Encapsulation and NMR on an aggregating peptide before fibrillogenesis

The early stages of peptide and protein aggregation include the formation of soluble oligomers, some of which may be cytotoxic. There is a paucity of structural information on these oligomers, however, because they are temporally unstable and tend to aggregate further into insoluble protofibrils and fibrils. To obtain structural information on soluble oligomers, we have developed a procedure for encapsulating a fibril-forming peptide, Peptide 1 (NH2-SDDYYYGFGSNKFGRPRDD-COOH), in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine single bilayer vesicles (POPC SBVs). We also encapsulated a non-fibril forming peptide, Peptide 2 (NH2-EEWEE-COOH), in POPC SBVs. The nominal concentration of Peptide 1 in the resulting 40 nm diameter SBVs was 2.4 +/- 0.1 mM, well above the concentration at which Peptide 1 forms fibrils. We demonstrated that these peptides had indeed been encapsulated by measuring longitudinal relaxation times (T1) in the presence and absence of a paramagnetic substance, 1 mM Gd-EDTA, by NMR spectroscopy. When the peptides were free in solution, they showed the expected shortening of T1 times and broadening of NMR peaks. In contrast, peptide encapsulated in POPC SBVs were shielded from the effects of Gd-EDTA and showed preservation of T1 values and NMR line widths. To demonstrate that encapsulation inhibits fibril formation, we measured one-dimensional proton (1D-1H) NMR spectra of the peptides in solution, and of the encapsulated peptides immediately after encapsulation, and 4 days after encapsulation, because Peptide 1 forms fibrils within 1 day. A 2.8 mM solution of Peptide 1 shows the loss of NMR signal expected for a fibrillizing peptide. In contrast, the 1D-1H spectra of encapsulated Peptide 1 measured immediately after encapsulation and 4 days after encapsulation were essentially identical, with preservation of line width at 4 days, i.e., well within the time frame of most high-resolution NMR experiments. Encapsulation may provide a means to obtain high-resolution NMR data on unstable soluble oligomers of peptides implicated in amyloidoses such as Alzheimer's Disease and provide the first detailed structural information about these possibly cytotoxic species that have hitherto been inaccessible to analysis.