Sensing Hydration of Biomimetic Cell Membranes

Abstract

Biological membranes play a vital role in cell functioning, providing structural integrity, controlling signal transduction, and controlling the transport of various chemical species. Owing to the complex nature of biomembranes, the self-assembly of lipids in aqueous media has been utilized to develop model systems mimicking the lipid bilayer structure, paving the way to elucidate the mechanisms underlying various biological processes, as well as to develop a number of biomedical and technical applications. The hydration properties of lipid bilayers are crucial for their activity in various cellular processes. Of particular interest is the local membrane dehydration, which occurs in membrane fusion events, including neurotransmission, fertilization, and viral entry. The lack of universal technique to evaluate the local hydration state of the membrane components hampers understanding of the molecular-level mechanisms of these processes. Here, we present a new approach to quantify the hydration state of lipid bilayers. It takes advantage of the change in the lateral diffusion of lipids that depends on the number of water molecules hydrating them. Using fluorescence recovery after photobleaching technique, we applied this approach to planar single and multicomponent supported lipid bilayers. The method enables the determination of the hydration level of a biomimetic membrane down to a few water molecules per lipid.

Nanoscale structural response of biomimetic cell membranes to controlled dehydration

“…One way of describing membrane dehydration would be to define a state where the amount of hydrating water decreases to a level where the dynamic or structural properties of the membrane begin to be affected. Based on the experiments reported here as well as our recent works, 37,50,51 both the dynamics and the structure of the membrane are gradually affected when the first hydration shell of the lipids is perturbed. In the case of the zwitterionic phosphocholine lipids, this happens when the hydration drops below about 12 molecules per lipid.…”

“…The observed changes in lipid partitioning are associated with the decreased hydrophobic mismatch, but unlike the height mismatch data, they are also strongly dependent on the lipid mobility. Since lipid mobility decreases strongly with decreasing membrane hydration, 37,50,51 it is not too surprising that we do not see a steady increase in the partitioning of the L d phase lipids into the L o phase. At lower membrane hydration states, we observed a competition between the lower hydrophobic mismatch, which favours lipid admixing, and the lower diffusivity, which hinders mixing.…”

“…From our previous studies, it is clear that as hydration decreases, the diffusion of lipids, as well as their mobile fraction, decreases abruptly when the number of water molecules hydrating the membrane falls below a certain value. 37,50 At 50% RH and below, the mobility of lipids is almost completely ceased, due to the breaking of the first hydration shell surrounding a single lipid head group moiety. In addition, other recent studies of ours have shown that the dynamics of lipids under dehydration conditions is strongly influenced by the ionic composition of the buffer hydrating the membrane.…”

Laurdan Discerns Lipid Membrane Hydration and Cholesterol Content

“…To perform a direct measurement of the effect of the hydration state of the lipid bilayer on the Laurdan fluorescence spectrum, we employed our home-built humidity control setup, , assuring a controlled drying process with a slow and sequential decrease in the relative humidity (RH) of the membrane environment. The setup consists of a nitrogen gas (N 2 ) cylinder, three flow meters, three manual valves, a reservoir with water (for water-vapor saturation), and an electronic hygrometer.…”

Nanoscale structural response of biomimetic cell membranes to controlled dehydration

“…It should be emphasized that the gradual decrease of the humidity did not lead to as prominent reorganization of the domains shape (domain borderline) over the whole cycle of dehydration and rehydration, as it was visible in the case of confocal imaging. From our previous studies, it is clear that upon a decrease of the hydration, diffusion of lipids, as well as their mobile fraction, abruptly drops down when the number of water molecules hydrating the membrane decreases below a certain value 37,49 . At 50% RH and less, the mobility of lipids is almost completely ceased, which results from the breaking of the first hydration shell surrounding a single lipid head group moiety.…”