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Neuronal plaques of amyloid β (Aβ) peptides of varying length carrying different posttranslational modifications represent a molecular hallmark of Alzheimer's disease. It is believed that transient oligomeric Aβ assemblies associating in early fibrillation events represent particularly cytotoxic peptide aggregates. Also, N-terminally truncated (in position 3 or 11) and pyroglutamate modified peptides exhibited an increased toxicity compared to the wildtype. In the current study, the molecular structure of oligomeric species of pGlu3-Aβ(3-40) and pGlu11-Aβ(11-40) was investigated using solid-state NMR spectroscopy. On the secondary structure level, for both modified peptides a large similarity between oligomers and mature fibrils of the modified peptides was found mainly based on 13C NMR chemical shift data. Some smaller structural differences were detected in the vicinity of the respective modification site. Also, the crucial early folding molecular contact between residues Phe19 and Leu34 could be observed for the oligomers of both modified peptide species. Therefore, it has to be concluded that the major secondary structure elements of Aβ are already present in oligomers of pGlu3-Aβ(3-40) and pGlu11-Aβ(11-40). These posttranslationally modified peptides arrange in a similar fashion as observed for wild type Aβ(1-40).

We studied the influence of globotriaosylceramide (Gb3) lipid molecules on the properties of phospholipid membranes composed of a liquid ordered (lo)/liquid disordered (ld) phase separated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/N-palmitoyl-d-erythro-sphingosylphosphorylcholine (PSM)/cholesterol mixture (40/35/20, mol/mol/mol) supplemented with 5 mol% of either short acyl chain palmitoyl-Gb3 or long acyl chain lignoceryl-Gb3 using 2H solid-state NMR spectroscopy. To this end, both globotriaosylceramides were chemically synthesized featuring a perdeuterated lipid acyl chain. The solid-state 2H NMR spectra support the phase separation into a POPC-rich ld phase and a PSM/cholesterol-rich lo phase. The long chain lignoceryl-Gb3 showed a rather unusual order parameter profile of the acyl chain, which flattens out for the last ∼6 methylene segments. Such an odd chain conformation can be explained by partial chain interdigitation and/or a very fluid midplane region of the membrane. Possibly, the Gb3 molecules may thus preferentially be localized at the lo/ld phase boundary. In contrast, the short chain palmitoyl-Gb3 was well associated with the PSM/cholesterol-rich lo phase. Gb3 molecules act as membrane receptors for the Shiga toxin (STx) produced by Shigella dysenteriae and by enterohemorrhagic strains of Escherichia coli (EHEC). The B-subunits of STx (STxB) forming a pentameric structure were produced recombinantly and incubated with the membrane mixtures leading to alterations in the lipid packing properties and lateral organization of the membranes. Typically, STxB binding led to a decrease in lipid chain order in agreement with partial immersion of protein segments into the lipid-water interface of the membrane. In the presence of STxB, Gb3 preferentially partitioned into the lo membrane phase. In particular the short acyl chain palmitoyl-Gb3 showed very similar chain order parameters to PSM. In the presence of STxB, all lipid species showed isotropic contributions to the 2H NMR powder spectra; this was most pronounced for the Gb3 molecules. Such isotropic contributions are caused by highly curved membrane structures, which have previously been detected as membrane invaginations in fluorescence microscopy. Our analysis estimated that STxB induced highly curved membrane structures with a curvature radius of less than ∼10 nm likely related to the insertion of STxB segments into the lipid-water interface of the membrane. peerReviewed

Tubular structures built from amphiphilic molecules are of interest for nano-sensing, drug delivery, and structuring of oils. In this study, we characterized the tubules built in aqueous suspensions of a cholesteryl nucleoside conjugate, cholesterylaminouridine (CholAU) and phosphatidylcholines (PCs). In mixtures with unsaturated PCs having chain lengths comparable to the length of CholAU, two different types of tubular structures were observed; nano- and micro-tubules had average diameters in the ranges 50–300 nm and 2–3 μm, respectively. Using cryo scanning electron microscopy (cryo-SEM) we found that nano- and micro-tubules differed in their morphology: the nano-tubules were densely packed, whereas micro-tubules consisted of loosely rolled undulated lamellas. Atomic force microscopy (AFM) revealed that the nano-tubules were built from 4 to 5 nm thick CholAU-rich bilayers, which were in the crystalline state. Solid-state 2H NMR spectroscopy also confirmed that about 25% of the total CholAU, being about the fraction of CholAU composing the tubules, formed the rigid crystalline phase. We found that CholAU/PC tubules can be functionalized by molecules inserted into lipid bilayers and fluorescently labeled PCs and lipophilic nucleic acids inserted spontaneously into the outer layer of the tubules. The tubular structures could be loaded and cross-linked, e.g. by DNA hybrids, and, therefore, are of interest for further development, e.g. as a depot scaffold for tissue regeneration.

Tau is a microtubule-associated protein and is involved in microtubule assembly and stabilization. It consists of four repeats that bind to the microtubule. The ΔK280 deletion mutation in the tau R2 repeat region is directly associated with the development of the frontotemporal dementia parkinsonism linked to chromosome 17 (FTDP-17). This deletion mutation is known to accelerate tau R2 repeat aggregation. However, the secondary and the tertiary structures of the self-assembled ΔK280 tau R2 repeat mutant aggregates are still controversial. Moreover, it is unclear whether extensions by one residue in the N- or the C-terminus of this mutant can influence the secondary or the tertiary structure. Herein, we combine solid-state NMR, atomic force microscopy, electron microscopy and all-atom explicit molecular dynamics simulations to investigate the effects of the deletion mutation and the N- and the C-terminal extension of this mutant on the structure. Our main findings show that the deletion mutation induces the formation of small aggregates, such as oligomers, and reduces the formation of fibrils. However, the extensions in the N- or the C-terminus revealed more fibril formation than small aggregates. Further, in the deletion mutation only one structure is preferred, while the N- and the C-terminal extensions strongly lead to polymorphic states. Finally, our broad and combined experimental and computational techniques provide direct structural information regarding ΔK280 tau R2 repeat mutant aggregates and their extensions in the N- and C-terminii by one residue.

We investigated the lamellar structure of ternary stratum corneum (SC) lipid model systems based on either the phytosphingosine type ceramide (CER) [NP] or CER[AP], supplemented by cholesterol and stearic acid as representative free fatty acid species. For the CER[NP] based membrane, neutron diffraction measurements revealed the coexistence of two lamellar phases, which markedly differ in their hydration properties. CER[NP] forms an extremely rigid and stable bilayer backbone and is at least partly sequestered in a separate phase which coexists with a second lamellar phase. At increased temperature, a structural re-organization of the lipids was observed. One of the lamellar phases disappeared, while the remaining phase increased its repeat distance by about 1 A. Such a behaviour has not been described for SC lipid model membranes based on CER[AP] so far. Further, 2H NMR spectroscopic measurements on two SC lipid model systems based on either CER[NP] or CER[AP] in addition to cholesterol and perdeuterated stearic acid revealed a state of high lamellar order present in both samples, emphasizing the importance of the phytosphingosine-type ceramides for the proper formation of stable SC bilayer structures. However, the CER[NP] based ternary model showed a state of higher lamellar order than the CER[AP] based system. Our results demonstrate that slight changes in the ceramides' head groups (CER[NP] with 3 hydroxyl groups vs. CER[AP] with 4 hydroxyl groups) have a dramatic influence on the morphology of the lipid structures formed by these lipids.