Amyloid Beta Peptide Membrane Perturbation Is the Basis for its Biological Effects

Experimental studies have indicated that the mechanisms offered for explaining the neurotoxicity of amyloid beta peptide (AP) are diverse, and include altered enzyme activities, disrupted calcium homeostasis, and increased free radical formation. AP appears to interact at the cell membrane with a multitude of receptor sites and also inserts physically into the membrane matrix. This membrane insertion affects the membrane fluidity and potentially influences the function of resident membrane proteins. We propose a unifying hypothesis to explain the experimental observations of the diverse cellular responses to AP. The indiscriminate physical insertion of AP into the cell membrane unspecifically activates a host of membrane processes by perturbation of the membrane proteins. This recurrent activation of membrane processes eventually culminates in neuronal cell death. We recommend that successful therapeutic interventions should be directed at reducing or preventing the interaction of AP with neuronal cell membranes.     

Drosophila Full-Length Amyloid Precursor Protein Is Required for Visual Working Memory and Prevents Age-Related Memory Impairment

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Perforin Promotes Amyloid Beta Internalisation in Neurons

Studies on the mechanisms of neuronal amyloid-β (Aβ) internalisation are crucial for understanding the neuropathological progression of Alzheimer’s disease (AD). We here investigated how extracellular Aβ peptides are internalised and focused on three different pathways: (i) via endocytic mechanisms, (ii) via the receptor for advanced glycation end products (RAGE) and (iii) via the pore-forming protein perforin. Both Aβ₄₀ and Aβ₄₂ were internalised in retinoic acid differentiated neuroblastoma (RA-SH-SY5Y) cells. A higher concentration was required for Aβ₄₀ (250 nM) compared with Aβ₄₂ (100 nM). The internalised Aβ₄₀ showed a dot-like pattern of distribution whereas Aβ₄₂ accumulated in larger and distinct formations. By confocal microscopy, we showed that Aβ₄₀ and Aβ₄₂ co-localised with mitochondria, endoplasmic reticulum (ER) and lysosomes. Aβ treatment of human primary cortical neurons (hPCN) confirmed our findings in RA-SH-SY5Y cells, but hPCN were less sensitive to Aβ; therefore, a 20 (Aβ₄₀) and 50 (Aβ₄₂) times higher concentration was needed for inducing uptake. The blocking of endocytosis completely inhibited the internalisation of Aβ peptides in RA-SH-SY5Y cells and hPCN, indicating that this is a major pathway by which Aβ enters the cells. In addition, the internalisation of Aβ₄₂, but not Aβ₄₀, was reduced by 55 % by blocking RAGE. Finally, for the first time we showed that pore formation in cell membranes by perforin led to Aβ internalisation in hPCN. Understanding how Aβ is internalised sheds light on the pathological role of Aβ and provides further ideas of inhibitory strategies for preventing Aβ internalisation and the spreading of neurodegeneration in AD.     

Interaction of Amyloid Inhibitor Proteins with Amyloid Beta Peptides: Insight from Molecular Dynamics Simulations

Knowledge of the detailed mechanism by which proteins such as human αB- crystallin and human lysozyme inhibit amyloid beta (Aβ) peptide aggregation is crucial for designing treatment for Alzheimer''s disease. Thus, unconstrained, atomistic molecular dynamics simulations in explicit solvent have been performed to characterize the Aβ_17–42 assembly in presence of the αB-crystallin core domain and of lysozyme. Simulations reveal that both inhibitor proteins compete with inter-peptide interaction by binding to the peptides during the early stage of aggregation, which is consistent with their inhibitory action reported in experiments. However, the Aβ binding dynamics appear different for each inhibitor. The binding between crystallin and the peptide monomer, dominated by electrostatics, is relatively weak and transient due to the heterogeneous amino acid distribution of the inhibitor surface. The crystallin-bound Aβ oligomers are relatively long-lived, as they form more extensive contact surface with the inhibitor protein. In contrast, a high local density of arginines from lysozyme allows strong binding with Aβ peptide monomers, resulting in stable complexes. Our findings not only illustrate, in atomic detail, how the amyloid inhibitory mechanism of human αB-crystallin, a natural chaperone, is different from that of human lysozyme, but also may aid de novo design of amyloid inhibitors.     

IRBIT: It Is Everywhere

IRBIT (IP₃Rs binding protein released with IP₃) is a protein originally identified by the Mikoshiba group as an inhibitor of IP₃ receptors function. Subsequently it was found to have multiple functions and regulate the activity of diverse proteins, including regulation of HCO₃ ⁻ transporters to coordinate epithelial HCO₃ ⁻ secretion and to determine localization of the Fip1 subunit of the CPSF complex to regulate mRNA processing. This review highlights the remarkably divers functions of IRBIT that are likely only a fraction of all the potential functions of this protein.     

The Proton-Pump Inhibitor Lansoprazole Enhances Amyloid Beta Production

A key event in the pathogenesis of Alzheimer’s disease (AD) is the accumulation of amyloid-β (Aβ) species in the brain, derived from the sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. Based on a systems biology study to repurpose drugs for AD, we explore the effect of lansoprazole, and other proton-pump inhibitors (PPIs), on Aβ production in AD cellular and animal models. We found that lansoprazole enhances Aβ37, Aβ40 and Aβ42 production and lowers Aβ38 levels on amyloid cell models. Interestingly, acute lansoprazole treatment in wild type and AD transgenic mice promoted higher Aβ40 levels in brain, indicating that lansoprazole may also exacerbate Aβ production in vivo. Overall, our data presents for the first time that PPIs can affect amyloid metabolism, both in vitro and in vivo.     

Minocycline Recovers MTT-Formazan Exocytosis Impaired by Amyloid Beta Peptide

Minocycline, a tetracycline antibiotic, has been reported to exert beneficial effects in models of Alzheimer’s disease (AD). To characterize the mechanisms underlying the putative minocycline-related neuroprotection, we studied its effect in an in vitro model of AD. Primary hippocampal cultures were treated with β-amyloid peptide (Aβ) and cell viability was assessed by standard MTT-assay. Incubation with 10 μM Aβ for 24 h significantly inhibits cellular MTT-reduction without inducing morphological signs of enhanced cell death or increase in release of lactate dehydrogenase. This indicates that cell viability was not affected. The inhibition of MTT-reduction by Aβ was due to an acceleration of MTT-formazan exocytosis. Intriguingly, the Aβ-triggered increase in MTT-formazan exocytosis was abolished by co-treatment with minocycline. In vehicle-treated cells minocycline had no effect on formazan exocytosis. This hitherto unrecognized property of minocycline has to be noticed in the elucidation of the underlying mechanism of this promising neuroprotectant.     

Relation of Plasma Homocysteine to Plasma Amyloid Beta Levels

Background Elevated plasma homocysteine and amyloid β (Aβ) have been associated with Alzheimer’s disease (AD). We investigated the cross-sectional association between these biomarkers. Methods We used linear regression to relate plasma homocysteine and Aβ adjusting for age, gender, creatinine, APOE-ε4, and ethnic group in 327 persons aged 78 ± 6.6 years. Results Plasma homocysteine correlated with age, serum creatinine, plasma Aβ40 and Aβ42, and was inversely correlated with serum vitamin B12, and folate. Aβ42, but not Aβ40, was related to later development of dementia. Homocysteine was related to higher Aβ40 levels (coefficient = 2.0; P < 0.0001) and this association was attenuated after adjustment for creatinine (coefficient = 1.0; P < 0.0001). The crude association between homocysteine and Aβ42 was weaker (coefficient = 0.5; P = 0.01) and became non-significant after adjustment for creatinine (coefficient = 0.4; P = 0.06). These associations were unrelated to ethnicity, the presence of APOE-ε4 or dementia. Analyses by quartiles of homocysteine showed that these association were driven primarily by the fourth quartile. Conclusions Plasma homocysteine is directly related to Aβ40. The association with Aβ42 is not significant. These results seem to indicate that homocysteine is related to aging but not specifically to AD. Special issue dedicated to John P. Blass.