Effects of antiparkinsonian agents on ß-amyloid and α-synuclein oligomer formation in vitro.

The aggregation of ß-amyloid protein (Aß) and α-synuclein (αS) are hypothesized to be the key pathogenic event in Alzheimer's disease (AD) and Lewy body diseases (LBD), with oligomeric assemblies thought to be the most neurotoxic. Inhibitors of oligomer formation, therefore, could be valuable therapeutics for patients with AD and LBD. Here, we examined the effects of antiparkinsonian agents (dopamine, levodopa, trihexyphenidyl, selegiline, zonisamide, bromocriptine, peroxide, ropinirole, pramipexole, and entacapone) on the in vitro oligomer formation of Aß40, Aß42, and αS using a method of photo-induced cross-linking of unmodified proteins (PICUP), electron microscopy, and atomic force microscopy. The antiparkinsonian agents except for trihexyphenidyl inhibited both Aß and αS oligomer formations, and, among them, dopamine, levodopa, pramipexole, and entacapone had the stronger in vitro activity. Circular dichroism and thioflavin T(S) assays showed that secondary structures of Aß and αS assemblies inhibited by antiparkinsonian agents were statistical coil state and that their seeding activities had disappeared. The antiparkinsonian agents could be potential therapeutic agents to prevent or delay AD and LBD progression.

Anti-amyloidogenic effects of soybean isoflavones in vitro: Fluorescence spectroscopy demonstrating direct binding to Aß monomers, oligomers and fibrils.

Alzheimer's disease is characterized by the presence of extracellular deposits of amyloid, primarily composed of the amyloid ß-protein (Aß). A growing body of evidence indicates that oligomeric forms of Aß play a critical role in disease causation. Soybean isoflavones are flavonoids with an isoflavone backbone. Isoflavones have been reported to protect against Aß-induced neurotoxicity in cultured cell systems, the molecular mechanisms remain unclear. Our previous studies demonstrated that red wine-related flavonoids with a flavone backbone are able to inhibit Aß assembly and destabilize preformed Aß aggregates. Here, we show that isoflavones, especially glycitein and genistein, have anti-fibrillization, anti-oligomerization and fibril-destabilizing effects on Aß(1-40) and Aß(1-42)in vitro at physiological pH and temperature, by using nucleation-dependent polymerization monitored by thioflavin T fluorescence, atomic force microscopy, electron microscopy, and photo-induced cross-linking of unmodified proteins followed by SDS-PAGE. Our three-dimensional fluorescence spectroscopic analyses demonstrated that glycitein interacted with Aß monomers, oligomers and fibrils, indicating specific binding of glycitein to these Aß species. Glycitein also interacted with different Aß fragments (Aß(1-42), Aß(1-40), Aß(1-16) and Aß(25-35)), exhibiting the highest fluorescence enhancement with Aß(25-35). We speculated that glycitein's anti-amyloidogenic properties are specifically mediated by its binding to Aß monomers, oligomers and fibrils. Isoflavones may hold promise as a treatment option for preventative strategies targeting amyloid formation in Alzheimer's disease.

Phenolic compounds prevent amyloid ß-protein oligomerization and synaptic dysfunction by site-specific binding.

Cerebral deposition of amyloid ß protein (Aß) is an invariant feature of Alzheimer disease (AD), and epidemiological evidence suggests that moderate consumption of foods enriched with phenolic compounds reduce the incidence of AD. We reported previously that the phenolic compounds myricetin (Myr) and rosmarinic acid (RA) inhibited Aß aggregation in vitro and in vivo. To elucidate a mechanistic basis for these results, we analyzed the effects of five phenolic compounds in the Aß aggregation process and in oligomer-induced synaptic toxicities. We now report that the phenolic compounds blocked Aß oligomerization, and Myr promoted significant NMR chemical shift changes of monomeric Aß. Both Myr and RA reduced cellular toxicity and synaptic dysfunction of the Aß oligomers. These results suggest that Myr and RA may play key roles in blocking the toxicity and early assembly processes associated with Aß through different binding.

Effect of melatonin on α-synuclein self-assembly and cytotoxicity.

α-Synuclein (αS) assembly has been implicated as a critical step in the development of Lewy body diseases such as Parkinson's disease and dementia with Lewy bodies. Melatonin (Mel), a secretory product of the pineal gland, is known to have beneficial effects such as an antioxidant function and neuroprotection. To elucidate whether Mel has an antiassembly effect, here we used circular dichroism spectroscopy, photoinduced crosslinking of unmodified proteins, thioflavin S fluorescence, size exclusion chromatography, electron microscopy and atomic force microscopy to examine the effects of Mel on the αS assembly. We also examined the effects of Mel on αS-induced cytotoxicity by assaying 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide metabolism in αS-treated, primary neuronal cells. Initial studies revealed that Mel blocked αS fibril formation as well as destabilizing preformed αS fibrils. Subsequent evaluation of the assembly-stage specificity of the effect showed that Mel was able to inhibit protofibril formation, oligomerization, and secondary structure transitions. Importantly, Mel decreased αS-induced cytotoxicity. These data suggest a mechanism of action for Mel, inhibition of assembly of toxic polymers and protection of neurons from their effect.

Familial Parkinson disease mutations influence α-synuclein assembly.

Lewy bodies composed of aggregates of α-synuclein (αS) in the brain are the main histopathological features of Lewy body diseases (LBD) such as Parkinson's disease and dementia with Lewy bodies. Mutations such as E46K, A30P and A53T in the αS gene cause autosomal dominant LBD in a number of kindreds. Although these mutations accelerate fibril formation, their precise effects at early stages of the αS aggregation process remain unknown. To answer this question, we examined the aggregation including monomer conformational dynamics and oligomerization of the E46K, A30P, A53T and A30P/A53T mutations and wild type (WT) using thioflavin S assay, circular dichroism spectroscopy, photo-induced cross-linking of unmodified proteins, electron microscopy, and atomic force microscopy. Relative to WT αS, E46K αS accelerated the kinetics of the secondary structure change and oligomerization, whereas A30P αS decelerated them. These effects were reflected in changes in average oligomer size. The mutant oligomers of E46K αS functioned as fibril seeds significantly more efficiently than those of WT αS, whereas the mutant oligomers of A30P αS were less efficient. Our results that mutations of familial LBD had opposite effects at early stages of αS assembly may provide new insight into the molecular mechanisms of LBD.