Abstract
Understanding of Alzheimer's disease (AD) pathophysiology requires molecular assessment of how key pathological factors, specifically amyloid β (Aβ) plaques, influence the surrounding microenvironment. Here, neuronal lipids have been implicated in Aβ plaque pathology, though the lipid microenvironment in direct proximity to Aβ plaques is still not fully resolved. A further challenge is the microenvironmental molecular heterogeneity, across structurally polymorphic Aβ features, such as diffuse, immature, and mature, fibrillary aggregates, whose resolution requires the integration of advanced, multimodal chemical imaging tools. Herein, we used matrix-assisted laser desorption/ionization trapped ion mobility spectrometry time-of-flight based mass spectrometry imaging (MALDI TIMS TOF MSI) in combination with hyperspectral confocal microscopy to probe the lipidomic microenvironment associated with structural polymorphism of Aβ plaques in transgenic Alzheimer's disease mice (tgAPPSWE ). Using on tissue and ex situ validation, TIMS MS/MS facilitated unambiguous identification of isobaric lipid species that showed plaque pathology-associated localizations. Integrated multivariate imaging data analysis revealed multiple, Aβ plaque-enriched lipid patterns for gangliosides (GM), phosphoinositols (PI), phosphoethanolamines (PE), and phosphatidic acids (PA). Conversely, sulfatides (ST), cardiolipins (CL), and polyunsaturated fatty acid (PUFA)-conjugated phosphoserines (PS), and PE were depleted at plaques. Hyperspectral amyloid imaging further delineated the unique distribution of PA and PE species to mature plaque core regions, while PI, LPI, GM2 and GM3lipids localized to immature Aβ aggregates present within the periphery of Aβ plaques. Finally, we followed AD pathology-associated lipid changes over time, identifying plaque- growth and maturation to be characterized by peripheral accumulation of PI (18:0/22:6). Together, these data demonstrate the potential of multimodal imaging approaches to overcome limitations associated with conventional advanced MS imaging applications. This allowed for the differentiation of both distinct lipid components in a complex micro-environment as well as their correlation to disease-relevant amyloid plaque polymorphs. Cover Image for this issue: https://doi.org/10.1111/jnc.15390.
Highlights
With the average age of the World's population increasing, the prevalence of age-associated diseases including neurodegenerative disorders is increasing as well
The major pathologic hallmark of Alzheimer's disease (AD) includes the formation of extracellular plaques consisting of amyloid-β (Aβ) peptides as well as neurofibrillary tangles formed by hyper-phosphorylated tau protein (Masters et al, 2015)
Our present study provides a clear separation and identification of these species facilitated through the ion mobility modality of trapped ion mobility spectrometry (TIMS) MSI
Summary
With the average age of the World's population increasing, the prevalence of age-associated diseases including neurodegenerative disorders is increasing as well. Matrix-assisted laser desorption ionization (MALDI)- based MSI has been demonstrated to be a valuable approach in retrieving novel chemical information on plaque pathology in both AD patient's brain and AD mouse models This includes molecular information on plaque-associated neuronal lipid dynamics, which is significant in that lipids have been implicated in AD pathology and more specific in Aβ plaque formation mechanisms before (Di Paolo & Kim, 2011). The MSI experiments were interfaced with fluorescent amyloid staining on the same section using structure sensitive, electrooptic amyloid probes, luminescent-conjugated oligothiophenes (LCOs) (Nilsson, 2009) that allow to delineate structurally different amyloid aggregates (Nilsson et al, 2007; Nystrom et al, 2013; Rasmussen et al, 2017) This hyperspectral imaging approach identified distinct lipid localization patterns specific to polymorphic amyloid structures including mass overlapping compounds, which could be further resolved by ion mobility separation. A matrix solution containing 7 mg/ml NEDC in 70% MeOH (Cat. no. 34860; Sigma–Aldrich) was sprayed onto the tissue sections with the following instrumental parameters: nitrogen flow (5 psi), spray temperature (30°C), nozzle height (40 mm), 14 passes with offsets and rotations, and spray velocity (1200 mm/min), and isocratic flow of 60 μl/min using 70% ACN as a pushing solvent
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