mHTT Seeding Activity: A Marker of Disease Progression and Neurotoxicity in Models of Huntington's Disease.

Self-propagating, amyloidogenic mutant huntingtin (mHTT) aggregates may drive progression of Huntington's disease (HD). Here, we report the development of a FRET-based mHTT aggregate seeding (FRASE) assay that enables the quantification of mHTT seeding activity (HSA) in complex biosamples from HD patients and disease models. Application of the FRASE assay revealed HSA in brain homogenates of presymptomatic HD transgenic and knockin mice and its progressive increase with phenotypic changes, suggesting that HSA quantitatively tracks disease progression. Biochemical investigations of mouse brain homogenates demonstrated that small, rather than large, mHTT structures are responsible for the HSA measured in FRASE assays. Finally, we assessed the neurotoxicity of mHTT seeds in an inducible Drosophila model transgenic for HTTex1. We found a strong correlation between the HSA measured in adult neurons and the increased mortality of transgenic HD flies, indicating that FRASE assays detect disease-relevant, neurotoxic, mHTT structures with severe phenotypic consequences in vivo.

Structural properties of EGCG-induced, nontoxic Alzheimer's disease Aß oligomers.

The green tea compound epigallocatechin-3-gallate (EGCG) inhibits Alzheimer's disease ß-amyloid peptide (Aß) neurotoxicity. Solution-state NMR allows probing initial EGCG-Aß interactions. We show that EGCG-induced Aß oligomers adopt a well-defined structure and are amenable for magic angle spinning solid-state NMR investigations. We find that EGCG interferes with the aromatic hydrophobic core of Aß. The C-terminal part of the Aß peptide (residues 22-39) adopts a ß-sheet conformation, whereas the N-terminus (residues 1-20) is unstructured. The characteristic salt bridge involving residues D23 and K28 is present in the structure of these oligomeric Aß aggregates as well. The structural analysis of small-molecule-induced amyloid aggregates will open new perspectives for Alzheimer's disease drug development.

Black tea theaflavins inhibit formation of toxic amyloid-ß and α-synuclein fibrils.

Causal therapeutic approaches for amyloid diseases such as Alzheimer's and Parkinson's disease targeting toxic amyloid oligomers or fibrils are still emerging. Here, we show that theaflavins (TF1, TF2a, TF2b, and TF3), the main polyphenolic components found in fermented black tea, are potent inhibitors of amyloid-ß (Aß) and α-synuclein (αS) fibrillogenesis. Their mechanism of action was compared to that of two established inhibitors of amyloid formation, (-)-epigallocatechin gallate (EGCG) and congo red (CR). All three compounds reduce the fluorescence of the amyloid indicator dye thioflavin T. Mapping the binding regions of TF3, EGCG, and CR revealed that all three bind to two regions of the Aß peptide, amino acids 12-23 and 24-36, albeit with different specificities. However, their mechanisms of amyloid inhibition differ. Like EGCG but unlike congo red, theaflavins stimulate the assembly of Aß and αS into nontoxic, spherical aggregates that are incompetent in seeding amyloid formation and remodel Aß fibrils into nontoxic aggregates. When compared to EGCG, TF3 was less susceptible to air oxidation and had an increased efficacy under oxidizing conditions. These findings suggest that theaflavins might be used to remove toxic amyloid deposits.

Bacterial inclusion bodies of Alzheimer's disease ß-amyloid peptides can be employed to study native-like aggregation intermediate states.

The structures of oligomeric intermediate states in the aggregation process of Alzheimer's disease ß-amyloid peptides have been the subject of debate for many years. Bacterial inclusion bodies contain large amounts of small heat shock proteins (sHSPs), which are highly homologous to those found in the plaques of the brains of Alzheimer's disease patients. sHSPs break down amyloid fibril structure in vitro and induce oligomeric assemblies. Prokaryotic protein overexpression thus mimics the conditions encountered in the cell under stress and allows the structures of Aß aggregation intermediate states to be investigated under native-like conditions, which is not otherwise technically possible. We show that IB40/IB42 fulfil all the requirements to be classified as amyloids: they seed fibril growth, are Congo red positive and show characteristic ß-sheet-rich CD spectra. However, IB40 and IB42 are much less stable than fibrils formed in vitro and contain significant amounts of non-ß-sheet regions, as seen from FTIR studies. Quantitative analyses of solution-state NMR H/D exchange rates show that the hydrophobic cores involving residues V18-F19-F20 adopt ß-sheet conformations, whereas the C termini adopt α-helical coiled-coil structures. In the past, an α-helical intermediate-state structure has been postulated, but could not be verified experimentally. In agreement with the current literature, in which Aß oligomers are described as the most toxic state of the peptides, we find that IB42 contains SDS-resistant oligomers that are more neurotoxic than Aß42 fibrils. E. coli inclusion bodies formed by the Alzheimer's disease ß-amyloid peptides Aß40 and Aß42 thus behave structurally like amyloid aggregation intermediate states and open the possibility of studying amyloids in a native-like, cellular environment.

Identification of VCP/p97, carboxyl terminus of Hsp70-interacting protein (CHIP), and amphiphysin II interaction partners using membrane-based human proteome arrays.

Proteins mediate their biological function through interactions with other proteins. Therefore, the systematic identification and characterization of protein-protein interactions have become a powerful proteomic strategy to understand protein function and comprehensive cellular regulatory networks. For the screening of valosin-containing protein, carboxyl terminus of Hsp70-interacting protein (CHIP), and amphiphysin II interaction partners, we utilized a membrane-based array technology that allows the identification of human protein-protein interactions with crude bacterial cell extracts. Many novel interaction pairs such as valosin-containing protein/autocrine motility factor receptor, CHIP/caytaxin, or amphiphysin II/DLP4 were identified and subsequently confirmed by pull-down, two-hybrid and co-immunoprecipitation experiments. In addition, assays were performed to validate the interactions functionally. CHIP e.g. was found to efficiently polyubiquitinate caytaxin in vitro, suggesting that it might influence caytaxin degradation in vivo. Using peptide arrays, we also identified the binding motifs in the proteins DLP4, XRCC4, and fructose-1,6-bisphosphatase, which are crucial for the association with the Src homology 3 domain of amphiphysin II. Together these studies indicate that our human proteome array technology permits the identification of protein-protein interactions that are functionally involved in neurodegenerative disease processes, the degradation of protein substrates, and the transport of membrane vesicles.

Sulfur oxidation in Paracoccus pantotrophus: interaction of the sulfur-binding protein SoxYZ with the dimanganese SoxB protein.

The central protein of the sulfur-oxidizing enzyme system of Paracoccus pantotrophus, SoxYZ, formed complexes with subunits associated and covalently bound. In denaturing SDS-polyacrylamide gel electrophoresis (PAGE) SoxY migrated at 12 and SoxZ at 16kDa. SDS-PAGE of homogeneous SoxYZ without reductant separated dimeric complexes of 25, 29, and 32kDa identified by the N-terminal amino acid sequences as SoxY-Y, SoxY-Z, and SoxZ-Z, and subunit cleavage by reduction suggested their linkage via protein disulfide bonds. SoxYZ was reversibly redox active between -0.25 and 0.2V, as monitored by a combined electrochemical and FTIR spectroscopic approach. The dimanganese SoxB protein (58.611Da) converted the covalently linked heterodimer SoxY-Z to SoxYZ with associated subunits which in turn aggregated to the heterotetramer Sox(YZ)(2). This reaction depended on time and the SoxB concentration, and demonstrated the interaction of these two Sox proteins.

EcoRII: a restriction enzyme evolving recombination functions?

The restriction endonuclease EcoRII requires the cooperative interaction with two copies of the sequence 5'CCWGG for DNA cleavage. We found by limited proteolysis that EcoRII has a two-domain structure that enables this particular mode of protein-DNA interaction. The C-terminal domain is a new restriction endonuclease, EcoRII-C. In contrast to the wild-type enzyme, EcoRII-C cleaves DNA specifically at single 5'CCWGG sites. Moreover, substrates containing two or more cooperative 5'CCWGG sites are cleaved much more efficiently by EcoRII-C than by EcoRII. The N-terminal domain binds DNA specifically and attenuates the activity of EcoRII by making the enzyme dependent on a second 5'CCWGG site. Therefore, we suggest that a precursor EcoRII endonuclease acquired an additional DNA-binding domain to enable the interaction with two 5'CCWGG sites. The current EcoRII molecule could be an evolutionary intermediate between a site-specific endonuclease and a protein that functions specifically with two DNA sites such as recombinases and transposases. The combination of these functions may enable EcoRII to accomplish its own propagation similarly to transposons.

Asymmetric photocross-linking pattern of restriction endonuclease EcoRII to the DNA recognition sequence.

The EcoRII homodimer engages two of its recognition sequences (5'-CCWGG) simultaneously and is therefore a type IIE restriction endonuclease. To identify the amino acids of EcoRII that interact specifically with the recognition sequence, we photocross-linked EcoRII with oligonucleotide substrates that contained only one recognition sequence for EcoRII. In this recognition sequence, we substituted either 5-iododeoxycytidine for each C or 5-iododeoxyuridine for A, G, or T. These iodo-pyrimidine bases were excited using a UV laser to result in covalent cross-linking products. The yield of EcoRII photocross-linked to the 5'-C of the 5'-CCAGG strand of the recognition sequence was 45%. However, we could not photocross-link EcoRII to the 5'-C of the 5'-CCTGG strand. Thus, the contact of EcoRII to the bases of the recognition sequence appears to be asymmetric, unlike that expected for most type II restriction endonucleases. Tryptic digestion of free and of cross-linked EcoRII, followed by high performance liquid chromatography (HPLC) separation of the individual peptides and Edman degradation, identified amino acids 25-49 of EcoRII as the cross-linking peptide. Mutational analysis of the electron-rich amino acids His(36) and Tyr(41) of this peptide indicates that Tyr(41) is the amino acid involved in the cross-link and that it therefore contributes to specific DNA recognition by EcoRII.