The use of 1,5-diaminonaphthalene for matrix-assisted laser desorption/ionization mass spectrometry imaging of brain in neurodegenerative disorders

Development of benzimidazole derivatives as efficient matrices for the analysis of acidic small-molecule compounds using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry in negative ion mode

RATIONALE

With the development of matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry (MS) in spatial localisation omics research on small molecules, the detection sensitivity of the matrix must increase. However, the types of matrices suitable for detecting acidic small molecules in (-) MALDI-MS mode are very limited and are either not sensitive enough or difficult to obtain.

METHODS

More than 10 commercially available benzimidazole and benzothiazole derivatives were selected as MALDI matrices in negative ion mode. MALDI-MS analysis was performed on 38 acidic small molecules and mouse serum, and the matrix effects were compared with those of the common commercial matrices 9-aminoacridine (9AA), 1,5-naphthalenediamine (DAN) and 3-aminoquinoline (3AQ). Moreover, the proton affinity (PA) of the selected potential matrix was calculated, and the relationships among the compound structure, PA value and matrix effect were discussed.

RESULTS

In (-) MALDI-MS mode, a higher PA value generally indicates a better matrix effect. Amino-substituted 2-phenyl-1H-benzo[d]imidazole derivatives had well-defined matrix effects on all analytes and were generally superior to the commonly used matrices 9AA, DAN and 3AQ. Among them, 2-(4-(dimethylamino-phenyl)-1H-benzo[d]imidazole-5-amine (E-4) has the best sensitivity and versatility for detecting different analytes and has the best ability to detect fatty acids in mouse serum; moreover, the limit of detection (LOD) of some analytes can reach as low as ng/L.

CONCLUSIONS

Compared to 9AA, DAN and 3AQ, matrix E-4 is more effective at detecting low-molecular-weight acidic compounds in (-) MALDI-MS mode, with higher sensitivity and better versatility. In addition, there is a clear correlation between compound structure, PA and matrix effects, which provides a basis for designing more efficient matrices.

Thermal evaporation as sample preparation for silver-assisted laser desorption/ionization mass spectrometry imaging of cholesterol in amyloid tissues

Cholesterol plays an important biological role in the body, and its disruption in homeostasis and synthesis has been implicated in several diseases. Mapping the locations of cholesterol is crucial for gaining a better understanding of these conditions. Silver deposition has proven to be an effective method for analyzing cholesterol using mass spectrometry imaging (MSI). We optimized and evaluated thermal evaporation as an alternative deposition technique to sputtering for silver deposition in MSI of cholesterol. A silver layer with a thickness of 6 nm provided an optimal combination of cholesterol signal intensity and mass resolution. The deposition of an ultrathin nanofilm of silver enabled high-resolution MSI with a pixel size of 10 μm. We used this optimized method to visualize the distribution of cholesterol in the senile plaques in the brains of APP/PS1 mice, a model that resembles Alzheimer's disease pathology. We found that cholesterol was evenly distributed across the frontal cortex tissue, with no evidence of plaque-like accumulation. Additionally, we investigated the presence and distribution of cholesterol in myocardial sections of a human heart affected by wild-type ATTR amyloidosis. We identified the presence of cholesterol in areas with amyloid deposition, but complete colocalization was not observed.

Brain areas lipidomics in female transgenic mouse model of Alzheimer's disease

Lipids are the major component of the brain with important structural and functional properties. Lipid disruption could play a relevant role in Alzheimer's disease (AD). Some brain lipidomic studies showed significant differences compared to controls, but few studies have focused on different brain areas related to AD. Furthermore, AD is more prevalent in females, but there is a lack of studies focusing on this sex. This work aims to perform a lipidomic study in selected brain areas (cerebellum, amygdala, hippocampus, entire cortex) from wild-type (WT, n = 10) and APPswe/PS1dE9 transgenic (TG, n = 10) female mice of 5 months of age, as a model of early AD, to identify alterations in lipid composition. A lipidomic mass spectrometry-based method was optimized and applied to brain tissue. As result, some lipids showed statistically significant differences between mice groups in cerebellum (n = 68), amygdala (n = 49), hippocampus (n = 48), and the cortex (n = 22). In addition, some lipids (n = 15) from the glycerolipid, phospholipid, and sphingolipid families were statistically significant in several brain areas simultaneously between WT and TG. A selection of lipid variables was made to develop a multivariate approach to assess their discriminant potential, showing high diagnostic indexes, especially in cerebellum and amygdala (sensitivity 70-100%, sensibility 80-100%).

Feeding High-Fat Diet Accelerates Development of Peripheral and Central Insulin Resistance and Inflammation and Worsens AD-like Pathology in APP/PS1 Mice

Alzheimer's disease (AD) is a progressive brain disorder characterized by extracellular amyloid-β (Aβ) plaques, intracellular neurofibrillary tangles formed by hyperphosphorylated Tau protein and neuroinflammation. Previous research has shown that obesity and type 2 diabetes mellitus, underlined by insulin resistance (IR), are risk factors for neurodegenerative disorders. In this study, obesity-induced peripheral and central IR and inflammation were studied in relation to AD-like pathology in the brains and periphery of APP/PS1 mice, a model of Aβ pathology, fed a high-fat diet (HFD). APP/PS1 mice and their wild-type controls fed either a standard diet or HFD were characterized at the ages of 3, 6 and 10 months by metabolic parameters related to obesity via mass spectroscopy, nuclear magnetic resonance, immunoblotting and immunohistochemistry to quantify how obesity affected AD pathology. The HFD induced substantial peripheral IR leading to central IR. APP/PS1-fed HFD mice had more pronounced IR, glucose intolerance and liver steatosis than their WT controls. The HFD worsened Aβ pathology in the hippocampi of APP/PS1 mice and significantly supported both peripheral and central inflammation. This study reveals a deleterious effect of obesity-related mild peripheral inflammation and prediabetes on the development of Aβ and Tau pathology and neuroinflammation in APP/PS1 mice.

Dissecting the brain with spatially resolved multi-omics

Recent studies have highlighted spatially resolved multi-omics technologies, including spatial genomics, transcriptomics, proteomics, and metabolomics, as powerful tools to decipher the spatial heterogeneity of the brain. Here, we focus on two major approaches in spatial transcriptomics (next-generation sequencing-based technologies and image-based technologies), and mass spectrometry imaging technologies used in spatial proteomics and spatial metabolomics. Furthermore, we discuss their applications in neuroscience, including building the brain atlas, uncovering gene expression patterns of neurons for special behaviors, deciphering the molecular basis of neuronal communication, and providing a more comprehensive explanation of the molecular mechanisms underlying central nervous system disorders. However, further efforts are still needed toward the integrative application of multi-omics technologies, including the real-time spatial multi-omics analysis in living cells, the detailed gene profile in a whole-brain view, and the combination of functional verification.

Robust quantitation of gangliosides and sulfatides in human brain using UHPLC-MRM-MS: Method development and application in Alzheimer's disease

Gangliosides (GAs) and sulfatides (STs) are acidic glycosphingolipids that are particularly abundant in the nervous system and are closely related to aging and neurodegenerative disorders. To explore their roles in brain diseases, in-depth molecular profiling, including structural variations of sphingoid backbone, fatty acyl group, and sugar chain of GAs and STs was performed. A total of 210 GAs and 38 STs were characterized in the inferior frontal gyrus (IFG) of human brain, with 90 GAs discovered in brain tissues for the first time. Influential MS parameters for detecting GAs and STs in multiple reaction monitoring (MRM) mode were systematically examined and optimized to minimize in-source fragmentation, resulting in remarkable signal intensity enhancement for GAs and STs, especially for polysialylated species. To eliminate analytical variations, isotopic interference-free internal standards were prepared by simple and fast reduction reaction. The final established method facilitated the simultaneous quantitation of 184 GAs and 30 STs from 25 subtypes, which represents the highest number of GAs quantitated among all quantitation methods recorded in literature so far. The method was further validated and applied to reveal the aberrant change of GAs and STs in the IFG of 12 Alzheimer's disease (AD) patients. Four GAs exhibited high classification capacity for AD (AUC ≥0.80) and were thereby considered the most promising signatures for AD. These findings suggested the close correlation between GAs and the pathogenesis of AD, highlighting the achievements of our robust method for investigating the roles of GAs and STs in various physiological states and diseases.

Systematic review of brain and blood lipidomics in Alzheimer's disease mouse models

Alzheimer's disease (AD) diagnosis is based on invasive and expensive biomarkers. Regarding AD pathophysiological mechanisms, there is evidence of a link between AD and aberrant lipid homeostasis. Alterations in lipid composition have been observed in blood and brain samples, and transgenic mouse models represent a promising approach. Nevertheless, there is great variability among studies in mice for the determination of different types of lipids in targeted and untargeted methods. It could be explained by the different variables (model, age, sex, analytical technique), and experimental conditions used. The aim of this work is to review the studies on lipid alteration in brain tissue and blood samples from AD mouse models, focusing on different experimental parameters. As result, great disparity has been observed among the reviewed studies. Brain studies showed an increase in gangliosides, sphingomyelins, lysophospholipids and monounsaturated fatty acids and a decrease in sulfatides. In contrast, blood studies showed an increase in phosphoglycerides, sterols, diacylglycerols, triacylglycerols and polyunsaturated fatty acids, and a decrease in phospholipids, lysophospholipids and monounsaturated fatty acids. Thus, lipids are closely related to AD, and a consensus on lipidomics studies could be used as a diagnostic tool and providing insight into the mechanisms involved in AD.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth 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 2020. 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. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

Spatial analysis of drug absorption, distribution, metabolism, and toxicology using mass spectrometry imaging

Mass spectrometry imaging (MSI) is emerging as a powerful analytical tool for detection, quantification, and simultaneous spatial molecular imaging of endogenous and exogenous molecules via in situ mass spectrometry analysis of thin tissue sections without the requirement of chemical labeling. The MSI generates chemically specific and spatially resolved ion distribution information for administered drugs and metabolites, which allows numerous applications for studies involving various stages of drug absorption, distribution, metabolism, excretion, and toxicity (ADMET). MSI-based pharmacokinetic imaging analysis provides a histological context and cellular environment regarding dynamic drug distribution and metabolism processes, and facilitates the understanding of the spatial pharmacokinetics and pharmacodynamic properties of drugs. Herein, we discuss the MSI's current technological developments that offer qualitative, quantitative, and spatial location information of small molecule drugs, antibody, and oligonucleotides macromolecule drugs, and their metabolites in preclinical and clinical tissue specimens. We highlight the macro and micro drug-distribution in the whole-body, brain, lung, liver, kidney, stomach, intestine tissue sections, organoids, and the latest applications of MSI in pharmaceutical ADMET studies.

MALDI Mass Spectrometry Imaging of Lipids on Free-Floating Brain Sections and Immunohistochemically Colocalized Markers of Neurodegeneration

In mass spectrometry imaging (MSI), the essential steps in sample preparation include collection and storage. The most widely used preservation procedure for MSI consists in freezing samples and storing them at temperatures below -80 °C. On the other hand, the most common method for preserving biological samples in clinical practice is their fixation in paraformaldehyde. The storage of free-floating sections is a particular type of the preservation of paraformaldehyde-fixed tissues that is used in immunohistochemistry. This chapter describes the approach of the multimodal imaging of free-floating brain sections using the MSI of lipids and the immunohistochemistry of neurodegeneration markers.

Applications of spatially resolved omics in the field of endocrine tumors

Endocrine tumors derive from endocrine cells with high heterogeneity in function, structure and embryology, and are characteristic of a marked diversity and tissue heterogeneity. There are still challenges in analyzing the molecular alternations within the heterogeneous microenvironment for endocrine tumors. Recently, several proteomic, lipidomic and metabolomic platforms have been applied to the analysis of endocrine tumors to explore the cellular and molecular mechanisms of tumor genesis, progression and metastasis. In this review, we provide a comprehensive overview of spatially resolved proteomics, lipidomics and metabolomics guided by mass spectrometry imaging and spatially resolved microproteomics directed by microextraction and tandem mass spectrometry. In this regard, we will discuss different mass spectrometry imaging techniques, including secondary ion mass spectrometry, matrix-assisted laser desorption/ionization and desorption electrospray ionization. Additionally, we will highlight microextraction approaches such as laser capture microdissection and liquid microjunction extraction. With these methods, proteins can be extracted precisely from specific regions of the endocrine tumor. Finally, we compare applications of proteomic, lipidomic and metabolomic platforms in the field of endocrine tumors and outline their potentials in elucidating cellular and molecular processes involved in endocrine tumors.

Mass spectrometry-based lipid analysis and imaging

Mass spectrometry imaging (MSI) is a powerful tool for in situ mapping of analytes across a sample. With growing interest in lipid biochemistry, the ability to perform such mapping without antibodies has opened many opportunities for MSI and lipid analysis. Herein, we discuss the basics of MSI with particular emphasis on MALDI mass spectrometry and lipid analysis. A discussion of critical advancements as well as protocol details are provided to the reader. In addition, strategies for improving the detection of lipids, as well as applications in biomedical research, are presented.

GM3 Ganglioside Linked to Neurofibrillary Pathology in a Transgenic Rat Model for Tauopathy

Glycosphingolipids (GSLs) are amphipathic lipids composed of a sphingoid base and a fatty acyl attached to a saccharide moiety. GSLs play an important role in signal transduction, directing proteins within the membrane, cell recognition, and modulation of cell adhesion. Gangliosides and sulfatides belong to a group of acidic GSLs, and numerous studies report their involvement in neurodevelopment, aging, and neurodegeneration. In this study, we used an approach based on hydrophilic interaction liquid chromatography (HILIC) coupled to high-resolution tandem mass spectrometry (HRMS/MS) to characterize the glycosphingolipid profile in rat brain tissue. Then, we screened characterized lipids aiming to identify changes in glycosphingolipid profiles in the normal aging process and tau pathology. Thorough screening of acidic glycosphingolipids in rat brain tissue revealed 117 ganglioside and 36 sulfatide species. Moreover, we found two ganglioside subclasses that were not previously characterized-GT1b-Ac2 and GQ1b-Ac2. The semi-targeted screening revealed significant changes in the levels of sulfatides and GM1a gangliosides during the aging process. In the transgenic SHR24 rat model for tauopathies, we found elevated levels of GM3 gangliosides which may indicate a higher rate of apoptotic processes.

Multimodal Imaging Mass Spectrometry: Next Generation Molecular Mapping in Biology and Medicine

Imaging mass spectrometry has become a mature molecular mapping technology that is used for molecular discovery in many medical and biological systems. While powerful by itself, imaging mass spectrometry can be complemented by the addition of other orthogonal, chemically informative imaging technologies to maximize the information gained from a single experiment and enable deeper understanding of biological processes. Within this review, we describe MALDI, SIMS, and DESI imaging mass spectrometric technologies and how these have been integrated with other analytical modalities such as microscopy, transcriptomics, spectroscopy, and electrochemistry in a field termed multimodal imaging. We explore the future of this field and discuss forthcoming developments that will bring new insights to help unravel the molecular complexities of biological systems, from single cells to functional tissue structures and organs.

Spatially resolved mass spectrometry analysis of amyloid plaque-associated lipids

Over the last 10 years, considerable technical advances in mass spectrometry (MS)-based bioanalysis have enabled the investigation of lipid signatures in neuropathological structures. In Alzheimer´s Disease (AD) research, it is now well accepted that lipid dysregulation plays a key role in AD pathogenesis and progression. This review summarizes current MS-based strategies, notably MALDI and ToF-SIMS imaging as well as laser capture microdissection combined with LC-ESI-MS. It also presents recent advances to assess lipid alterations associated with Amyloid-β plaques, one of the hallmarks of AD. Collectively, these methodologies offer new opportunities for the study of lipids, thus pushing forward our understanding of their role in such a complex and still untreatable disease as AD.

Mass spectrometry imaging of free-floating brain sections detects pathological lipid distribution in a mouse model of Alzheimer's-like pathology

Mass spectrometry imaging (MSI) is a modern analytical technique capable of monitoring the spatial distribution of compounds within target tissues. Collection and storage are important steps in sample preparation. The recommended and most widely used preservation procedure for MSI is freezing samples in isopentane and storing them at temperatures below -80 °C. On the other hand, the most common and general method for preserving biological samples in clinical practice is fixation in paraformaldehyde. Special types of samples prepared from these fixed tissues that are used for histology and immunohistochemistry are free-floating sections. It would be very beneficial if the latter procedure could also be applicable for the samples intended for subsequent MSI analysis. In the present work, we optimized and evaluated paraformaldehyde-fixed free-floating sections for the analysis of lipids in mouse brains and used the sections for the study of lipid changes in double transgenic APP/PS1 mice, a model of Alzheimer's-like pathology. Moreover, we examined the neuroprotective properties of palm¹¹-PrRP31, an anorexigenic and glucose-lowering analog of prolactin-releasing peptide, and liraglutide, a type 2 diabetes drug. From the free-floating sections, we obtained lipid images without interference or delocalization, and we demonstrated that free-floating sections can be used for the MSI of lipids. In the APP/PS1 mice, we observed a changed distribution of various lipids compared to the controls. The most significant changes in lipids in the brains of APP/PS1 mice compared to wild-type controls were related to gangliosides (GM2 36:1, GM3 36:1) and phosphatidylinositols (PI 38:4, 36:4) in regions where the accumulation of senile plaques occurred. In APP/PS1 mice peripherally treated with palm¹¹-PrRP31 or liraglutide for 2 months, we found that both peptides reduced the amount and space occupied by lipids, which were linked to the senile plaques. These results indicate that palm¹¹-PrRP31 as well as liraglutide might be potentially useful in the treatment of neurodegenerative diseases.

Imaging mass spectrometry allows for neuroanatomic-specific detection of gangliosides in the healthy and diseased brain

Gangliosides have a wide variety of biological functions due to their location on the outer leaflet of plasma membranes. They form a critical component of membrane rafts, or ganglioside-enriched microdomains, where they influence the physical properties of the membrane as well as its function. Gangliosides can change their structure to meet their external and internal environmental demands. This ability to change structure makes gangliosides both fascinating and technologically challenging targets to identify and understand. A full understanding on how gangliosides are regulated within the central nervous system (CNS) is critical, as ganglioside dysregulation is observed in the aging brain as well as in several neurodegenerative injuries and diseases such as stroke, Alzheimer's disease, Parkinson's disease, Huntington's disease and several lysosomal storage disorders diseases, including Tay Sach's disease. Mass spectrometry (MS) has become a useful means to better understand ganglioside composition and function. Imaging mass spectrometry (IMS) provides the added benefit of placing analytical information within an anatomical context. This review article will discuss recent advances in MS-based detection methods, with a focus on IMS-based approaches to help understand the spatial-specific role gangliosides in the healthy brain as in CNS injuries and disease.

Spatial Lipidomics Reveals Region and Long Chain Base Specific Accumulations of Monosialogangliosides in Amyloid Plaques in Familial Alzheimer's Disease Mice (5xFAD) Brain

Ganglioside metabolism is significantly altered in Alzheimer's disease (AD), which is a progressive neurodegenerative disease prominently characterized by one of its pathological hallmarks, amyloid deposits or "senile plaques". While the plaques mainly consist of aggregated variants of amyloid-β protein (Aβ), recent studies have revealed a number of lipid species including gangliosides in amyloid plaques along with Aβ peptides. It has been widely suggested that long chain (sphingosine) base (LCBs), C18:1-LCB and C20:1-LCB, containing gangliosides might play different roles in neuronal function in vivo. In order to elucidate region-specific aspects of amyloid-plaque associated C18:1-LCB and C20:1-LCB ganglioside accumulations, high spatial resolution (10 μm per pixel) matrix assisted laser desorption ionization imaging mass spectrometry (MALDI-IMS) of gangliosides in amyloid plaques was performed in hippocampal and adjacent cortical regions of 12 month old 5xFAD mouse coronal brain sections from two different stereotaxic coordinates (bregma points, -2.2 and -2.7 mm). MALDI-IMS uncovered brain-region (2 and 3D) and/or LCB specific accumulations of monosialogangliosides (GMs): GM1, GM2, and GM3 in the hippocampal and cortical amyloid plaques. The results reveal monosialogangliosides to be an important component of amyloid plaques and the accumulation of different gangliosides is region and LCB specific in 12 month old 5xFAD mouse brain. This is discussed in relation to amyloid-associated AD pathogenesis such as lipid related immune changes in amyloid plaques, AD specific ganglioside metabolism, and, notably, AD-associated impaired neurogenesis in the subgranular zone.

Multimodal Imaging of Amyloid Plaques: Fusion of the Single-Probe Mass Spectrometry Image and Fluorescence Microscopy Image

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases. The formation of amyloid plaques by aggregated amyloid beta (Aβ) peptides is a primary event in AD pathology. Understanding the metabolomic features and related pathways is critical for studying plaque-related pathological events (e.g., cell death and neuron dysfunction). Mass spectrometry imaging (MSI), due to its high sensitivity and ability to obtain the spatial distribution of metabolites, has been applied to AD studies. However, limited studies of metabolites in amyloid plaques have been performed due to the drawbacks of the commonly used techniques such as matrix-assisted laser desorption/ionization MSI. In the current study, we obtained high spatial resolution (∼17 μm) MS images of the AD mouse brain using the Single-probe, a microscale sampling and ionization device, coupled to a mass spectrometer under ambient conditions. The adjacent slices were used to obtain fluorescence microscopy images to locate amyloid plaques. The MS image and the fluorescence microscopy image were fused to spatially correlate histological protein hallmarks with metabolomic features. The fused images produced significantly improved spatial resolution (∼5 μm), allowing for the determination of fine structures in MS images and metabolomic biomarkers representing amyloid plaques.