Cholesterol is an amphiphilic molecule present in cellular membranes at different concentrations depending on the cell’s role and functionality. It is uniquely structured as its hydrophilic head is much smaller than its bulky hydrophobic tail. When present in a phospholipid leaflet cholesterol behaves according to the umbrella model, filling the gaps in between phospholipid molecules and reducing the area per lipid. This provides an ordering and condensing effect for the membrane. The effect of cholesterol on cell membranes has been investigated and it has been seen to minimize its permeability and move the membrane from a liquid disordered to a liquid ordered state. These mechanics may be measured through the measurement of the interfacial tensions of lipid monolayers and bilayers with and without cholesterol.
In this work, the droplet interface bilayer (DIB) technique is used to investigate cholesterol’s effect on surface tension and the membrane structure. Based on lipid-coated aqueous droplets adhering in an oil medium, the DIB method enables linking droplets mechanics and membrane mechanics for interpreting changes in the membrane structure through droplet-droplet adhesion and the angles of contact at the droplet intersections. It is a purely fluidic method where the droplets are minimally constrained and the membrane is free to expand dependent on the interfacial tensions. By observing the changes in the droplets’ adherence while measuring the membrane properties through electrophysiology, one can calculate membrane’s tension, specific capacitance, applied stress and resulting strain.
Pendant drop tensiometry was used to investigate the changes in the monolayer surface tension with increasing amount of cholesterol. Higher monolayer tensions were observed with increasing cholesterol concentrations, along with increasing bilayer tension and adhesion energy. Next, membrane properties with varying cholesterol were measured. The membranes exhibited a non-linear reduction in thickness with the increasing cholesterol mole fractions. To understand cholesterol’s impact on membrane’s structure and rigidity, an electric field was gradually increased across the membrane until failure providing membrane compression beyond the equilibrium dimensions. The membrane thickness along with the corresponding electrical stress were measured for each voltage. Plotting the pressure versus membrane thickness for different cholesterol concentrations shows that cholesterol does not affect the critical stress at failure. However, it significantly reduces membrane thinning. Adding 30% cholesterol reduces membrane thinning by more than half. This indicated that cholesterol enhances the stiffness of the membrane, making it more rigid and resistant to transverse compression.
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E