Tandem Mass Spectrometry Multiplex Analysis of Glucosylceramide and Galactosylceramide Isoforms in Brain Tissues at Different Stages of Parkinson Disease

Sphingolipids: membrane microdomains in brain development, function and neurological diseases

Sphingolipids are highly enriched in the nervous system where they are pivotal constituents of the plasma membranes and are important for proper brain development and functions. Sphingolipids are not merely structural elements, but are also recognized as regulators of cellular events by their ability to form microdomains in the plasma membrane. The significance of such compartmentalization spans broadly from being involved in differentiation of neurons and synaptic transmission to neuronal–glial interactions and myelin stability. Thus, perturbations of the sphingolipid metabolism can lead to rearrangements in the plasma membrane, which has been linked to the development of various neurological diseases. Studying microdomains and their functions has for a long time been synonymous with studying the role of cholesterol. However, it is becoming increasingly clear that microdomains are very heterogeneous, which among others can be ascribed to the vast number of sphingolipids. In this review, we discuss the importance of microdomains with emphasis on sphingolipids in brain development and function as well as how disruption of the sphingolipid metabolism (and hence microdomains) contributes to the pathogenesis of several neurological diseases.

Augmented frontal cortex diacylglycerol levels in Parkinson's disease and Lewy Body Disease

Background

Research from our laboratory, and that of other investigators, has demonstrated augmented levels of diacylglycerols (DAG) in the frontal cortex and plasma of subjects with Alzheimer’s disease (AD) and Mild Cognitive Impairment (MCI). We have extended these observations to investigate the frontal cortex of subjects with Parkinson’s disease (PD) and Lewy Body Disease (LBD), with and without coexisting pathologic features of AD.

Methods/Principal findings

Utilizing a high-resolution mass spectrometry analytical platform, we clearly demonstrate that DAG levels are significantly increased in the frontal cortex of subjects with PD, LBD with intermediate neocortical AD neuropathology, and in LBD with established neocortical AD neuropathology. In the case of the PD cohort, increases in cortical DAG levels were detected in cases with no neocortical pathology but were greater in subjects with neocortical pathology. These data suggest that DAG changes occur early in the disease processes and are amplified as cortical dysfunction becomes more established.

Conclusions

These findings suggest that altered DAG synthesis/metabolism is a common feature of neurodegenerative diseases, characterized by proteinopathy, that ultimately result in cognitive deficits. With regard to the mechanism responsible for these biochemical alterations, selective decrements in cortical levels of phosphatidylcholines in LBD and PD suggest that augmented degradation and/or decreased synthesis of these structural glycerophospholipids may contribute to increases in the pool size of free DAGs. The observed augmentation of DAG levels may be phospholipase-driven since neuroinflammation is a consistent feature of all disease cohorts. If this conclusion can be validated it would support utilizing DAG levels as a biomarker of the early disease process and the investigation of early intervention with anti-inflammatory agents.

The lysosomal enzyme alpha-Galactosidase A is deficient in Parkinson's disease brain in association with the pathologic accumulation of alpha-synuclein

The aberrant accumulation of alpha-synuclein (α-syn) is believed to contribute to the onset and pathogenesis of Parkinson's disease (PD). The autophagy-lysosome pathway (ALP) is responsible for the high capacity clearance of α-syn. ALP dysfunction is documented in PD and pre-clinical evidence suggests that inhibiting the ALP promotes the pathological accumulation of α-syn. We previously identified the pathological accumulation of α-syn in the brains of mice deficient for the soluble lysosomal enzyme alpha-Galactosidase A (α-Gal A), a member of the glycosphingolipid metabolism pathway. In the present study, we quantified α-Gal A activity and levels of its glycosphingolipid metabolites in postmortem temporal cortex specimens from control individuals and in PD individuals staged with respect to α-syn containing Lewy body pathology. In late-state PD temporal cortex we observed significant decreases in α-Gal A activity and the 46kDa "active" species of α-Gal A as determined respectively by fluorometric activity assay and western blot analysis. These decreases in α-Gal A activity/levels correlated significantly with increased α-syn phosphorylated at serine 129 (p129S-α-syn) that was maximal in late-stage PD temporal cortex. Mass spectrometric analysis of 29 different isoforms of globotriaosylceramide (Gb3), a substrate of α-Gal A indicated no significant differences with respect to different stages of PD temporal cortex. However, significant correlations were observed between increased levels of several Gb3 isoforms and with decreased α-Gal A activity and/or increased p129S-α-syn. Deacylated Gb3 (globotriaosylsphingosine or lyso-Gb3) was also analyzed in PD brain tissue but was below the limit of detection of 20pmol/g. Analysis of other lysosomal enzymes revealed a significant decrease in activity for the lysosomal aspartic acid protease cathepsin D but not for glucocerebrosidase (GCase) or cathepsin B in late-stage PD temporal cortex. However, a significant correlation was observed between decreasing GCase activity and increasing p129S-α-syn. Together our findings indicate α-Gal A deficiency in late-stage PD brain that correlates significantly with the pathological accumulation of α-syn, and further suggest the potential for α-Gal A and its glycosphingolipid substrates as putative biomarkers for PD.

Lipid biomarkers for the peroxisomal and lysosomal disorders: their formation, metabolism and measurement

Lipid biomarkers play important roles in the diagnosis of and monitoring of treatment in peroxisomal disorders and lysosomal storage disorders. Today, a variety of lipids, including very long chain fatty acids, glycolipids, bile acids and the oxidation products of cholesterol, have been considered as biomarkers for these disorders. In this brief review, the authors summarized the recent advances regarding these lipid biomarkers in terms of their formation, metabolism and measurement in these disorders. An understanding of these biomarkers will offer a key to the development of novel diagnoses and help create more effective therapies in the future.

Separation and Analysis of Lactosylceramide, Galabiosylceramide, and Globotriaosylceramide by LC-MS/MS in Urine of Fabry Disease Patients

Fabry disease is an X-linked lysosomal storage disorder caused by α-galactosidase A (α-GAL A) deficiency. This enzyme contributes to the cellular recycling of glycosphingolipids such as galabiosylceramide (Ga₂), globotriaosylceramide (Gb₃), and globotriaosylsphingosine (lyso-Gb₃) by hydrolyzing the terminal α-galactosyl moiety. Urine and plasma α-GAL A substrates are currently analyzed as biomarkers for the detection, monitoring, and follow-up of Fabry disease patients. The sensitivity of the analysis of Ga₂ is decreased by the co-analysis of its structural isomer, lactosylceramide (LacCer), which is not an α-GAL A substrate. A normal-phase ultraperformance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS) methodology, allowing the baseline separation of 12 Ga₂ isoforms/analogues from their lactosylceramide counterparts, was developed and validated in urine. The method was multiplexed with the analysis of 12 Gb₃ isoforms/analogues having the same fatty acid moieties as those of Ga₂ for comparison, and with creatinine for sample normalization. Urine samples were studied from 34 untreated and 33 Fabry males treated by enzyme replacement therapy (ERT) and 54 untreated and 19 ERT-treated Fabry females, along with 34 male and 25 female healthy controls. The chromatographic separation of Ga₂ from LacCer increased the sensitivity of analysis, especially in women. One untreated Fabry female and two treated Fabry females presented abnormal levels of Ga₂ but normal levels of Gb₃, supporting the importance of analyzing Ga_2, in addition to Gb₃. Our results show that urine LacCer levels from females were significantly higher than those from males. Moreover, LacCer levels were not affected by Fabry disease for both males and females.

Structural Analysis of Unsaturated Glycosphingolipids Using Shotgun Ozone-Induced Dissociation Mass Spectrometry

Glycosphingolipids are essential biomolecules widely distributed across biological kingdoms yet remain relatively underexplored owing to both compositional and structural complexity. While the glycan head group has been the subject of most studies, there is paucity of reports on the lipid moiety, particularly the location of unsaturation. In this paper, ozone-induced dissociation mass spectrometry (OzID-MS) implemented in a traveling wave-based quadrupole time-of-flight (Q-ToF) mass spectrometer was applied to study unsaturated glycosphingolipids using shotgun approach. Resulting high resolution mass spectra facilitated the unambiguous identification of diagnostic OzID product ions. Using [M+Na]+ adducts of authentic standards, we observed that the long chain base and fatty acyl unsaturation had distinct reactivity with ozone. The reactivity of unsaturation in the fatty acyl chain was about 8-fold higher than that in the long chain base, which enables their straightforward differentiation. Influence of the head group, fatty acyl hydroxylation, and length of fatty acyl chain on the oxidative cleavage of double bonds was also observed. Application of this technique to bovine brain galactocerebrosides revealed co-isolated isobaric and regioisomeric species, which otherwise would be incompletely identified using contemporary collision-induced dissociation (CID) alone. These results highlight the potential of OzID-MS in glycosphingolipids research, which not only provides complementary structural information to existing CID technique but also facilitates de novo structural determination of these complex biomolecules. Graphical Abstract ᅟ.

Tissue Localization of Glycosphingolipid Accumulation in a Gaucher Disease Mouse Brain by LC-ESI-MS/MS and High-Resolution MALDI Imaging Mass Spectrometry

To better understand regional brain glycosphingolipid (GSL) accumulation in Gaucher disease (GD) and its relationship to neuropathology, a feasibility study using mass spectrometry and immunohistochemistry was conducted using brains derived from a GD mouse model (4L/PS/NA) homozygous for a mutant GCase (V394L [4L]) and expressing a prosaposin hypomorphic (PS-NA) transgene. Whole brains from GD and control animals were collected using one hemisphere for MALDI FTICR IMS analysis and the other for quantitation by LC-ESI-MS/MS. MALDI IMS detected several HexCers across the brains. Comparison with the brain hematoxylin and eosin (H&E) revealed differential signal distributions in the midbrain, brain stem, and CB of the GD brain versus the control. Quantitation of serial brain sections with LC-ESI-MS/MS supported the imaging results, finding the overall HexCer levels in the 4L/PS-NA brains to be four times higher than the control. LC-ESI-MS/MS also confirmed that the elevated hexosyl isomers were glucosylceramides rather than galactosylceramides. MALDI imaging also detected differential analyte distributions of lactosylceramide species and gangliosides in the 4L/PS-NA brain, which was validated by LC-ESI-MS/MS. Immunohistochemistry revealed regional inflammation, altered autophagy, and defective protein degradation correlating with regions of GSL accumulation, suggesting that specific GSLs may have distinct neuropathological effects.

Diastereomer-specific quantification of bioactive hexosylceramides from bacteria and mammals

Mammals synthesize, cell-type specifically, the diastereomeric hexosylceramides, β-galactosylceramide (GalCer) and β-glucosylceramide (GlcCer), which are involved in several diseases, such as sphingolipidosis, diabetes, chronic kidney diseases, or cancer. In contrast, Bacteroides fragilis, a member of the human gut microbiome, and the marine sponge, Agelas mauritianus, produce α-GalCer, one of the most potent stimulators for invariant natural killer T cells. To dissect the contribution of these individual stereoisomers to pathologies, we established a novel hydrophilic interaction chromatography-based LC-MS² method and separated (R > 1.5) corresponding diastereomers from each other, independent of their lipid anchors. Testing various bacterial and mammalian samples, we could separate, identify (including the lipid anchor composition), and quantify endogenous β-GlcCer, β-GalCer, and α-GalCer isomers without additional derivatization steps. Thereby, we show a selective decrease of β-GlcCers versus β-GalCers in cell-specific models of GlcCer synthase-deficiency and an increase of specific β-GlcCers due to loss of β-glucoceramidase 2 activity. Vice versa, β-GalCer increased specifically when cerebroside sulfotransferase (Gal3st1) was deleted. We further confirm β-GalCer as substrate of globotriaosylceramide synthase for galabiaosylceramide synthesis and identify additional members of the human gut microbiome to contain immunogenic α-GalCers. Finally, this method is shown to separate corresponding hexosylsphingosine standards, promoting its applicability in further investigations.

Intracellular metabolite β-glucosylceramide is an endogenous Mincle ligand possessing immunostimulatory activity

Sensing and reacting to tissue damage is a fundamental function of immune systems. Macrophage inducible C-type lectin (Mincle) is an activating C-type lectin receptor that senses damaged cells. Notably, Mincle also recognizes glycolipid ligands on pathogens. To elucidate endogenous glycolipids ligands derived from damaged cells, we fractionated supernatants from damaged cells and identified a lipophilic component that activates reporter cells expressing Mincle. Mass spectrometry and NMR spectroscopy identified the component structure as β-glucosylceramide (GlcCer), which is a ubiquitous intracellular metabolite. Synthetic β-GlcCer activated myeloid cells and induced production of inflammatory cytokines; this production was abrogated in Mincle-deficient cells. Sterile inflammation induced by excessive cell death in the thymus was exacerbated by hematopoietic-specific deletion of degrading enzyme of β-GlcCer (β-glucosylceramidase, GBA1). However, this enhanced inflammation was ameliorated in a Mincle-deficient background. GBA1-deficient dendritic cells (DCs) in which β-GlcCer accumulates triggered antigen-specific T-cell responses more efficiently than WT DCs, whereas these responses were compromised in DCs from GBA1 × Mincle double-deficient mice. These results suggest that β-GlcCer is an endogenous ligand for Mincle and possesses immunostimulatory activity.

Effects of dietary glucocerebrosides from sea cucumber on the brain sphingolipid profiles of mouse models of Alzheimer's disease

Alterations of sphingolipid levels in the hippocampus and cortex of normal, AD model, and SCG-treated mice.

LRRK2 deficiency impacts ceramide metabolism in brain

Mutations in LRRK2 gene cause inherited Parkinson's disease (PD) and variations around LRRK2 act as risk factor for disease. Similar to sporadic disease, LRRK2-linked cases show late onset and, typically, the presence of proteinaceous inclusions named Lewy bodies (LBs) in neurons. Recently, defects on ceramide (Cer) metabolism have been recognized in PD. In particular, heterozygous mutations in the gene encoding for glucocerebrosidase (GBA1), a lysosomal enzyme converting glucosyl-ceramides (Glc-Cer) into Cer, increase the risk of developing PD. Although several studies have linked LRRK2 with membrane-related processes and autophagic-lysosomal pathway regulation, whether this protein impinges on the Cer pathway has not been addressed. Here, using a targeted lipidomics approach, we report an altered sphingolipid composition in Lrrk2(-/-) mouse brains. In particular, we observe a significant increase of Cer levels in Lrrk2(-/-) mice and direct effects on GBA1. Collectively, our results suggest a link between LRRK2 and Cer metabolism, providing new insights into the possible role of this protein in sphingolipids metabolism, with implications for PD therapeutics.