Influence of denaturants on amyloid ß42 aggregation kinetics.

Amyloid formation is linked to devastating neurodegenerative diseases, motivating detailed studies of the mechanisms of amyloid formation. For Aß, the peptide associated with Alzheimer's disease, the mechanism and rate of aggregation have been established for a range of variants and conditions in vitro and in bodily fluids. A key outstanding question is how the relative stabilities of monomers, fibrils and intermediates affect each step in the fibril formation process. By monitoring the kinetics of aggregation of Aß42, in the presence of urea or guanidinium hydrochloride (GuHCl), we here determine the rates of the underlying microscopic steps and establish the importance of changes in relative stability induced by the presence of denaturant for each individual step. Denaturants shift the equilibrium towards the unfolded state of each species. We find that a non-ionic denaturant, urea, reduces the overall aggregation rate, and that the effect on nucleation is stronger than the effect on elongation. Urea reduces the rate of secondary nucleation by decreasing the coverage of fibril surfaces and the rate of nucleus formation. It also reduces the rate of primary nucleation, increasing its reaction order. The ionic denaturant, GuHCl, accelerates the aggregation at low denaturant concentrations and decelerates the aggregation at high denaturant concentrations. Below approximately 0.25 M GuHCl, the screening of repulsive electrostatic interactions between peptides by the charged denaturant dominates, leading to an increased aggregation rate. At higher GuHCl concentrations, the electrostatic repulsion is completely screened, and the denaturing effect dominates. The results illustrate how the differential effects of denaturants on stability of monomer, oligomer and fibril translate to differential effects on microscopic steps, with the rate of nucleation being most strongly reduced.

Increased Secondary Nucleation Underlies Accelerated Aggregation of the Four-Residue N-Terminally Truncated Aß42 Species Aß5-42.

Aggregation of the amyloid-ß (Aß) peptide into plaques is believed to play a crucial role in Alzheimer's disease. Amyloid plaques consist of fibrils of full length Aß peptides as well as N-terminally truncated species. ß-Site amyloid precursor protein-cleaving enzyme (BACE1) cleaves amyloid precursor protein in the first step in Aß peptide production and is an attractive therapeutic target to limit Aß generation. Inhibition of BACE1, however, induces a unique pattern of Aß peptides with increased levels of N-terminally truncated Aß peptides starting at position 5 (Aß5-X), indicating that these peptides are generated through a BACE1-independent pathway. Here we elucidate the aggregation mechanism of Aß5-42 and its influence on full-length Aß42. We find that, compared to Aß42, Aß5-42 is more aggregation prone and displays enhanced nucleation rates. Aß5-42 oligomers cause nonspecific membrane disruption to similar extent as Aß42 but appear at earlier time points in the aggregation reaction. Noteworthy, this implies similar toxicity of Aß42 and Aß5-42 and the toxic species are generated faster by Aß5-42. The increased rate of secondary nucleation on the surface of existing fibrils originates from a higher affinity of Aß5-42 monomers for fibrils, as compared to Aß42: an effect that may be related to the reduced net charge of Aß5-42. Moreover, Aß5-42 and Aß42 peptides coaggregate into heteromolecular fibrils and either species can elongate existing Aß42 or Aß5-42 fibrils but Aß42 fibrils are more catalytic than Aß5-42 fibrils. Our findings highlight the importance of the N-terminus for surface-catalyzed nucleation and thus the production of toxic oligomers.

Protein stabilization with retained function of monellin using a split GFP system.

Sweet proteins are an unexploited resource in the search for non-carbohydrate sweeteners mainly due to their low stability towards heating. Variants of the sweet protein monellin, with increased stability, were derived by an in vivo screening method based on the thermodynamic linkage between fragment complementation and protein stability. This approach depends on the correlation between mutational effects on the affinity between protein fragments and the stability of the intact protein. By linking the two fragments of monellin to the split GFP (green fluorescent protein) system, reconstitution of GFP was promoted and moderately fluorescent colonies were obtained. Two separate random libraries were produced for the monellin chains and the mutant clones were ranked based on fluorescence intensity. Mutants with increased affinity between the fragments, and subsequently increased stability, caused increased fluorescence intensity of split GFP. Single chain monellin variants of the top-ranked mutants for each chain, S76Y in the A-chain and W3C + R39G in the B-chain and all combinations thereof, were expressed and the increase in stability was verified by temperature denaturation studies using circular dichroism spectroscopy. Functionality studies showed that mutant S76Y has retained sweetness and has potential use within the food industry.

Phage display and kinetic selection of antibodies that specifically inhibit amyloid self-replication.

The aggregation of the amyloid ß peptide (Aß) into amyloid fibrils is a defining characteristic of Alzheimer's disease. Because of the complexity of this aggregation process, effective therapeutic inhibitors will need to target the specific microscopic steps that lead to the production of neurotoxic species. We introduce a strategy for generating fibril-specific antibodies that selectively suppress fibril-dependent secondary nucleation of the 42-residue form of Aß (Aß42). We target this step because it has been shown to produce the majority of neurotoxic species during aggregation of Aß42. Starting from large phage display libraries of single-chain antibody fragments (scFvs), the three-stage approach that we describe includes (i) selection of scFvs with high affinity for Aß42 fibrils after removal of scFvs that bind Aß42 in its monomeric form; (ii) ranking, by surface plasmon resonance affinity measurements, of the resulting candidate scFvs that bind to the Aß42 fibrils; and (iii) kinetic screening and analysis to find the scFvs that inhibit selectively the fibril-catalyzed secondary nucleation process in Aß42 aggregation. By applying this approach, we have identified four scFvs that inhibit specifically the fibril-dependent secondary nucleation process. Our method also makes it possible to discard antibodies that inhibit elongation, an important factor because the suppression of elongation does not target directly the production of toxic oligomers and may even lead to its increase. On the basis of our results, we suggest that the method described here could form the basis for rationally designed immunotherapy strategies to combat Alzheimer's and related neurodegenerative diseases.

Calcium Binding and Disulfide Bonds Regulate the Stability of Secretagogin towards Thermal and Urea Denaturation.

Secretagogin is a calcium-sensor protein with six EF-hands. It is widely expressed in neurons and neuro-endocrine cells of a broad range of vertebrates including mammals, fishes and amphibia. The protein plays a role in secretion and interacts with several vesicle-associated proteins. In this work, we have studied the contribution of calcium binding and disulfide-bond formation to the stability of the secretagogin structure towards thermal and urea denaturation. SDS-PAGE analysis of secretagogin in reducing and non-reducing conditions identified a tendency of the protein to form dimers in a redox-dependent manner. The denaturation of apo and Calcium-loaded secretagogin was studied by circular dichroism and fluorescence spectroscopy under conditions favoring monomer or dimer or a 1:1 monomer: dimer ratio. This analysis reveals significantly higher stability towards urea denaturation of Calcium-loaded secretagogin compared to the apo protein. The secondary and tertiary structure of the Calcium-loaded form is not completely denatured in the presence of 10 M urea. Reduced and Calcium-loaded secretagogin is found to refold reversibly after heating to 95°C, while both oxidized and reduced apo secretagogin is irreversibly denatured at this temperature. Thus, calcium binding greatly stabilizes the structure of secretagogin towards chemical and heat denaturation.