Nanoscale Packing Differences in Sphingomyelin andPhosphatidylcholine Revealed by BODIPY Fluorescence in Monolayers:Physiological Implications

摘要

Phosphatidycholines (PC) with two saturated acyl chains (e.g., dipalmitoyl) mimic natural sphingomyelin (SM) by promoting raft formation in model membranes. However, sphingoid-based lipids, such as SM, rather than saturated-chain PCs have been implicated as key components of lipid rafts in biomembranes. These observations raise questions about the physical packing properties of the phase states that can be formed by these two major plasma membrane lipids with identical phosphocholine headgroups. To investigate, we developed a monolayer platform capable of monitoring changes in surface fluorescence by acquiring multiple spectra during measurement of a lipid force–area isotherm. We relied on the concentration-dependent emission changes of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-labeled PC to detect nanoscale alterations in lipid packing and phase state induced by monolayer lateral compression. The BODIPY-PC probe contained an indacene ring with four symmetrically located methyl (Me) substituents to enhance localization to the lipid hydrocarbon region. Surface fluorescence spectra indicated changes in miscibility even when force–area isotherms showed no deviationfrom ideal mixing behavior in the surface pressure versus cross-sectionalmolecular area response. We detected slightly better mixing of Me4-BODIPY-8-PC with the fluid-like, liquid expanded phase of1-palmitoyl-2-oleoyl-PC compared to N-oleoyl-SM.Remarkably, in the gel-like, liquid condensed phase, Me4-BODIPY-8-PC mixed better with N-palmitoyl-SM thandipalmitoyl-PC, suggesting naturally abundant SMs with saturated acylchains form gel-like lipid phase(s) with enhanced ability to accommodatedeeply embedded components compared to dipalmitoyl-PC gel phase. Thefindings reveal a fundamental difference in the lateral packing propertiesof SM and PC that occurs even when their acyl chains match.