Inhibitory Effects of Sulfated Polysaccharides from the Sea Cucumber Cucumaria Frondosa against Aβ40 Aggregation and Cytotoxicity

Abnormal aggregation and deposition of Aβ is one of the causative agents for Alzheimer's disease. The development of inhibitors for Aβ aggregation has been considered a possible method to prevent and treat Alzheimer's disease. Edible sea cucumbers contain many bioactive molecules, including saponins, phospholipids, peptides, and polysaccharides. Herein, we report that polysaccharides extracted from sea cucumber Cucumaria frondosa could reduce the aggregation and cytotoxicity of Aβ40. By utilizing multiple biochemical and biophysical instruments, we found that the polysaccharides could inhibit the aggregation of Aβ40. A chemical kinetics analysis further suggested that the major inhibitory effects of the polysaccharides were achieved by disassembling mature fibrils, which in turn reduced the cytotoxicity of Aβ. These results suggested that the polysaccharides extracted from sea cucumber could be used as an effective inhibitor for Aβ.

Site-Selective Lysine Reactions Guided by Protein-Peptide Interaction

Site-selective lysine post-translational modifications such as acetylation, methylation, hydroxylation, and isopeptide formation mediate the precise control of important signaling events in cells with unmistakable accuracy. This unparalleled site selectivity (modification of a single lysine in a particular protein in the proteome) is still a challenge for non-enzymatic protein reactions; the difficulty lies in the differentiation of the lysine ε-amino group from other reactive groups and in the precise pinpointing of one particular lysine ε-amino group out of many other lysine ε-amino groups and the N-terminal amine of the protein that have similar chemical reactivity. Here, we have explored proximal lysine conjugation reactions through peptide-guided fluorodinitrobenzene, isothiocyanate, and phenyl ester reactions and have validated the site-specific targeting of the ε-amino group of one single lysine in natural proteins that contain multiple lysine residues. This precise site selectivity is a result of the proximity-induced reactivity guided by a specific protein-peptide interaction: the binding interaction preorganizes an amine-reactive group in the peptide and one of the lysine side chain ε-amino groups of the protein into close proximity, thereby confining the reactivity to a selected area of the target protein. The binding-guide lysine reactions were first examined on an SH3 domain and then tested on several ubiquitin-like proteins such as SUMO, Atg8 protein family, plant ATG8, and mammalian LC3 proteins that contain at least seven lysine residues on the surface. Exquisite site selectivity was confirmed in all of the proteins tested. A set of amine reactions were tested for their feasibility in the site-selective lysine reaction. Selected amine-reactive groups were optimized, and the reaction sites on the LC3 protein were confirmed by mass spectrometry.

Stabilizing amyloid-β peptide by the N-terminus capture is capable of preventing and eliminating amyloid-β oligomers

A novel strategy to prevent and eliminate amyloid-β (Aβ) oligomers from either the early aggregation or the fibril dissolution pathway is described.

Stabilizing the monomeric amyloid-β peptide by tyrocidine A prevents and reverses amyloidogenesis without the accumulation of oligomers

Stabilizing the monomeric amyloid-β peptide by tyrocidine A prevents accumulation of oligomers.

Sulforaphane interaction with amyloid beta 1-40 peptide studied by electrospray ionization mass spectrometry

RATIONALE

Aggregation of amyloid beta 1-40 (Aβ) in the brain causes Alzheimer's disease (AD) and several small molecules are known to inhibit the aggregation process. Sulforaphane (SFN) is a natural isothiocyanate which is known to prevent various neurodegenerative processes. However, its interaction with Aβ is yet to be explored. Such studies could provide new mechanistic insights for its neuroprotective properties.

METHODS

Liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) and in-source fragmentation experiments were performed on an Orbitrap mass spectrometer. The solution of Aβ and SFN was incubated and analyzed by mass spectrometry. Isotopic distribution patterns, accurate mass values and theoretical product ions were used to analyze the mass spectrometry data. The nature of binding of SFN and its binding sites with Aβ were evaluated by LC/MS and trypsin digestion experiments.

RESULTS

ESI-MS analysis of the incubated solution of Aβ and SFN showed a 1:1 complex of [Aβ+SFN]. LC/MS analysis revealed that the solution contains three different [Aβ+SFN] complexes due to covalent binding of SFN to Aβ at three different sites. The in-source fragmentation experiments revealed that SFN is binding to free NH(2) groups (N-terminal amino acid and lysines) in Aβ. Trypsin digestion experiments further confirmed the SFN binding sites in Aβ.

CONCLUSIONS

The interaction of SFN, an anticancer agent, with Aβ was studied using ESI-MS. SFN is found to bind covalently and specifically with the free NH(2) group of N-terminal aspartic acid and the ε-amino group of lysine at positions 16 and 28. Aggregation assay studies showed a lesser inclination of Aβ to aggregate when SFN is present. Hence the present study helps in understanding the mechanism of the action of SFN on the Aβ peptide.