Congo red reverses amyloid beta protein-induced cellular stress in astrocytes

Diminished amyloid-β uptake by mouse microglia upon treatment with quantum dots, silver or cerium oxide nanoparticles: Nanoparticles and amyloid-β uptake by microglia

Alzheimer’s disease (AD) is a chronic neurodegenerative disease leading to progressive dementia in elderly people. The disease is characterized, among others, by formation of amyloid- β (A β) polypeptide plaques in the brain. Although etiology of the disease is not fully understood, recent research suggest that nanomaterials may affect AD development. Here, we described the consequences of exposure of mouse BV-2 microglia to silver nanoparticles (AgNPs, 50 µg/mL), cerium oxide nanoparticles (CeO2NPs, 100 µg/mL), and cadmium telluride quantum dots (CdTeQDs, 3 or 10 µg/mL) in the context of its ability to clear A β plaques. The brain microglial cells play an important role in removing A β plaques from the brain. Cell viability and cycle progression were assessed by trypan blue test and propidium iodide binding, respectively. The uptake of A β and NPs was measured by flow cytometry. Secretion of proinflammatory cytokines was measured with the use of cytometric bead array. A β (0.1 μM) did not affect viability, whereas NPs decreased microglia growth by arresting the cells in G1 phase (CdTeQDs) or in S phase (AgNPs and CeO2NPs) of cell cycle. The uptake of A β was significantly reduced in the presence of AgNPs and CeO2NPs. In addition, the least toxic CeO2NPs induced the release of proinflammatory cytokine, tumor necrosis factor α. In summary, each of the NPs tested affected either the microglia phagocytic activity (AgNPs and CeO2NPs) and/or its viability (AgNPs and CdTeQDs) that may favor the occurrence of AD and accelerate its development.

Binding of ACE-inhibitors to in vitro and patient-derived amyloid-β fibril models

Currently, no drugs exist that can prevent or reverse Alzheimer's disease, a neurodegenerative disease associated with the presence, in the brain, of plaques that are composed of β-amyloid (Aβ) peptides. Recent studies suggest that angiotensin-converting enzyme (ACE) inhibitors, a set of drugs used to treat hypertension, may inhibit amyloid formation in vitro. In the present study, we investigate through computer simulations the binding of ACE inhibitors to patient-derived Aβ fibrils and contrast it with that of ACE inhibitors binding to in vitro generated fibrils. The binding affinities of the ACE inhibitors are compared with that of Congo red, a dye that is used to identify amyloid structures and that is known to be a weak inhibitor of Aβ aggregation. We find that ACE inhibitors have a lower binding affinity to the patient-derived fibrils than to in vitro generated ones. For patient-derived fibrils, their binding affinities are even lower than that of Congo red. Our observations raise doubts on the hypothesis that these drugs inhibit fibril formation in Alzheimer patients by interacting directly with the amyloids.

A marine-derived acidic oligosaccharide sugar chain specifically inhibits neuronal cell injury mediated byβ-amyloid-induced astrocyte activationin vitro

In Alzheimer's disease (AD), beta-amyloid (Abeta) plaques are surrounded by activated astrocytes and microglia. A growing body of evidence suggests that these activated astrocytes contribute to neurotoxicity through the induction of inflammatory cytokines and the production of oxidative stress mediators. Thus, a compound inhibiting Abeta-induced activation of astrocytes may lead to a novel therapy for AD. Our current work investigates the roles of acidic oligosaccharide sugar chain (AOSC), derived from brown algae Echlonia Kurome Okam, on Abeta-induced inflammatory responses and cytotoxicity. We observed that AOSC inhibited the toxicity and apoptosis in SH-SY5Y cell line induced by Abeta-stimulated astrocytes conditioned medium. We found that AOSC inhibited the reactive phenotype of astrocytes, blocked cellular oxidative stress, reduced the production of tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 and prevented the influx of Ca2+. Thus, our results indicate that AOSC might be a potentially therapeutic compound for AD.

Abeta mediated diminution of MTT reduction--an artefact of single cell culture?

The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazoliumbromide (MTT) reduction assay is a frequently used and easily reproducible method to measure beta-amyloid (Abeta) toxicity in different types of single cell culture. To our knowledge, the influence of Abeta on MTT reduction has never been tested in more complex tissue. Initially, we reproduced the disturbed MTT reduction in neuron and astroglia primary cell cultures from rats as well as in the BV2 microglia cell line, utilizing four different Abeta species, namely freshly dissolved Abeta (25-35), fibrillar Abeta (1-40), oligomeric Abeta (1-42) and oligomeric Abeta (1-40). In contrast to the findings in single cell cultures, none of these Abeta species altered MTT reduction in rat organotypic hippocampal slice cultures (OHC). Moreover, application of Abeta to acutely isolated hippocampal slices from adult rats and in vivo intracerebroventricular injection of Abeta also did not influence the MTT reduction in the respective tissue. Failure of Abeta penetration into the tissue cannot explain the differences between single cells and the more complex brain tissue. Thus electrophysiological investigations disclosed an impairment of long-term potentiation (LTP) in the CA1 region of hippocampal slices from rat by application of oligomeric Abeta (1-40), but not by freshly dissolved Abeta (25-35) or fibrillar Abeta (1-40). In conclusion, the experiments revealed a glaring discrepancy between single cell cultures and complex brain tissue regarding the effect of different Abeta species on MTT reduction. Particularly, the differential effect of oligomeric versus other Abeta forms on LTP was not reflected in the MTT reduction assay. This may indicate that the Abeta oligomer effect on synaptic function reflected by LTP impairment precedes changes in formazane formation rate or that cells embedded in a more natural environment in the tissue are less susceptible to damage by Abeta, raising cautions against the consideration of single cell MTT reduction activity as a reliable assay in Alzheimer's drug discovery studies.

Insight into the kinetic of amyloid beta (1-42) peptide self-aggregation: elucidation of inhibitors' mechanism of action

The initial transition of amyloid beta (1-42) (Abeta42) soluble monomers/small oligomers from unordered/alpha-helix to a beta-sheet-rich conformation represents a suitable target to design new potent inhibitors and to obtain effective therapeutics for Alzheimer's disease. Under optimized conditions, this reliable and reproducible CD kinetic study showed a three-step sigmoid profile that was characterized by a lag phase (prevailing unordered/alpha-helix conformation), an exponential growth phase (increasing beta-sheet secondary structure) and a plateau phase (prevailing beta-sheet secondary structure). This kinetic analysis brought insight into the inhibitors' mechanism of action. In fact, an increase in the duration of the lag phase can be related to the formation of an inhibitor-Abeta complex, in which the non-amyloidogenic conformation is stabilized. When the exponential rate is affected exclusively, such as in the case of Congo red and tetracycline, then the inhibitor affinity might be higher for the pleated beta-sheet structure. Finally, by adding the inhibitor at the end of the exponential phase, the soluble protofibrils can be disrupted and the Abeta amyloidogenic structure can revert into monomers/small oligomers. Congo red and tetracycline preferentially bind to amyloid in the beta-sheet conformation because both decreased the slope of the exponential growth, even if to a different extent, whereas no effect was observed for tacrine and galantamine. Some very preliminary indications can be derived about the structural requirements for binding to nonamyloidogenic or beta-sheet amyloid secondary structure for the development of potent antiaggregating agents. On these premises, memoquin, a multifunctional molecule that was designed to become a drug candidate for the treatment of Alzheimer's disease, was investigated under the reported circular dichroism assay and its anti-amyloidogenic mechanism of action was elucidated.

Congo red and protein aggregation in neurodegenerative diseases

Congo red is a commonly used histological dye for amyloid detection. The specificity of this staining results from Congo red's affinity for binding to fibril proteins enriched in beta-sheet conformation. Unexpectedly, recent investigations indicate that the dye also possesses the capacity to interfere with processes of protein misfolding and aggregation, stabilizing native protein monomers or partially folded intermediates, while reducing concentration of more toxic protein oligomers. Inhibitory effects of Congo red upon amyloid toxicity may also range from blockade of channel formation and interference with glycosaminoglycans binding or immune functions, to the modulation of gene expression. Particularly, Congo red exhibits ameliorative effect in models of neurodegenerative disorders, such as Alzheimer's, Parkinson's, Huntington's and prion diseases. Another interesting application of Congo red analogues is the development of imaging probes. Based on their small molecular size and penetrability through blood-brain barrier, Congo red congeners can be used for both antemortem and in vivo visualization and quantification of brain amyloids. Therefore, understanding mechanisms involved in dye-amyloidal fibril binding and inhibition of aggregation will provide instructive guides for the design of future compounds, potentially useful for monitoring and treating neurodegenerative diseases.

Curcumin inhibits formation of amyloid beta oligomers and fibrils, binds plaques, and reduces amyloid in vivo

Alzheimer's disease (AD) involves amyloid beta (Abeta) accumulation, oxidative damage, and inflammation, and risk is reduced with increased antioxidant and anti-inflammatory consumption. The phenolic yellow curry pigment curcumin has potent anti-inflammatory and antioxidant activities and can suppress oxidative damage, inflammation, cognitive deficits, and amyloid accumulation. Since the molecular structure of curcumin suggested potential Abeta binding, we investigated whether its efficacy in AD models could be explained by effects on Abeta aggregation. Under aggregating conditions in vitro, curcumin inhibited aggregation (IC(50) = 0.8 microM) as well as disaggregated fibrillar Abeta40 (IC(50) = 1 microM), indicating favorable stoichiometry for inhibition. Curcumin was a better Abeta40 aggregation inhibitor than ibuprofen and naproxen, and prevented Abeta42 oligomer formation and toxicity between 0.1 and 1.0 microM. Under EM, curcumin decreased dose dependently Abeta fibril formation beginning with 0.125 microM. The effects of curcumin did not depend on Abeta sequence but on fibril-related conformation. AD and Tg2576 mice brain sections incubated with curcumin revealed preferential labeling of amyloid plaques. In vivo studies showed that curcumin injected peripherally into aged Tg mice crossed the blood-brain barrier and bound plaques. When fed to aged Tg2576 mice with advanced amyloid accumulation, curcumin labeled plaques and reduced amyloid levels and plaque burden. Hence, curcumin directly binds small beta-amyloid species to block aggregation and fibril formation in vitro and in vivo. These data suggest that low dose curcumin effectively disaggregates Abeta as well as prevents fibril and oligomer formation, supporting the rationale for curcumin use in clinical trials preventing or treating AD.

Cholesterol does not affect the toxicity of amyloid beta fragment but mimics its effect on MTT formazan exocytosis in cultured rat hippocampal neurons

It has recently been reported that methyl-beta-cyclodextrin-solubilized cholesterol protects PC12 cells from amyloid beta protein (Abeta) toxicity. To ask if this is the case in brain neurons, we investigated its effect in primary cultured rat hippocampal neurons. In basal culture conditions with no addition of Abeta, methyl-beta-cyclodextrin-solubilized cholesterol at concentrations of 30-100 microM was toxic to neurons, but at concentrations of 1-10 microM promoted neuronal survival. Methyl-beta-cyclodextrin-solubilized cholesterol at 1-10 microM was also effective in protecting neurons from toxicity of 20 microM Abeta. However, these effects were all mimicked by methyl-beta-cyclodextrin alone, but not by cholesterol solubilized by dimethylsulfoxide or ethanol. The effects of methyl-beta-cyclodextrin-solubilized cholesterol on neuronal survival and Abeta toxicity are probably attributed to the action of methyl-beta-cyclodextrin, but not cholesterol. Alternatively, we found that methyl-beta-cyclodextrin-solubilized cholesterol at lower concentrations ( > 10 nM) inhibited cellular reduction of 3-(4,5-dimethylthiazol-2-yl)-2.5-diphenyltetrazolium bromide (MTT) by promoting the exocytosis of MTT formazan. This effect was shared by dimethylsulfoxide- or ethanol-solubilized cholesterol, but not by methyl-beta-cyclodextrin, supporting that it is attributed to the action of cholesterol. These results suggest that cholesterol does not protect neurons from Abeta toxicity, or rather inhibits cellular MTT reduction in a similar manner to Abeta.

Amyloid beta protein inhibits cellular MTT reduction not by suppression of mitochondrial succinate dehydrogenase but by acceleration of MTT formazan exocytosis in cultured rat cortical astrocytes

Alzheimer's disease amyloid beta protein (Abeta) inhibits cellular reduction of the dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Kaneko et al. have previously hypothesized that Abeta works by suppressing mitochondrial succinate dehydrogenase (SDH), but Liu and Schubert have recently demonstrated that Abeta decreases cellular MTT reduction by accelerating the exocytosis of MTT formazan in neuronal cells. To ask which is the case in astrocytes, we compared the effects of Abeta and 3-nitropropionic acid (3-NP), a specific SDH inhibitor, on MTT reduction in cultured rat cortical astrocytes. Treatment with 3-NP (10 mM) decreased cellular activity of MTT reduction, regardless of the time of incubation with MTT. On the other hand. Abeta-induced inhibition of cellular MTT reduction was dependent on the time of incubation with MTT. The cells treated with Abeta (0.1-1000 nM) exhibited normal capacity for MTT reduction at an early stage of incubation ( < 30 min), but ceased to reduce MTT at the late stage (> 1 h). Microscopic examination revealed that Abeta treatment accelerated the appearance of needle-like MTT formazan crystals at the cell surface. These observations support that Abeta accelerates the exocytosis of MTT formazan in astrocytes. In addition to inhibition of MTT reduction, Abeta is known to induce morphological changes in astrocytes. Following addition of Abeta (20 microM), polygonal astrocytes changed into process-bearing stellate cells. To explore a possible linkage between these two effects of Abeta, we tested if astrocyte stellation is induced by agents that mimic the effect of Abeta on MTT reduction. Cholesterol (5 5000 nM) and lysophosphatidic acid (0.2-20 microg/ml) were found to accelerate the exocytosis of MTT formazan in a similar manner to Abeta, but failed to induce astrocyte stellation. Therefore, Abeta-induced inhibition of MTT reduction is unlikely to be directly linked to its effect on astrocyte morphology.