Abstract
The four major phospholipids found in eye lens membrane are phosphatidylcholine (PC), sphingomyelin (SM), phosphatidylethanolamine (PE), and phosphatidylserine (PS). Lens lipid composition of the fiber-cell membrane significantly changes with species. In lower-age animals, phosphatidylcholine (PC) is dominant, whereas, in humans, sphingomyelin (SM) is more dominant. We have used atomic force microscopy (AFM) to study morphological and nanomechanical properties of the lens lipid membrane models from mouse, pig, and humans. Small unilamellar vesicles (SUVs) were prepared from the mixture of four phospholipids (PC, SM, PS, and PE) that resemble mouse, pig, and human phospholipid composition lens membranes. The SUVs phospholipids mixtures were prepared with 0 mol% and 50 mol% of cholesterol. The topographical image and the mechanical properties of animal and human lens model membranes were accessed in a supported lipid membrane (SLM) formed on mica. The topographical images of phospholipid mixtures with 0 mol% cholesterol for both animal and human lens membrane models exhibited coexisting solid-ordered and liquid-disordered phase, where the proportion of solid-ordered phase increases with an increase in SM content in phospholipid mixture. The height difference between the two phases was ∼ 1nm. However, in the presence of 50 mol% of cholesterol, the membrane exhibited only a liquid-ordered phase, both for animal and human phospholipid mixtures. The breakthrough force on the SLM of the animals and human lens model membrane obtained with 50 mol% of cholesterol follow the trends: F (human) > F (pig) > F (mouse). Similarly, the membrane area compressibility modulus (Ka) of the membrane with 50 mol% cholesterol was accessed using the force curve's elastic regime. We found: Ka (human) > Ka (pig) > Ka (mouse). Our results show that phospholipid composition plays a significant role in modulating membrane mechanical properties.
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