Yanqin Liu

Gallic acid is the major component of grape seed extract that inhibits amyloid fibril formation

Many protein misfolding diseases, for example, Alzheimers, Parkinsons and Huntingtons, are characterised by the accumulation of protein aggregates in an amyloid fibrillar form. Natural products which inhibit fibril formation are a promising avenue to explore as therapeutics for the treatment of these diseases. In this study we have shown, using in vitro thioflavin T assays and transmission electron microscopy, that grape seed extract inhibits fibril formation of kappa-casein (κ-CN), a milk protein which forms amyloid fibrils spontaneously under physiological conditions. Among the components of grape seed extract, gallic acid was the most active component at inhibiting κ-CN fibril formation, by stabilizing κ-CN to prevent its aggregation. Concomitantly, gallic acid significantly reduced the toxicity of κ-CN to pheochromocytoma12 cells. Furthermore, gallic acid effectively inhibited fibril formation by the amyloid-beta peptide, the putative causative agent in Alzheimers disease. It is concluded that the gallate moiety has the fibril-inhibitory activity.

The dissociated form of κ-casein is the precursor to its amyloid fibril formation

Bovine milk kappa-casein forms a self-associating oligomeric micelle-like species, in equilibrium with dissociated forms. In its native form, intra- and inter-molecular disulfide bonds lead to the formation of multimeric species ranging from monomers to decamers. When incubated under conditions of physiological pH and temperature, both reduced and non-reduced kappa-casein form highly structured beta-sheet amyloid fibrils. We investigated whether the precursor to kappa-casein fibril formation is a dissociated state of the protein or its oligomeric micelle-like form. We show that reduced kappa-casein is capable of forming fibrils well below its critical micelle concentration, i.e. at concentrations where only dissociated forms of the protein are present. Moreover, by regulating the degree of disulfide linkages, we were able to investigate how oligomerization of kappa-casein influences its propensity for fibril formation under conditions of physiological pH and temperature. Thus, using fractions containing different proportions of multimeric species, we demonstrate that the propensity of the disulfide-linked multimers to form fibrils is inversely related to their size, with monomeric kappa-casein being the most aggregation prone. We conclude that dissociated forms of kappa-casein are the amyloidogenic precursors to fibril formation rather than oligomeric micelle-like species. The results highlight the role of oligomerization and natural binding partners in preventing amyloid fibril formation by disease-related proteins in vivo.

Gallic acid interacts with α-synuclein to prevent the structural collapse necessary for its aggregation.

The accumulation of protein aggregates containing amyloid fibrils, with α-synuclein being the main component, is a pathological hallmark of Parkinsons disease (PD). Molecules which prevent the formation of amyloid fibrils or disassociate the toxic aggregates are touted as promising strategies to prevent or treat PD. In the present study, in vitro Thioflavin T fluorescence assays and transmission electron microscopy imaging results showed that gallic acid (GA) potently inhibits the formation of amyloid fibrils by α-synuclein. Ion mobility-mass spectrometry demonstrated that GA stabilises the extended, native structure of α-synuclein, whilst NMR spectroscopy revealed that GA interacts with α-synuclein transiently.

Carboxymethylated-κ-casein: a convenient tool for the identification of polyphenolic inhibitors of amyloid fibril formation

Reduced and carboxymethylated-kappa-casein (RCM-kappa-CN) is a milk-derived amyloidogenic protein that readily undergoes nucleation-dependent aggregation and amyloid fibril formation via a similar pathway to disease-specific amyloidogenic peptides like amyloid beta (Abeta), which is associated with Alzheimers disease. In this study, a series of flavonoids, many known to be inhibitors of Abeta fibril formation, were screened for their ability to inhibit RCM-kappa-CN fibrilisation, and the results were compared with literature data on Abeta inhibition. Flavonoids that had a high degree of hydroxylation and molecular planarity gave good inhibition of RCM-kappa-CN fibril formation. IC(50) values were between 10- and 200-fold higher with RCM-kappa-CN than literature results for Abeta fibril inhibition, however, with few exceptions, they showed a similar trend in potency. The convenience and reproducibility of the RCM-kappa-CN assay make it an economic alternative first screen for Abeta inhibitory activity, especially for use with large compound libraries.

Hemin as a generic and potent protein misfolding inhibitor

Protein misfolding causes serious biological malfunction, resulting in diseases including Alzheimers disease, Parkinsons disease and cataract. Molecules which inhibit protein misfolding are a promising avenue to explore as therapeutics for the treatment of these diseases. In the present study, thioflavin T fluorescence and transmission electron microscopy experiments demonstrated that hemin prevents amyloid fibril formation of kappa-casein, amyloid beta peptide and α-synuclein by blocking β-sheet structure assembly which is essential in fibril aggregation. Further, inhibition of fibril formation by hemin significantly reduces the cytotoxicity caused by fibrillar amyloid beta peptide in vitro. Interestingly, hemin degrades partially formed amyloid fibrils and prevents further aggregation to mature fibrils. Light scattering assay results revealed that hemin also prevents protein amorphous aggregation of alcohol dehydrogenase, catalase and γs-crystallin. In summary, hemin is a potent agent which generically stabilises proteins against aggregation, and has potential as a key molecule for the development of therapeutics for protein misfolding diseases.

Ion Mobility Mass Spectrometry Studies of the Inhibition of Alpha Synuclein Amyloid Fibril Formation by (–)-Epigallocatechin-3-Gallate

Ion mobility-mass spectrometry (IM-MS) is emerging as an important biophysical technique for the structural analysis of proteins and their assemblies, in particular for structurally heterogeneous systems such as those on the protein misfolding and aggregation pathway. Using IM-MS we have monitored amyloid fibril formation of A53T α-synuclein, a mutant synuclein protein associated with Parkinson’s disease, and identified that a conformational change towards a more compact structure occurs during the initial stages of aggregation. Binding of A53T α-synuclein to a flavenoid based amyloid fibril inhibitor, (–)-epigallocatechin-3-gallate, has been observed with a 1:1 stoichiometry. By analysis of ion collision cross-sections, we show epigallocatechin gallate binding prevents protein conformational change, and in turn decreases the formation of fibrillar aggregates.

Disulfide-containing peptides from the glandular skin secretions of froglets of the genus Crinia: Structure, activity and evolutionary trends

The skin secretions of Crinia signifera, C. riparia and C. deserticola contain bioactive disulfide-containing peptides. Signiferin 1 (RLCIPYIIPC-OH) from C. signifera and C. deserticola) contracts smooth muscle at a concentration of 10(-9) M, and effects proliferation of lymphocytes at 10(-6) M. In contrast, riparin 1.1 (RLCIPVIFC-OH) and riparin 1.2 (FLPPCAYKGTC-OH) from C. riparia show lymphocyte activity but do not contract smooth muscle. The lymphocyte and smooth muscle activities involve CCK2R. 3D structures of signiferin 1 and riparin 1.1 have been established using 2D NMR methods: these studies show significant differences in the shapes of the disulfide rings and with the orientations of the N-terminal residues. cDNA cloning establishes that the pre sections of the precursor pre-pro-riparin 1.4-1.6 peptides are different from the conserved pre regions of disulfide-containing antimicrobial peptides from species of the genus Rana found in the northern hemisphere and caerin antimicrobial peptides isolated from Australian tree frogs of the genus Litoria. This suggests that (i) either that riparins 1 have converged to similar structure and function to the ranid and hyloid prepropeptides which were lost initially from the myobatrachid lineage, or (ii) the prepropeptides in all three groups were derived from a single ancestral form that has remained relatively conserved in the hyloid and ranoid lineages but has undergone substantial divergent evolution in the myobatrachids.

The Effect of Milk Constituents and Crowding Agents on Amyloid Fibril Formation by κ-Casein.

When not incorporated into the casein micelle, κ-casein, a major milk protein, rapidly forms amyloid fibrils at physiological pH and temperature. In this study, the effects of milk components (calcium, lactose, lipids, and heparan sulfate) and crowding agents on reduced and carboxymethylated (RCM) κ-casein fibril formation was investigated using far-UV circular dichroism spectroscopy, thioflavin T binding assays, and transmission electron microscopy. Longer-chain phosphatidylcholine lipids, which form the lining of milk ducts and milk fat globules, enhanced RCM κ-casein fibril formation irrespective of whether the lipids were in a monomeric or micellar state, whereas shorter-chain phospholipids and triglycerides had little effect. Heparan sulfate, a component of the milk fat globule membrane and catalyst of amyloid deposition in extracellular tissue, had little effect on the kinetics of RCM κ-casein fibril formation. Major nutritional components such as calcium and lactose also had no significant effect. Macromolecular crowding enhances protein-protein interactions, but in contrast to other fibril-forming species, the extent of RCM κ-casein fibril formation was reduced by the presence of a variety of crowding agents. These data are consistent with a mechanism of κ-casein fibril formation in which the rate-determining step is dissociation from the oligomer to give the highly amyloidogenic monomer. We conclude that the interaction of κ-casein with membrane-associated phospholipids along its secretory pathway may contribute to the development of amyloid deposits in mammary tissue. However, the formation of spherical oligomers such as casein micelles is favored over amyloid fibrils in the crowded environment of milk, within which the occurrence of amyloid fibrils is low.

Ion mobility-mass spectrometry-based screening for inhibition of α- synuclein aggregation.

Aberrant protein folding and formation of amyloid fibrils are associated with numerous debilitating human diseases, for which there are currently no suitable therapeutic treatments. For instance, Parkinsons disease is characterised pathologically by the intraneural accumulation of the amyloid protein α-synuclein. In order to search for new therapeutic agents that are effective in preventing the early conformational changes that precede protein aggregation, it is necessary to devise new analytical screening approaches. Here we demonstrate the use of ion mobility–mass spectrometry for screening of molecules capable of inhibiting the misfolding and aggregation of α-synuclein (specifically, the A53T human mutant). Importantly, this assay allows for the analysis of conformational changes that precede aggregation, and therefore is unique in its ability to identify inhibitors working at the earliest stages of amyloid formation. In addition, we use complementary mass spectrometry methods to probe selected protein–ligand interactions responsible for fibril inhibition.

Amyloid aggregation and membrane activity of the antimicrobial peptide uperin 3.5

Amyloid fibrils are highly ordered, β‐sheet rich forms of aggregated peptides and proteins that are associated with a variety of pathological human disorders, including Alzheimers and Parkinsons diseases. Amyloid fibril‐forming peptides may be functionally related to antimicrobial peptides, despite differing significantly in sequence and structure. Specifically, their interaction with lipid membranes has mechanistic similarities. The 17‐amino acid peptide uperin 3.5 (U3.5) from an Australian amphibian is antimicrobial and amyloidogenic. Using a quartz crystal microbalance, we investigated the interaction of U3.5 with artificial membranes and found that (i) the membrane interaction of U3.5 is independent of the peptides aggregation state, (ii) the presence of cholesterol in the membrane dramatically alters peptide–membrane interaction leading to a transmembrane pore‐like arrangement of U3.5, and (iii) electrostatic interaction is important for the membrane activity of U3.5 whereby removal of the positive charge at position 7 of U3.5 enhanced its fibrillar aggregation and ablated its membrane interaction, i.e. there is an inverse relationship between the antimicrobial and amyloidogenic properties of U3.5.