Structural mass spectrometry analysis of lipid changes in a Drosophila epilepsy model brain

Mass Spectrometric Imaging of Anionic Phospholipids Desorbed from Human Hippocampal Sections: Discrimination between Temporal and Nontemporal Lobe Epilepsies

Temporal lobe epilepsy (TLE) is one of the most common neurological disorders, often accompanied by hippocampal sclerosis. The molecular processes underlying this epileptogenesis are poorly understood. To examine the lipid profile, 39 fresh frozen sections of the human hippocampus obtained from epilepsy surgery for TLE (n = 14) and non-TLE (control group; n = 25) patients were subjected to desorption electrospray ionization mass spectrometry imaging in the negative ion mode. In contrast to our earlier report that showed striking downregulation of positively charged phospholipids (e.g., phosphatidylcholine and phosphatidylethanolamine, etc.) in the TLE hippocampus, this study finds complementary upregulation of negatively charged phospholipids, notably, phosphatidylserine and phosphatidylglycerol. This result may point to an active metabolic pool in the TLE hippocampus that produces these anionic phospholipids at the expense of the cationic phospholipids. This metabolic shift could be due to the dysregulation of the Kennedy and CDP-DG pathways responsible for biosynthesizing these lipids. Thus, this study further opens up opportunities to investigate the molecular hallmarks and potential therapeutic targets for TLE.

Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging of Phospholipid Changes in a Drosophila Model of Early Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a degenerative disease caused by motor neuron damage in the central nervous system, and it is difficult to diagnose early. Drosophila melanogaster is widely used to investigate disease mechanisms and discover biomarkers because it is easy to induce disease in Drosophila through genetic engineering. We performed matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) to investigate changes in phospholipid distribution in the brain tissue of an ALS-induced Drosophila model. Fly brain tissues of several hundred micrometers or less were sampled using a fly collar to obtain reproducible tissue sections of similar sizes. MSI of brain tissues of Drosophila cultured for 1 or 10 days showed that the distribution of phospholipids, including phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), phosphatidylserine (PS), and phosphatidylinositol (PI), was significantly different between the control group and the ALS group. In addition, the lipid profile according to phospholipids differed as the culture time increased from 1 to 10 days. These results suggest that disease indicators based on lipid metabolites can be discovered by performing MALDI-MSI on very small brain tissue samples from the Drosophila disease model to ultimately assess the phospholipid changes that occur in early-stage ALS.

Integrative analysis of non-targeted lipidomic data and brain structural imaging identifies phosphatidylethanolamine associated with epileptogenesis

INTRODUCTION

Epilepsy is a chronic disease, while epileptogenesis is a latent period where brain will be transformed into an epileptic one. Mechanisms of epileptogenesis remain unclear.

OBJECTIVES

We aim to provide information of hippocampal lipidomic changes related with epileptogenesis in two kindling models. Combining hippocampal structural imaging indices, our study also attempts to assess biochemical alterations as a function of epileptogenesis in a non-invasive way.

METHODS

We constructed two kinds of chemical kindling models, which have long been used as models of epileptogenesis. Two kindling and one control groups were all subjected to structural imaging acquisition after successfully kindled. Voxel-based morphometry, a postprocessing method for brain imaging data, was used to segment and extract hippocampal gray matter volume for subsequent integrative analysis. LC-MS based lipidomic analysis was applied to identify distinct hippocampal lipidomic profiles between kindling and control groups. Further, we regress hippocampal structural indices on lipids to identify those associated with both epileptogenesis and brain structural changes.

RESULTS

We report distinct lipidomic profiles between kindling groups and control. A total of 638 lipids were detected in all three groups. Among them were 98 individual lipids, showing significant alterations, in particular lipid class of phosphatidylethanolamine (PE), glucosylceramide and phosphatidylcholine. Hippocampal gray matter volumes were found significant different between groups (P = 0.0223). After combining brain imaging data, we demonstrate several individual PE, namely PE(O-18:1_22:3), PE(O-18:1_22:6) and PE(18:1_18:1), are associated with both epileptogenesis and hippocampal gray matter volume.

CONCLUSION

This study suggests metabolic pathway of PE might involve in epileptogenesis. Also, for the first time, we link level of PE with structural brain imaging indices, in an attempt to potentiate the futuristic application of noninvasive brain imaging techniques to identify epileptogenesis in its latent period.

Mapping the fly Malpighian tubule lipidome by imaging mass spectrometry

Matrix-assisted laser/desorption ionization imaging mass spectrometry (MALDI IMS) is an analytical technique for understanding the spatial distribution of biomolecules across a sample surface. Originally employed for mammalian tissues, this technology has been adapted to study specimens as diverse as microbes and cell cultures, food such as strawberries, and invertebrates including the vinegar fly Drosophila melanogaster. As an ideal model organism, Drosophila has brought greater understanding about conserved biological processes, organism development, and diseased states and even informed management practices of agriculturally and environmentally important species. Drosophila displays anatomically separated renal (Malpighian) tubules that are the physiological equivalent to the vertebrate nephron. Insect Malpighian tubules are also responsible for pesticide detoxification. In this article, we first describe an effective workflow and sample preparation method to study the phospholipid distribution of the Malpighian tubules that initially involves the manual microdissection of the tubules in saline buffer followed by a series of washes to remove excess salt and enhances the phospholipid signals prior to matrix deposition and IMS at 25-μm spatial resolution. We also established a complementary methodology for lipid IMS analysis of whole-body fly sections using a dual-polarity data acquisition approach at the same spatial resolution after matrix deposition by sublimation. Both procedures yield rich signal profiles from the major phospholipid classes. The reproducibility and high-quality results offered by these methodologies enable cohort studies of Drosophila through MALDI IMS.

Mass Spectrometry Imaging Shows Cocaine and Methylphenidate Have Opposite Effects on Major Lipids in Drosophila Brain

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to study the effects of cocaine versus methylphenidate administration on both the localization and abundance of lipids in Drosophila melanogaster brain. A J105 ToF-SIMS with a 40 keV gas cluster primary ion source enabled us to probe molecular ions of biomolecules on the fly with a spatial resolution of ∼3 μm, giving us unique insights into the effect of these drugs on molecular lipids in the nervous system. Significant changes in phospholipid composition were observed in the central brain for both. Principal components image analysis revealed that changes occurred mainly for phosphatidylcholines, phosphatidylethanolamines, and phosphatidylinositols. When the lipid changes caused by cocaine were compared with those induced by methylphenidate, it was shown that these drugs exert opposite effects on the brain lipid structure. We speculate that this might relate to the molecular mechanism of cognition and memory.

Phospholipid Homeostasis Regulates Dendrite Morphogenesis in Drosophila Sensory Neurons

Disruptions in lipid homeostasis have been observed in many neurodevelopmental disorders that are associated with dendrite morphogenesis defects. However, the molecular mechanisms of how lipid homeostasis affects dendrite morphogenesis are unclear. We find that easily shocked (eas), which encodes a kinase with a critical role in phospholipid phosphatidylethanolamine (PE) synthesis, and two other enzymes in this synthesis pathway are required cell autonomously in sensory neurons for dendrite growth and stability. Furthermore, we show that the level of Sterol Regulatory Element-Binding Protein (SREBP) activity is important for dendrite development. SREBP activity increases in eas mutants, and decreasing the level of SREBP and its transcriptional targets in eas mutants largely suppresses the dendrite growth defects. Furthermore, reducing Ca²⁺ influx in neurons of eas mutants ameliorates the dendrite morphogenesis defects. Our study uncovers a role for EAS kinase and reveals the in vivo function of phospholipid homeostasis in dendrite morphogenesis.

The potential of Ion Mobility Mass Spectrometry for high-throughput and high-resolution lipidomics

Lipids are a large and highly diverse family of biomolecules, which play essential structural, storage and signalling roles in cells and tissues. Although traditional mass spectrometry (MS) approaches used in lipidomics are highly sensitive and selective, lipid analysis remains challenging due to the chemical diversity of lipid structures, multiple isobaric species and incomplete separation using many forms of chromatography. Ion mobility (IM) separates ions in the gas phase based on their physicochemical properties. Addition of IM to the traditional lipidomic workflow both enhances separation of complex lipid mixtures, beneficial for lipid identification, and improves isomer resolution. Herein, we discuss the recent developments in IM-MS for lipidomics.

Lipid Regulation of Sodium Channels

The lipid landscapes of cellular membranes are complex and dynamic, are tissue dependent, and can change with the age and the development of a variety of diseases. Researchers are now gaining new appreciation for the regulation of ion channel proteins by the membrane lipids in which they are embedded. Thus, as membrane lipids change, for example, during the development of disease, it is likely that the ionic currents that conduct through the ion channels embedded in these membranes will also be altered. This chapter provides an overview of the complex regulation of prokaryotic and eukaryotic voltage-dependent sodium (Nav) channels by fatty acids, sterols, glycerophospholipids, sphingolipids, and cannabinoids. The impact of lipid regulation on channel gating kinetics, voltage-dependence, trafficking, toxin binding, and structure are explored for Nav channels that have been examined in heterologous expression systems, native tissue, and reconstituted into artificial membranes. Putative mechanisms for Nav regulation by lipids are also discussed.

Lipid structural effects of oral administration of methylphenidate in Drosophila brain by secondary ion mass spectrometry imaging

We use time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging to investigate the effects of orally administrated methylphenidate on lipids in the brain of Drosophila melanogaster (fruit fly), a major invertebrate model system in biological study and neuroscience. TOF-SIMS imaging was carried out using a recently designed high energy 40 keV Ar4000(+) gas cluster ion gun which demonstrated improved sensitivity for intact lipids in the fly brain compared to the 40 keV C60(+) primary ion gun. In addition, correlation of TOF-SIMS and SEM imaging on the same fly brain showed that there is specific localization that is related to biological functions of various biomolecules. Different lipids distribute in different parts of the brain, central brain, optical lobes, and proboscis, depending on the length of the carbon chain and saturation level of fatty acid (FA) branches. Furthermore, data analysis using image principal components analysis (PCA) showed that methylphenidate dramatically affected both the distribution and abundance of lipids and their derivatives, particularly fatty acids, diacylglycerides, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol in the fly brains. Our approach using TOF-SIMS imaging successfully visualizes the effects of methylphenidate on the chemical structure of the fly brain.

Rescue of easily shocked mutant seizure sensitivity in Drosophila adults

Genetic factors that influence seizure susceptibility can act transiently during the development of neural circuits or might be necessary for the proper functioning of existing circuits. We provide evidence that the Drosophila seizure-sensitive mutant easily shocked (eas) represents a neurological disorder in which abnormal functioning of existing neural circuits leads to seizure sensitivity. The eas(+) gene encodes for the protein Ethanolamine Kinase, involved in phospholipid biosynthesis. We show that induction of eas(+) in adult mutant flies rescues them from seizure sensitivity despite previously known developmental defects in brain morphology. Additionally, through cell-type-specific rescue, our results suggest a specific role for eas(+) in excitatory rather than inhibitory neural transmission. Overall, our findings emphasize an important role for proper phospholipid metabolism in normal brain function and suggest that certain classes of epilepsy syndromes could have the potential to be treated with gene therapy techniques.

Biomolecular signatures of diabetic wound healing by structural mass spectrometry

Wound fluid is a complex biological sample containing byproducts associated with the wound repair process. Contemporary techniques, such as immunoblotting and enzyme immunoassays, require extensive sample manipulation and do not permit the simultaneous analysis of multiple classes of biomolecular species. Structural mass spectrometry, implemented as ion mobility-mass spectrometry (IM-MS), comprises two sequential, gas-phase dispersion techniques well suited for the study of complex biological samples because of its ability to separate and simultaneously analyze multiple classes of biomolecules. As a model of diabetic wound healing, poly(vinyl alcohol) sponges were inserted subcutaneously into nondiabetic (control) and streptozotocin-induced diabetic rats to elicit a granulation tissue response and to collect acute wound fluid. Sponges were harvested at days 2 or 5 to capture different stages of the early wound-healing process. Utilizing IM-MS, statistical analysis, and targeted ultraperformance liquid chromatography analysis, biomolecular signatures of diabetic wound healing have been identified. The protein S100-A8 was highly enriched in the wound fluids collected from day 2 diabetic rats. Lysophosphatidylcholine (20:4) and cholic acid also contributed significantly to the differences between diabetic and control groups. This report provides a generalized workflow for wound fluid analysis demonstrated with a diabetic rat model.

Combined elemental and biomolecular mass spectrometry imaging for probing the inventory of tissue at a micrometer scale

Several complementary mass spectrometric imaging techniques allow mapping of various analytes within biological tissue sections. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) quantitatively detects elements and isotopes with very high sensitivity and a particularly high dynamical range. Matrix-assisted laser desorption/ionization ion mobility mass spectrometry (MALDI-IM-MS) allows a pixel-by-pixel classification and identification of biomolecules. In order to dispose of the healthy hemisphere as an internal calibrant in addition to routinely used external standards, adjacent brain sections of mice with a unilateral 6-OHDA lesion of the medial forebrain bundle were chosen as exemplary samples. We demonstrate a comprehensive way of data acquisition and analysis by coregistering mass spectrometric data on photomicrographs as common reference space and thus providing trimodal spatial information. Registering subsequent planar element maps yielded continuous 3-dimensional data sets. Furthermore, we introduce a correction of MSI data for variable slice thickness applicable to all MSI techniques. In the present case, we observed increased concentrations of iron, manganese, and copper in the lesioned substantia nigra while monounsaturated lipid levels were decreased in the identical region of interest. Our techniques provide new insights into the intricate spatial relationship of morphology and chemistry within tissue.

Lipid analysis and lipidomics by structurally selective ion mobility-mass spectrometry

Recent advances in mass spectrometry approaches to the analysis of lipids include the ability to incorporate both lipid class identification with lipid structural information for increased characterization capabilities. The detailed examination of lipids and their biosynthetic and biochemical pathways made possible by novel instrumental and bioinformatics approaches is advancing research in fundamental cellular and medical studies. Recently, high-throughput structural analysis has been demonstrated through the use of rapid gas-phase separation on the basis of the ion mobility (IM) analytical technique combined with mass spectrometry (IM-MS). While IM-MS has been extensively utilized in biochemical research for peptide, protein and small molecule analysis, the role of IM-MS in lipid research is still an active area of development. In this review of lipid-based IM-MS research, we begin with an overview of three contemporary IM techniques which show great promise in being applied towards the analysis of lipids. Fundamental concepts regarding the integration of IM-MS are reviewed with emphasis on the applications of IM-MS towards simplifying and enhancing complex biological sample analysis. Finally, several recent IM-MS lipid studies are highlighted and the future prospects of IM-MS for integrated omics studies and enhanced spatial profiling through imaging IM-MS are briefly described.

New applications of mass spectrometry in lipid analysis

Mass spectrometry has emerged as a powerful tool for the analysis of all lipids. Lipidomic analysis of biological systems using various approaches is now possible with a quantitative measurement of hundreds of lipid molecular species. Although availability of reference and internal standards lags behind the field, approaches using stable isotope-labeled derivative tagging permit precise determination of specific phospholipids in an experimental series. The use of reactivity of ozone has enabled assessment of double bond positions in fatty acyl groups even when species remain in complex lipid mixtures. Rapid scanning tandem mass spectrometers are capable of quantitative analysis of hundreds of targeted lipids at high sensitivity in a single on-line chromatographic separation. Imaging mass spectrometry of lipids in tissues has opened new insights into the distribution of lipid molecular species with promising application to study pathophysiological events and diseases.