Electrostatically driven lipid-lysozyme mixed fibers display a multilamellar structure without amyloid features

The Association of Lipids with Amyloid Fibrils

Amyloid formation continues to be a widely studied area because of its association with numerous diseases, such as Alzheimer’s and Parkinson’s diseases. Despite a large body of work on protein aggregation and fibril formation, there are still significant gaps in our understanding of the factors that differentiate toxic amyloid formation in vivo from alternative misfolding pathways. In addition to proteins, amyloid fibrils are often associated in their cellular context with several types of molecule, including carbohydrates, polyanions, and lipids. This review focuses in particular on evidence for the presence of lipids in amyloid fibrils and the routes by which those lipids may become incorporated. Chemical analyses of fibril composition, combined with studies to probe the lipid distribution around fibrils, provide evidence that in some cases, lipids have a strong association with fibrils. In addition, amyloid fibrils formed in the presence of lipids have distinct morphologies and material properties. It is argued that lipids are an integral part of many amyloid deposits in vivo, where their presence has the potential to influence the nucleation, morphology, and mechanical properties of fibrils. The role of lipids in these structures is therefore worthy of further study.

Lipid Hydroperoxide Compromises the Membrane Structure Organization and Softens Bending Rigidity

Lipid hydroperoxides are key mediators of diseases and cell death. In this work, the structural and dynamic perturbations induced by the hydroperoxidized POPC lipid (POPC-OOH) in fluid POPC membranes, at both 23 and 37 °C, were addressed using advanced small-angle X-ray scattering (SAXS) and fluorescence methodologies. Notably, SAXS reveals that the hydroperoxide group decreases the lipid bilayer bending rigidity. This alteration disfavors the bilayer stacking and increases the swelling in-between stacked bilayers. We further investigated the changes in the apolar/polar interface of hydroperoxide-containing membranes through time-resolved fluorescence/anisotropy experiments of the probe TMA-DPH and time-dependent fluorescence shifts of Laurdan. A shorter mean fluorescence lifetime for TMA-DPH was obtained in enriched POPC-OOH membranes, revealing a higher degree of hydration near the membrane interface. Moreover, a higher microviscosity near TMA-DPH and lower order are predicted for these oxidized membranes, at variance with the usual trend of variation of these two parameters. Finally, the complex relaxation process of Laurdan in pure POPC-OOH membranes also indicates a higher membrane hydration and viscosity in the close vicinity of the -OOH moiety. Altogether, our combined approach reveals that the hydroperoxide group promotes alterations in the membrane structure organization, namely, at the level of membrane order, viscosity, and bending rigidity.

Liposomes modified with cardiolipin can act as a platform to regulate the potential flux of NADP+-dependent isocitrate dehydrogenase

Cardiolipin (CL) is a phospholipid found in the outer mitochondrial membrane (OMM) and inner mitochondrial membrane (IMM) in animal cells. Isocitrate dehydrogenase (ICDH) is an important catalytic enzyme that is localized at the cytosol and mitochondria; the metabolic pathway catalyzed by ICDH differs between the OMM and IMM. To estimate the possible role of lipid membrane in the enzymatic activity of NADP⁺-dependent ICDH, CL-modified liposomes were prepared using CL/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/cholesterol (Ch), and their characteristics were analyzed based on the fluorescent probe method. The relative enzymatic activity of ICDH decreased in the presence of CL/DPPC/Ch=(30/50/20) liposome, whereas activity increased in the presence of CL/DPPC/Ch=(5/75/20) liposome. NADP⁺ had the greatest substrate affinity and was dominant in the regulation of ICDH activity. Analysis of membrane properties indicated that membranes in CL-modified liposomes were dehydrated by ICDH binding. Using circular dichroism analysis, CL/DPPC/Ch=(30/50/20) liposome induced a conformational change in ICDH, indicating that CL-rich membrane domains could inhibit ICDH activity. These results suggest that lipid membranes, including CL molecules, could act as a platform to regulate ICDH-related metabolic pathways such as the tricarboxylic acid cycle and lipid synthesis.

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Phosphatidylcholine liposomes were modified with cardiolipin and characterized.

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DPPC liposomes did not affect the activity of ICDH.

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ICDH activity was enhanced with liposomes at 5 mol% cardiolipin.

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ICDH activity was lowered with liposomes at 30 mol% cardiolipin.

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Liposomes with high content of cardiolipin led to conformational changes of ICDH.

Exploring homo-FRET to quantify the oligomer stoichiometry of membrane-bound proteins involved in a cooperative partition equilibrium

The establishment of protein-protein interactions between membrane-bound proteins is associated with several biological functions and dysfunctions. Here, an analytical framework that uses energy homo transfer to directly probe quantitatively the oligomerization state of membrane-bound proteins engaged in a three-state cooperative partition is presented. Briefly, this model assumes that monomeric protein molecules partition into the bilayer surface and reversibly assemble into oligomers with k subunits. A general equation relating the overall steady-state fluorescence anisotropy of the sample to its fractional labeling was derived by considering explicitly that the anisotropy of mixed oligomers containing i-labeled monomers is inversely proportional to the number of labeled subunits per oligomer (Runnels and Scarlata limit). This method was very robust in describing the electrostatic interaction of Alexa Fluor 488 fluorescently labeled lysozyme (Lz-A488) with phosphatidylserine-containing membranes. The pronounced decrease detected in the fluorescence anisotropy of Lz-A488 always correlated with the system reaching a high membrane surface density of the protein (at a low lipid-to-protein (L/P) molar ratio). The occurrence of energy homo transfer-induced fluorescence depolarization was further confirmed by measuring the anisotropy decays of Lz-A488 under these conditions. A global analysis of the steady-state anisotropy data obtained under a wide range of experimental conditions (variable anionic lipid content of the liposomes, L/P molar ratios and protein fractional labeling) confirmed that membrane-bound Lz-A488 assembled into oligomeric complexes, possibly with a stoichiometry of k = 6 ± 1. This study illustrates that even in the presence of a coupled partition-oligomerization equilibrium, steady-state anisotropy measurements provide a simple and reliable tool to monitor the self-assembly of membrane-bound proteins.

Electrostatically driven lipid-protein interaction: Answers from FRET

Electrostatics govern the association of a large number of proteins with cellular membranes. In some cases, these proteins present specialized lipid-binding modules or membrane targeting domains while in other cases association is achieved through nonspecific interaction of unstructured clusters of basic residues with negatively charged lipids. Given its spatial resolution in the nanometer range, Förster resonance energy transfer (FRET) is a powerful tool to give insight into protein-lipid interactions and provide molecular level information which is difficult to retrieve with other spectroscopic techniques. In this review we present and discuss the basic formalisms of both hetero- and homo-FRET pertinent to the most commonly encountered problems in lipid-protein interaction studies and highlight some examples of implementations of different FRET methodologies to characterize lipid/protein systems in which electrostatic interactions play a crucial role. This article is part of a Special Issue entitled: Lipid-protein interactions.

Protein/lipid coaggregates are formed during α-synuclein-induced disruption of lipid bilayers

Amyloid formation is associated with neurodegenerative diseases such as Parkinson's disease (PD). Significant α-synuclein (αSN) deposition in lipid-rich Lewy bodies is a hallmark of PD. Nonetheless, an unraveling of the connection between neurodegeneration and amyloid fibrils, including the molecular mechanisms behind potential amyloid-mediated toxic effects, is still missing. Interaction between amyloid aggregates and the lipid cell membrane is expected to play a key role in the disease progress. Here, we present experimental data based on hybrid analysis of two-photon-microscopy, solution small-angle X-ray scattering and circular dichroism data. Data show in real time changes in liposome morphology and stability upon protein addition and reveal that membrane disruption mediated by amyloidogenic αSN is associated with dehydration of anionic lipid membranes and stimulation of protein secondary structure. As a result of membrane fragmentation, soluble αSN:-lipid coaggregates are formed, hence, suggesting a novel molecular mechanism behind PD amyloid cytotoxicity.