Membrane Elasticity Modulated by Cholesterol in Model of Porcine Eye Lens-Lipid Membrane

Experimental evidence shows that the eye lens loses its elasticity dramatically with age. It has also been reported that the cholesterol (Chol) content in the eye lens fiber cell plasma membrane increases significantly with age. High Chol content leads to the formation of cholesterol bilayer domains (CBDs) in the lens membrane. The role of high Chol associated with lens elasticity is unclear. The purpose of this research is to investigate the membrane elasticity of the model of porcine lens-lipid (MPLL) membrane with increasing Chol content to elucidate the role of high Chol in lens membrane elasticity. In this study, we used atomic force microscopy (AFM) to study the mechanical properties (breakthrough force and area compressibility modulus (KA)) of the MPLL membrane with increasing Chol content where KA is the measure of membrane elasticity. We varied Chol concentration in Chol/MPLL membrane from 0 to ∼71 mol%. Supported Chol/MPLL membranes were prepared by fusion of small unilamellar vesicles (SUVs) on top of a flat mica surface. SUVs of the Chol/MPLL lipid mixture were prepared with the rapid solvent exchange method followed by probe-tip sonication. For the Chol/MPLL mixing ratio of 0, AFM image showed the formation of two distinct phases of the membrane, i.e., liquid-disordered phase (ld) and solid-ordered phase (so) membrane. However, with Chol/MPLL mixing ratio of 0.5 and above, only liquid-ordered phase (lo) membrane was formed. Also, two distinct breakthrough forces corresponding to ld and so were observed for Chol/MPLL mixing ratio of 0, whereas only one breakthrough force was observed for membranes with Chol/MPLL mixing ratio of 0.5 and above. No significant difference in the membrane surface roughness was measured with increasing Chol content for these membranes; however, breakthrough force and KA for lo membrane increased when Chol/MPLL mixing ratio was increased from 0.5 to 1. Interestingly above the Chol/MPLL mixing ratio of 1, both breakthrough force and KA decreased, indicating the formation of CBDs. Furthermore, these results showed that membrane elasticity increases at high Chol content, suggesting that high Chol content in lens membrane might be responsible for maintaining lens membrane elasticity

AFM Study of Model Membrane Made of Mouse Lens Phospholipids

The lipid composition of the eye lens membrane differs between the species. In the lower life span animals, eye lenses have phosphatidylcholine as a major component, while lenses have sphingomyelins as a major phospholipid in higher life span animals. Here, we study the mechanical properties of model membrane made of mouse phospholipid composition with Chol/PL mixing ratio of 0 and 1, respectively using Atomic force microscopy (AFM). The model membrane\u27s height image with 0 mol% Chol shows two phases: solid ordered (so) and liquid disordered (ld). The height difference between the two phases is ~ 1 nm. The model membrane with Chol/PL mixing ratio of 1 exhibit only a single-phase known as liquid-ordered phase (lo). Thus, the membrane roughness of Chol/PL mixing ratio of 1 is 152±8 pm which is significantly lower than the roughness of membrane with Chol/PL mixing ratio of 1, which is 241±36pm. Similarly, the average breakthrough force for the Chol/PL mixing ratio of 0 is 5.4±0.41 nN, while for Chol/PL mixing ratio of 1, the average breakthrough force is 4.88±0.58 nN. We speculate that this slight difference in the membrane breakthrough force may be due to two phases in the 0 mol% Chol-containing membrane