Effects of oligomeric lysozyme on structural state of model membranes

Lysozyme Interaction with Phospholipid Nanodroplets Probed by Sum Frequency Scattering Vibrational Spectroscopy

When a nanoparticle (NP) is introduced into a biological environment, its identity and interactions are immediately attributed to the dense layer of proteins that quickly covers the particle. The formation of this layer, dubbed the protein corona, is in general a combination of proteins interacting with the surface of the NP and a contest between other proteins for binding sites either at the surface of the NP or upon the dense layer. Despite the importance for surface engineering and drug development, the molecular mechanisms and structure behind interfacial biomolecule action have largely remained elusive. We use ultrafast sum frequency scattering (SFS) spectroscopy to determine the structure and the mode of action by which these biomolecules interact with and manipulate interfaces. The majority of work in the field of sum frequency generation has been done on flat model interfaces. This limits some important membrane properties such as membrane fluidity and dimensionality─important factors in biomolecule-membrane interactions. To move toward three-dimensional (3D) nanoscopic interfaces, we utilize SFS spectroscopy to interrogate the surface of 3D lipid monolayers, which can be used as a model lipid-based nanocarrier system. In this study, we have utilized SFS spectroscopy to follow the action of lysozyme. SFS spectra in the amide I region suggest that there is lysozyme at the interface and that the lysozyme induces an increased lipid monolayer order. The binding of lysozyme with the NP is demonstrated by an increase in acyl chain order determined by the ratio of the CH₃ symmetric and CH₂ symmetric peak amplitudes. Furthermore, the lipid headgroup orientation s-PO₂^- change strongly supports lysozyme insertion into the lipid layer causing lipid disruption and reorientation. Altogether, with SFS, we have made a huge stride toward understanding the binding and structure change of proteins within the protein corona.

Direct matrix-assisted laser desorption ionization time-of-flight mass spectrometric analysis of lysozyme contained in hen egg white

As a natural antibacterial peptide, lysozyme (LZ) is widely used in medicine and the food industry. Despite many years of research on this compound, its new antibacterial properties are still to be determined. The primary aim of this work is to demonstrate the application of the matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometric analysis of LZ directly in hen egg white samples without extraction thereof. The egg white samples were kept over 10 weeks at room temperature and measured every week. The resulting positive and negative ion mass spectra were then compared to determine the intensity of the LZ mass peak. Storage of the egg white for over 10 weeks did not influence the LZ mass peak intensity (both positive and negative). It can be concluded that the LZ concentration in the egg white samples did not vary with time. The effect of the matrix/sample ratio on LZ detection was also examined, and it was found to be different in the case of positive and negative ionization. The mass peaks of LZ oligomeric forms were observed in all mass spectra, so the MALDI method could be used in subsequent studies.

Interactions of Lipid Membranes with Fibrillar Protein Aggregates

Amyloid fibrils are an intriguing class of protein aggregates with distinct physicochemical, structural and morphological properties. They display peculiar membrane-binding behavior, thus adding complexity to the problem of protein-lipid interactions. The consensus that emerged during the past decade is that amyloid cytotoxicity arises from a continuum of cross-β-sheet assemblies including mature fibrils. Based on literature survey and our own data, in this chapter we address several aspects of fibril-lipid interactions, including (i) the effects of amyloid assemblies on molecular organization of lipid bilayer; (ii) competition between fibrillar and monomeric membrane-associating proteins for binding to the lipid surface; and (iii) the effects of lipids on the structural morphology of fibrillar aggregates. To illustrate some of the processes occurring in fibril-lipid systems, we present and analyze fluorescence data reporting on lipid bilayer interactions with fibrillar lysozyme and with the N-terminal 83-residue fragment of amyloidogenic mutant apolipoprotein A-I, 1-83/G26R/W@8. The results help understand possible mechanisms of interaction and mutual remodeling of amyloid fibers and lipid membranes, which may contribute to amyloid cytotoxicity.

Lysozyme, a mediator of sepsis that deposits in the systemic vasculature and kidney as a possible mechanism of acute organ dysfunction

In septic shock (SS), dysfunction of many organ systems develops during the course of the illness, although the mechanisms are not clear. In earlier studies, we reported that lysozyme-c (Lzm-S), a protein that is released from leukocytes and macrophages, was a mediator of the myocardial depression and vasodilation that develop in a canine model of Pseudomonas aeruginosa SS. Whereas both of these effects of Lzm-S are dependent on its ability to intrinsically generate hydrogen peroxide, we subsequently showed that Lzm-S can also deposit within the vascular smooth muscle layer of the systemic arteries in this model. In the present study, we extend our previous findings. We used a canine carotid artery organ bath preparation to study the time course and dose dependence of Lzm-S deposition within the vascular smooth muscle layer. We used a human aortic vascular smooth muscle cell preparation to determine whether Lzm-S can persistently inhibit contraction in this preparation. We also used a canine P. aeruginosa model to determine whether Lzm-S deposition might occur in other organs such as the kidney, liver, and small intestine. The results showed that, in the carotid artery organ bath preparation, Lzm-S deposition occurred within minutes of instillation and there was a dose-response effect. In the human aortic vascular smooth muscle cell preparation, Lzm-S inhibited contraction during a 4-day period. In the in vivo model, Lzm-S accumulated in the kidney and the superior mesenteric artery. In a canine renal epithelial preparation, we further showed that Lzm-S can be taken up by the renal tubules to activate inflammatory pathways. We conclude that Lzm-S can deposit in the systemic vasculature and kidneys in SS, where this deposition could lead to acute organ dysfunction.

Formation of lysozyme oligomers at model cell membranes monitored with sum frequency generation spectroscopy

A growing number of studies suggest that the formation of toxic oligomers, precursors of amyloid fibrils, is initiated at the cell membrane and not in the cytosolic compartments of the cell. Studies of membrane-induced protein oligomerization are challenging due to the difficulties of probing small numbers of proteins present at membrane surfaces. Here, we employ surface-sensitive vibrational sum frequency generation (VSFG) to investigate the secondary structure of lysozyme at the surface of lipid monolayers. We investigate lysozyme aggregation at negatively charged 1,2-dipalmitoyl-sn-glycero-3-(phospho-rac-1-glycerol) (DPPG) lipid monolayers under different pH conditions. The changes in the molecular vibrations of lipids, proteins, and water as a function of pH and surface pressure allow us to simultaneously monitor details of the conformation state of lysozyme, the organization of lipids, and the state of lipid-bound water. At pH = 6 lysozyme induces significant disordering of the lipid layer, and it exists in two states: a monomeric state with a predominantly α-helix content and an oligomeric (za-mer) state. At pH ≤ 3, all membrane-bound lysozyme self-associates into oligomers characterized by an antiparallel β-sheet structure. This is different from the situation in bulk solution, for which circular dichroism (CD) shows that the protein maintains an α-helix conformation, under both neutral and acidic pH conditions. The transition from monomers to oligomers is also associated with a decreased hydration of the lipid monolayer resulting in an increase of the lipid acyl chains ordering. The results indicate that oligomerization requires cooperative action between lysozyme incorporated into the lipid membrane and peripherally adsorbed lysozyme and is associated with the membrane dehydration and lipid reorganization. Membrane-bound oligomers with antiparallel β-sheet structure are found to destabilize lipid membranes.

Development of a label-free aptasensor for monitoring the self-association of lysozyme

A novel aptamer and surface plasmon resonance (SPR)-based sensor was developed for the label-free detection of lysozyme. The aptasensor is characterised by a detection limit of 1 μg mL(-1) and a linear range of 5-50 μg mL(-1). As an application, we examined the usefulness of the aptasensor for monitoring the early stages of the aggregation of lysozyme. It was surprisingly found that, despite a significant decrease in monomer content during aggregation, the response of the aptasensor for protein solutions aged for 12 hours was similar to that for the fresh protein. To correlate the results obtained with the aptasensor with the composition of lysozyme solutions at various time points, we examined them in detail by atomic force microscopy (AFM), thioflavin T fluorescence, size-exclusion chromatography (SEC) and Matrix Assisted Laser Desorption Ionisation Time of Flight Mass Spectrometry (MALDI-TOF-MS). All methods together indicated that during the initial hours of aggregation, the protein solutions contained small lysozyme oligomers (mainly dimers) and decreasing amounts of monomers. Our results thus suggest that the aptamer also recognizes lysozyme dimers/oligomers. A higher non-specific binding was observed for the aggregated lysozyme at the surface of the aptasensor as compared to the native protein. This was attributed to the hydrophobic patches which are exposed by the unfolded lysozyme and/or oligomer species, allowing for different adsorption and organisation at the surface of the aptasensor. This hypothesis is supported by square wave voltammetry (SWV) studies using solutions of aggregated lysozyme. A higher electrochemical signal due to the direct oxidation of tyrosine/tryptophan residues was observed for aged protein solutions as compared to the fresh solution, indicative of an increased number of such exposed electroactive residues and of overall increased surface hydrophobicity of the protein. Our work presents a label-free lysozyme aptasensor that is useful not only for the detection of the protein monomer but also for observing the onset of aggregation. The approach can be extended to other proteins which are prone to aggregation.

Fluorescence study on aggregated lysozyme and lipid bilayer interactions

Fluorescent probes 1,6-diphenyl-1,3,5-hexatriene (DPH), pyrene, 4-dimethylaminochalcone (DMC) and 4-p-(dimethylaminostyryl)-1-dodecylpyridinium (DSP-12) have been utilized to monitor the impact of lysozyme (Lz) oligomers on physicochemical properties of phosphatidylcholine/cardiolipin (PC/CL) membranes. Analysis of spectral responses of the employed probes revealed the reduction of membrane free volume and dehydration of lipid bilayer surface upon incorporation of Lz self-assemblies. Hydrophobic interactions were found to control the binding of Lz oligomers to the lipid bilayer. Comparison of the effects of Lz monomers, oligomers and fibrils showed that soluble oligomeric intermediates exert the most destructive influence on membrane properties.

Modulation of physiological and pathological activities of lysozyme by biological membranes

The molecular details of interactions between lipid membranes and lysozyme (Lz), a small polycationic protein with a wide range of biological activities, have long been the focus of numerous studies. The biological consequences of this process are considered to embrace at least two aspects: i) correlation between antimicrobial and membranotropic properties of this protein, and ii) lipid-mediated Lz amyloidogenesis. The mechanisms underlying the lipid-assisted protein fibrillogenesis and membrane disruption exerted by Lz in bacterial cells are believed to be similar. The present investigation was undertaken to gain further insight into Lz-lipid interactions and explore the routes by which Lz exerts its antimicrobial and amyloidogenic actions. Binding and Förster resonance energy transfer studies revealed that upon increasing the content of anionic lipids in lipid vesicles, Lz forms aggregates in a membrane environment. Total internal reflection fluorescence microscopy and pyrene excimerization reaction were employed to study the effect of Lz on the structural and dynamic properties of lipid bilayers. It was found that Lz induces lipid demixing and reduction of bilayer free volume, the magnitude of this effect being much more pronounced for oligomeric protein.