Pyroglutamation of amyloid-βx-42 (Aβx-42) followed by Aβ1-40 deposition underlies plaque polymorphism in progressing Alzheimer's disease pathology

Amyloid-β (Aβ) pathology in Alzheimer's disease (AD) is characterized by the formation of polymorphic deposits comprising diffuse and cored plaques. Because diffuse plaques are predominantly observed in cognitively unaffected, amyloid-positive (CU-AP) individuals, pathogenic conversion into cored plaques appears to be critical to AD pathogenesis. Herein, we identified the distinct Aβ species associated with amyloid polymorphism in brain tissue from individuals with sporadic AD (s-AD) and CU-AP. To this end, we interrogated Aβ polymorphism with amyloid conformation–sensitive dyes and a novel in situ MS paradigm for chemical characterization of hyperspectrally delineated plaque morphotypes. We found that maturation of diffuse into cored plaques correlated with increased Aβ1–40 deposition. Using spatial in situ delineation with imaging MS (IMS), we show that Aβ1–40 aggregates at the core structure of mature plaques, whereas Aβ1–42 localizes to diffuse amyloid aggregates. Moreover, we observed that diffuse plaques have increased pyroglutamated Aβx-42 levels in s-AD but not CU-AP, suggesting an AD pathology–related, hydrophobic functionalization of diffuse plaques facilitating Aβ1–40 deposition. Experiments in tgAPP_Swe mice verified that, similar to what has been observed in human brain pathology, diffuse deposits display higher levels of Aβ1–42 and that Aβ plaque maturation over time is associated with increases in Aβ1–40. Finally, we found that Aβ1–40 deposition is characteristic for cerebral amyloid angiopathy deposition and maturation in both humans and mice. These results indicate that N-terminal Aβx-42 pyroglutamation and Aβ1–40 deposition are critical events in priming and maturation of pathogenic Aβ from diffuse into cored plaques, underlying neurotoxic plaque development in AD.

Novel Trimodal MALDI Imaging Mass Spectrometry (IMS3) at 10 μm Reveals Spatial Lipid and Peptide Correlates Implicated in Aβ Plaque Pathology in Alzheimer's Disease

Multimodal chemical imaging using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) can provide comprehensive molecular information in situ within the same tissue sections. This is of relevance for studying different brain pathologies such as Alzheimer's disease (AD), where recent data suggest a critical relevance of colocalizing Aβ peptides and neuronal lipids. We here developed a novel trimodal, high-resolution (10 μm) MALDI imaging MS (IMS) paradigm for negative and positive ion mode lipid analysis and subsequent protein ion imaging on the same tissue section. Matrix sublimation of 1,5-diaminonaphthalene (1,5-DAN) enabled dual polarity lipid MALDI IMS on the same pixel points at high spatial resolutions (10 μm) and with high spectral quality. This was followed by 10 μm resolution protein imaging on the same measurement area, which allowed correlation of lipid signals with protein distribution patterns within distinct cerebellar regions in mouse brain. The demonstrated trimodal imaging strategy (IMS3) was further shown to be an efficient approach for simultaneously probing Aβ plaque-associated lipids and Aβ peptides within the hippocampus of 18 month-old transgenic AD mice (tgArcSwe). Here, IMS3 revealed a strong colocalization of distinct lipid species including ceramides, phosphatidylinositols, sulfatides (Cer 18:0, PI 38:4, ST 24:0) and lysophosphatidylcholines (LPC 16:0, LPC 18:0) with plaque-associated Aβ isoforms (Aβ 1-37, Aβ 1-38, Aβ 1-40). This highlights the potential of IMS3 as an alternative, superior approach to consecutively performed immuno-based Aβ staining strategies. Furthermore, the IMS3 workflow allowed for multimodal in situ MS/MS analysis of both lipids and Aβ peptides. Altogether, the here presented IMS3 approach shows great potential for comprehensive, high-resolution molecular analysis of histological features at cellular length scales with high chemical specificity. It therefore represents a powerful approach for probing the complex molecular pathology of, e.g., neurodegenerative diseases that are characterized by neurotoxic protein aggregation.

Histology-Compatible MALDI Mass Spectrometry Based Imaging of Neuronal Lipids for Subsequent Immunofluorescent Staining

Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) enables acquisition of spatial distribution maps for molecular species in situ. This can provide comprehensive insights on the pathophysiology of different diseases. However, current sample preparation and MALDI-IMS acquisition methods have limitations in preserving molecular and histological tissue morphology, resulting in interfered correspondence of MALDI-IMS data with subsequently acquired immunofluorescent staining results. We here investigated the histology compatibility of MALDI-IMS to image neuronal lipids in rodent brain tissue with subsequent immunohistochemistry and fluorescent staining of histological features. This was achieved by sublimation of a low ionization energy matrix compound, 1,5-diaminonapthalene (1,5-DAN), minimizing the number of low-energy laser shots. This yielded improved lipid spectral quality and speed of data acquisition and reduced matrix cluster formation along with preservation of specific histological information at cellular levels. This gentle, histology-compatible MALDI-IMS protocol also diminished thermal effects and mechanical stress created during nanosecond laser ablation processes that were prominent in subsequent immunofluorescent staining images but not with classical hematoxylin and eosin (H&E) staining on the same tissue section. Furthermore, this methodology proved to be a powerful strategy for investigating β-amyloid (Aβ) plaque-associated neuronal lipids as exemplified by performing high-resolution MALDI-IMS with subsequent fluorescent amyloid staining in a transgenic mouse model of Alzheimer's disease (tgSwe).

Delineating Amyloid Plaque Associated Neuronal Sphingolipids in Transgenic Alzheimer's Disease Mice (tgArcSwe) Using MALDI Imaging Mass Spectrometry

The major pathological hallmarks of Alzheimer’s disease (AD) are the progressive aggregation and accumulation of beta-amyloid (Aβ) and hyperphosphorylated tau protein into neurotoxic deposits. Aβ aggregation has been suggested as the critical early inducer, driving the disease progression. However, the factors that promote neurotoxic Aβ aggregation remain elusive. Imaging mass spectrometry (IMS) is a powerful technique to comprehensively elucidate the spatial distribution patterns of lipids, peptides, and proteins in biological tissue sections. In the present study, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS)-based imaging was used on transgenic Alzheimer’s disease mouse (tgArcSwe) brain tissue to investigate the sphingolipid microenvironment of individual Aβ plaques and elucidate plaque-associated sphingolipid alterations. Multivariate data analysis was used to interrogate the IMS data for identifying pathologically relevant, anatomical features based on their lipid chemical profile. This approach revealed sphingolipid species that distinctly located to cortical and hippocampal deposits, whose Aβ identity was further verified using fluorescent amyloid staining and immunohistochemistry. Subsequent multivariate statistical analysis of the spectral data revealed significant localization of gangliosides and ceramides species to Aβ positive plaques, which was accompanied by distinct local reduction of sulfatides. These plaque-associated changes in sphingolipid levels implicate a functional role of sphingolipid metabolism in Aβ plaque pathology and AD pathogenesis. Taken together, the presented data highlight the potential of imaging mass spectrometry as a powerful approach for probing Aβ plaque-associated lipid changes underlying AD pathology.

Designed Glycopeptidomimetics Disrupt Protein-Protein Interactions Mediating Amyloid β-Peptide Aggregation and Restore Neuroblastoma Cell Viability

How anti-Alzheimer's drug candidates that reduce amyloid 1-42 peptide fibrillization interact with the most neurotoxic species is far from being understood. We report herein the capacity of sugar-based peptidomimetics to inhibit both Aβ1-42 early oligomerization and fibrillization. A wide range of bio- and physicochemical techniques, such as a new capillary electrophoresis method, nuclear magnetic resonance, and surface plasmon resonance, were used to identify how these new molecules can delay the aggregation of Aβ1-42. We demonstrate that these molecules interact with soluble oligomers in order to maintain the presence of nontoxic monomers and to prevent fibrillization. These compounds totally suppress the toxicity of Aβ1-42 toward SH-SY5Y neuroblastoma cells, even at substoichiometric concentrations. Furthermore, demonstration that the best molecule combines hydrophobic moieties, hydrogen bond donors and acceptors, ammonium groups, and a hydrophilic β-sheet breaker element provides valuable insight for the future structure-based design of inhibitors of Aβ1-42 aggregation.

An improved capillary electrophoresis method for in vitro monitoring of the challenging early steps of Aβ1-42 peptide oligomerization: application to anti-Alzheimer's drug discovery

We report an improved CE method to monitor in vitro the self-assembly of monomeric amyloid β-peptide (42 amino acids amyloid β-peptide, Aβ1-42 ) and in particular the crucial early steps involved in the formation of the neurotoxic oligomers. In order to start the kinetics from the beginning, sample preparation was optimized to provide samples containing exclusively the monomeric form. The CE method was also improved using a dynamic coating and by reducing the separation distance. Using this method, the disappearance of the monomer as well as the progressive formation of four species during the self-assembly process can now be monitored and quantified over time. The hydrodynamic radius of the species present at the initial kinetics step was estimated around 1.8 nm by Taylor dispersion analysis while SDS-PAGE analyses showed the predominance of the monomer. These results confirmed that the Aβ1-42 species present at this initial time was the monomer. Methylene blue, an anti-Alzheimer disease candidate, was then evaluated. In spite of an oligomerization inhibition, the enhanced disappearance of the Aβ1-42 monomer provoked by methylene blue was demonstrated for the first time. This method, allowing the monomeric and smallest oligomeric species to be monitored, represents a new accurate and precise way to evaluate compounds for drug discovery.

Palladium-catalyzed direct arylation of polysubstituted benzofurans

An efficient access to 2-substituted 3-arylbenzofurans through a palladium-catalyzed C3 direct arylation of 2-substituted benzofurans with aryl bromides is described. The scope and limitation of this reaction was studied. The method tolerates a variety of functional groups on the aryl halide and has been successfully extended to polysubstituted benzofurans to obtain the corresponding 3-arylbenzofurans with good to excellent yields.