Mechanical properties of milk sphingomyelin bilayer membranes in the gel phase: Effects of naturally complex heterogeneity, saturation and acyl chain length investigated on liposomes using AFM

Milk polar lipids composition and functionality: a systematic review

Polar lipids including glycerophospholipids and sphingophospholipids are important nutrients and milk is a major source, particularly for infants. This systematic review describes the human and bovine milk polar lipid composition, structural organization, sources for formulation, and physiological functionality. A total of 2840 records were retrieved through Scopus, 378 were included. Bovine milk is a good source of polar lipids, where yield and composition are highly dependent on the choice of dairy streams and processing. In milk, polar lipids are organized in the milk fat globule membrane as a tri-layer encapsulating triglyceride. The overall polar lipid concentration in human milk is dependent on many factors including lactational stage and maternal diet. Here, reasonable ranges were determined where possible. Similar for bovine milk, where differences in milk lipid concentration proved the largest factor determining variation. The role of milk polar lipids in human health has been demonstrated in several areas and critical review indicated that brain, immune and effects on lipid metabolism are best substantiated areas. Moreover, insights related to the milk fat globule membrane structure-function relation as well as superior activity of milk derived polar lipid compared to plant-derived sources are emerging areas of interest regarding future research and food innovations.

Dietary Sphingomyelin Metabolism and Roles in Gut Health and Cognitive Development

Sphingomyelin (SM) is a widely occurring sphingolipid that is a major plasma membrane constituent. Milk and dairy products are rich SM sources, and human milk has high SM content. Numerous studies have evaluated the roles of SM in maintaining cell membrane structure and cellular signal transduction. There has been a growing interest in exploring the role of dietary SM, especially from human milk, in imparting health benefits. This review focuses on recent publications regarding SM content in several dietary sources and dietary SM metabolism. SM digestion and absorption are slow and incomplete and mainly occur in the middle sections of the small intestine. This review also evaluates the effect of dietary SM on gut health and cognitive development. Studies indicate that SM may promote gut health by reducing intestinal cholesterol absorption in adults. However, there has been a lack of data supporting clinical trials. An association between milk SM and neural development is evident before childhood. Hence, additional studies and well-designed randomized controlled trials that incorporate dietary SM evaluation, SM metabolism, and its long-term functions on infants and children are required.

Loading of lutein in egg-sphingomyelin vesicles as lipid carriers: Thermotropic phase behaviour, structure of sphingosome membranes and lutein crystals

Lutein is a xanthophyll carotenoid provided exclusively by the diet, that has protective functions and beneficial effects on human health. Supplementation in lutein is necessary to reach the recommended daily dietary intake. However, the introduction of lutein into foods and beverages is a real challenge since this lipophilic nutrient has a poor aqueous solubility and a low bioavailability. In this study, we investigated the capacity of egg-sphingomyelin (ESM) vesicles called sphingosomes to solubilise lutein into the bilayers. The thermal and structural properties of ESM bilayers were examined in presence of various amounts of lutein by differential scanning calorimetry (DSC) and temperature-controlled X-ray diffraction (XRD), the structures of sphingosomes and lutein crystals were observed by microscopic techniques. ESM bilayers were in the fluid L_α phase above the phase transition temperature T_m = 39.6 °C and in the lamellar ripple P_β' phase below T_m where ESM sphingosomes exhibited ondulations and were facetted. Lutein molecules were successfully incorporated into the ESM bilayers where they induced a structural disorganisation. For ESM/lutein 90/10 %mol (91.8/8.2 %wt; 89 mg lutein / g ESM), lutein partitioning occured with the formation of lutein crystals in the aqueous phase together with lutein-loaded ESM vesicles. This study highlighted the capacity of new lipid carriers such as egg-sphingosomes to solubilise lutein and opens perspectives for the formulation of effective lutein-fortified functionnal foods and beverages providing health benefits.

Normal and shear forces between boundary sphingomyelin layers under aqueous conditions

Sphingomyelin is one of the predominant phospholipid groups in synovial joints, where lipids have been strongly implicated in the boundary lubrication of articular cartilage; however, little attention has been paid to its lubrication behavior. In this study, we demonstrate that sphingomyelin is an excellent boundary lubricant by measuring the normal and shear forces between sphingomyelin-layer-coated surfaces with a surface force balance under aqueous conditions. Slightly negatively-charged egg sphingomyelin vesicles were adsorbed on mica either by calcium bridging or by charge screening with high concentration monovalent salt. The normal force profiles between opposing egg sphingomyelin layers (vesicles or bilayers) show long-ranged weak repulsion and short-ranged strong repulsion on approaching. Friction coefficients, calculated from the highest load, were (7.2 ± 1.7) × 10-4 at contact stresses of 9.1 ± 0.7 MPa across 0.3 mM liposome dispersion in 0.03 mM Ca2+, and (0.8-3.5) × 10-3 at contact stresses of 7.6 ± 0.8 MPa across 0.3 mM liposome dispersion in 150 mM NaNO3. Similar or slightly lower friction coefficients of (5.3 ± 0.8) × 10-4 at 9.8 ± 0.2 MPa were obtained by replacing the liposome dispersion in 0.03 mM Ca2+ by water. Such low friction coefficients, attributed to the hydration lubrication mechanism, are comparable to those of phosphatidylcholine lipids, which have been widely recognized as excellent aqueous biolubricants. Therefore, we believe that sphingomyelin, in parallel with phosphatidylcholine, contributes to the remarkably good boundary lubrication in synovial joints.

Size-dependent interaction of hydrophilic/hydrophobic ligand functionalized cationic and anionic nanoparticles with lipid bilayers

We study the nature of nanoparticle (NPs)-membrane interaction as a function of nanoparticle size for different functionalization using molecular dynamics simulation. Zinc sulphide quantum dots of size, 2 nm and 4 nm are used as model NPs, and DLPC and DPPC lipid bilayers are used as model membranes. We use coarse-grained polarizable MARTINI model (MPW) to simulate the NPs and lipid bilayers. Our simulation results show that uncharged bare NPs penetrate the lipid bilayers and embed themselves within the hydrophobic core of the bilayer both in the gel and fluid phases. NPs of size 4 nm are shown to disrupt the bilayer. The bilayer recovers from the damages caused by smaller NPs of size 2 nm. In case of either purely hydrophilic or hybrid (with hydrophilic/hydrophobic ratio of 2:1) ligand-functionalized NPs of smaller size (shell size 2 nm), only cationic NPs bind to the bilayer. However, for larger NPs with a shell size of 4 nm, both anionic and cationic hybrid functionalized NPs bind to the bilayer. The performance of standard Martini (SM) force field for the charged NP/bilayer systems has also been tested and compared with the results obtained using MPW model. Although the overall trend that the cationic NPs interact strongly with the bilayers than their anionic counterparts has been captured correctly using SM, the adsorption behaviour of the functionalized NPs differ significantly in the SM force field. The interaction of anionic NPs with both fluid and gel bilayers has been observed to be least accurately represented in the SM force field.

Biophysical approaches for exploring lipopeptide-lipid interactions

In recent years, lipopeptides (LPs) have attracted a lot of attention in the pharmaceutical industry due to their broad-spectrum of antimicrobial activity against a variety of pathogens and their unique mode of action. This class of compounds has enormous potential for application as an alternative to conventional antibiotics and for pest control. Understanding how LPs work from a structural and biophysical standpoint through investigating their interaction with cell membranes is crucial for the rational design of these biomolecules. Various analytical techniques have been developed for studying intramolecular interactions with high resolution. However, these tools have been barely exploited in lipopeptide-lipid interactions studies. These biophysical approaches would give precise insight on these interactions. Here, we reviewed these state-of-the-art analytical techniques. Knowledge at this level is indispensable for understanding LPs activity and particularly their potential specificity, which is relevant information for safe application. Additionally, the principle of each analytical technique is presented and the information acquired is discussed. The key challenges, such as the selection of the membrane model are also been briefly reviewed.

Fatty acid profile of phospholipids and sphingomyelin in milk and regulation of sphingomyelin synthesis of mammary glands in cows receiving increasing levels of crushed sunflower seeds

The objective of this study was to investigate the effect of increasing dietary supplementation of crushed sunflower seed (CSS) in the diet of dairy cows on the fatty acid (FA) composition of phospholipids and sphingomyelin in milk, and on mammary transcription of genes that are important for sphingomyelin de novo synthesis. Four groups of 6 cows received diets supplemented with CSS at 0% (control), or 5, 10, or 15% of dry matter for a 5-wk experimental period. Milk samples and mammary biopsies were collected at the end of the experiment. Phospholipid concentration in milk fat decreased linearly with CSS supplementation. Sphingomyelin concentration in milk fat was unaffected by CSS supplementation. Daily yield of phospholipids decreased linearly with CSS supplementation. Daily yield of sphingomyelin was not significantly affected. The CSS supplementation linearly increased the proportion of monounsaturated FA in milk phospholipids. The major isomer incorporated into phospholipids was C18:1 (n-9 cis), which showed a linear increase with CSS supplementation. The C22:0 proportion in sphingomyelin increased linearly with CSS supplementation and constituted between 15.2 to 25.4% of total FA in sphingomyelin. However, CSS supplementation linearly decreased C23:0 sphingomyelin. Mammary transcription of serine palmitoyl transferase, long chain subunit 1 and subunit 2, the rate-limiting enzymes in ceramide synthesis, showed a linear decrease with increasing CSS supplementation. In conclusion, the data showed that dietary supplementation of CSS linearly increased the proportion of unsaturated FA and monounsaturated FA in milk phospholipids with no effect on phospholipid concentration. In addition, CSS supplementation linearly decreased n-3 polyunsaturated fatty acid proportion in sphingomyelin. The results further showed that mammary transcription of important genes for sphingomyelin de novo synthesis is regulated by lipid supplementation.

Chain-Length- and Saturation-Tuned Mechanics of Fluid Nanovesicles Direct Tumor Delivery

Small unilamellar vesicles (SUVs), ubiquitous in organisms, play key and active roles in various biological processes. Although the physical properties of the constituent lipid molecules (i.e., the acyl chain length and saturation) are known to affect the mechanical properties of SUVs and consequently regulate their biological behaviors and functions, the underlying mechanism remains elusive. Here, we combined theoretical modeling and experimental investigation to probe the mechanical behaviors of SUVs with different lipid compositions. The membrane bending rigidity of SUVs increased with increasing chain length and saturation, resulting in differences in the vesicle rigidity and deformable capacity. Furthermore, we tested the tumor delivery capacity of liposomes with low, intermediate, and high rigidity as typical models for SUVs. Interestingly, liposomes with intermediate rigidity exhibited better tumor extracellular matrix diffusion and multicellular spheroid (MCS) penetration and retention than that of their stiffer or softer counterparts, contributing to improved tumor suppression. Stiff SUVs had superior cellular internalization capacity but intermediate tumor delivery efficacy. Stimulated emission depletion microscopy directly showed that the optimal formulation was able to transform to a rod-like shape in MCSs, which stimulated fast transport in tumor tissues. In contrast, stiff liposomes hardly deformed, whereas soft liposomes changed their shape irregularly, which slowed their MCS penetration. Our findings introduce special perspectives from which to map the detailed mechanical properties of SUVs with different compositions, provide clues for understanding the biological functions of SUVs, and suggest that liposome mechanics may be a design parameter for enhancing drug delivery.

Quantitative atomic force microscopy provides new insight into matrix vesicle mineralization

Matrix vesicles (MVs) are a class of extracellular vesicles that initiate mineralization in cartilage, bone, and other vertebrate tissues by accumulating calcium ions (Ca²⁺) and inorganic phosphate (P_i) within their lumen and forming a nucleation core (NC). After further sequestration of Ca²⁺ and P_i, the NC transforms into crystalline complexes. Direct evidence of the existence of the NC and its maturation have been provided solely by analyses of dried samples. We isolated MVs from chicken embryo cartilage and used atomic force microscopy peak force quantitative nanomechanical property mapping (AFM-PFQNM) to measure the nanomechanical and morphological properties of individual MVs under both mineralizing (+Ca²⁺) and non-mineralizing (-Ca²⁺) fluid conditions. The elastic modulus of MVs significantly increased by 4-fold after incubation in mineralization buffer. From AFM mapping data, we inferred the morphological changes of MVs as mineralization progresses: prior to mineralization, a punctate feature, the NC, is present within MVs and this feature grows and stiffens during mineralization until it occupies most of the MV lumen. Dynamic light scattering showed a significant increase in hydrodynamic diameter and no change in the zeta potential of hydrated MVs after incubation with Ca²⁺. This validates that crystalline complexes, which are strongly negative relative to MVs, were forming within the lumen of MVs. These data were substantiated by transmission electron microscopy energy dispersive X-ray and Fourier transform infrared spectroscopic analyses of dried MVs, which provide evidence that the complexes increased in size, crystallinity, and Ca/P ratio within MVs during the mineralization process.

Spatially resolved small-angle X-ray scattering for characterizing mechanoresponsive liposomes using microfluidics

Atherosclerosis gives rise to blood vessel occlusion associated with blood flow alteration and substantial increase of average wall shear stress. This modification was proved acting as a purely physical trigger for targeted vasodilator release from a particular type of liposomes composed of 1,3-diaminophospholipids (Pad-PC-Pad). The flow-induced structural changes of these faceted liposomes, however, are completely unknown. Therefore, spatially resolved small-angle X-ray scattering was combined with microfluidics to uniquely study the purely physical mechanisms, which give rise to the highly efficient drug release from mechanoresponsive liposomes of nanometer size. The microfluidic device, designed to mimic a stenotic blood vessel, consisted of a 1-mm-wide channel with a constriction, 125 ​μm in diameter. Here, the changes of the average bilayer thickness and the mean size of the mechanoresponsive liposomes have been locally detected under flow conditions. Overall shape and bilayer thickness do change already near the constriction inlet, but the alteration is dominant near the outlet. At a flow rate of 0.2 ​μL/s, the liposome's bilayer thickness increased by 30 % compared to the situation well before the constriction and under static condition. The detected bilayer thickness increase of the faceted liposomes is in line with the mechanically induced loss of interdigitation between the phospholipid amide chains. These results imply that rather the gradient force than the wall shear stress provokes structural changes of Pad-PC-Pad liposomes and the related drug release at stenoses. The approach, i.e. the combination of microfluidics and spatially resolved small-angle X-ray scattering, paves the way to design highly efficient and specific systems for the targeted drug delivery at constrictions with predefined morphology.