

MALDI mass spectrometry imaging (MSI) enables analysis of peptides along with histology. However, there are several critical steps in MALDI MSI of peptides, 1 of which is spectral quality. Suppression of MALDI matrix clusters by the aid of ammonium salts in MALDI experiments is well known. It is asserted that addition of ammonium salts dissociates potential matrix adducts and thereafter decreases matrix cluster formation. Consequently, MALDI MS sensitivity and mass accuracy increase. Up to our knowledge, a limited number of MALDI MSI studies used ammonium salts as matrix additives to suppress matrix clusters and enhance peptide signals. In this work, we investigated the effect of ammonium phosphate monobasic (AmP) as alpha-cyano-4-hydroxycinnamic acid (α-CHCA) matrix additive in MALDI MSI of peptides. Prior to MALDI MSI, the effect of varying concentrations of AmP in α-CHCA was assessed in bovine serum albumin tryptic digests and compared with the control (α-CHCA without AmP). Based on our data, the addition of AmP as matrix additive decreased matrix cluster formation regardless of its concentration, and specifically, 8 mM AmP and 10 mM AmP increased bovine serum albumin peptide signal intensities. In MALDI MSI of peptides, both 8 and 10 mM AmP in α-CHCA improved peptide signals especially in the mass range of m/z 2000 to 3000. In particular, 9 peptide signals were found to have differential intensities within the tissues deposited with AmP in α-CHCA (AUC > 0.60). To the best of our knowledge, this is the first MALDI MSI of peptides work investigating different concentrations of AmP as α-CHCA matrix additive to enhance peptide signals in formalin-fixed paraffin-embedded (FFPE) tissues. Further, AmP as part of α-CHCA matrix could enhance protein identifications and support MALDI MSI-based proteomic approaches.

Visualization of Brain Gangliosides Using MALDI Imaging Mass Spectrometry

Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) is a histological method used for various molecules including gangliosides. The method is based on mass spectrometry, and thus target molecules with small structural differences are directly and independently detected. Here we describe a general procedure and related key notes for analyzing major brain gangliosides by MALDI-IMS.

Ion mobility mass spectrometry provides novel insights into the expression and structure of gangliosides in the normal adult human hippocampus

General work-flow for ganglioside analysis by IM-MS.

Quantitative Screening of Cell-Surface Gangliosides by Nondestructive Extraction and Hydrophobic Collection

A screening strategy involving designed extractors and collectors was used for the nondestructive quantitation of gangliosides on cell surfaces. The extractors were constructed by functionalizing maleimide silica bubbles with a DNA probe, which contains an endonuclease cleavage site and a boronic acid end to extract cell-surface sialic acid-containing compounds through simple centrifugation. After the extractors containing the extracted compounds were incubated with endonuclease, the released oligonucleotide-gangliosides were selectively collected by silanized collector bubbles through hydrophobic interactions. The in vitro fluorescent signals from the collectors were used for the quantitation of cell-surface gangliosides. By combining with sialidase cleavage, a protocol for the identification of ganglioside subtypes was developed. The successful monitoring of the regeneration of cell-surface gangliosides demonstrates the potential of this strategy in probing related biological processes.

Imaging mass spectrometry identifies prognostic ganglioside species in rodent intracranial transplants of glioma and medulloblastoma

Matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (MALDI-MSI) allows us to investigate the distribution of lipid molecules within tissues. We used MALDI-MSI to identify prognostic gangliosides in tissue sections of rat intracranial allografts of rat glioma and mouse intracranial xenografts of human medulloblastoma. In the healthy adult rodent brain, GM1 and GD1 were the main types of glycolipids. Both gangliosides were absent in both intracranial transplants. The ganglioside GM3 was not present in the healthy adult brain but was highly expressed in rat glioma allografts. In combination with tandem mass spectrometry GM3 (d18:1/C24:0) was identified as the most abundant ganglioside species in the glioma allotransplant. By contrast, mouse xenografts of human medulloblastoma were characterized by prominent expression of the ganglioside GM2 (d18:0/C18:0). Together, these data demonstrate that tissue-based MALDI-MSI of gangliosides is able to discriminate between different brain tumors and may be a useful clinical tool for their classification and grading.

Improved MALDI imaging MS analysis of phospholipids using graphene oxide as new matrix

Matrix assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) is an increasingly important technique for detection and spatial localization of phospholipids on tissue. Due to the high abundance and being easy-to-ionize of phosphatidylcholine (PC), therefore, selecting matrix to yield signals of other lipids has become the most crucial factor for a successful MALDI-IMS analysis of phospholipids. Herein, graphene oxide (GO) was proposed as a new matrix to selectively enhance the detection of other types of phospholipids that are frequently suppressed by the presence of PC in positive mode. Compared to the commonly used matrix DHB, GO matrix significantly improved signal-to-noise ratios of phospholipids as a result of its high desorption/ionization efficiency for nonpolar compounds. Also, GO afforded homogeneous crystallizations with analytes due to its monolayer structure and good dispersion, resulting in better reproducibility of shot-to-shot (CV < 13%) and spot-to-spot (CV < 14%) analysis. Finally, GO matrix was successfully applied to simultaneous imaging of PC, PE, PS and glycosphingolipid in the mouse brain, with a total of 65 phospholipids identified.

MALDI imaging delineates hippocampal glycosphingolipid changes associated with neurotoxin induced proteopathy following neonatal BMAA exposure

The environmental toxin β-N-methylamino-L-alanine (BMAA) has been proposed to contribute to neurodegenerative diseases. We have previously shown that neonatal exposure to BMAA results in dose-dependent cognitive impairments, proteomic alterations and progressive neurodegeneration in the hippocampus of adult rats. A high BMAA dose (460mg/kg) also induced intracellular fibril formation, increased protein ubiquitination and enrichment of proteins important for lipid transport and metabolism. The aim of this study was therefore to elucidate the role of neuronal lipids in BMAA-induced neurodegeneration. By using matrix assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS), we characterized the spatial lipid profile in the hippocampus of six month-old rats that were treated neonatally (postnatal days 9-10) with 460mg/kg BMAA. Multivariate statistical analysis revealed long-term changes in distinct ganglioside species (GM, GD, GT) in the dentate gyrus. These changes could be a consequence of direct effects on ganglioside biosynthesis through the b-series (GM3-GD3-GD2-GD1b-GT1b) and may be linked to astrogliosis. Complementary immunohistochemistry experiments towards GFAP and S100β further verified the role of increased astrocyte activity in BMAA-induced brain damage. This highlights the potential of imaging MS for probing chemical changes associated with neuropathological mechanisms in situ. This article is part of a Special Issue entitled: MALDI Imaging, edited by Dr. Corinna Henkel and Prof. Peter Hoffmann.