Biophysical Insights into the Role of Amyloid-Beta Misfolding in Alzheimer’s Disease Pathogenesis.

Amyloids are protein aggregates that build up as plaques in various tissues in the body and are associated with a number of diseases. Of the amyloidoses, Alzheimer’s disease (AD) is the most known and socially distressing. Amyloid-beta (Abeta) is the amyloidogenic protein associated with AD and is implicated in the etiology of the disease. Abeta aggregation is highly heterogeneuos, giving rise to a number of possible aggregation pathways and intermediate oligomeric structures. The mechanism of Abeta aggregation was studied here in the presence and absence of a model cell membranes employing fluorescence spectrosopy, light scattering, atomic force microscopy, and NMR spectroscopy. First, Abeta aggregation is investigated in the presence of a lipid bilayer, exploring the particular role of lipid composition on the mechanism of membrane disruption. It was shown that membrane disruption by Abeta occurs by a two-step process: (i) intial formation of ion-selective pores followed by (ii) non-specific fragmentation of the lipid membrane during amyloid fiber formation. Moreover, the presence of gangliosides enhances pore formation and is necessary for fiber-dependent membrane fragmentation. Next, magic angle spinning (MAS) NMR is used to gain structural insights on an Abeta oligomer, providing atomic-level characterization on a non-fibrillar product of Abeta. Importantly, it is demonstrated that MAS NMR and 1H-1H dipolar interactions can be used as a spectral filter to detect Abeta oligomers without a purification procedure. In comparison to other solid-state NMR techniques, the experiment is extraordinarily selective and sensitive, as it can resolve spectra on a small population of oligomers (7% of the total Abeta concentration). Using this method, it was shown that a stable, primarily disordered Abeta oligomer forms and coexists with amyloid fibers. Finally, a real-time 2D NMR method is implemented to study the mechanism of Abeta fiber elongation. It is demonstrated that monomeric Abeta undergoes a conformational conversion after binding to the fiber surface to complete the elongation step, with the strongest interaction occurring in the central region of the peptide (residues Phe19- Glu22). To our knowledge, this is the first high-resolution account of the fiber elongation process and provides residue-specific details of amyloid fiber polymorphism.PhDBiophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116684/1/kotlesam_1.pd

Corporate image: a service recovery perspective

This article explores direct and indirect antecedents that contribute to corporate image formation in a service recovery context. Two studies were carried out in Egypt. Study 1 comprises 29 semistructured interviews with complainants of mobile phone network service providers in Egypt. Study 2 encompasses a mail survey of another 437 complainants. Findings reveal the importance of perceived justice, namely, interactional justice, in corporate image formation, as well as the mediating role of satisfaction with service recovery in the perceived justice-corporate image relationship. Results also reveal two empirical relationships: problem solving as a determinant of distributive justice and follow-up as a driver of procedural justice. Accordingly, this study contributes to the service field by providing the first empirical evaluation of new direct and indirect antecedents of corporate image formation in a service recovery context. Managerial recommendations are provided that encourage service practitioners to emphasize perceived justice and satisfaction with a service recovery process to enhance the company’s image. Additionally, companies should invest in implementing problem solving and follow-up as service recovery strategies since both significantly enhance perceived justice

All-d-Enantiomer of β-Amyloid Peptide Forms Ion Channels in Lipid Bilayers

Alzheimer’s disease (AD) is the most common type of senile dementia in aging populations. Amyloid β (Aβ)-mediated dysregulation of ionic homeostasis is the prevailing underlying mechanism leading to synaptic degeneration and neuronal death. Aβ-dependent ionic dysregulation most likely occurs either directly via unregulated ionic transport through the membrane or indirectly via Aβ binding to cell membrane receptors and subsequent opening of existing ion channels or transporters. Receptor binding is expected to involve a high degree of stereospecificity. Here, we investigated whether an Aβ peptide enantiomer, whose entire sequence consists of d-amino acids, can form ion-conducting channels; these channels can directly mediate Aβ effects even in the absence of receptor–peptide interactions. Using complementary approaches of planar lipid bilayer (PLB) electrophysiological recordings and molecular dynamics (MD) simulations, we show that the d-Aβ isomer exhibits ion conductance behavior in the bilayer indistinguishable from that described earlier for the l-Aβ isomer. The d isomer forms channel-like pores with heterogeneous ionic conductance similar to the l-Aβ isomer channels, and the d-isomer channel conductance is blocked by Zn2+, a known blocker of l-Aβ isomer channels. MD simulations further verify formation of β-barrel-like Aβ channels with d- and l-isomers, illustrating that both d- and l-Aβ barrels can conduct cations. The calculated values of the single-channel conductance are approximately in the range of the experimental values. These findings are in agreement with amyloids forming Ca2+ leaking, unregulated channels in AD, and suggest that Aβ toxicity is mediated through a receptor-independent, nonstereoselective mechanism

Detergent-Type Membrane Fragmentation by MSI-78, MSI-367, MSI-594, and MSI-843 Antimicrobial Peptides and Inhibition by Cholesterol: A Solid-State Nuclear Magnetic Resonance Study

Multidrug resistance against the existing antibiotics is becoming a global threat, and any potential drug that can be designed using cationic antimicrobial peptides (AMP) could be an alternate solution to alleviate this existing problem. The mechanism of action of killing bacteria by an AMP differs drastically in comparison to that of small molecule antibiotics. The main target of AMPs is to interact with the lipid bilayer of the cell membrane and disrupt it to kill bacteria. Consequently, the modes of membrane interaction that lead to the selectivity of an AMP are very important to understand. Here, we have used different membrane compositions, such as negatively charged, zwitterionic, or mixed large unilamellar vesicles (LUVs), to study the interaction of four different synthetically designed cationic, linear antimicrobial peptides: MSI-78 (commercially known as pexiganan), MSI-367, MSI-594, and MSI-843. Our solid-state nuclear magnetic resonance (NMR) experiments confirmed that the MSI peptides fragmented LUVs through a detergent-like carpet mechanism depending on the amino acid sequence of the MSI peptide and/or the membrane composition of LUVs. Interestingly, the fragmented lipid aggregates such as SUVs or micelles are sufficiently small to produce an isotropic peak in the ³¹P NMR spectrum. These fragmented lipid aggregates contain only MSI peptides bestowed with lipid molecules as confirmed by NMR in conjunction with circular dichroism spectroscopy. Our results also demonstrate that cholesterol, which is present only in the eukaryotic cell membrane, inhibits the MSI-induced fragmentation of LUVs, suggesting that the MSI peptides can discriminate the bacteria and the eukaryotic cell membranes, and this selectivity could be used for further development of novel antibiotics

All-d-Enantiomer of β-Amyloid Peptide Forms Ion Channels in Lipid Bilayers.

Alzheimer's disease (AD) is the most common type of senile dementia in aging populations. Amyloid β (Aβ)-mediated dysregulation of ionic homeostasis is the prevailing underlying mechanism leading to synaptic degeneration and neuronal death. Aβ-dependent ionic dysregulation most likely occurs either directly via unregulated ionic transport through the membrane or indirectly via Aβ binding to cell membrane receptors and subsequent opening of existing ion channels or transporters. Receptor binding is expected to involve a high degree of stereospecificity. Here, we investigated whether an Aβ peptide enantiomer, whose entire sequence consists of d-amino acids, can form ion-conducting channels; these channels can directly mediate Aβ effects even in the absence of receptor-peptide interactions. Using complementary approaches of planar lipid bilayer (PLB) electrophysiological recordings and molecular dynamics (MD) simulations, we show that the d-Aβ isomer exhibits ion conductance behavior in the bilayer indistinguishable from that described earlier for the l-Aβ isomer. The d isomer forms channel-like pores with heterogeneous ionic conductance similar to the l-Aβ isomer channels, and the d-isomer channel conductance is blocked by Zn(2+), a known blocker of l-Aβ isomer channels. MD simulations further verify formation of β-barrel-like Aβ channels with d- and l-isomers, illustrating that both d- and l-Aβ barrels can conduct cations. The calculated values of the single-channel conductance are approximately in the range of the experimental values. These findings are in agreement with amyloids forming Ca(2+) leaking, unregulated channels in AD, and suggest that Aβ toxicity is mediated through a receptor-independent, nonstereoselective mechanism