A mix-and-click method to measure amyloid-β concentration with sub-micromolar sensitivity

Nanotechnology for microglial targeting and inhibition of neuroinflammation underlying Alzheimer's pathology

BACKGROUND

Alzheimer's disease (AD) is considered to have a multifactorial etiology. The hallmark of AD is progressive neurodegeneration, which is characterized by the deepening loss of memory and a high mortality rate in the elderly. The neurodegeneration in AD is believed to be exacerbated following the intercoupled cascades of extracellular amyloid beta (Aβ) plaques, uncontrolled microglial activation, and neuroinflammation. Current therapies for AD are mostly designed to target the symptoms, with limited ability to address the mechanistic triggers for the disease. In this study, we report a novel nanotechnology based on microglial scavenger receptor (SR)-targeting amphiphilic nanoparticles (NPs) for the convergent alleviation of fibril Aβ (fAβ) burden, microglial modulation, and neuroprotection.

METHODS

We designed a nanotechnology approach to regulate the SR-mediated intracellular fAβ trafficking within microglia. We synthesized SR-targeting sugar-based amphiphilic macromolecules (AM) and used them as a bioactive shell to fabricate serum-stable AM-NPs via flash nanoprecipitation. Using electron microscopy, in vitro approaches, ELISA, and confocal microscopy, we investigated the effect of AM-NPs on Aβ fibrilization, fAβ-mediated microglial inflammation, and neurotoxicity in BV2 microglia and SH-SY5Y neuroblastoma cell lines.

RESULTS

AM-NPs interrupted Aβ fibrilization, attenuated fAβ microglial internalization via targeting the fAβ-specific SRs, arrested the fAβ-mediated microglial activation and pro-inflammatory response, and accelerated lysosomal degradation of intracellular fAβ. Moreover, AM-NPs counteracted the microglial-mediated neurotoxicity after exposure to fAβ.

CONCLUSIONS

The AM-NP nanotechnology presents a multifactorial strategy to target pathological Aβ aggregation and arrest the fAβ-mediated pathological progression in microglia and neurons.

Preparation and Fractionation of Heterogeneous Aβ42 Oligomers with Different Aggregation Properties

Deposition of amyloid-β (Aβ) aggregates in the form of amyloid plaques is a central feature of Alzheimer's disease. The end products of Aβ aggregation are amyloid fibrils. Soluble Aβ aggregates called oligomers are also formed either on or off the pathway of fibril formation. The amyloid fibrils from different clinical subtypes of Alzheimer's disease have been found to adopt different structures, a phenomenon called fibril polymorphism. Meanwhile, different types of Aβ oligomers have also been found. Recently, it has been shown that different types of Aβ42 oligomers may form fibrils of different structures, linking oligomer heterogeneity to fibril polymorphism. In this chapter, we describe methods to prepare heterogeneous Aβ42 oligomers and to quantify the concentration of these oligomers at a low micromolar range using a fluorescamine method. Fractionation of these oligomers by size using ultrafiltration filters allows for the formation of Aβ42 fibrils with different structural properties.

Lipid membranes induce structural conversion from amyloid oligomers to fibrils

Formation of amyloid oligomers and fibrils underlies the pathogenesis of a number of neurodegenerative diseases such as Alzheimer's. One mechanism of action by which Aβ aggregates cause neuronal toxicity is through interactions with cellular membranes. Aβ aggregates have been shown to disrupt membrane integrity via pore formation, membrane thinning, or lipid extraction. At the same time, lipid membranes also affect the rate of Aβ aggregation and remodel pre-formed Aβ fibrils. Here we show that Aβ42 globulomers, a type of well-characterized and stable Aβ oligomers, convert to amyloid fibrils in the presence of DOPC liposomes. Electron paramagnetic resonance studies show that the fibrils converted from Aβ42 globulomers adopt the same structure as fibrils formed directly from monomers. Our results suggest that the interactions between Aβ oligomers and cellular membranes are dynamic. By converting Aβ oligomers to fibrils, the lipid membrane can reduce the membrane-disrupting activities caused by these oligomers. Modulation of Aβ-membrane interactions as a therapeutic strategy should take into account the dynamic nature of these interactions.

Segmental structural dynamics in Aβ42 globulomers

Aβ42 aggregation plays a central role in the pathogenesis of Alzheimer's disease. In addition to the insoluble fibrils that comprise the amyloid plaques, Aβ42 also forms soluble aggregates collectively called oligomers, which are more toxic and pathogenic than fibrils. Understanding the structure and dynamics of Aβ42 oligomers is critical for developing effective therapeutic interventions against these oligomers. Here we studied the structural dynamics of Aβ42 globulomers, a type of Aβ42 oligomers prepared in the presence of sodium dodecyl sulfate, using site-directed spin labeling. Spin labels were introduced, one at a time, at all 42 residue positions of Aβ42 sequence. Electron paramagnetic resonance spectra of spin-labeled samples reveal four structural segments based on site-dependent spin label mobility pattern. Segment-1 consists of residues 1-6, which have the highest mobility that is consistent with complete disorder. Segment-3 is the most immobilized region, including residues 31-34. Segment-2 and -4 have intermediate mobility and are composed of residues 7-30 and 35-42, respectively. Considering the inverse relationship between protein dynamics and stability, our results suggest that residues 31-34 are the most stable segment in Aβ42 oligomers. At the same time, the EPR spectral lineshape suggests that Aβ42 globulomers lack a well-packed structural core akin to that of globular proteins.

Alzheimer's Aβ42 and Aβ40 form mixed oligomers with direct molecular interactions

Formation of Aβ oligomers and fibrils plays a central role in the pathogenesis of Alzheimer's disease. There are two major forms of Aβ in the brain: Aβ42 and Aβ40. Aβ42 is the major component of the amyloid plaques, but the overall abundance of Aβ40 is several times that of Aβ42. In vitro experiments show that Aβ42 and Aβ40 affect each other's aggregation. In mouse models of Alzheimer's disease, overexpression of Aβ40 has been shown to reduce the plaque pathology, suggesting that Aβ42 and Aβ40 also interact in vivo. Here we address the question of whether Aβ42 and Aβ40 interact with each other in the formation of oligomers using electron paramagnetic resonance (EPR) spectroscopy. When the Aβ42 oligomers were formed using only spin-labeled Aβ42, the dipolar interaction between spin labels that are within 20 Å range broadened the EPR spectrum and reduced its amplitude. Oligomers formed with a mixture of spin-labeled Aβ42 and wild-type Aβ42 gave an EPR spectrum with higher amplitude due to weakened spin-spin interactions, suggesting molecular mixing of labeled and wild-type Aβ42. When spin-labeled Aβ42 and wild-type Aβ40 were mixed to form oligomers, the resulting EPR spectrum also showed reduced amplitude, suggesting that wild-type Aβ40 can also form oligomers with spin-labeled Aβ42. Therefore, our results suggest that Aβ42 and Aβ40 form mixed oligomers with direct molecular interactions. Our results point to the importance of investigating Aβ42-Aβ40 interactions in the brain for a complete understanding of Alzheimer's pathogenesis and therapeutic interventions.

Distinguishing the Effect on the Rate and Yield of Aβ42 Aggregation by Green Tea Polyphenol EGCG

Deposition of Aβ42 aggregates in the form of amyloid plaques is a pathological hallmark of Alzheimer's disease. A desired avenue of intervention is the inhibition of Aβ42 aggregation. Epigallocatechin gallate (EGCG), the main polyphenol in green tea, has been generally considered an inhibitor of Aβ aggregation. However, previous experiments focused on the reduction of the amount of Aβ42 aggregates, while the effect of EGCG on the rate of Aβ42 aggregation was not critically analyzed. Here we performed an experimental evaluation of Aβ42 aggregation kinetics in the absence and presence of EGCG at a wide range of concentrations. We found that EGCG reduced thioflavin T fluorescence in an EGCG concentration-dependent manner, suggesting that EGCG reduced the amount of Aβ42 fibrils. The effect of EGCG on the rate of Aβ42 aggregation appears to be bimodal. We found that higher EGCG-to-Aβ42 ratios promoted the rate of Aβ42 aggregation, while lower EGCG-to-Aβ42 ratios inhibited the aggregation rate. To confirm that the reduction of thioflavin T fluorescence is due to the lowered aggregate quantity, but not due to perturbation of thioflavin T binding to Aβ42 fibrils, we probed the effect of EGCG on Aβ42 aggregation using site-directed spin labeling. Electron paramagnetic resonance of spin-labeled Aβ42 aggregates suggests that high EGCG-to-Aβ42 ratios led to a greatly reduced amount of Aβ42 fibrils, and these aggregates adopt similar structures as the fibrils in the no-EGCG sample. Potential implications of this work in designing prevention or therapeutic strategies using EGCG are discussed.

Early Effects of Aβ Oligomers on Dendritic Spine Dynamics and Arborization in Hippocampal Neurons

Alzheimer’s disease (AD) is a neurodegenerative disorder that leads to impaired memory and cognitive deficits. Spine loss as well as changes in spine morphology correlates with cognitive impairment in this neurological disorder. Many studies in animal models and ex vivo cultures indicate that amyloid β-peptide (Aβ) oligomers induce synaptic damage early during the progression of the disease. Here, in order to determine the events that initiate synaptic alterations, we acutely applied oligomeric Aβ to primary hippocampal neurons and an ex vivo model of organotypic hippocampal cultures from a mouse after targeted expression of EGFP to allow high-resolution imaging and algorithm-based evaluation of spine changes. Dendritic spines were classified as thin, stubby or mushroom, based on morphology. In vivo, time-lapse imaging showed that the three spine types were relatively stable, although their stability significantly decreased after treatment with Aβ oligomers. Unexpectedly, we observed that the density of total dendritic spines increased in organotypic hippocampal slices treated with Aβ compared to control cultures. Specifically, the fraction of stubby spines significantly increased, while mushroom and thin spines remained unaltered. Pharmacological tools revealed that acute Aβ oligomers induced spine changes through mechanisms involving CaMKII and integrin β1 activities. Additionally, analysis of dendritic complexity based on a 3D reconstruction of the whole neuron morphology showed an increase in the apical dendrite length and branching points in CA1 organotypic hippocampal slices treated with Aβ. In contrast to spines, the morphological changes were affected by integrin β1 but not by CaMKII inhibition. Altogether, these data indicate that the Aβ oligomers exhibit early dual effects by acutely enhancing dendritic complexity and spine density.

Aβ42 fibril formation from predominantly oligomeric samples suggests a link between oligomer heterogeneity and fibril polymorphism

Amyloid-β (Aβ) oligomers play a central role in the pathogenesis of Alzheimer's disease. Oligomers of different sizes, morphology and structures have been reported in both in vivo and in vitro studies, but there is a general lack of understanding about where to place these oligomers in the overall process of Aβ aggregation and fibrillization. Here, we show that Aβ42 spontaneously forms oligomers with a wide range of sizes in the same sample. These Aβ42 samples contain predominantly oligomers, and they quickly form fibrils upon incubation at 37°C. When fractionated using ultrafiltration filters, the samples enriched with smaller oligomers form fibrils at a faster rate than the samples enriched with larger oligomers, with both a shorter lag time and faster fibril growth rate. This observation is independent of Aβ42 batches and hexafluoroisopropanol treatment. Furthermore, the fibrils formed by the samples enriched with larger oligomers are more readily solubilized by epigallocatechin gallate, a main catechin component of green tea. These results suggest that the fibrils formed by larger oligomers may adopt a different structure from fibrils formed by smaller oligomers, pointing to a link between oligomer heterogeneity and fibril polymorphism.

A mix-and-click method to measure amyloid-β concentration with sub-micromolar sensitivity

Aggregation of amyloid-β (Aβ) protein plays a central role in Alzheimer's disease. Because protein aggregation is a concentration-dependent process, rigorous investigations require accurate concentration measurements. Owing to the high aggregation propensity of Aβ protein, working solutions of Aβ are typically in the low micromolar range. Therefore, an ideal Aβ quantification method requires high sensitivity without sacrificing speed and accuracy. Absorbance at 280 nm is frequently used to measure Aβ concentration, but the sensitivity is low with only one tyrosine and no tryptophan residues in the Aβ sequence. Here we present a fluorescence method for Aβ quantification using fluorescamine, which gives high fluorescence upon reaction with primary amines. We show that, using hen egg white lysozyme as a standard, fluorescence correlates linearly with primary amine concentration across a wide range of fluorescamine concentrations, from 62.5 to 1000 µM. The maximal sensitivity of detection is achieved at a fluorescamine concentration of 250 µM or higher. The fluorescamine method is compatible with the presence of dimethyl sulfoxide, which is commonly used in the preparation of Aβ oligomers, and limits the use of absorbance at 280 nm due to its high background reading. Using aggregation kinetics, we show that the fluorescamine method gives accurate concentration measurements at low micromolar range and leads to highly consistent aggregation data. We recommend the fluorescamine assay to be used for routine and on-the-fly concentration determination in Aβ oligomerization and fibrillization experiments.