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Riku Paananen, Ilpo Vattulainen, Jukka Moilanen; Structure of tear film lipid layer models containing cholesteryl esters and lysophosphatidylcholine. Invest. Ophthalmol. Vis. Sci. 2018;59(9):4906.
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© ARVO (1962-2015); The Authors (2016-present)
Cholesteryl esters (CEs) are a major component of the tear film lipid layer (TFLL), but their role in its function remains unclear. The purpose of the study was to evaluate the organization of TFLL CEs at the air-water interface. Since methyl-branched CEs, which make up the majority of TFLL CEs are not readily available, we used mixtures of monounsaturated and normal saturated CEs to mimic the physical properties of TFLL CEs.
Lipid films containing varying ratios of cholesteryl nervonate (CN), cholesterol lignocerate, cholesteryl arachidate, cholesteryl palmitate (CP) and lysophosphatidylcholine (LPC), were studied on water surface using surface pressure and surface potential measurements combined with Brewster angle microscopy.
Pure CEs studied formed three-dimensional aggregates that did not spread on water surface. Adding LPC to saturated CE films did not induce spreading of CEs, instead LPC formed a separate monolayer phase. In contrast, mixtures of CN and LPC spread as a fluid “double layer”, with a layer of CN molecules overlying a monolayer at the water surface. Compression of the double layer caused the formation of additional CN lamellae. A single LPC molecule formed a double layer with approximately 9 CN molecules, and increasing the LPC:CN ratio caused LPC-rich monolayer regions to form. Depending on the ratio of the components, mixtures of LPC, CN, and saturated CEs formed either a fluid double layer, a film composed of solid and fluid double-layer regions, or a completely solid film.
Only mixed films of LPC and CEs were stable at the water surface. The amount of LPC needed was close to the levels of polar lipids found in the tear fluid. It is possible that excess polar lipids are detrimental to the structure of TFLL, since excess LPC separated into monolayer domains that did not support formation of overlying CE layers. Further studies are needed to determine whether TFLL CEs predominantly form solid or fluid films as both were observed for CE mixtures.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
Figure 1. Brewster angle microscopy images of mixed LPC and CE films on water. All measurements were conducted at 35 °C. Scale bar is 1 mm.
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