Purpose: Instability of the tear film with rapid tear break-up time is a common feature of aqueous deficient and evaporative dry eye diseases, suggesting there may be a shared structural abnormality of the tear film that is responsible for the instability. A change in the normal meibum lipid composition and conformation is possibly this event. Principle Component Analysis of infrared spectra of human meibum indicate that human meibum collected from normal donors (Mn) is more ordered than meibum from donors with meibomian gland dysfunction (Md). In this study we quantified the conformation of Md to test this finding.
Methods: Changes in lipid conformation with temperature were measured using infrared spectroscopy. There were two phases to our study. In phase 1 we measured the phase transitions of human samples, Mn and Md. In phase 2, we measured phase transitions of model lipid standards composed of different waxes and cholesterol ester.
Results: The phase transition temperature was significantly higher by 4°C for Md compared to Mn of age-matched donors with no history of dry-eye symptoms. Most (82%) of the phase-transition temperatures measured for Md were above the values for Mn. The small change in the transition temperature was amplified in the average lipid order (stiffness) at 33.4(o)C. The average lipid order at 33.4(o)C for Md was significantly higher (30%, p = 0.004), than for Mn. The strength of lipid-lipid interactions was 72% higher for Md compared to Mn. The ability of one lipid to influence the melting of adjacent lipids is termed cooperativity. There was no significant differences between Mn and Md in the phase transition cooperativity. Nor was there a difference between Mn and Md in the minimum order or maximum order that Mn and Md can achieve at very low and very high temperatures, respectively. From our model wax studies, we found that the phase transition of complex mixtures of natural lipids is 'set' by the level of unsaturation. A double bond decreased the phase transition temperature by about 40 °C. The addition of a second CH=CH moiety decreased the phase transition temperature by about 19 °C. Unsaturated waxes were miscible with saturated waxes. When a saturated wax was mixed with an unsaturated one, the saturated wax disproportionately increased the phase transition of the mixture by about 30 °C compared to the saturated wax alone. Cholesterol ester had little affect on the phase transition temperature of waxes. Model studies indicate that changes in the amount of lipid saturation rather than the amount of cholesterol esters could be a factor in the observed conformational changes.
Concluisions: Meibum lipid compositional changes with meibomian gland dysfunction reflect changes in hydrocarbon chain conformation and lipid-lipid interaction strength. Spectroscopic techniques are useful to study the lipid-lipid interactions and conformation of lipid from individual patients.
Invest Ophthalmol Vis Sci. 2011 Mar 11. [Epub ahead of print]
Borchman D, Foulks GN, Yappert MC, Bell J, Wells E, Neravetla S, Greenstone V.
Department of Ophthalmology and Visual Sciences, University of Louisville, 301 E. Muhammad Ali Blvd., Louisville, KY 40202, US;