BIOPHYSICAL STUDY OF TEAR FILM LIPID LAYER
BIOPHYSICAL STUDY OF TEAR FILM LIPID LAYER
Tear film lipid layer (TFLL) is the outmost layer of the tear film. The current consensus is that the 40-nm thick TFLL consists of two sublayers, i.e., a polar lipid layer covering the air-water surface of the cornea, and a nonpolar lipid layer that resides upon the polar lipids and is directly exposed to air. The polar lipids account for 20 mol% of the TFLL, including ~ 12 mol% phospholipids, and ~ 4 mol% (O-acyl)-ω-hydroxy fatty acids (OAHFAs), which belongs to a newly discovered class of endogenous lipids termed fatty acid esters of hydroxy fatty acids (FAHFAs). The nonpolar lipids account for 80 mol% of the TFLL, with wax esters (WEs, accounting for ~ 43 mol%) and cholesteryl esters (CEs, accounting for ~39 mol%) being the most prevalent nonpolar lipid classes. The major physiological function of the TFLL is to stabilize the tear film by reducing surface tension and retarding evaporation of the aqueous layer. Dysfunction of the TFLL leads to dysfunctional tear syndrome, with the dry eye disease (DED) being the most prevalent eye disease affecting 10-30% of the world population. It is estimated that the DED directly and indirectly causes a $55 billion annual economic burden in the United States alone. To date, except for treatments alleviating the dry eye symptoms, effective therapeutic interventions in treating the DED are still lacking. Therefore, there is an urgent need to better understand the biophysical function of the TFLL and to develop translational solutions in effectively managing the DED. The focus of this dissertation is to study biophysical properties of the TFLL using a newly developed experimental methodology called constrained drop surfactometry (CDS). Main contributions fell into the following four headings: 1. Study of the composition-functional correlations of a model TFLL, under physiologically relevant conditions. For the first time, this study unveiled that the primary biophysical function of FAHFAs is to optimize the interfacial rheological properties of the TFLL. 2. Study of the polymorphism and collapse mechanism of FAHFA monolayers. This study revealed that FAHFA molecules at the air-water surface demonstrate unique polymorphic behaviors, which can be explained by configurational transitions of the molecules under various lateral pressures. 3. Development of a novel ventilated, closed-chamber, droplet evaporimeter with a constant surface area. This droplet-based evaporimeter is capable of a rigorous control of environmental conditions, including the temperature, relative humidity, airflow rate, surface area, and surface pressure, thus allowing for reproducible water evaporation measurements over a time period of only 5 minutes. The volumetric evaporation rate of this droplet evaporimeter is less than 2.7 μL/min, comparable to the basal tear production of healthy adults. This study demonstrated that the TFLL resists water evaporation with a combined mechanism by increasing film compactness of the polar lipid film at the air-water surface, and, to a lesser extent, by increasing film thickness of the nonpolar lipid film. 4. Comparative study of the dynamic surface activity, interfacial rheology, evaporation resistance, and ultrastructure of the meibomian lipid films extracted from wild type (WT) and Soat1 knockout (KO) mice. Inactivation of Soat1 gene led to a complete stoppage of CE production in meibomian glands and a severe change in the eye phenotype in experimental animals. Lipidomic analysis with ultrahigh-pressure liquid chromatography ̶ mass spectrometry showed that the pool of cholesterol rose seven times in the KO mice compared with their WT siblings, and, an almost complete ablation of CEs longer than C18-C20 was observed. This study revealed novel experimental evidence about the composition-structure-functional correlations of the meibomian lipid films. Overall, research in this dissertation advanced the biophysical understanding of the TFLL, and provided novel implications in the pathophysiological and translational understanding of DED.
Table of Contents
All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Email firstname.lastname@example.org if you need this content in ADA-compliant format.