Milk Sphingomyelin Domains in Biomimetic Membranes and the Role of Cholesterol: Morphology and Nanomechanical Properties Investigated Using AFM and Force Spectroscopy

Abstract

Milk sphingomyelin (MSM) and cholesterol segregate into domains in the outer bilayer membrane surrounding milk fat globules. To elucidate the morphology and mechanical properties of theses domains, supported lipid bilayers with controlled molar proportions of MSM, dioleoylphosphatidylcholine (DOPC) and cholesterol were produced in buffer mimicking conditions of the milk aqueous phase. Atomic force microscopy imaging showed that (i) for T < 35 °C MSM segregated in gel phase domains protruding above the fluid phase, (ii) the addition of 20 mol % cholesterol resulted in smaller and more elongated l(o) phase domains than in equimolar MSM/DOPC membranes, (iii) the MSM/cholesterol-enriched l(o) phase domains were less salient than the MSM gel phase domains. Force spectroscopy measurements furthermore showed that cholesterol reduced the resistance of MSM/DOPC membrane to perforation. The results are discussed with respect to the effect of cholesterol on the biophysical properties of lipid membranes. The combination of AFM imaging and force mapping provides unprecedented insight into the structural and mechanical properties of milk lipid membranes, and opens perspectives for investigation of the functional properties of MSM domains during milk fat processing or digestion.

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

“…MSM/dioleylphosphatidylcholine (DOPC) at 25°C (i.e., below Tm of MSM [16]) or with that of other SM species in model bilayers [37][38][39][40]. The characterization of high-Tm lipid rich domains below 35°C is also in agreement with previous studies performed with dried Langmuir-Blodgett monolayers [8,9].…”

“…This is mainly due to AFM high lateral and vertical resolutions. Recently, AFM investigation of binary MSM/dioleylphosphatidylcholine (DOPC) model membranes of the MFGM showed the formation of MSM domains, for temperatures below 35°C[16]. Moreover, AFM provides force spectroscopy ability that allows association of mechanical information on the nanometer lateral scale with structural information corresponding to phases of the lipid bilayer[17][18][19][20][21].…”

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

“…A slight decrease in the nanomechanical stability of both coexisting phases, but more evidenced for the l o domains, was directly related to the increment of Chol content. A similar effect has been reported for DOPC:milk sphingomyelin (MSM) bilayers, where Chol not only affects the morphology of the MSM domains but also decreases their nanomechanical stability [87]. While DOPC:MSM (50:50 molar ratio) SLBs displayed F b of around 1.7 nN for the DOPC-rich continuous phase and 3–5.5 nN for the MSM-rich domains, upon 20 mol % Chol addition, the mean F b decreased to values lower than 1 nN.…”

Polymer resonators sensors for detection of sphingolipid gel/fluid phase transition and melting temperature measurement

“…SPH is especially prominent in myelin, a membranous sheath that surrounds and insulates the axons of numerous neurons [29][30][31][32]. In humans, SPH represents 85% of all sphingolipids, and typically make up 10-20 mol % of plasma membrane lipids with higher concentrations found in nerve tissues, red blood cells, and the ocular lenses… Such a plasma membrane component participates in many signaling pathways.…”