A Game-Theoretic Approach to Deciphering the Dynamics of Amyloid-Beta Aggregation Along Competing Pathways

Aggregation of amyloid-β (Aβ) peptides is a significant event that underpins Alzheimer\u27s disease (AD). Aβ aggregates, especially the low-molecular weight oligomers, are the primary toxic agents in AD pathogenesis. Therefore, there is increasing interest in understanding their formation and behaviour. In this paper, we use our previously established results on heterotypic interactions between Aβ and fatty acids (FAs) to investigate off-pathway aggregation under the control of FA concentrations to develop a mathematical framework that captures the mechanism. Our framework to define and simulate the competing on- and off-pathways of Aβ aggregation is based on the principles of game theory. Together with detailed simulations and biophysical experiments, our models describe the dynamics involved in the mechanisms of Aβ aggregation in the presence of FAs to adopt multiple pathways. Specifically, our reduced-order computations indicate that the emergence of off- or on-pathway aggregates are tightly controlled by a narrow set of rate constants, and one could alter such parameters to populate a particular oligomeric species. These models agree with the detailed simulations and experimental data on using FA as a heterotypic partner to modulate the temporal parameters. Predicting spatio-temporal landscape along competing pathways for a given heterotypic partner such as lipids is a first step towards simulating scenarios in which the generation of specific ‘conformer strains’ of Aβ could be predicted. This approach could be significant in deciphering the mechanisms of amyloid aggregation and strain generation, which are ubiquitously observed in many neurodegenerative diseases

IN VITRO TOXICITY STUDIES ON THE EXTRACT OF MEDICINAL PLANT EVOLVULUS NUMMULARIUS AS A POTENT MICROBICIDAL CANDIDATE

The herb Evolvulus nummularius (L). L generally grown as an ornamental plant. This herb has found many applications in traditional folk medicine. There was however insufficient scientific data to back its safety to be used on humans. Methanolic extract of E. nummularius was used to check for its safety as a vaginal microbicide through various safety tests such as cell viability using MTT assay on three female genital tract epithelial cell lines, vaginal (VK2/E6E7), endocervical (End1/E6E7) and endometrial (HEC-1-A). Quantification of hemolytic activity was done on human red blood cells (RBCs). Determination of cellular integrity was checked by transepithelial electrical resistance (TER) assay and permeability by fluorescent microsphere assay. At 24 hours, application of the extract for cell viability assay showed extensive cell death with cell disruption. IC50 of VK2/E6E7 and HEC-1-A cells were found to be 2 mg/ml, IC50of End1/E6E7 was 1 mg/ml. For hemolytic assay, with treatment of the extract for one hour did not show hemolysis till the concentration of 2.5mg/ml. In TER and microsphere permeability assays, polarized HEC-1-A monolayer 24 hours post treatment had significant drop in TER and enhanced fluorescence from passage of microspheres implying disruption of the epithelial monolayer. The study revealed the crude methanolic extract appeared to be toxic towards human RBCs and female genital tract epithelial cells. Due to its toxic nature, its direct applications to the human vaginal tissue in vivo should be done with caution. Keywords: Medicinal plants; Microbicide; Evolvulus nummularius (L). L; MTT assay; Transepithelial electrical resistance; Fluorescent microsphere assay.   Â&nbsp

Biophysical Characteristics of Lipid-Induced Aβ Oligomers Correlate to Distinctive Phenotypes In Transgenic Mice

Alzheimer\u27s disease (AD) is a progressive neurodegenerative disorder that affects cognition and memory. Recent advances have helped identify many clinical sub-types in AD. Mounting evidence point toward structural polymorphism among fibrillar aggregates of amyloid-β (Aβ) to being responsible for the phenotypes and clinical manifestations. In the emerging paradigm of polymorphism and prion-like propagation of aggregates in AD, the role of low molecular weight soluble oligomers, which are long known to be the primary toxic agents, in effecting phenotypes remains inconspicuous. In this study, we present the characterization of three soluble oligomers of Aβ42, namely 14LPOs, 16LPOs, and GM1Os with discreet biophysical and biochemical properties generated using lysophosphatidyl glycerols and GM1 gangliosides. The results indicate that the oligomers share some biophysical similarities but display distinctive differences with GM1Os. Unlike the other two, GM1Os were observed to be complexed with the lipid upon isolation. It also differs mainly in detection by conformation-sensitive dyes and conformation-specific antibodies, temperature and enzymatic stability, and in the ability to propagate morphologically-distinct fibrils. GM1Os also show distinguishable biochemical behavior with pronounced neuronal toxicity. Furthermore, all the oligomers induce cerebral amyloid angiopathy (CAA) and plaque burden in transgenic AD mice, which seems to be a consistent feature among all lipid-derived oligomers, but 16LPOs and GM1Os displayed significantly higher effect than the others. These results establish a correlation between molecular features of Aβ42 oligomers and their distinguishable effects in transgenic AD mice attuned by lipid characteristics, and therefore help bridge the knowledge gap in understanding how oligomer conformers could elicit AD phenotypes

EFFECTS OF MEMBRANE COMPONENTS ON THE OLIGOMERIZATION OF AMYLOID-β (Aβ) IN ALZHEIMER DISEASE

Deposition of extracellular proteinaceous plaques of amyloid-β (Aβ) is one of the major hallmarks of Alzheimer disease (AD). Although, insoluble high molecular fibrils of Aβ are the main constituents of the senile plaques in AD, low molecular weight soluble Aβ oligomers have emerged as the key toxic species involved in memory impairment and neuronal cell death in AD. The process for generation of oligomers can be highly diverse and can involve homotypic interactions of Aβ as well as heterotypic interaction of Aβ with other macromolecules. Previously, the generation of Aβ oligomers with distinct biochemical and biophysical characteristics catalyzed non-esterified fatty acid micelles (large fatty acid derived oligomers (LFAO)) was observed by our lab. In the work presented here the generation of Aβ oligomer strains have been demonstrated in the presence of lyso-phospholipid micelles of different chain lengths and gangliosides with different sugar distributions in their head groups. It has been shown that lipids of different characteristics can assist the generation of biophysically distinct oligomers with differences in toxicity in AD brains. We also furthered this study to investigate oligomerization of Aβ in liposomal model membrane systems with varied lipid composition and found GM1 as a key mediator in Aβ oligomerization and its cooperative membrane damage. Lastly, this work reveals that Aβ oligomerization by gangliosides is mediated by their sugar distribution and positively charged amino-acid in the N-terminal region of Aβ. Overall, this study brings forth molecular insights of key components responsible for oligomer strain generation and hence propagation of disease in the AD pathology

αs Oligomers Generated from Interactions with a Polyunsaturated Fatty Acid and a Dopamine Metabolite Differentially Interact with Aβ to Enhance Neurotoxicity

It is increasingly becoming clear that neurodegenerative diseases are not as discrete as originally thought to be but display significant overlap in histopathological and clinical presentations. For example, nearly half of the patients with Alzheimer\u27s disease (AD) and synucleinopathies such as Parkinson\u27s disease (PD) show symptoms and pathological features of one another. Yet, the molecular events and features that underlie such comorbidities in neurodegenerative diseases remain poorly understood. Here, inspired to uncover the molecular underpinnings of the overlap between AD and PD, we investigated the interactions between amyloid-β (Aβ) and α-synuclein (αS), aggregates of which form the major components of amyloid plaques and Lewy bodies, respectively. Specifically, we focused on αS oligomers generated from the dopamine metabolite called dihydroxyphenylacetaldehyde (DOPAL) and a polyunsaturated fatty acid docosahexaenoic acid (DHA). The two αS oligomers showed structural and conformational differences as confirmed by the disparity in size, secondary structure, susceptibility to proteinase K digestion, and cytotoxicity. More importantly, the two oligomers differentially modulated Aβ aggregation; while both inhibited Aβ aggregation to varying extents, they also induced structurally different Aβ assemblies. Furthermore, Aβ seeded with DHA-derived αS oligomers showed greater toxicity than DOPAL-derived αS oligomers in SH-SY5Y neuroblastoma cells. These results provide insights into the interactions between two amyloid proteins with empirically distinctive biophysical and cellular manifestations, enunciating a basis for potentially ubiquitous cross-amyloid interactions across many neurodegenerative diseases

Ganglioside-Enriched Phospholipid Vesicles Induce Cooperative Aβ Oligomerization and Membrane Disruption

A major hallmark of Alzheimer’s disease (AD) is the accumulation of extracellular aggregates of amyloid-β (Aβ). Structural polymorphism observed among Aβ fibrils in AD brains seem to correlate with the clinical subtypes suggesting a link between fibril polymorphism and pathology. Since fibrils emerge from a templated growth of low-molecular-weight oligomers, understanding the factors affecting oligomer generation is important. Membrane lipids are key factors to influence early stages of Aβ aggregation and oligomer generation, which cause membrane disruption. We have previously demonstrated that conformationally discrete Aβ oligomers can be generated by modulating the charge, composition, and chain length of lipids and surfactants. Here, we extend our studies into liposomal models by investigating Aβ oligomerization on large unilamellar vesicles (LUVs) of total brain extracts (TBE), reconstituted lipid rafts (LRs), or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Varying the vesicle composition by specifically increasing the amount of GM1 gangliosides as a constituent, we found that only GM1-enriched liposomes induce the formation of toxic, low-molecular-weight oligomers. Furthermore, we found that the aggregation on liposome surface and membrane disruption are highly cooperative and sensitive to membrane surface characteristics. Numerical simulations confirm such a cooperativity and reveal that GM1-enriched liposomes form twice as many pores as those formed in the absence GM1. Overall, this study uncovers mechanisms of cooperativity between oligomerization and membrane disruption under controlled lipid compositional bias, and refocuses the significance of the early stages of Aβ aggregation in polymorphism, propagation, and toxicity in AD

Ganglioside-Enriched Phospholipid Vesicles Induce Cooperative Aβ Oligomerization and Membrane Disruption

A major hallmark of Alzheimer’s disease (AD) is the accumulation of extracellular aggregates of amyloid-β (Aβ). Structural polymorphism observed among Aβ fibrils in AD brains seem to correlate with the clinical subtypes suggesting a link between fibril polymorphism and pathology. Since fibrils emerge from a templated growth of low-molecular-weight oligomers, understanding the factors affecting oligomer generation is important. Membrane lipids are key factors to influence early stages of Aβ aggregation and oligomer generation, which cause membrane disruption. We have previously demonstrated that conformationally discrete Aβ oligomers can be generated by modulating the charge, composition, and chain length of lipids and surfactants. Here, we extend our studies into liposomal models by investigating Aβ oligomerization on large unilamellar vesicles (LUVs) of total brain extracts (TBE), reconstituted lipid rafts (LRs), or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Varying the vesicle composition by specifically increasing the amount of GM1 gangliosides as a constituent, we found that only GM1-enriched liposomes induce the formation of toxic, low-molecular-weight oligomers. Furthermore, we found that the aggregation on liposome surface and membrane disruption are highly cooperative and sensitive to membrane surface characteristics. Numerical simulations confirm such a cooperativity and reveal that GM1-enriched liposomes form twice as many pores as those formed in the absence GM1. Overall, this study uncovers mechanisms of cooperativity between oligomerization and membrane disruption under controlled lipid compositional bias, and refocuses the significance of the early stages of Aβ aggregation in polymorphism, propagation, and toxicity in AD