Specific changes of sulfatide levels in individuals with pre-clinical Alzheimer's disease: an early event in disease pathogenesis

APP Function and Lipids: A Bidirectional Link

Extracellular neuritic plaques, composed of aggregated amyloid-β (Aβ) peptides, are one of the major histopathological hallmarks of Alzheimer's disease (AD), a progressive, irreversible neurodegenerative disorder and the most common cause of dementia in the elderly. One of the most prominent risk factor for sporadic AD, carrying one or two aberrant copies of the apolipoprotein E (ApoE) ε4 alleles, closely links AD to lipids. Further, several lipid classes and fatty acids have been reported to be changed in the brain of AD-affected individuals. Interestingly, the observed lipid changes in the brain seem not only to be a consequence of the disease but also modulate Aβ generation. In line with these observations, protective lipids being able to decrease Aβ generation and also potential negative lipids in respect to AD were identified. Mechanistically, Aβ peptides are generated by sequential proteolytic processing of the amyloid precursor protein (APP) by β- and γ-secretase. The α-secretase appears to compete with β-secretase for the initial cleavage of APP, preventing Aβ production. All APP-cleaving secretases as well as APP are transmembrane proteins, further illustrating the impact of lipids on Aβ generation. Beside the pathological impact of Aβ, accumulating evidence suggests that Aβ and the APP intracellular domain (AICD) play an important role in regulating lipid homeostasis, either by direct effects or by affecting gene expression or protein stability of enzymes involved in the de novo synthesis of different lipid classes. This review summarizes the current literature addressing the complex bidirectional link between lipids and AD and APP processing including lipid alterations found in AD post mortem brains, lipids that alter APP processing and the physiological functions of Aβ and AICD in the regulation of several lipid metabolism pathways.

Lipid mapping of the rat brain for models of disease

Lipids not only constitute the primary component of cellular membranes and contribute to metabolism but also serve as intracellular signaling molecules and bind to specific membrane receptors to control cell proliferation, growth and convey neuroprotection. Over the last several decades, the development of new analytical techniques, such as imaging mass spectrometry (IMS), has contributed to our understanding of their involvement in physiological and pathological conditions. IMS allows researchers to obtain a wide range of information about the spatial distribution and abundance of the different lipid molecules that is crucial to understand brain functions. The primary aim of this study was to map the spatial distribution of different lipid species in the rat central nervous system (CNS) using IMS to find a possible relationship between anatomical localization and physiology. The data obtained were subsequently applied to a model of neurological disease, the 192IgG-saporin lesion model of memory impairment. The results were obtained using a LTQ-Orbitrap XL mass spectrometer in positive and negative ionization modes and analyzed by ImageQuest and MSIReader software. A total of 176 different molecules were recorded based on the specific localization of their intensities. However, only 34 lipid species in negative mode and 51 in positive were assigned to known molecules with an error of 5ppm. These molecules were grouped by different lipid families, resulting in: Phosphatidylcholines (PC): PC (34: 1)+K⁺ and PC (32: 0)+K⁺ distributed primarily in gray matter, and PC (36: 1)+K⁺ and PC (38: 1)+Na⁺ distributed in white matter. Phosphatidic acid (PA): PA (38: 3)+K⁺ in white matter, and PA (38: 5)+K⁺ in gray matter and brain ventricles. Phosphoinositol (PI): PI (18: 0/20: 4)-H⁺ in gray matter, and PI (O-30: 1) or PI (P-30: 0)-H⁺ in white matter. Phosphatidylserines (PS): PS (34: 1)-H⁺ in gray matter, and PS (38: 1)-H⁺ in white matter. Sphingomyelin (SM) SM (d18: 1/16: 0)-H⁺ in ventricles and SM (d18: 1/18: 0)-H⁺ in gray matter. Sulfatides (ST): ST (d18: 1/24: 1)-H⁺ in white matter. The specific distribution of different lipids supports their involvement not only in structural and metabolic functions but also as intracellular effectors or specific receptor ligands and/or precursors. Moreover, the specific localization in the CNS described here will enable us to analyze lipid distribution to identify their physiological conditions in rat models of neurodegenerative pathologies, such as Alzheimer's disease. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Recent advances in expanding the coverage of the lipidome

The lipidome comprises a large array of molecules with diverse physicochemical properties. Lipids are structural components of cells, act as a source of energy, and function as signaling mediators. Alterations in lipid metabolism are involved in the onset and progression of a variety of diseases, including metabolic syndrome and cancer. Because of this, interest in lipidomics, the comprehensive characterization of the lipidome by mass spectrometry, has intensified in recent years. However, obtaining a truly complete overview of all lipids in a sample has remained very challenging due to their enormous structural diversity. Here, we provide an overview of the collection of analytical approaches used to study various lipid classes, emphasizing innovations in sample preparation and liquid chromatography-mass spectrometry (LC-MS). Additionally, we provide practical suggestions for increasing the coverage of the lipidome.

Sulfatides Primarily Exist in the Substantia Nigra Region of Mouse Brain Tissue

Lipid distribution in the brain is important for many biological functions and has been associated with some brain diseases. The aim of this study was to investigate lipid distribution in different regions of brain tissue in mice. To this end, substantia nigra (SN), caudate putamen (CPu), hippocampus (Hip), hypothalamus (Hyp), and cortex (Cx) tissues of mice were analyzed using direct infusion nanoelectrospray-ion trap mass spectrometry and multivariate analyses. The SN, CPu, Hip, Hyp, and Cx groups showed clear differences in lipid distribution using principal component analysis and a partial least-squares discriminant analysis score plot, and lipid levels were significantly different in different brain regions. In particular, sulfatides were mainly distributed in the SN region. Our results could be used to help understand the functions and mechanisms of lipids in various brain diseases.

Direct infusion mass spectrometry for metabolomic phenotyping of diseases

Metabolomics based on direct mass spectrometry (MS) analysis, either by direct infusion or flow injection of crude sample extracts, shows a great potential for metabolic fingerprinting because of its high-throughput screening capability, wide metabolite coverage and reduced time of analysis. Considering that numerous metabolic pathways are significantly perturbed during the initiation and progression of diseases, these metabolomic tools can be used to get a deeper understanding about disease pathogenesis and discover potential biomarkers for early diagnosis. In this work, we describe the most common metabolomic platforms used in biomedical research, with special focus on strategies based on direct MS analysis. Then, a comprehensive review on the application of direct MS fingerprinting in clinical issues is provided.

Novel sulfoglycolipid IG20 causes neuroprotection by activating the phase II antioxidant response in rat hippocampal slices

Compound IG20 is a newly synthesised sulphated glycolipid that promotes neuritic outgrowth and myelinisation, at the time it causes the inhibition of glial proliferation and facilitates exocytosis in chromaffin cells. Here we have shown that IG20 at 0.3-10 μM afforded neuroprotection in rat hippocampal slices stressed with veratridine, glutamate or with oxygen plus glucose deprivation followed by reoxygenation (OGD/reox). Excess production of reactive oxygen species (ROS) elicited by glutamate or ODG/reox was prevented by IG20 that also restored the depressed tissue levels of GSH and ATP in hippocampal slices subjected to OGD/reox. Furthermore, the augmented iNOS expression produced upon OGD/reox exposure was also counteracted by IG20. Additionally, the IG20 elicited neuroprotection was prevented by the presence of inhibitors of the signalling pathways Jak2/STAT3, MEK/ERK1/2, and PI3K/Akt, consistent with the ability of the compound to increase the phosphorylation of Jak2, ERK1/2, and Akt. Thus, the activation of phase II response and the Nrf2/ARE pathway could explain the antioxidant and anti-inflammatory effects and the ensuing neuroprotective actions of IG20.

A comprehensive profiling of sulfatides in myelin from mouse brain using liquid chromatography coupled to high-resolution accurate tandem mass spectrometry

Sulfatides are sulfoglycolipids found in the myelin sheath. The composition ratio of sulfatide molecular species changes with age, and it has also been associated with the pathogenesis of various human central nervous system diseases. However, profiling sulfatides in biological samples is difficult, due to the great variety of molecular species. In this work, a new, easy and reliable liquid chromatography-electrospray tandem mass spectrometry (LC-ESI(+)-MS/MS) method has been developed to profile sulfatide content in biological samples of myelin. The 'wrong-way-round' ionization effect has been described for this type of molecules for the first time, making it possible to correctly identify as many as 37 different sulfatides in mouse brain myelin samples, including molecules with different fatty acid chain lengths and varying degrees of unsaturation and hydroxylation. A chemometric analysis of their relative abundances showed that the main difference among individuals of different ages was the content of sulfatides with odd-numbered fatty acid chains, in addition to hydroxylated species.

The Impact of Vitamin E and Other Fat-Soluble Vitamins on Alzheimer´s Disease

Alzheimer’s disease (AD) is the most common cause of dementia in the elderly population, currently affecting 46 million people worldwide. Histopathologically, the disease is characterized by the occurrence of extracellular amyloid plaques composed of aggregated amyloid-β (Aβ) peptides and intracellular neurofibrillary tangles containing the microtubule-associated protein tau. Aβ peptides are derived from the sequential processing of the amyloid precursor protein (APP) by enzymes called secretases, which are strongly influenced by the lipid environment. Several vitamins have been reported to be reduced in the plasma/serum of AD-affected individuals indicating they have an impact on AD pathogenesis. In this review we focus on vitamin E and the other lipophilic vitamins A, D, and K, and summarize the current knowledge about their status in AD patients, their impact on cognitive functions and AD risk, as well as their influence on the molecular mechanisms of AD. The vitamins might affect the generation and clearance of Aβ both by direct effects and indirectly by altering the cellular lipid homeostasis. Additionally, vitamins A, D, E, and K are reported to influence further mechanisms discussed to be involved in AD pathogenesis, e.g., Aβ-aggregation, Aβ-induced neurotoxicity, oxidative stress, and inflammatory processes, as summarized in this article.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The lipidome associated with the γ-secretase complex is required for its integrity and activity

γ-Secretase is a multi-subunit membrane protease complex that catalyses the final intramembrane cleavage of the β-amyloid precursor protein (APP) during the neuronal production of amyloid-β peptides (Aβ), which are implicated as the causative agents of Alzheimer's disease (AD). In the present study, we report the reconstitution of a highly purified, active γ-secretase complex into proteoliposomes without exogenous lipids and provide the first direct evidence for the existence of a microenvironment of 53 molecular species from 11 major lipid classes specifically associated with the γ-secretase complex, including phosphatidylcholine and cholesterol. Importantly, we demonstrate that the pharmacological modulation of certain phospholipids abolishes both the integrity and the enzymatic activity of the intramembrane protease. Together, our findings highlight the importance of a specific lipid microenvironment for the structure and function of γ-secretase.

Differential effects on glial activation by a direct versus an indirect thrombin inhibitor

Thrombin is a potent regulator of brain function in health and disease, modulating glial activation and brain inflammation. Thrombin inhibitors, several of which are in clinical use as anti-coagulants, can reduce thrombin-dependent neuroinflammation in pathological conditions. However, their effects in a healthy CNS are largely unknown. In adult healthy mice, we compared the effects of treatment by the direct thrombin inhibitor dabigatran etexilate (DE), to those of warfarin, which acts by preventing vitamin K recycling essential for coagulation. After 4weeks, warfarin increased both astrocyte GFAP and microglia Iba-1 staining throughout the CNS; whereas DE reduced expression of both markers. Warfarin, but not DE, reduced sulfatide levels; and warfarin showed longer lasting changes in cerebellar gene expression. DE also reduced glial activation in a mouse model of Alzheimer's disease, although no changes in amyloid plaque burden were observed. These results suggest that treatment with direct thrombin inhibitors may be preferable to those agents which reduce vitamin K levels and have the potential to increase glial activation.

Advanced Shotgun Lipidomics for Characterization of Altered Lipid Patterns in Neurodegenerative Diseases and Brain Injury

Multi-dimensional mass spectrometry-based shotgun lipidomics (MDMS-SL) is a powerful technology platform among current lipidomics practices due to its high efficiency, sensitivity, and reproducibility, as well as its broad coverage. This platform has been widely used to determine the altered lipid profiles induced by diseases, injury, genetic manipulations, drug treatments, and aging, among others. Herein, we summarize the principles underlying this platform and present a protocol for analysis of many of the lipid classes and subclasses covered by MDMS-SL directly from lipid extracts of brain samples. We believe that this protocol can aid researchers in the field to determine altered lipid patterns in neurodegenerative diseases and brain injury.

Novel advances in shotgun lipidomics for biology and medicine

The field of lipidomics, as coined in 2003, has made profound advances and been rapidly expanded. The mass spectrometry-based strategies of this analytical methodology-oriented research discipline for lipid analysis are largely fallen into three categories: direct infusion-based shotgun lipidomics, liquid chromatography-mass spectrometry-based platforms, and matrix-assisted laser desorption/ionization mass spectrometry-based approaches (particularly in imagining lipid distribution in tissues or cells). This review focuses on shotgun lipidomics. After briefly introducing its fundamentals, the major materials of this article cover its recent advances. These include the novel methods of lipid extraction, novel shotgun lipidomics strategies for identification and quantification of previously hardly accessible lipid classes and molecular species including isomers, and novel tools for processing and interpretation of lipidomics data. Representative applications of advanced shotgun lipidomics for biological and biomedical research are also presented in this review. We believe that with these novel advances in shotgun lipidomics, this approach for lipid analysis should become more comprehensive and high throughput, thereby greatly accelerating the lipidomics field to substantiate the aberrant lipid metabolism, signaling, trafficking, and homeostasis under pathological conditions and their underpinning biochemical mechanisms.

Alzheimer's as a Systems-Level Disease Involving the Interplay of Multiple Cellular Networks

Alzheimer's disease (AD), and many neurodegenerative disorders, are multifactorial in nature. They involve a combination of genomic, epigenomic, interactomic and environmental factors. Progress is being made, and these complex diseases are beginning to be understood as having their origin in altered states of biological networks at the cellular level. In the case of AD, genomic susceptibility and mechanisms leading to (or accompanying) the impairment of the central Amyloid Precursor Protein (APP) processing and tau networks are widely accepted as major contributors to the diseased state. The derangement of these networks may result in both the gain and loss of functions, increased generation of toxic species (e.g., toxic soluble oligomers and aggregates) and imbalances, whose effects can propagate to supra-cellular levels. Although well sustained by empirical data and widely accepted, this global perspective often overlooks the essential roles played by the main counteracting homeostatic networks (e.g., protein quality control/proteostasis, unfolded protein response, protein folding chaperone networks, disaggregases, ER-associated degradation/ubiquitin proteasome system, endolysosomal network, autophagy, and other stress-protective and clearance networks), whose relevance to AD is just beginning to be fully realized. In this chapter, an integrative perspective is presented. Alzheimer's disease is characterized to be a result of: (a) intrinsic genomic/epigenomic susceptibility and, (b) a continued dynamic interplay between the deranged networks and the central homeostatic networks of nerve cells. This interplay of networks will underlie both the onset and rate of progression of the disease in each individual. Integrative Systems Biology approaches are required to effect its elucidation. Comprehensive Systems Biology experiments at different 'omics levels in simple model organisms, engineered to recapitulate the basic features of AD may illuminate the onset and sequence of events underlying AD. Indeed, studies of models of AD in simple organisms, differentiated cells in culture and rodents are beginning to offer hope that the onset and progression of AD, if detected at an early stage, may be stopped, delayed, or even reversed, by activating or modulating networks involved in proteostasis and the clearance of toxic species. In practice, the incorporation of next-generation neuroimaging, high-throughput and computational approaches are opening the way towards early diagnosis well before irreversible cell death. Thus, the presence or co-occurrence of: (a) accumulation of toxic Aβ oligomers and tau species; (b) altered splicing and transcriptome patterns; (c) impaired redox, proteostatic, and metabolic networks together with, (d) compromised homeostatic capacities may constitute relevant 'AD hallmarks at the cellular level' towards reliable and early diagnosis. From here, preventive lifestyle changes and tailored therapies may be investigated, such as combined strategies aimed at both lowering the production of toxic species and potentiating homeostatic responses, in order to prevent or delay the onset, and arrest, alleviate, or even reverse the progression of the disease.

Neurometabolic roles of ApoE and Ldl-R in mouse brain

Polymorphisms in ApoE are highly correlated with the progression of neurodegenerative disease, in particular Alzheimer's disease. Little is known, however, about the role of ApoE or cholesterol metabolism on brain neurochemistry in general. To better understand the role of lipoprotein and cholesterol metabolism in the brain, we profiled 6 and 12-week old Apoe KO and Ldlr KO mouse models via unbiased metabolomics to determine which metabolites were affected at an early age to identify those that may play a role in triggering pathology later in life. Steady-state metabolomics revealed only subtle differences among Apoe KO, Ldlr KO and WT mouse brains. Ldlr KO mice exhibited alterations in metabolites involved in neurotransmitter, amino acid and cholesterol metabolism. In contrast, Apoe KO mice only showed subtle changes in amino acid and neurotransmitter metabolism. These subtle changes in a broad range of metabolites indicate that ApoE and Ldl-R alone may not play a significant role in these mouse models at an early age, but instead require the cumulative effect from different pathways that lead to dysfunction at a much later stage of life.

Recent advances in lipidomics for disease research

Lipidomics is an important branch of metabolomics, which aims at the detailed analysis of lipid species and their multiple roles in the living system. In recent years, the development of various analytical methods for effective identification and characterization of lipids has greatly promoted the process of lipidomics. Meanwhile, as many diseases demonstrate a remarkable alteration in lipid profiles compared with that of healthy people, lipidomics has been extensively introduced to disease research. The comprehensive lipid profiling provides a chance to discover novel biomarkers for specific disease. In addition, it plays a crucial role in the study of lipid metabolism, which could illuminate the pathogenesis of diseases. In this review, after brief discussion of analytical methods for lipidomics in clinical research, we focus on the recent advances of lipidomics related to four types of diseases, including cancer, atherosclerosis, diabetes mellitus, and Alzheimer's disease.

Non-targeted lipidomics of CSF and frontal cortex grey and white matter in control, mild cognitive impairment, and Alzheimer's disease subjects

OBJECTIVE

We undertook a non-targeted lipidomics analysis of post-mortem cerebrospinal fluid (CSF), frontal cortex grey matter, and subjacent white matter to define potential biomarkers that distinguish cognitively intact subjects from those with incipient or established dementia. Our objective was to increase our understanding of the role of brain lipids in pathophysiology of aging and age-related cognitive impairment.

METHODS

Levels of 650 individual lipids, across 26 lipid subclasses, were measured utilising a high-resolution mass spectrometric analysis platform.

RESULTS

Monoacylglycerols (MAG), diacylglycerols (DAG), and the very-long-chain fatty acid 26:0 were elevated in the grey matter of the mild cognitive impairment (MCI) and old dementia (OD) cohorts. Ethanolamine plasmalogens (PlsEtn) were decreased in the grey matter of the young dementia (YD) and OD cohorts while and phosphatidylethanolamines (PtdEth) were lower in the MCI, YD and OD cohorts. In the white matter, decrements in sulphatide levels were detected in the YD group, DAG levels were elevated in the MCI group, and MAG levels were increased in the YD and OD groups.

CONCLUSION

The parallel changes in grey matter MAGs and DAGs in the MCI and OD groups suggest that these two cohorts may have a similar underlying pathophysiology; consistent with this, MCI subjects were more similar in age to OD than to YD subjects. While PlsEtn and phosphatidylethanolamine were decreased in the YD and OD groups they were unaltered in the MCI group indicating that alterations in plasmalogen synthesis are unlikely to represent an initiating event in the transition from MCI to dementia.

Lipidomics of human brain aging and Alzheimer's disease pathology

Lipids stimulated and favored the evolution of the brain. Adult human brain contains a large amount of lipids, and the largest diversity of lipid classes and lipid molecular species. Lipidomics is defined as "the full characterization of lipid molecular species and of their biological roles with respect to expression of proteins involved in lipid metabolism and function, including gene regulation." Therefore, the study of brain lipidomics can help to unravel the diversity and to disclose the specificity of these lipid traits and its alterations in neural (neurons and glial) cells, groups of neural cells, brain, and fluids such as cerebrospinal fluid and plasma, thus helping to uncover potential biomarkers of human brain aging and Alzheimer disease. This review will discuss the lipid composition of the adult human brain. We first consider a brief approach to lipid definition, classification, and tools for analysis from the new point of view that has emerged with lipidomics, and then turn to the lipid profiles in human brain and how lipids affect brain function. Finally, we focus on the current status of lipidomics findings in human brain aging and Alzheimer's disease pathology. Neurolipidomics will increase knowledge about physiological and pathological functions of brain cells and will place the concept of selective neuronal vulnerability in a lipid context.

A monophasic extraction strategy for the simultaneous lipidome analysis of polar and nonpolar retina lipids

Lipid extraction using a monophasic chloroform/methanol/water mixture, coupled with functional group selective derivatization and direct infusion nano-ESI-high-resolution/accurate MS, is shown to facilitate the simultaneous analysis of both highly polar and nonpolar lipids from a single retina lipid extract, including low abundance highly polar ganglioside lipids, nonpolar sphingolipids, and abundant glycerophospholipids. Quantitative comparison showed that the monophasic lipid extraction method yielded similar lipid distributions to those obtained from established "gold standard" biphasic lipid extraction methods known to enrich for either highly polar gangliosides or nonpolar lipids, respectively, with only modest relative ion suppression effects. This improved lipid extraction and analysis strategy therefore enables detailed lipidome analyses of lipid species across a broad range of polarities and abundances, from minimal amounts of biological samples and without need for multiple lipid class-specific extractions or chromatographic separation prior to analysis.

Alterations in mouse brain lipidome after disruption of CST gene: a lipidomics study

To investigate the effects of a critical enzyme, cerebroside sulfotransferase (CST), involving sulfatide biosynthesis on lipid (particularly sphingolipid) homeostasis, herein, we determined the lipidomes of brain cortex and spinal cord from CST null and heterozygous (CST(-/-) and CST(+/-), respectively) mice in comparison to their wild-type littermates by multi-dimensional mass spectrometry-based shotgun lipidomics. As anticipated, we demonstrated the absence of sulfatide in the tissues from CST(-/-) mice and found that significant reduction of sulfatide mass levels was also present, but in an age-dependent manner, in CST(+/-) mice. Unexpectedly, we revealed that the profiles of sulfatide species in CST(+/-) mice were significantly different from that of littermate controls with an increase in the composition of species containing saturated and hydroxylated fatty acyl chains. Contrary to the changes of sulfatide levels, shotgun lipidomics analysis did not detect significant changes of the mass levels of other lipid classes examined. Taken together, shotgun lipidomics analysis demonstrated anticipated sulfatide mass deficiency in CST defect mouse brain and revealed novel brain lipidome homeostasis in these mice. These results might provide new insights into the role of CST in myelin function.