Gel-gel phase separation within milk sphingomyelin domains revealed at the nanoscale using atomic force microscopy
Résumé
The milk sphingomyelin (MSM) is involved in the formation of ordered lipid domains in the biological milk
fat globule membrane (MFGM), where it accounts for about 30%wt of the polar lipids. Moreover, MSM exhibits
a large variety in saturated acyl chain lengths (from C16:0 to C24:0-SM) compared to other natural
sphingomyelins, which may impact the packing of MSM molecular species in the gel phase domains and
the topography of the MFGM. To investigate this, supported lipid bilayers of synthetic sphingomyelins or
of MSM-containing mixtures, including a MFGM polar lipid extract, were imaged at temperatures below
the Tm of MSM (i.e. b34 °C for which MSM is in the gel phase) in hydrated conditions using atomic force
microscopy. In all compositions containing MSM, the MSM-rich gel phase domains exhibited lower and
upper height levels H, interpreted as two distinct gel phases with ΔH ~ 0.5–1.1 nm. Two (lower and upper)
gel phases were also found for pure C24:0-SM bilayers or for bilayers of a C16:0-SM/C24:0-SM equimolar mixture, while C16:0-SM bilayers were uniformly flat and less thick than C24:0-SM bilayers. The upper gel phase of MSM-containing bilayers was interpreted as mixed interdigitated C24:0-SM molecules, while the lower gel phase was attributed both to fully interdigitated C24:0-SM molecules and non-interdigitated C16:0-SM molecules.
These results show that the composition of natural sphingomyelins, inducing a mismatch between the
d18:1 sphingosine and the acyl chains, is important in both the internal organization and the topography of
biological membranes, especially that of the MFGM. This organization could be involved in specific biological
functions, e.g. the insertion of proteins.
