Reduction of glass-surface charge density slows the lipid diffusion in the proximal leaflet of a supported lipid bilayer

A comparison of lipid diffusive dynamics in monolayers and bilayers in the context of interleaflet coupling

Cellular membranes are composed of lipids typically organized in a double-leaflet structure. Interactions between these two leaflets - often referred to as interleaflet coupling - play a crucial role in various cellular processes. Despite extensive study, the mechanisms governing such interactions remain incompletely understood. Here, we investigate the effects of interleaflet coupling from a specific point of view, i.e. by comparing diffusive dynamics in bilayers and monolayers, focusing on potential lipid-specific interactions between opposing leaflets. Through quantitative fluorescence microscopy techniques, we characterize lipid diffusion and mean molecular area in monolayers and bilayers composed of different lipids. Our results suggest that the observed decrease in bilayer lipid diffusion compared to monolayers depends on lipid identity. Furthermore, our analysis suggests that lipid acyl chain structure and spatial configuration at the bilayer may strongly influence interleaflet interactions and dynamics in bilayers. These findings provide insights into the role of lipid structure in mediating interleaflet coupling and underscore the need for further experimental investigations to elucidate the underlying mechanisms.

Diffusion of Multiple Species Resolved by Fluorescence Lifetime Recovery after Photobleaching (FLRAP)

Fluorescence recovery after photobleaching (FRAP) is now an indispensable tool to analyze the diffusion of molecules in vivo and in vitro. However, a conventional fluorescence intensity-based approach has difficulty in analyzing the diffusion of multiple species simultaneously. Here, we report fluorescence lifetime recovery after photobleaching (FLRAP) that incorporates fluorescence lifetime information into FRAP. By using FLRAP, the fluorescence intensity-recovery curves of each species can be successfully extracted from the ensemble photon data by utilizing their species-specific fluorescence decay curves, which are verified by applying FLRAP to two heterogeneous systems. Thus, FLRAP can be a powerful tool to quantitatively elucidate the molecular diffusion of multiple species in complex systems such as in living cells.

Peripheral adsorption of polylysine on one leaflet of a lipid bilayer reduces the lipid diffusion of both leaflets

Understanding polycation-lipid interaction is essential not only in molecular biology but also in the biomedical industry and pharmacology. However, the effect of the polycation-lipid interaction on the molecular properties of lipids in biomembranes remains elusive. Here, two fluorescence correlation spectroscopies (FCSs), pulse-interleaved excitation (PIE) FCS and lifetime-based FCS, were performed to elucidate the change in the lipid diffusion of a model biomembrane induced by polylysine (PLL) adsorption. The results of PIE-FCS showed that the diffusions of both anionic and zwitterionic lipids become slower in the presence of PLL but the mobility of the anionic lipids is much reduced, suggesting the preferential interaction between the PLL and the anionic lipids due to the electrostatic attraction. Furthermore, leaflet-specific lipid diffusion analysis by lifetime-based FCS clearly showed that PLL adsorption on one leaflet of the membrane reduces the lipid diffusion of both leaflets in the same manner. This clearly indicates that the interleaflet coupling is strong in the presence of PLL.

Interleaflet Coupling of Lipid Nanodomains - Insights From in vitro Systems

The plasma membrane is a complex system, consisting of two layers of lipids and proteins compartmentalized into small structures called nanodomains. Despite the asymmetric composition of both leaflets, coupling between the layers is surprisingly strong. This can be evidenced, for example, by recent experimental studies performed on phospholipid giant unilamellar vesicles showing that nanodomains formed in the outer layer are perfectly registered with those in the inner leaflet. Similarly, microscopic phase separation in one leaflet can induce phase separation in the opposing leaflet that would otherwise be homogeneous. In this review, we summarize the current theoretical and experimental knowledge that led to the current view that domains are – irrespective of their size – commonly registered across the bilayer. Mechanisms inducing registration of nanodomains suggested by theory and calculations are discussed. Furthermore, domain coupling is evidenced by experimental studies based on the sparse number of methods that can resolve registered from independent nanodomains. Finally, implications that those findings using model membrane studies might have for cellular membranes are discussed.

Effect of electrostatic interaction on the leaflet-specific diffusion in a supported lipid bilayer revealed by fluorescence lifetime correlation analysis

Lipid–support electrostatic interaction determines the lipid dynamics in the proximal leaflet of a SLB.