A ß-barrel-like tetramer formed by a ß-hairpin derived from Aß.

ß-Hairpins formed by the ß-amyloid peptide Aß are building blocks of Aß oligomers. Three different alignments of ß-hairpins have been observed in the structures of Aß oligomers or fibrils. Differences in ß-hairpin alignment likely contribute to the heterogeneity of Aß oligomers and thus impede their study at high-resolution. Here, we designed, synthesized, and studied a series of ß-hairpin peptides derived from Aß12-40 in one of these three alignments and investigated their solution-phase assembly and folding. These assays reveal the formation of tetramers and octamers that are stabilized by intermolecular hydrogen bonding interactions between Aß residues 12-14 and 38-40 as part of an extended ß-hairpin conformation. X-ray crystallographic studies of one peptide from this series reveal the formation of ß-barrel-like tetramers and octamers that are stabilized by edge-to-edge hydrogen bonding and hydrophobic packing. Dye-leakage and caspase 3/7 activation assays using tetramer and octamer forming peptides from this series reveal membrane-damaging and apoptotic properties. A molecular dynamics simulation of the ß-barrel-like tetramer embedded in a lipid bilayer shows membrane disruption and water permeation. The tetramers and octamers described herein provide additional models of how Aß may assemble into oligomers and supports the hypothesis that ß-hairpin alignment and topology may contribute directly to oligomer heterogeneity.

Exploring amyloid oligomers with peptide model systems.

The assembly of amyloidogenic peptides and proteins, such as the ß-amyloid peptide, α-synuclein, huntingtin, tau, and islet amyloid polypeptide, into amyloid fibrils and oligomers is directly linked to amyloid diseases, such as Alzheimer's, Parkinson's, and Huntington's diseases, frontotemporal dementias, and type II diabetes. Although amyloid oligomers have emerged as especially important in amyloid diseases, high-resolution structures of the oligomers formed by full-length amyloidogenic peptides and proteins have remained elusive. Investigations of oligomers assembled from fragments or stabilized ß-hairpin segments of amyloidogenic peptides and proteins have allowed investigators to illuminate some of the structural, biophysical, and biological properties of amyloid oligomers. Here, we summarize recent advances in the application of these peptide model systems to investigate and understand the structures, biological properties, and biophysical properties of amyloid oligomers.

Phenylalanine Mutation to Cyclohexylalanine Facilitates Triangular Trimer Formation by ß-Hairpins Derived from Aß.

Oligomers of the ß-amyloid peptide, Aß, play a central role in the pathogenesis and progression of Alzheimer's disease. Trimers and higher-order oligomers composed of trimers are thought to be the most neurotoxic Aß oligomers. To gain insights into the structure and assembly of Aß oligomers, our laboratory has previously designed and synthesized macrocyclic peptides derived from Aß17-23 and Aß30-36 that fold to form ß-hairpins and assemble to form trimers. In this study, we found that mutating Phe20 to cyclohexylalanine (Cha) in macrocyclic Aß-derived peptides promotes crystallization of an Aß-derived peptide containing the Aß24-29 loop (peptide 3F20Cha) and permits elucidation of its structure and assembly by X-ray crystallography. X-ray crystallography shows that peptide 3F20Cha forms a hexamer. X-ray crystallography and SDS-PAGE further show that trimer 4F20Cha, a covalently stabilized trimer derived from peptide 3F20Cha, forms a dodecamer. Size exclusion chromatography shows that trimer 4F20Cha forms higher-order assemblies in solution. Trimer 4F20Cha exhibits cytotoxicity against the neuroblastoma cell line SH-SY5Y. These studies demonstrate the use of the F20Cha mutation to further stabilize oligomers of Aß-derived peptides that contain more of the native sequence and thus better mimic the oligomers formed by full-length Aß.

Effects of N-Terminal Residues on the Assembly of Constrained ß-Hairpin Peptides Derived from Aß.

This paper describes the synthesis, solution-phase biophysical studies, and X-ray crystallographic structures of hexamers formed by macrocyclic ß-hairpin peptides derived from the central and C-terminal regions of Aß, which bear "tails" derived from the N-terminus of Aß. Soluble oligomers of the ß-amyloid peptide, Aß, are thought to be the synaptotoxic species responsible for neurodegeneration in Alzheimer's disease. Over the last 20 years, evidence has accumulated that implicates the N-terminus of Aß as a region that may initiate the formation of damaging oligomeric species. We previously studied, in our laboratory, macrocyclic ß-hairpin peptides derived from Aß16-22 and Aß30-36, capable of forming hexamers that can be observed by X-ray crystallography and SDS-PAGE. To better mimic oligomers of full length Aß, we use an orthogonal protecting group strategy during the synthesis to append residues from Aß1-14 to the parent macrocyclic ß-hairpin peptide 1, which comprises Aß16-22 and Aß30-36. The N-terminally extended peptides N+1, N+2, N+4, N+6, N+8, N+10, N+12, and N+14 assemble to form dimers, trimers, and hexamers in solution-phase studies. X-ray crystallography reveals that peptide N+1 assembles to form a hexamer that is composed of dimers and trimers. These observations are consistent with a model in which the assembly of Aß oligomers is driven by hydrogen bonding and hydrophobic packing of the residues from the central and C-terminal regions, with the N-terminus of Aß accommodated by the oligomers as an unstructured tail.

X-ray Crystallography Reveals Parallel and Antiparallel ß-Sheet Dimers of a ß-Hairpin Derived from Aß16-36 that Assemble to Form Different Tetramers.

High-resolution structures of oligomers formed by the ß-amyloid peptide, Aß, are important for understanding the molecular basis of Alzheimer's disease. Dimers of Aß are linked to the pathogenesis and progression of Alzheimer's disease, and tetramers of Aß are neurotoxic. This paper reports the X-ray crystallographic structures of dimers and tetramers, as well as an octamer, formed by a peptide derived from the central and C-terminal regions of Aß. In the crystal lattice, the peptide assembles to form two different dimers-an antiparallel ß-sheet dimer and a parallel ß-sheet dimer-that each further self-assemble to form two different tetramers-a sandwich-like tetramer and a twisted ß-sheet tetramer. The structures of these dimers and tetramers derived from Aß serve as potential models for dimers and tetramers of full-length Aß that form in vitro and in Alzheimer's disease-afflicted brains.

Phage-Based Structural Color Sensors and Their Pattern Recognition Sensing System.

The mammalian olfactory system provides great inspiration for the design of intelligent sensors. To this end, we have developed a bioinspired phage nanostructure-based color sensor array and a smartphone-based sensing network system. Using a M13 bacteriophage (phage) as a basic building block, we created structural color matrices that are composed of liquid-crystalline bundled nanofibers from self-assembled phages. The phages were engineered to express cross-responsive receptors on their major coat protein (pVIII), leading to rapid, detectable color changes upon exposure to various target chemicals, resulting in chemical- and concentration-dependent color fingerprints. Using these sensors, we have successfully detected 5-90% relative humidity with 0.2% sensitivity. In addition, after modification with aromatic receptors, we were able to distinguish between various structurally similar toxic chemicals including benzene, toluene, xylene, and aniline. Furthermore, we have developed a method of interpreting and disseminating results from these sensors using smartphones to establish a wireless system. Our phage-based sensor system has the potential to be very useful in improving national security and monitoring the environment and human health.