Deposition of protein fibers with a characteristic cross-β sheet structure is the molecular marker associated with human disorders ranging from Alzheimer's disease to type II diabetes and spongiform encephalopathy. Given the large number of non-disease related proteins and peptides that have been shown to form amyloid fibrils in vitro, it has been suggested that amyloid fibril formation represents a generic protein phase transition. In the last two decades it has become clear that the same protein/peptide can assemble into distinct morphologically and structurally amyloid aggregates depending on the solution conditions. Moreover, recent studies have shown that the early stage, oligomeric amyloid assemblies are the main culprit in vivo. We have investigated the amyloid assemblies formed under denaturing conditions for Hen Egg White Lysozyme (HewL) whose human homologue is directly implicated in hereditary non-neuropathic systemic amyloidosis. Our early investigations showed that HewL can aggregate via at least two distinct assembly pathways depending on solution ionic strength at fixed pH, temperature, and protein concentration. By combining Dynamic Light Scattering (DLS), Static Light Scattering (SLS) and Atomic Force Microscopy (AFM) we showed that at low ionic strength, the pathway is characterized by the nucleation and growth of long (several micron), rigid fibrils (RF) via monomers assembly. A second, high ionic strength pathway is characterized by the rapid assembly of monomers into globular oligomers that further polymerize into curvilinear fibrils (aO/CF). At NaCl concentrations above 400 mM, aggregation resulted in precipitate formation.
Next, we used Foureir Transform Infrared spectroscopy (FTIR) and an amyloid-specific dye, Thioflavin T (ThT), to show that both RF and (a)O/CF are amyloidogenic species, but they have detectable structural differences. Moreover, we have determined that each assembly pathway has unique SLS, DLS, FTIR and ThT response signatures that help determine the assembly type prior to AFM imaging of aggregates.
Taking advantage of the morphological, structural and kinetic signatures for the two distinct HewL amyloid aggregates I mapped out their amyloid aggregates phase diagram spanning over two orders of magnitude in protein concentration and from 50 to 800 mM NaCl in ionic strength. This is the most complete phase diagram for amyloid aggregates of a given protein up to date. The phase diagram has three distinct regions delineated by sharp boundaries. The RF- aO/CF was called Critical Oligomer Concentration, and we commonly refer to “above the COC” as the region were aO/CF are kinetically favored.. In the region of low salt/high protein concentrations, RF were the only amyloid species to nucleate and grow. As both salt and protein concentrations increase, aO/CF become the kinetically favored species, and RF nucleate and grow after several days of incubation. At high protein and high salt concentrations, aO/CF form very fast and eventually lose solubility forming a precipitate (Ppt). Cross-seeding experiments showed that RF is the thermodynamically stable aggregate phase, while the O/CF are the metastable species.
Finally, we used the phase diagram to design experiments that would allow us to reveal the RF nucleation mechanism in presence of aO/CF. RF nucleation above the COC can undergo either via internal restructuring of aO/CF (NCC) or through a random coalescence of monomers into a nucleus (NP). The experimental results obtained so far strongly indicate that RF nucleate via NP mechanism both below and above the COC.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-8622 |
| Date | 01 November 2017 |
| Creators | Miti, Tatiana |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
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