Influence of Free Fatty Acids on Lipid Membrane-Nisin Interaction

Synergistic antimicrobial effect of nisin-octanoic acid nanoemulsions against E. coli and S. aureus

Food safety is a major public health concern all over the world. Therefore, the prevention of food contamination is becoming extremely crucial. In this study, an antimicrobial nanoemulsion composed of water-soluble nisin and fat-soluble octanoic acid was successfully prepared. The results showed that the average particle size and the polymer dispersity index of the nisin-octanoic acid (NOA) nanoemulsion were around 52.21 nm and 0.253, respectively. The NOA nanoemulsion required less amounts of nisin and octanoic acid to achieve the effective antimicrobial effect against Escherichia coli and Staphylococcus aureus. In addition, the growth curves of E. coli and S. aureus were determined. The OD₆₀₀ of NOA nanoemulsion was significantly lower than free nisin after being incubated for 24 h (p < 0.001), indicating that the antimicrobial effect of NOA nanoemulsion was outstanding. Meanwhile, the synergistic antimicrobial property of NOA nanoemulsion against E. coli and S. aureus was significantly better than free nisin under nonacid conditions (p < 0.05). Overall, the results of this study may provide guidance for the further application of nisin in more forms.

Conformation and Orientation of Antimicrobial Peptides MSI-594 and MSI-594A in a Lipid Membrane

There is significant interest in the development of antimicrobial compounds to overcome the increasing bacterial resistance to conventional antibiotics. Studies have shown that naturally occurring and de novo-designed antimicrobial peptides could be promising candidates. MSI-594 is a synthetic linear, cationic peptide that has been reported to exhibit a broad spectrum of antimicrobial activities. Investigation into how MSI-594 disrupts the cell membrane is important for better understanding the details of this antimicrobial peptide (AMP)'s action against bacterial cells. In this study, we used two different synthetic lipid bilayers: zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and anionic 7:3 POPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho(1'-rac-glycerol) (POPG). Sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) were used to determine the orientations of MSI-594 and its analogue MSI-594A associated with zwitterionic POPC and anionic 7:3 POPC/POPG lipid bilayers. The simulated ATR-FTIR and SFG spectra using nuclear magnetic resonance (NMR)-determined structures were compared with experimental spectra to optimize the bent angle between the N- (1-11) and C- (12-24) termini helices and the membrane orientations of the helices; since the NMR structure of the peptide was determined from lipopolysaccharide (LPS) micelles, the optimization was needed to find the most suitable conformation and orientation in lipid bilayers. The reported experimental results indicate that the optimized MSI-594 helical hairpin structure adopts a complete lipid bilayer surface-bound orientation (denoted "face-on") in both POPC and 7:3 POPC/POPG lipid bilayers. The analogue peptide, MSI-584A, on the other hand, exhibited a larger bent angle between the N- (1-11) and C- (12-24) termini helices with the hydrophobic C-terminal helix inserted into the hydrophobic region of the bilayer (denoted "membrane-inserted") when interacting with both POPC and 7:3 POPC/POPG lipid bilayers. These experimental findings on the membrane orientations suggest that both peptides are likely to disrupt the cell membrane through the carpet mechanism.

Linoleic acid improves PIEZO2 dysfunction in a mouse model of Angelman Syndrome

Angelman syndrome (AS) is a neurogenetic disorder characterized by intellectual disability and atypical behaviors. AS results from loss of expression of the E3 ubiquitin-protein ligase UBE3A from the maternal allele in neurons. Individuals with AS display impaired coordination, poor balance, and gait ataxia. PIEZO2 is a mechanosensitive ion channel essential for coordination and balance. Here, we report that PIEZO2 activity is reduced in Ube3a deficient male and female mouse sensory neurons, a human Merkel cell carcinoma cell line and female human iPSC-derived sensory neurons with UBE3A knock-down, and de-identified stem cell-derived neurons from individuals with AS. We find that loss of UBE3A decreases actin filaments and reduces PIEZO2 expression and function. A linoleic acid (LA)-enriched diet increases PIEZO2 activity, mechano-excitability, and improves gait in male AS mice. Finally, LA supplementation increases PIEZO2 function in stem cell-derived neurons from individuals with AS. We propose a mechanism whereby loss of UBE3A expression reduces PIEZO2 function and identified a fatty acid that enhances channel activity and ameliorates AS-associated mechano-sensory deficits.

Effect of polyunsaturated free fatty acids on the membrane fusion mechanism

Membrane fusion is one of the important processes for the survival of eukaryotic cells and the entry of enveloped viruses into the host cells. Lipid composition plays a crucial role by modulating the organization and dynamics of the membrane, as well as the structure and conformation of membrane proteins. The diversity of the lipid acyl chain in its length and degree of unsaturation originates from the variation in free fatty acids (FFAs). We have studied the effect of linoleic (LA) and alpha-linolenic (ALA) acids on the depth-dependent organization, dynamics, and fusion of DOPC/DOPE (70/30 mol%) membranes utilizing steady-state and time-resolved fluorescence spectroscopic methods. Our results suggest that membranes with 5 mol% LA stabilize the stalk-intermediate and promote lipid mixing at the early stage of the process, i.e., the fusion follows the classical stalk model. Conversely, the extents of lipid and content mixing at the stalk intermediate are similar in the presence of 5 mol% of ALA, indicating the fusion mechanism as a nonclassical one like in the DOPC/DOPE (70/30 mol%) membranes. Our results provide an in-depth insight into the effect of the increasing degree of fatty acid tail unsaturation on membrane organization and dynamics and their impact on the membrane fusion mechanism.

Depth-Dependent Segmental Melting of the Sphingomyelin Alkyl Chain in Lipid Bilayers

The chain melting of lipid bilayers has often been investigated in detail using calorimetric methods, such as differential scanning calorimetry (DSC), and the resultant main transition temperature is regarded as one of the most important parameters in model membrane experiments. However, it is not always clear whether the hydrocarbon chains of lipids are gradually melting along the depth of the lipid bilayer or whether they all melt concurrently in a very narrow temperature range, as implied by DSC. In this study, we focused on stearoyl-d-sphingomyelin (SSM) as an example of raft-forming lipids. We synthesized deuterium-labeled SSMs at the 4', 10', and 16' positions, and their depth-dependent melting was measured using solid-state deuterium NMR by changing the temperature by 1.0 °C, and comparing with that observed from a saturated lipid, palmitoylstearoylphosphatidylcholine (PSPC). The results showed that SSM exhibited a characteristic depth-dependent melting, which was not observed for PSPC. The strong intermolecular hydrogen bonds between the sphingomyelin amide moiety probably caused the chain melting to start from the chain terminus through the middle part and end in the upper part. This depth-dependent melting implies that the small gel-like domains of SSM remain at temperatures slightly above the main transition temperature. These sphingomyelin features may be responsible for the biological properties of SM-based lipid rafts.

A unifying framework for amyloid-mediated membrane damage: The lipid-chaperone hypothesis

Over the past thirty years, researchers have highlighted the role played by a class of proteins or polypeptides that forms pathogenic amyloid aggregates in vivo, including i) the amyloid Aβ peptide, which is known to form senile plaques in Alzheimer's disease; ii) α-synuclein, responsible for Lewy body formation in Parkinson's disease and iii) IAPP, which is the protein component of type 2 diabetes-associated islet amyloids. These proteins, known as intrinsically disordered proteins (IDPs), are present as highly dynamic conformational ensembles. IDPs can partially (mis) fold into (dys) functional conformations and accumulate as amyloid aggregates upon interaction with other cytosolic partners such as proteins or lipid membranes. In addition, an increasing number of reports link the toxicity of amyloid proteins to their harmful effects on membrane integrity. Still, the molecular mechanism underlying the amyloidogenic proteins transfer from the aqueous environment to the hydrocarbon core of the membrane is poorly understood. This review starts with a historical overview of the toxicity models of amyloidogenic proteins to contextualize the more recent lipid-chaperone hypothesis. Then, we report the early molecular-level events in the aggregation and ion-channel pore formation of Aβ, IAPP, and α-synuclein interacting with model membranes, emphasizing the complexity of these processes due to their different spatial-temporal resolutions. Next, we underline the need for a combined experimental and computational approach, focusing on the strengths and weaknesses of the most commonly used techniques. Finally, the last two chapters highlight the crucial role of lipid-protein complexes as molecular switches among ion-channel-like formation, detergent-like, and fibril formation mechanisms and their implication in fighting amyloidogenic diseases.

Grapevine stilbenoids as natural food preservatives: calorimetric and spectroscopic insights into the interaction with model cell membranes

Food contamination with pathogenic microorganisms, such as Listeria monocytogenes, Salmonella enterica, Staphylococcus aureus and Bacillus cereus, is a common health concern. Natural products, which have been the main source of antimicrobials for centuries, may represent a turning point in alleviating the antibiotic crisis, and plant polyphenolic compounds are considered a promising source for new antibacterial agents. Resveratrol and resveratrol-derived monomers and oligomers (stilbenoids) have been shown to exert a variegated pattern of efficacy as antimicrobials depending on both the polyphenols' structure and the nature of the microorganisms, and the bacterial cell membrane seems to be one of their primary targets.In this scenario and based on the thermodynamic information reported in the literature about cell membranes, this study aimed at the investigation of the direct interaction of selected stilbenoids with a simple but informative model cell membrane. Three complete stilbenoid "monomer/dimer/dehydro-dimer" sets were chosen according to different geometries and substitution patterns. Micro-DSC was performed on 2 : 3 DPPC : DSPC small unilamellar vesicles with incorporated polyphenols at physiological pH and the results were integrated using complementary NMR data. The study highlighted the molecular determinants and mechanisms involved in the stilbenoid-membrane interaction, and the results were well correlated with the microbiological evidence previously assessed.

Clustering of Stearic Acids in Model Phospholipid Membranes Revealed by Double Electron-Electron Resonance

Free fatty acids play various important roles in biological membranes. Double electron-electron resonance spectroscopy (DEER, also known as PELDOR) of spin-labeled biomolecules is capable of studying magnetic dipole-dipole (d-d) interactions between spin labels at the nanoscale range of distances. Here, DEER is applied to study intermolecular d-d interactions between doxyl-spin-labeled stearic acids (DSA) in gel-phase phospholipid bilayers composed either of an equimolecular mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine or of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. DEER data obtained for different DSA concentrations showed that DSA molecules at their concentration in the bilayer χ larger than 0.5 mol % are assembled into lateral lipid-mediated clusters, with a characteristic intermolecular distance of 2 nm. Some evidences were obtained indicating that clusters may consist of "subclusters", alternatively appearing in two opposite leaflets. Conventional electron paramagnetic resonance (EPR) spectra for the gel-phase bilayers showed that for χ larger than 2 mol % the molecules in the clusters stick together, forming oligomers. Room-temperature EPR spectra for the liquid-crystalline phase were found to change noticeably for χ larger than 0.5 mol %, which may indicate the clustering in a liquid-crystalline phase similar to that observed by DEER in the gel phase.