Abstract
Purpose:
The adsorption of tear proteins from the aqueous layer to the lipid layer of the tear film is known to play a role in tear stability. Unstable tear film is a characteristic of the dry-eye condition. Tears also become hyperosmolar in dry-eye due to increased evaporation. Our previous research has shown that hyperosmolarity does not affect the stability of meibomian lipids but it is not clear whether it affects the adsorption of tear proteins to lipids. The aim of this study was to investigate the effect of hyperosmolarity on the adsorption of tear proteins to human meibomian lipids.
Methods:
Human meibomian lipids were spread on an artificial tear (AT) solution (normal 305 mOsm/Kg or hyperosmolar 478 mOsm/Kg) at 35⁰C in a Langmuir trough. The lipid film was compressed and expanded to record pre-adsorption surface pressure-area (Π-A) isocycles. The film was compressed to a pressure of 10mN/m and the area of the film was held constant. A protein solution (lysozyme or lactoferrin) was injected into the AT solution and changes in surface pressure with time (Π-T) were recorded. The protein adsorbed lipid film was compressed and expanded to record post-adsorption Π-A isocycles.
Results:
Lysozyme adsorbed to human meibomian lipids showing a similar increase in the surface pressure (7mN/m) on both normal and hyperosmolar AT but the Π-T curve was smoother on hyperosmolar AT. Lactoferrin adsorbed to human meibomian lipids showing a small increase in the surface pressure (2mM/m on normal AT and 5mN/m on hyperosmolar AT) and the Π-T curves were similar. Protein adsorbed lipid films showed higher surface pressures than the lipid-only film for both lysozyme and lactoferrin. Lysozyme adsorbed lipid films showed similar Π-A profiles on normal and hyperosmolar AT while lactoferrin adsorbed lipid films showed slightly higher surface pressure on hyperosmolar AT.
Conclusions:
Tear proteins adsorb to the lipid layer increasing its stability in both normal and hyperosmolar conditions. The adsorption of lysozyme to meibomian lipids is not affected by hyperosmolar conditions while that of lactoferrin increases slightly. This indicates that hyperosmolarity may affect adsorption of different proteins differently. Given that lysozyme, the most abundant tear protein, stays unaffected and lactoferrin adsorbs poorly, the overall effect of hyperosmolarity on the adsorption of tear proteins to the lipid layer may not be substantial.