Lipidomic analysis of meibomian gland secretions from the tree shrew: Identification of candidate tear lipids critical for reducing evaporation

In-vitro evaluation of the evaporation retardation by Meibomian lipids in homogeneous and non-homogeneous evaporation

HYPOTHESIS

An important function of the Tear Film Lipid Layer (TFLL) is the retardation of evaporation. We propose two micro-scaled systems to quantify the influence of the TFLL in evaporation for single patients, which may contribute as an improvement on the diagnosis of Meibomian Gland Dysfunctions (MGD).

EXPERIMENTS

Meibum was extracted from 10 patients with hypersecretory MGD and 9 healthy controls. The lipids were placed over water, and the evaporation was determined in the case of homogeneous evaporation over a surface (pendant drop), and the case where the evaporation depends on a pinned triple contact line (meniscus).

FINDINGS

For the homogeneous case, the presence of Meibum reduced evaporation in 30%, although there was no significant difference between controls and MGD patients. However, evaporation induced by menisci was 25 % higher in MGD patients. Our results contribute to the evidence of the inhibition of evaporation by Meibum. Our study also suggests that the evaporation induced by contact points may be a more relevant model to measure differences in evaporation due to the composition of Meibum. This model may also have connotations in the occurrence of internal stresses in the tear film, inducing its instability.

Lipid conformational order and the etiology of cataract and dry eye

Lens and tear film lipids are as unique as the systems they reside in. The major lipid of the human lens is dihydrosphingomylein, found in quantity only in the lens. The lens contains a cholesterol to phospholipid molar ratio as high as 10:1, more than anywhere else in the body. Lens lipids contribute to maintaining lens clarity, and alterations in lens lipid composition due to age are likely to contribute to cataract. Lens lipid composition reflects adaptations to the unique characteristics of the lens: no turnover of lens lipids or proteins; the lowest amount of oxygen of any tissue; and contains almost no intracellular organelles. The tear film lipid layer (TFLL) is also unique. The TFLL is a thin (100 nm) layer of lipid on the surface of tears covering the cornea that contributes to tear film stability. The major lipids of the TFLL are wax esters and cholesterol esters that are not found in the lens. The hydrocarbon chains associated with the esters are longer than those found anywhere else in the body (as long as 32 carbons), and many are branched. Changes in the composition and structure of the 30,000 different moieties of TFLL contribute to the instability of tears. The focus of the current review is how spectroscopy has been used to elucidate the relationships between lipid composition, conformational order and function, and the etiology of cataract and dry eye.

Mass Spectrometric Analysis of Meibomian Gland Lipids

The precorneal tear film keeps the eye surface moist and helps to maintain normal eye function. The outermost lipid layer of the tear film, which attenuates tear film evaporation, contains meibum secreted from the meibomian gland. Most meibum lipids are neutral, including wax esters (WEs), cholesteryl esters (CEs), and diesters (DEs), along with some polar lipids including free fatty acids (FFAs), O-acyl-ω-hydroxy fatty acids (OAHFAs), and trace phospholipids. Detection of neutral lipids by mass spectrometry (MS) is challenging due to interference from impurities, particularly when working with minute-volume meibum samples. Here, we describe procedures for sample preparation and MS analysis of these elusive meibum lipids that can be used to examine dry eye disease mechanisms. Because the method described here minimizes impurity peaks for lipids generally, neutral and otherwise, it may be applied to high-sensitivity analysis of other biological samples.

Deficiency in Acyl-CoA:Wax Alcohol Acyltransferase 2 causes evaporative dry eye disease by abolishing biosynthesis of wax esters

Lipids secreted by the meibomian glands (MGs) of the eyelids are essential to the protection of the eye's surface. An altered meibum composition represents the primary cause of evaporative dry eye disease (DED). Despite the critical importance of the meibum, its biosynthetic pathways and the roles of individual lipid components remain understudied. Here, we report that the genetic deletion of Acyl-CoA:wax alcohol acyltransferase 2 (AWAT2) causes the obstruction of MGs and symptoms of evaporative DED in mice. The lipid composition of the meibum isolated from Awat2^-/- mice revealed the absence of wax esters, which was accompanied by a compensatory overproduction of cholesteryl esters. The resulting increased viscosity of meibum led to the dilation of the meibomian ducts, and the progressive degeneration of the MGs. Overall, we provide evidence for the main physiological role of AWAT2 and establish Awat2^-/- mice as a model for DED syndrome that can be used in studies on tear film-oriented therapies.

A simple and reproducible method for quantification of human tear lipids with ultrahigh-performance liquid chromatography-mass spectrometry

Purpose

The purpose was to select a simple and reproducible method for lipid measurements of human tears with ultrahigh-performance liquid chromatography-mass spectrometry (UHPLC-MS). Two sample preparation procedures were evaluated and compared: the Bligh and Dyer (BD) liquid-liquid extraction method with chloroform and methanol and protein precipitation with isopropanol (IPA).

Methods

Reproducibility and recovery efficiencies of 20 non-endogenous internal lipid standards were tested in 10-µl tear samples from healthy subjects. The lipid coverage and the simplicity of execution were also assessed. Lipid profiles of the tear extracts were acquired with UHPLC-MS, uhpland the lipids were identified using SimLipid software.

Results

Both methods were robust producing good lipid coverage and reproducibility and high recovery efficiencies. The two protocols identified a 69-feature tear lipidome that covered 11 lipid classes from six different lipid categories. The main differences in recovery were due to the intrinsic lipid selectivity of each solvent. Although both methods were similarly efficient in recovering O-acyl-ω-hydroxy fatty acid (OAHFAs) and non-polar lipids, polar lipids were more efficiently recovered with IPA precipitation, which, in turn, exhibited higher reproducibility. In addition, IPA precipitation is automatable and simpler than the BD approach.

Conclusions

IPA precipitation is an excellent procedure for extracting lipids from small tear volumes for quantitative large-scale, untargeted lipid profiling, which may be useful for identifying lipid biomarkers in tears from patients with different ocular surface pathologies, allowing personalized therapies to be designed.