Amyloid

Misfolding and aggregation of Cellular Prion Protein (PrPc) is a major molecular process involved in the pathogenesis of Prion diseases. An N-terminal portion of the Prion protein, PrP106-128, is a 23-residue peptide fragment characterized by an amphipathic structure with two domains: a hydrophilic N-terminal domain and a hydrophobic C-terminal domain. Here, we studied the aggregation properties of the prion fragment peptide PrP106-128. The results show that the peptide aggregates in a concentration-dependent manner in an aqueous solution and that the aggregation is sensitive to pH and the preformed amyloid seeds.Furthermore, we show that the zwitterionic POPC liposomes moderately inhibit the aggregation of PrP(106–128), whereas POPC/cholesterol (8:2) vesicles facilitate peptide aggregation likely due to the increase of the lipid packing order and membrane rigidity in the presence of cholesterol. In addition, anionic lipid vesicles of POPG and POPG/cholesterol above a certain concentration accelerate the aggregation of the peptide remarkably. The strong electrostatic interactions between the N-terminal region of the peptide and POPG may constrain the conformational plasticity of the peptide, preventing insertion of the peptide into the inner side of the membrane and thus promoting fibrillation on the membrane surface. The results suggest that the charge properties of the membrane, the composition of the liposomes, and the rigidity of lipid packing are critical in determining peptide adsorption on the membrane surface and the efficiency of the membrane in catalyzing peptide oligomeric nucleation and amyloid formation.

Unintentional weight loss in older adults often precedes Alzheimer’s disease (AD). Positron emission tomography (PET) scan reveals that AD patients exhibit reduced uptake of fluorodeoxyglucose into brain cells, defined as ‘hypometabolism’. However, cellular mechanisms underlying weight loss and hypometabolism have not received much attention. The primary goal of the study was to test the hypothesis that cells become starved in confrontation with amyloid beta proteins (Aβ), which are increasingly aggregated in the AD brain. Cellular ATP is known as a biomarker indicating for cell starvation. We found that Aβ caused a dose-dependent reduction in ATP of astrocytes. This effect was similar to those of cells being deprived from nutrients (i.e., glucose, pyruvate and glutamine). Together, the data of the present study support the hypothesis that cell starvation is likely associated with weight loss and hypometabolism in AD patients.

Human calcitonin (hCT) is a peptide hormone that is produced by the thyroid gland where it regulates blood calcium and stimulates bone formation. However, increased concentrations can cause hCT to aggregate into amyloid fibrils where they can cause cellular toxicity. In this dissertation, we investigated the role of the N-terminal intramolecular disulfide bond, the effects cholesterol derivatives, the inhibitory effects of a group of polyphenolic molecules, and membrane interactions on hCT amyloid formation.
To better understand hCT amyloid formation, we investigated the role of the N-terminal intramolecular disulfide bond has on the aggregation kinetics of hCT. Our results demonstrated that the presence of the disulfide bond is key to the formation of the oligomeric nucleus that is needed for amyloid formation. We also investigated the role of cholesterol, cholesterol sulfate, and 3β-[N-(dimethylaminoethane)carbamoyl]-cholesterol (DC-cholesterol) in moderating hCT fibril formation. We showed that cholesterol does not significantly affect hCT fibrillization while high concentrations of cholesterol sulfate has a moderate inhibiting effect. However, DC-cholesterol strongly inhibits hCT fibril formation in a concentration-dependent manner suggesting the role of electrostatic and hydrogen bonding interactions have in moderating the interactivity between hCT and the surface of DC-cholesterol vesicles. We also probed the inhibitory effects of a group of polyphenolic molecules on hCT fibril formation. Our results showed that molecules containing vicinal hydroxyl groups on the phenyl ring effectively inhibits hCT fibril formation though a plausible covalent linkage between the oxidized polyphenol and hCT.

The misfolding of native, cellular prion protein (PrPc) to a conformationally altered pathogenic isoform, designated scrapie PrPsc, is the main molecular process involved in the pathogenesis of prion diseases. Prion diseases are marked by the accumulation of conformationally modified forms of cellular prion protein. An N-terminal portion of the prion protein, PrP (106-128), is a 23-residue peptide fragment and is characterized by an amphipathic structure with two domains: a hydrophilic N-terminal domain and a hydrophobic C-terminal domain. In this study, the aggregation characteristics of the PrP (106-128) peptide were investigated using a combination of biophysical approaches. We investigated the effect of different factors including concentrations, pH, and metal ions, on the aggregation of the peptide. Our results demonstrated that the peptide steadily aggregates at concentrations higher than 25 M. The aggregation propensity and fibril formation is higher at pH 7.4 and pH 8.1, and the aggregation is inhibited at pH lower than 6. Furthermore, our results indicate that the Cu2+ has much less effect on the peptide amyloidogenesis, while Zn2+ has a significant influence on the PrP (106-128) amyloidogenesis. We further presented a systematic analysis of the impact of phospholipid liposomes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1’-racglycerol) (POPG) in the absence or presence of cholesterol, on the amyloidogenesis of PrP (106-128). The results showed that POPC vesicles does not significantly influence the aggregation kinetics of the peptide. However, the anionic lipid POPG delays the aggregation in a concentration-dependent manner, whereas the addition of POPG with the cholesterol shows fast kinetics of fibrillization, thus reducing the lag time of the aggregation kinetics. We also monitored the effect of cholesterol and its derivatives including cholesterol-SO4 and DC-cholesterol on PrP (106-128) amyloidogenesis. Our results showed that the cholesterol inhibits the peptide aggregation and delays the formation of fibrils in a concentration-dependent manner. Cholesterol-SO4 dramatically facilitates the aggregation at high concentrations but has the potential to slow down the fibrillization at low concentrations, whereas cationic DC-cholesterol vesicles can effectively inhibit peptide fibril formation at high concentrations.

Alzheimer’s disease (AD) is a deleterious neurodegenerative disease caused in major part by the aberrant processing and accumulation of amyloid beta peptides. In this dissertation, we systematically investigated the role of N-terminal region (NTR) residues of amyloid (1-40) (Aβ40) peptide in amyloidogenesis, lipid bilayer membrane interaction and damage, as well as neurotoxicity. Herein, we investigated the role of NTR residues on the aggregation and amyloid fibril formation process, to gain understanding on the electrostatic and hydrophobic constituents of the mechanism. This was achieved by substituting specific charged residues located in the NTR of Aβ40 and investigating their effects through a variety of techniques. We also investigated the role of NTR charged residues in their interaction with supported phospholipid bilayer membranes through the use of Quartz Crystal Microbalance with Dissipation (QCM-D) monitoring to gain insight on the mechanistic details of the interaction. To further understand the implications of substituting charged NTR residues on membrane interaction, pore formation and damage, we utilized a carboxyfluorescein dye leakage assay to quantify the membrane damage caused by Aβ40 and the NTR mutants. We also performed neurotoxicity assay with SH-SY5Y neuroblastoma cells to shed light on the effects of NTR substitutions on cellular toxicity. Finally, we synthesized a polymer, trimethyl chitosan (TMC), and utilized it as a polyelectrolyte monitor of electrostatic interactions occurring between TMC and the NTR of Aβ40. Our results demonstrate that the NTR charged residues of Aβ40 contribute significantly to the aggregation process, amyloidogenesis, and phospholipid membrane interaction and perturbation by means of electrostatic, thermodynamic and hydrophobic forces.

Alzheimer’s disease (AD) has been defined as a type of dementia that causes
problems with memory, thinking, and behavior. AD is characterized by tau tangles and
Aβ plaques in and around neurons, respectively. The impact this disease has on its
victims’ health, both physically and mentally, is unimaginable and the rate of progression
is not expected to decrease any time soon. This threat to our minds encourages the
importance of understanding AD. Amongst the theories as to what bio mechanisms cause
the brain to intertwine is the amyloid cascade hypothesis. The purpose of this thesis is to
review the amyloid cascade hypothesis and discuss treatments which utilize this model.
We also wish to examine social aspects such as loneliness and socioeconomic factors
which are associated with the progression of AD. Research presented provides evidence
that targeting the accumulation of Aβ in the brain will prevent further biochemical
responses to form neurodegenerative pathology. From the collected data, we observe that
therapies targeting the amyloidogenic pathway have received positive feedback in the
medical community. Amongst them, an Aβ synthetic peptide vaccine which made history
in vaccine development due to their responder rate. The impact of social factors such as
loneliness in the advancement of AD is also supported by research. While it is
acknowledged that any neurodegenerative disease is far too complex to narrow its cause
specifically, this thesis provides an association with multiple aspects that can be
understood and applied to future research in this field.

The types of intrusion errors (Prior List, Semantically Related, and Unrelated)
made on the LASSI-L verbal memory task were compared across three diagnostic groups
(N = 160, 61 % female), Cognitively Normal (CN), amnestic Mild Cognitive Impairment
(aMCI), and Alzheimer’s disease (AD). Errors related to Proactive, Recovery from
Proactive, and Retroactive Interference were also analyzed, as well as the relationship of
errors to Amyloid load, a biomarker of AD. Results suggest that the types of error made
indicated the level of cognitive decline. It appears that as deficits increase, impaired
semantic networks result in the simultaneous activation of items that are semantically
related to LASSI-L words. In the aMCI group, providing a semantic cue resulted in an
increased production of Semantically Related intrusions. Unrelated intrusions occurred
rarely, although, a small number occurred even in the CN group, warranting further
investigation. Amyloid load correlated with all intrusion errors.