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
Résumé
The polar lipid assembly and biophysical properties of the biological membrane enveloping the milk fat globules
(theMFGM) are yet poorly known, especially in connectionwith the temperature history thatmilk 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 1 nm 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).