The temperature-dependent physical state of polar lipids and their miscibility impact the topography and mechanical properties of bilayer models of the milk fat globule membrane

Abstract

The polar lipid assembly and biophysical properties of the biological membrane enveloping the milk fat globules (the MFGM) are yet poorly known, especially in connection with the temperature history that milk can experience after its secretion. However, bioactive mechanisms depend on biological structure, which itself highly depend on temperature. The objectives of this study were to investigate polar lipid packing in hydrated bilayers, models of the MFGM, and to follow at intermolecular level temperature-induced changes in the range 60-6°C, using the combination of differential scanning calorimetry, X-ray diffraction, atomic force microscopy (AFM) imaging and force spectroscopy. MFGM polar lipids, especially sphingomyelin, contain long chain saturated fatty acids with high phase transition temperatures. On cooling, the liquid disordered ld to solid ordered so (gel) phase transition of MFGM polar lipids started at about 40°C, leading to phase separation and formation of so phase domains protruding by about 1nm from the ld phase. Indentation measurements using AFM revealed that the resistance of the so phase domains to rupture was significantly higher than that of the ld phase and that it increased for both the domain and fluid phases with decreasing temperature. However, packing and stability of the bilayers were adversely affected by fast cooling to 6°C or by cooling-rewarming cycle. This study showed that MFGM polar lipid bilayers are dynamic systems. Heterogeneity in the structure and mechanical properties of the membrane was induced by temperature-dependent so/ld phase immiscibility of the lipid components. This could have consequences on the MFGM technological and biological functions (e.g. immunity and milk lipid digestion).

Young modulus of supported lipid membranes containing milk sphingomyelin in the gel, fluid or liquid-ordered phase, determined using AFM force spectroscopy

“…These results are in agreement with Picas et al [34] who reported Young modulus values of 19 and 28 MPa, respectively, for the fluid and gel phase of a DOPC/DPPC bilayer. As the Young modulus E and the breakthrough force of lipid membrane systems can be somewhat compared [28,35], the present results also agree with previous findings showing that the gel phase domains of MFGM polar lipids bilayers ruptured at higher force compared to the surrounding continuous fluid phase [48]. This is also in line with breakthrough force measurements on a large range of simple systems [49].…”

“…These results show that the physical state of the polar lipids, which evolves as a function of temperature, governs the lateral packing, phase separation and local stiffness of membranes. This is coherent with previous studies reporting low and homogeneous breakthrough force values in bilayers of MFGM polar lipids at 40°C, compared to 26°C [48]. Changes in the mechanical properties of the membrane with their phase state is a direct consequence of the order parameter of lateral lipid packing [54].…”

“…The presented results were also in line with the sharp decrease of the bending modulus of phosphatidylcholine vesicles as they undertake gel ➔ fluid phase transition [57][58][59]. Furthermore, the decrease in Young modulus (or in breakthrough force - [48]) of the fluid phase with temperature is likely due to the increased molecular agitation and increased molecular distance between lipids upon heating of the bilayer [60]. Another reason could be that the melting of the high-Tm polar lipids modifies the composition of the fluid phase, thereby increasing the complexity for optimal lateral packing of the molecules.…”

Structure and Nanomechanics of Model Membranes by Atomic Force Microscopy and Spectroscopy: Insights into the Role of Cholesterol and Sphingolipids

“…It allows for obtaining relevant information about the structural and physical changes of the membrane occurring during the phospholipid phase transitions [9,53,66,67]. Recently, insights on the dynamics of the DMPC (1,2-dimyristoyl- sn -glycero-3-phosphocholine, 14:0 PC; T m = 24 °C) transition from ripple phase to fluid phase reversibly in real time by HS-AFM have also been reported [68].…”

Changes in milk fat globules and membrane lipids under the shear fields of microfiltration and centrifugation

“…Moreover, it has been noted that major losses, especially in components located in the outer layers of the MFGM, occur upon increasing the number of centrifugal processing cycles. In addition, cream washing may selectively remove MFGM components from the interface and decrease stability [16,19,20]. The extent of MFGM rupture is expected to depend on the specific conditions applied during handling and processing.…”