Lipidomics of human Meibomian gland secretions: Chemistry, biophysics, and physiological role of Meibomian lipids

Comparison of non-pharmaceutical treatments for evaporative dry eye: a randomised controlled study protocol

INTRODUCTION

The lipid layer of the tear film is critical to maintaining the integrity of the tear film and absence in the tear film lipid layer (TFLL) is one of the main causes of evaporative dry eye (EDE) in dry eye disease patients, resulting in excessive evaporation (so-called hyperevaporative dry eye). This study protocol will be designed to assess and compare the effects of intense pulsed light (IPL), heated eye mask (HEM), vectored thermal pulsation system (VTPS) and eyelid massage device (EMD) for improving signs and symptoms of EDE.

METHODS AND ANALYSIS

Patients with EDE will be randomly divided into IPL, HEM, VTPS and EMD groups and will be followed up for 6 weeks. The primary outcome measure will be non-invasive tear breakup time (NITBUT). The secondary outcome measures will include, TFLL score, meibomian gland quality and expressibility change from baseline conjunctivocorneal staining with fluorescein and lissamine, tear meniscus height, conjunctival hyperaemia (redness score) and ocular surface disease index questionnaire. Additionally, adverse events will be monitored and documented.

ETHICS AND DISSEMINATION

Ethics approval number: IRB(2023)K019.01. The findings will be shared regardless of the effect's direction.

TRIAL REGISTRATION NUMBER

NCT05923528.

Measurement of Anions in Tear Fluid Using Ion Chromatography

PURPOSE

Tear fluid (TF) contains a variety of electrolytes that exhibit a strong correlation with its osmotic pressure. These electrolytes are also related to the etiology of diseases on ocular surfaces such as dry eye syndromes and keratopathy. Although positive ions (cations) in TF have been investigated to understand their roles, negative ions (anions) have hardly been studied because applicable analytical methods are restricted to a few kinds. In this study, we established a method to analyze the anions involved in a sufficiently small amount of TF for in situ diagnosis of a single subject.

METHODS

Twenty healthy volunteers (10 men and 10 women) were recruited. Anions in their TF were measured on a commercial ion chromatograph (IC-2010, Tosoh, Japan). Tear fluid (5 μL or more) was collected from each subject with a glass capillary, diluted with 300 μL of pure water, and conveyed to the chromatograph. We successfully monitored the concentrations of bromide, nitrate, phosphate, and sulfate anions (Br - , NO 3- , HPO 42- , and SO 42- , respectively) in TF.

RESULTS

Br - and SO 42- were universally detected in all samples, whereas NO 3- was found in 35.0% and HPO 42- in 30.0% of them. The mean concentrations (mg/L) of each anion were Br - , 4.69 ± 0.96; NO 3- , 0.80 ± 0.68; HPO 42- , 17.48 ± 7.60; and SO 42- , 3.34 ± 2.54. As for SO 42- , no sex differences or diurnal variations were observed.

CONCLUSIONS

We established an efficient protocol to quantitate various inorganic anions involved in a small amount of TF using a commercially available instrument. This is the first step to elucidate the role of anions in TF.

Urban Particulate Matter Triggers Meibomian Gland Dysfunction

Purpose

The meibomian gland (MG), as the largest modified sebaceous gland, is potentially damaged by urban particulate matter (UPM) based on epidemiological evidence, but the specific experimental mechanisms remain unknown. This study investigated the effects of UPM on MG dysfunction (MGD) in rodent models.

Methods

Female C57BL/6J mice received eye drops containing UPM suspension or PBS for 14 days. The proliferative capacity and progenitor of MG were evaluated by immunofluorescence. Cell apoptosis was confirmed by TUNEL assay, along with the analysis of caspase family expression. Lipid accumulation was visualized by Oil Red O staining and LipidTox staining. Ductal hyperkeratinization, neutrophil infiltration, and pyroptosis activation were detected through immunostaining. The relative gene expression and signaling pathway activation were determined by Western blot analysis.

Results

Administration of UPM caused MGD-like clinical signs, manifested as distinct corneal epithelial erosion, increased MG orifice occlusion, and glandular dropout. UPM exposure significantly induced progenitor loss, cellular apoptosis, and lipogenic disorder in MG, by reducing P63/Lrig1 expression and increasing cleaved caspase-8, -9, and -3 and meibum lipogenic protein (HMGCR/SREBP-1) expression. UPM-treated mice exhibited ductal hyperkeratinization and neutrophil recruitment. Simultaneously, pyroptosis was motivated, as indicated by the heightened expression of NLRP3 and the cleavage of caspase-1 and -4 and gasdermin D, as well as the increase in IL-1β and IL-18 downstream. The underlying pathological mechanisms of UPM involve the phosphorylation of mitogen-activated protein kinase and nuclear factor-κB.

Conclusions

These results provided direct evidence for the toxicity of UPM in MG. UPM-induced activation of pyroptosis and mitogen-activated protein kinase/nuclear factor-κB signaling pathway might account for the inflammatory MGD.

Elucidating the Molecular Interactions between Lipids and Lysozyme: Evaporation Resistance and Bacterial Barriers for Dry Eye Disease

Dry eye disease (DED) is a chronic condition characterized by ocular dryness and inflammation. The tear film lipid layer (TFLL) is the outermost layer composed of lipids and proteins that protect the ocular surface. However, environmental contaminants can disrupt its structure, potentially leading to DED. Although the importance of tear proteins in the TFLL functionality has been clinically recognized, the molecular mechanisms underlying TFLL-protein interactions remain unclear. In this study, we investigated tear protein-lipid interactions and analyzed their role in the TFLL functionality. The results show that lysozyme (LYZ) increases the stability of the TFLL by reducing its surface tension and increasing its surface pressure, resulting in increased TFLL evaporation and bacterial invasion resistance, with improved wettability and lubrication performance. These findings highlight the critical role of LYZ in maintaining ocular health and provide potential avenues for investigating novel approaches to DED treatment and patient well-being.

Relationship between Dry Eye Disease and Dyslipidemia: A Systematic Review

BACKGROUND

Dyslipidemia has been suggested to be associated with the occurrence of dry eye disease (DED). However, whether dyslipidemia is responsible for the development of DED remains unclear. In this systematic review, we explored the relationship between DED and dyslipidemia by using quantitative data.

METHODS

Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, we conducted a comprehensive literature search in several databases, including PubMed, Embase, Cochrane Library, Web of Science, and Google Scholar, and obtained six relevant studies.

RESULTS

Our findings indicated that the majority of the selected studies reported a statistically significant association between dyslipidemia and DED, particularly in women. However, our quantitative analysis revealed that only two studies reported statistically significant differences in total cholesterol and high-density lipoprotein cholesterol values.

CONCLUSION

No statistically significant differences exist in the majority of lipid profile parameters between individuals with and without DED, but there is a statistically significant association between dyslipidemia and DED.

Latanoprost incorporates in the tear film lipid layer: An experimental and computational model study

Glaucoma is a leading cause of blindness worldwide, with elevated intraocular pressure being a major risk factor for its development and progression. First-line treatment for glaucoma relies on the administration of prostaglandin analogs, with latanoprost being the most widely used. However, before latanoprost reaches the cornea, it must pass through the tear film and tear film lipid layer (TFLL) on the ocular surface. Given the significant lipophilicity of latanoprost, we hypothesize that TFLL could, to a certain extent, act as a reservoir for latanoprost, releasing it on longer time scales, apart from the fraction being directly delivered to the cornea in a post-instillation mechanism. We investigated this possibility by studying latanoprost behavior in acellular in vitro TFLL models. Furthermore, we employed in silico molecular dynamics simulations to rationalize the experimental results and obtain molecular-level insight into the latanoprost-TFLL interactions. Our experiments demonstrated that latanoprost indeed accumulates in the TFLL models, and our simulations explain the basis of the accumulation mechanism. These results support the hypothesis that TFLL can serve as a reservoir for latanoprost, facilitating its prolonged release. This finding could have significant implications for optimizing glaucoma treatment, especially in the development of new drug delivery systems targeting the TFLL.

Alteration of Meibum Lipidomics Profiling in Patients With Chronic Ocular Graft-Versus-Host Disease

Purpose

To investigate the characteristics of the lipid profiling in meibum of patients with chronic ocular graft-versus-host disease (coGVHD) and to detect the potential influence of anti-inflammatory therapy on these differential lipids.

Methods

This cross-sectional study included 25 coGVHD patients and 13 non-coGVHD after allogeneic hematopoietic stem cell transplantation. Among those with coGVHD, 14 had prior topical treatment (coGVHD(T)), and 11 did not (coGVHD(WT)). All participants completed ocular surface disease index questionnaire and received slit lamp examination, Schirmer's test without anesthesia, ocular surface interferometer, and meibography. Binocular meibum was collected and pooled for lipidomic analysis by liquid chromatography-mass spectrometry.

Results

One hundred and twenty differential lipid species were found among the three groups (96 of coGVHD(WT) vs. non-coGVHD, 78 of coGVHD(WT) vs. coGVHD(T), and three of non-coGVHD vs. coGVHD(T)). Compared with non-coGVHD group, coGVHD(WT) group had a significant abnormality of meibum composition, showing a significant decrease in glycerolipids, and an increase in glycerophospholipids and sphingolipids. Similar changes were also observed when coGVHD(WT) versus coGVHD(T). CoGVHD severity was negatively associated with mono-unsaturated triglycerides (TG), (β = -214.7; 95% CI, -363.9 to -65.5; P = 0.006) and poly-unsaturated TG (β = -4019.9; 95% CI, -7758.1 to -281.6; P = 0.036). Intensity of immunosuppression was negatively associated with mono-unsaturated TG (β = -162.4; 95% CI, -268.6 to -56.2; P = 0.004) and positively associated with phosphatidylcholine (β = 332.0; 95% CI, 19.2-644.8; P = 0.038).

Conclusions

Altered meibum in coGVHD is characterized by a decrease of glycerolipids and an increase of glycerophospholipids and may be significantly reversed by topical anti-inflammatory therapy.

Tear Film Lipid Layer Changes Following Combined Effect of Heated Eye Mask with Intense Pulsed Light Therapy for Evaporative Dry Eye: A Randomized Control Study

Background: Dry eye disease (DED) is a complex ocular surface inflammatory disorder with a multifactorial etiology. Therapies such as intense pulsed light (IPL) and heated eye mask (HEM) have been reported to improve the tear film lipid layer (TFLL) and signs and symptoms of DED. Methods: This randomized study aimed to compare the effects of IPL combined with HEM (IPL+HEM) group, IPL group, and control group in participants with evaporative DED. All participants were examined at baseline (D0), day 21 (D21), day 42 (D42), and day 84 (D84) for noninvasive tear breakup time (NITBUT), TFLL, corneal conjunctival staining (CS), meibomian gland quality (MGQ), meibomian gland expressibility (MGEx), and Ocular Surface Disease Index (OSDI). Results: The mean age of participants was IPL+HEM: 28.06 ± 3.88 years, IPL: 29.88 ± 4.68 years, and control: 28.52 ± 3.77 years. At D84, significant improvements in TFLL (p < 0.05), noninvasive tear breakup time (NITBUT) (p < 0.05), corneoconjunctival staining (CS) (p < 0.05), MGQ (p < 0.05), MGEx (p < 0.05), and OSDI (p < 0.05) were found in the IPL+HEM and IPL groups, whereas the control group had no significant improvements. Furthermore, ΔTFLL significantly correlated with ΔNITBUT (r = -0.678, p < 0.001), ΔCS (r = 0.321, p < 0.001), ΔMGQ (r = 0.669, p < 0.001), ΔMGEx (r = 0.598, p < 0.001), and ΔOSDI score (r = 0.649, p < 0.001). Conclusions: IPL therapy in combination with HEM and IPL therapy only can significantly improve the quality of TFLL and clinically reduce the sign and symptoms of evaporative DED. However, IPL therapy in combination with HEM was found to be more effective than IPL therapy alone.

Meibomian gland development: Where, when and how?

The Meibomian gland (MG) is an indispensable adnexal structure of eye that produces meibum, an important defensive component for maintaining ocular homeostasis. Normal development and maintenance of the MGs is required for ocular health since atrophic MGs and disturbances in composition and/or secretion of meibum result in major ocular pathologies, collectively termed as Meibomian gland dysfunction (MGD). Currently available therapies for MGD merely provide symptomatic relief and do not treat the underlying deficiency of the MGs. Hence, a thorough understanding of the timeline of MG development, maturation and aging is required for regenerative purposes along with signaling molecules & pathways controlling proper differentiation of MG lineage in mammalian eye. Understanding the factors that contribute to the development of MGs, developmental abnormalities of MGs, and changes in the quality & quantity of meibum with developing phases of MGs are essential for developing potential treatments for MGD. In this review, we compiled a timeline of events and the factors involved in the structural and functional development of MGs and the associated developmental defects of MGs during development, maturation and aging.

Immune Fingerprint in Diabetes: Ocular Surface and Retinal Inflammation

Diabetes is a prevalent global health issue associated with significant morbidity and mortality. Diabetic retinopathy (DR) is a well-known inflammatory, neurovascular complication of diabetes and a leading cause of preventable blindness in developed countries among working-age adults. However, the ocular surface components of diabetic eyes are also at risk of damage due to uncontrolled diabetes, which is often overlooked. Inflammatory changes in the corneas of diabetic patients indicate that inflammation plays a significant role in diabetic complications, much like in DR. The eye's immune privilege restricts immune and inflammatory responses, and the cornea and retina have a complex network of innate immune cells that maintain immune homeostasis. Nevertheless, low-grade inflammation in diabetes contributes to immune dysregulation. This article aims to provide an overview and discussion of how diabetes affects the ocular immune system's main components, immune-competent cells, and inflammatory mediators. By understanding these effects, potential interventions and treatments may be developed to improve the ocular health of diabetic patients.

Biophysical profiling of synthetic ultra-long tear film lipids

The tear film lipid layer (TFLL) is a unique biological membrane of importance to the maintenance of ocular surface health. The underlying factors at play, e.g. the ability to retard evaporation and offer protection from the environment, are all closely connected to the properties of individual lipid components and their interplay. The TFLL contains unique ultra-long polar lipid species such as O-acyl-ω-hydroxy fatty acids, type I-St diesters and type II diesters, which are considered important for its proper function. Herein, we have synthesized model compounds from these categories and studied their biophysical and surface rheological properties at the aqueous interface. Altogether, we provide insights on the distinct biophysical profiles of these lipid classes and discuss how their interplay may affect the structure and function of the TFLL.

Effect of Model Tear Film Lipid Layer on Water Evaporation

Purpose

A majority of in vitro models were incapable of reproducing the evaporation resistance of tear film lipid layer (TFLL) in vivo. The purpose of this research is to develop a novel in vitro model to study the effect of TFLL on water evaporation.

Methods

A ventilated, closed-chamber, droplet evaporimeter with a constant surface area has been invented to study the evaporation resistance of TFLL. This evaporimeter ensures 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. Together with direct film imaging using atomic force microscopy (AFM), we have studied the effect of a model TFLL on water evaporation, as a function of the lipid composition and surface pressure.

Results

A model TFLL composed of 40% wax esters, 40% cholesteryl esters, and 20% polar lipids was capable of reducing the water evaporation rate by 11% at surface pressure 47 mN/m. AFM revealed that the model TFLL at high surface pressures consists of discrete droplets/aggregates of the nonpolar lipids residing atop a polar lipid monolayer with phase separation.

Conclusions

The TFLL may resist 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.

Nonpolar Lipids Contribute to Midday Fogging During Scleral Lens Wear

Purpose

To determine correlations between lipids in the fluid reservoir (FR) and the severity of midday fogging (MDF) in scleral lens (SL) wear.

Methods

SL neophytes were recruited to wear custom SL for 4 days, examined after 8 hours on days 1 and 4. Lens vault and MDF were quantified from anterior segment optical coherence tomography (AS-OCT), and the FR was collected and analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Relative abundance of lipids was compared to MDF scores using nonparametric correlation testing (Spearman rank). Ocular surface and SL fitting characteristics (lens vault, fitting curves) were likewise compared to MDF.

Results

Thirteen participants (26 eyes, 69% female, 28 ± 9 years old) were included in this study. MDF severity after 8 hours of SL wear was 33 ± 29 units on day 1 and 28 ± 24 units on day 4 (r = .94; P < 0.01). Twelve samples were analyzed using LC-MS/MS, and a total of 170 distinct lipid species were detected. The lipid classes with greatest correlation to MDF were the wax esters (r = .73, P = 0.01), cholesteryl esters (r = .59; P = 0.049), and triacylglycerols (r = .64, P = 0.03). Polar lipids were observed abundantly in all samples. None of the measured ocular surface or fitting outcomes were correlated to MDF.

Conclusions

Nonpolar lipids were the greatest contributors to MDF among these normal participants. Polar lipids may be due to cellular debris, although they do not appear contributory to MDF.

Optimization of the Sulfo-Phospho-Vanillin Assay for Total Lipid Normalization in Untargeted Quantitative Lipidomic LC-MS/MS Applications

Liquid chromatography (LC)-mass spectrometry (MS)/MS lipidomic normalization is generally performed by equalizing pre-extraction sample materials or via DNA or protein pre-quantitation methods, which have known measurement inaccuracies. We propose the use of the sulfo-phospho-vanillin assay (SPVA), a total lipid colorimetric analysis, as a pre-quantitation method to normalize lipids in lipidomic LC-MS/MS applications. The assay has been applied to a 300 μL well volume in a 96-well plate and tested using Avanti total lipid standards of porcine brain and E. coli. Assay parameters for lipid sample volume, sulfuric acid, vanillin/phosphoric acid, post-reaction incubation time, and wavelength are optimized for robust application to biologically sourced lipid samples. Standard test samples were prepared using three concentrations covering approximately 100 μg/mL range. The optimized assay yielded test sample errors less than 10%, indicating a precise and accurate assay performance. The test samples were then analyzed by LC-MS/MS and normalized using SPVA pre-quantitation and pseudo-mass normalization. The detected lipids showed smaller standard deviations and greater relative concentration differences compared to the pseudo-mass normalized lipids, showing promise as a normalization method.

Comparison Between Heated Eye Mask and Intense Pulsed Light Treatment for Contact Lens-Related Dry Eye

Background: Contact lens-related dry eye (CLDE) is common in contact lens (CL) wearers and can lead to ocular pain, decreased visual quality, and reduced quality of life. Objective: The purpose of this prospective, randomized, examiner masked study was to compare the effectiveness of intense pulsed light (IPL) and heated eye mask (HEM) for relieving CLDE. Methods: The final analysis included 60 patients (30 in the IPL group and 30 in the HEM group) who were using CL and had been suffering contact lens discomfort for more than a year. The IPL treatment group had two IPL treatments spaced 3 weeks apart, whereas the HEM group received HEM daily for 6 weeks (42 days). Non-invasive tear break-up time (NITBUT), tear-film lipid layer (TFLL), conjunctival fluorescein staining, meibum gland quality and expression, conjunctival redness score, ocular surface disease index (OSDI), best-corrected visual acuity, endothelial cell count, and intraocular pressure were assessed at baseline: first visit (V1), second visit (V2), and third visit (V3). Results: The mean age of the IPL group was 28.6 ± 4.3 years (16 females, 53%) and that of the HEM group was 28.6 ± 4.2 years (16 females, 53%). Mean NITBUT for the IPL group at V2 was 5.53 ± 0.77 sec (p < 0.001) and at V3 was 7.72 ± 0.88 sec (p < 0.001); the IPL group demonstrated a clinically and statistically significant improvement in mean NITBUT. In addition, the HEM group showed a clinically and statistically significant improvement in mean NITBUT at V3 (5.86 ± 0.76 sec, p < 0.001). At V3, the C-OSDI, TFLL score, and meibum gland quality and expressibility all increased considerably (p < 0.05) in both groups. Conclusions: Our results imply that IPL or HEM treatment of CLDE can be safely used to relieve symptoms of CLDE. In addition, IPL treatment is more effective in improving the general stability of the tear-film and decreasing the need for artificial tears in CLDE.

Application of Animal Models in Interpreting Dry Eye Disease

Different pathophysiologic mechanisms are involved in the initiation, development, and outcome of dry eye disease (DED). Animal models have proven valuable and efficient in establishing ocular surface microenvironments that mimic humans, thus enabling better understanding of the pathogenesis. Several dry eye animal models, including lacrimal secretion insufficiency, evaporation, neuronal dysfunction, and environmental stress models, are related to different etiological factors. Other models may be categorized as having a multifactorial DED. In addition, there are variations in the methodological classification, including surgical lacrimal gland removal, drug-induced models, irradiation impairment, autoimmune antibody-induced models, and transgenic animals. The aforementioned models may manifest varying degrees of severity or specific pathophysiological mechanisms that contribute to the complexity of DED. This review aimed to summarize various dry eye animal models and evaluate their respective characteristics to improve our understanding of the underlying mechanism and identify therapeutic prospects for clinical purposes.

Biophysical properties of tear film lipid layer II. Polymorphism of FAHFA

Fatty acid esters of hydroxy fatty acids (FAHFAs) are a newly discovered class of endogenous lipids that consist of two acyl chains connected through a single ester bond. Being a unique species of FAHFAs, (O-acyl)-ω-hydroxy fatty acids (OAHFAs) differ from other FAHFAs in that their hydroxy fatty acid backbones are ultralong and their hydroxy esterification is believed to be solely at the terminal (ω-) position. Only in recent years with technological advances in lipidomics have OAHFAs been identified as an important component of the tear film lipid layer (TFLL). It was found that OAHFAs account for approximately 4 mol% of the total lipids and 20 mol% of the polar lipids in the TFLL. However, their biophysical function and contribution to the TFLL is still poorly understood. Here we studied the molecular biophysical mechanisms of OAHFAs using palmitic-acid-9-hydroxy-stearic-acid (PAHSA) as a model. PAHSA and OAHFAs share key structural similarities that could result in comparable biophysical properties and molecular mechanisms. With combined biophysical experiments, atomic force microscopy observations, and all-atom molecular dynamics simulations, we found that the biophysical properties of a dynamic PAHSA monolayer under physiologically relevant conditions depend on a balance between kinetics and thermal relaxation. PAHSA molecules at the air-water surface demonstrate unique polymorphic behaviors, which can be explained by configurational transitions of the molecules under various lateral pressures. These findings could have novel implications in understanding biophysical functions that FAHFAs, in general, or OAHFAs, specifically, play in the TFLL.

Biophysical properties of tear film lipid layer I. Surface tension and surface rheology

Tear film lipid layer (TFLL) is the outmost layer of the tear film. It plays a crucial role in stabilizing the tear film by reducing surface tension and retarding evaporation of the aqueous layer. Dysfunction of the TFLL leads to dysfunctional tear syndrome, with dry eye disease (DED) being the most prevalent eye disease, affecting 10%-30% of the world population. To date, except for treatments alleviating dry eye symptoms, effective therapeutic interventions in treating DED are still lacking. Therefore, there is an urgent need to understand the biophysical properties of the TFLL with the long-term goal to develop translational solutions in effectively managing DED. Here, we studied the composition-function correlations of an artificial TFLL, under physiologically relevant conditions, using a novel experimental methodology called constrained drop surfactometry. This artificial TFLL was composed of 40% behenyl oleate and 40% cholesteryl oleate, representing the most abundant wax ester and cholesteryl ester in the natural TFLL, respectively, and 15% phosphatidylcholine and 5% palmitic-acid-9-hydroxy-stearic-acid (PAHSA), which represent the two predominant polar lipid classes in the natural TFLL. Our study suggests that the major biophysical function of phospholipids in the TFLL is to reduce the surface tension, whereas the primary function of PAHSA is to optimize the rheological properties of the TFLL. These findings have novel implications in better understanding the physiological and biophysical functions of the TFLL and may offer new translational insight to the treatment of DED.

Metabolomics and lipidomics approaches in human tears: A systematic review

The human tear film is at the interface between the ocular surface and the external environment. Although investigation has been hindered by its small volume, improvements in preanalytical and analytical methods have allowed the omics approach to represent an innovative biomarker search strategy. There is still a significant lack of standardization, representing a barrier for performing between-studies comparisons and transferring experimental findings into clinical use and trials. We summarize the preanalytical and analytical procedures, describe the biomarkers that can be found using the metabo-lipidomics approach, and provide our expert opinion for omics investigations in human tears. For this systematic review of 38 studies, we searched PubMed by combining Boolean operators with the following keywords: tear, metabolomic, lipidomic, -omics. The human tear metabo-lipidome has been well-characterized in normal individuals using high-resolution liquid chromatography coupled with mass spectrometry. Lipid and metabolite profiles were influenced by ocular (e.g. dry eye disorders; Meibomian gland dysfunction; contact lens wear; glaucoma; keratoconus; pterygium) and systemic conditions (e.g. multiple sclerosis). Investigating the tear metabo-lipidome could improve our understanding of the pathogenesis of both ocular and systemic diseases, but also provide diagnostic as well as prognostic biomarkers.

Human meibomian gland epithelial cell culture models: Current progress, challenges, and future directions

The widely used immortalised human meibomian gland epithelia cell (iHMGEC) line has made possible extensive studies of the biology and pathophysiology of meibomian glands (MG). Tissue culture protocols for iHMGEC have been revised and modified to optimise the growth conditions for cell differentiation and lipid accumulation. iHMGEC proliferate in serum-free medium but require serum or other appropriate exogenous factors to differentiate. Several supplements can enhance differentiation and neutral lipid accumulation in iHMGEC grown in serum-containing medium. In serum-free medium, rosiglitazone, a peroxisome proliferator activator receptor-γ (PPARγ) agonist, is reported to induce iHMGEC differentiation, neutral lipid accumulation and expression of key biomarkers of differentiation. iHMGEC cultured in serum-containing medium under hypoxia or with azithromycin increases DNAse 2 activity, a biomarker of terminal differentiation in sebocytes. The production of lipids with composition similar to meibum has not been observed in vitro and this remains a major challenge for iHMGEC culture. Innovative methodologies such as 3D ex vivo culture of MG and generation of MG organoids from stem cells are important for further developing a model that more closely mimics the in vivo biology of human MG and to facilitate the next generation of studies of MG disease and dry eye.

Effects of Aging on Human Meibum

Purpose

The purpose of this study was to determine if aging affects meibum lipid composition in non-meibomian gland dysfunction (MGD)/non-dry eye (DE) population. Aging has been repeatedly linked to pathological changes in various tissues and organs, including the onset of MGD and DE, in a number of clinical and population-wide surveys. Both conditions have been associated with abnormal meibum secretion and composition, among other factors. However, the chemical basis for such a connection has not been established yet.

Methods

To identify and characterize possible changes in the meibum and meibogenesis with aging, lipidomic analyses of meibum samples collected from human subjects of two age groups - young (29 ± 5 years, n = 21) and elderly (68 ± 7 years, n = 29) - with similar male to female ratios in each group were conducted. Intact lipid species from major lipid groups of meibum (such as wax esters, cholesteryl esters, free cholesterol, triacylglycerols, etc.) were compared using lipidome-wide untargeted (such as Principal Component Analysis) and targeted (such as Orthogonal Projections to Latent Structures Discriminant Analysis) approaches, along with focused analyses of specific lipid species in liquid-chromatography mass spectrometry (LC-MS) and tandem mass spectrometry (MS-MS) experiments.

Results

Extremely high similarities of meibum lipids in the two age groups were observed, with only minor changes in the individual lipid species. The magnitude of the intergroup variability for tested lipid species was comparable to the intragroup variability for the same meibum components. No statistically significant differences in the lipid esterification, elongation, and unsaturation patterns were observed.

Conclusions

Chronological aging itself seems to have only minor effect on meibogenesis in healthy, non-MGD/non-DE subjects.

Differential effects of dietary cholesterol and triglycerides on the lipid homeostasis in Meibomian glands

Exocrine Meibomian glands (MG) play a central role in the ocular surface physiology by producing meibum - a lipid secretion composed of cholesteryl esters (CE), cholesterol (Chl), triacylgycerols (TAG), waxes and other types of lipids. MG were previously shown to synthesize Meibomian lipids (ML) in situ via a complex array of reactions termed meibogenesis. However, questions remain about the role of dietary lipids in meibogenesis. To establish if dietary Chl (DC) and TAG (DT) can participate in meibogenesis, we studied mice whose diet was supplemented with trace amounts of deuterated Chl (2H-Chl) and 13C-labeled triolein (13C-TO), and the products of their biosynthetic transformations were analyzed using LC/MS. We demonstrated that 2H-Chl, but not 13C-TO, could be directly incorporated into meibum. Furthermore, 2H-Chl was esterified into MG-specific ultra long 2H-CE, which were vastly different from plasma CE and 2H-CE. The measured 2H-Chl/Chl and 2H-CE/CE ratios in meibum increased in a time-dependent manner reaching ∼5% and ∼1.2 %, respectively. The 2H-Chl/2H-CE ratio was about 3.5x higher than that for endogenous unlabeled Chl and CE, indicating accumulation of 2H-Chl in meibum. The elongation pattern of Meibomian 2H-CE closely replicated that of unlabeled CE. On the other hand, 13C-TO was not detected in any of the ML samples as an intact lipid or its metabolized/hydrolyzed products. We conclude that DC can be directly esterified into MG-specific CE, while DT undergo extensive catabolic transformations before reaching MG. These findings demonstrate that DC can have a direct impact on MG and ocular surface lipid homeostasis and pathophysiology.

Human meibum and tear film derived (O-acyl)-omega-hydroxy fatty acids in meibomian gland dysfunction

PURPOSE

The molecular basis of the tear film and lipid layer alterations in meibomian gland dysfunction (MGD) is unknown. This study aimed to identify and compare (O-acyl)-omega-hydroxy fatty acids (OAHFAs) derived from human meibum and tears in MGD.

METHODS

Of 195 eligible subjects (18-84 years, 62.6% female), 183 and 174 provided samples for tears and meibum, respectively. Subjects were classified into four groups: Normal, Asymptomatic MGD, MGD, and Mixed. Samples from the right eye of each subject were infused into the SCIEX 5600 TripleTOF mass spectrometer in negative ion mode. Lipid intensities identified with Analyst1.7 TF and SCIEX LipidView1.3 were normalized by an internal standard and total ion current, then statistically compared in MetaboAnalyst 4.0.

RESULTS

In meibum and tears, 76 and 78 unique OAHFAs were identified, respectively. The five most frequent and abundant OAHFAs were 18:2/16:2, 18:1/32:1, 18:1/30:1, 18:2/32:1, and 18:1/34:1. Two OAHFAs, 18:2/20:2 and 18:2/20:1, were identified only in tears. Initial univariate analysis revealed three differently regulated OAHFAs in meibum and eight in tears. Partial Least Square Discriminant Analysis showed 18:1/32:1, 18:2/16:2, 18:1/34:1 and 18:0/32:1 in tears, and 18:2/16:2, 18:1/32:1 and 18:2/32:2 in meibum, had variable importance in projection scores >1.5 and contributed the most to the separation of groups. In both meibum and tears, all OAHFAS except 18:2/16:2 were reduced in MGD compared to the normal group.

CONCLUSION

MGD is accompanied by differential expression of specific OAHFAs in meibum and tears. These results suggest OAHFAs play a role in the altered biochemical profile of the tear film lipid layer in humans with MGD.

Triacylglycerol lipidome from human meibomian gland epithelial cells: Description, response to culture conditions, and perspective on function

Preliminary work has shown that select triacylglycerols (TAGs) are upregulated in a preclinical model of MGD, suggesting that TAGs may be an important outcome variable in research involving human meibomian gland epithelial cells (HMGECs). The purpose of this study was to explore the HMGEC TAG lipidome in culture conditions known to influence differentiation. HMGECs were differentiated in DMEM/F12 with 10 ng/ml EGF, FBS (2% or 10%), and rosiglitazone (0, 20, or 50 μM) for two or five days. Following culture, lipids were extracted, processed, and directly infused into a Triple TOF 5600 mass spectrometer (SCIEX, Framingham, MA) with electrospray ionization. MS and MS/MS^ALL spectra were acquired in the positive ion mode and performed with the SWATH technology. Only the TAGs that were present in all 48 samples were included in the analysis. Multiple regression techniques were utilized to assess the effects of each factor (FBS, rosiglitazone, and culture duration) on each expressed TAG. The HMGEC TAG lipidome consisted of 115 TAGs with 42-62 carbons and zero to 10 double bonds. Fatty acyl chains had 14 to 26 carbons and zero to five double bonds. C18:1 (oleic acid, 25/115, 21.7%) and C16:0 (palmitic acid, 16/115, 13.9%) were the most common fatty acids. FBS, rosiglitazone, and culture duration were significant predictors for 93 TAGs (80.9%) with R² values ranging from 0.20 to 0.77 (p < 0.05). FBS and rosiglitazone achieved significance (p < 0.05) for 80 (69.6%) and 67 TAGs (58.3%), respectively. Rosiglitazone demonstrated a selective upregulation of TAGs containing 16 or 18 carbons. Culture duration reached significance (p < 0.05) for only 36 TAGs (31.3%). When comparing the 10 most abundant C18:1-containing TAGs in meibum, FBS was a negative predictor for five TAGs (mean standardized coefficient [SC] = -0.58, p < 0.001), rosiglitazone was a positive predictor for six TAGs (mean SC = 0.41, p ≤ 0.03), and culture duration weakly influenced one TAG (SC = 0.27, p = 0.008). FBS and rosiglitazone, unlike culture duration, are powerful modulators of the TAG profile. Rosiglitazone induces changes that could be consistent with fatty acid synthesis, suggesting that quantifying the TAG lipidome could be an indirect measure of lipogenesis. Though both have been described as differentiating agents, FBS and rosiglitazone induce opposing effects on meibum-relevant TAGs. Culturing with rosiglitazone is associated with a TAG profile that is more consistent with the expected outcome of lipogenesis and with the profile observed in normal human meibum.

Meibum lipid composition in type 2 diabetics with dry eye

PURPOSE

The purpose of this investigation was to analyze and compare the composition of meibum between type 2 diabetics with dry eye disease (DED) and control subjects to better reveal the pathologic mechanisms of the meibomian gland degeneration (MGD) and DED in type 2 diabetes mellitus (T2DM).

METHODS

90 subjects were divided into the following 4 groups: DM-DED group: T2DM patients with DED (n = 30); DM control group: DM patients without DED (n = 18); DED group: DED patients without DM (n = 26); naive control group: normal subjects (n = 16). The lipid composition of meibum samples collected from these subjects was analyzed by high-pressure liquid chromatography-mass spectrometry (HPLC-MS) system. The content of lipid features from 12 major lipid classes was compared among the 4 groups.

RESULTS

A significantly lower level of triacylglycerols (TG) and wax esters (WE) was found between DM-DED patients and normal controls (P < 0.01), whereas the level of Cholesteryl Ester (CE) in DM-DED patients increased compared with DED patients (P < 0.05). The level of (O-acyl)-omega-hydroxy fatty acids (OAHFA) in DM-DED patients was significantly lower than that in normal controls (P < 0.01). An opposite higher level of phospholipids (PLs) was observed in DM-DED patients than that in normal controls (P < 0.01).

CONCLUSIONS

T2DM could influence the expression of meibum lipids to further aggravate DED and MGD. Lower expression of TG,WE and OAHFA, higher expression of CE and PLs were discovered in meibum lipids of T2DM-DED.

A Randomized Double-Masked Phase 2a Trial to Evaluate Activity and Safety of Topical Ocular Reproxalap, a Novel RASP Inhibitor, in Dry Eye Disease

Purpose: To determine whether reproxalap, a novel reactive aldehyde species (RASP) inhibitor, is safe and effective for the treatment of the signs and symptoms of dry eye disease (DED).

Methods: In a randomized double-masked parallel-group Phase 2a trial of 3 topical ocular reproxalap formulations (0.1% ophthalmic solution, 0.5% ophthalmic solution, and 0.5% lipid ophthalmic solution), 51 patients with DED were randomly assigned 1:1:1 at a single US site. Eyes were treated bilaterally 4 times daily for 28 days, and standard DED signs and symptoms were assessed at baseline and after 7 and 28 days of dosing. Tear RASP levels were assessed at baseline and at day 28.

Results: The effect of treatment on DED signs and symptoms was similar across the treatment arms, and pooled data from the 28-day treatment period demonstrated significant improvement from baseline in Symptom Assessment in Dry Eye Disease score (P = 0.003), Ocular Discomfort Scale score (P < 0.0001), Ocular Discomfort Score and 4-Symptom Questionnaire overall score (P = 0.0004), Schirmer's test (P = 0.008), tear osmolarity (P = 0.003), and lissamine green total staining score (P = 0.002). Improvements in DED symptoms were evident within 1 week of therapy, and effect sizes generally approached or exceeded 0.5. No significant changes in safety measures were observed.

Conclusion: The results suggest that the novel RASP inhibitor reproxalap has the potential to mitigate the signs and symptoms of DED, and may represent a new, rapidly and broadly active treatment approach for DED (NCT03162783).

Obese Mice with Dyslipidemia Exhibit Meibomian Gland Hypertrophy and Alterations in Meibum Composition and Aqueous Tear Production

BACKGROUND

Dyslipidemia may be linked to meibomian gland dysfunction (MGD) and altered meibum lipid composition. The purpose was to determine if plasma and meibum cholesteryl esters (CE), triglycerides (TG), ceramides (Cer) and sphingomyelins (SM) change in a mouse model of diet-induced obesity where mice develop dyslipidemia.

METHODS

Male C57/BL6 mice (8/group, age = 6 wks) were fed a normal (ND; 15% kcal fat) or an obesogenic high-fat diet (HFD; 42% kcal fat) for 10 wks. Tear production was measured and meibography was performed. Body and epididymal adipose tissue (eAT) weights were determined. Nano-ESI-MS/MS and LC-ESI-MS/MS were used to detect CE, TG, Cer and SM species. Data were analyzed by principal component analysis, Pearson's correlation and unpaired t-tests adjusted for multiple comparisons; significance set at p ≤ 0.05.

RESULTS

Compared to ND mice, HFD mice gained more weight and showed heavier eAT and dyslipidemia with higher levels of plasma CE, TG, Cer and SM. HFD mice had hypertrophic meibomian glands, increased levels of lipid species acylated by saturated fatty acids in plasma and meibum and excessive tear production.

CONCLUSIONS

The majority of meibum lipid species with saturated fatty acids increased with HFD feeding with evidence of meibomian gland hypertrophy and excessive tearing. The dyslipidemia is associated with altered meibum composition, a key feature of MGD.

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.

Comprehensive profiling of Asian and Caucasian meibomian gland secretions reveals similar lipidomic signatures regardless of ethnicity

Meibum-a lipid secretion that is produced by Meibomian glands (MG) in a process termed meibogenesis-plays a critical role in ocular surface physiology. Abnormalities in the chemical composition of meibum were linked to widespread ocular pathologies-dry eye syndrome (DES) and MG dysfunction (MGD). Importantly, in epidemiologic studies the Asian population was shown to be prone to these pathologies more than the Caucasian one, which was tied to differences in their meibomian lipids. However, biochemical data to support these observations and conclusions are limited. To determine if non-DES/non-MGD Asian meibum was significantly different from that of Caucasians, individual samples of meibum collected from ethnic Asian population living in Japan were compared with those of Caucasians living in the USA. These experiments revealed that composition of major lipid classes, such as wax esters (WE), cholesteryl esters (CE), triacylglycerols, (O)-acylated ω-hydroxy fatty acids (OAHFA), cholesteryl sulfate, cholesteryl esters of OAHFA, and diacylated α,ω-dihydroxy fatty alcohols remained invariable in both races, barring a minor (< 10%; p < 0.01) increase in the Asian CE/WE ratio. Considering the natural variability range for most meibomian lipids (app. ± 15% of the Mean), these differences in meibogenesis were deemed to be minimal and unlikely to have a measurable physiological impact.

Delineating a novel metabolic high triglycerides-low waxes syndrome that affects lipid homeostasis in meibomian and sebaceous glands

Meibomian glands that are embedded in tarsal plates of human eyelids, and sebaceous glands found in the skin, including that of eyelids, are two related types of holocrine glands that produce lipid-rich secretions called meibum and sebum. Pervasive ocular disorders, such as Meibomian gland dysfunction and dry eye, have been linked to changes in the lipid composition of meibum. However, in most described cases the changes were either small, or random, or insufficiently characterized on the molecular level. Here, we present results of comprehensive lipidomic analyses of meibum, tears and sebum of a patient whose secretions were highly abnormal (abnormal meibum, tears and sebum, or AMTS, patient). The lipidomes were characterized on the level of individual lipid species using ultra-high performance liquid chromatography and high resolution mass spectrometry. The major differences between the AMTS patient and normal age- and gender-matched subjects included, among others, severely diminished pools of normal meibomian lipids such as wax esters and cholesteryl esters in meibum and tears, a 2x increase in total cholesteryl esters to wax esters ratio, their skewed molecular profiles, a ~3x increase in free cholesterol to cholesteryl esters ratio, and, most importantly, a 20x to 30x increase in the triglicerides fraction over the norm. Sebum of the AMTS patient was also highly abnormal lacking major wax esters. Notably, the routine blood lipid panel test of the AMTS patient showed no abnormalities. The data imply that the AMTS patient had a severe, previously unreported, metabolic disorder that affected meibogenesis in Meibomian glands and sebogenesis in sebaceous glands. This is, to the best of our knowledge, a first observation of the condition that we have termed High Triglycerides/Low Waxes (HTLW) syndrome.

Lipid polarity gradient formed by ω-hydroxy lipids in tear film prevents dry eye disease

Meibum lipids form a lipid layer on the outermost side of the tear film and function to prevent water evaporation and reduce surface tension. (O-Acyl)-ω-hydroxy fatty acids (OAHFAs), a subclass of these lipids, are thought to be involved in connecting the lipid and aqueous layers in tears, although their actual function and synthesis pathway have to date remained unclear. Here, we reveal that the fatty acid ω-hydroxylase Cyp4f39 is involved in OAHFA production. Cyp4f39-deficient mice exhibited damaged corneal epithelium and shortening of tear film break-up time, both indicative of dry eye disease. In addition, tears accumulated on the lower eyelid side, indicating increased tear surface tension. In Cyp4f39-deficient mice, the production of wax diesters (type 1ω and 2ω) and cholesteryl OAHFAs was also impaired. These OAHFA derivatives show intermediate polarity among meibum lipids, suggesting that OAHFAs and their derivatives contribute to lipid polarity gradient formation for tear film stabilization.

Mixed polar-nonpolar lipid films as minimalistic models of Tear Film Lipid Layer: A Langmuir trough and fluorescence microscopy study

The Tear Film Lipid Layer (TFLL) covering the surface of the aqueous film at human cornea forms a first barrier between the eye and environment. Its alterations are related to dry eye disease. TFLL is formed by a complex mixture of lipids, with an excess of nonpolar components and a minor fraction of polar molecules. Its thickness is up to 160 nm, hence a multilayer-like structure of TFLL is assumed. However, details of TFLL organization are mostly unavailable in vivo due to the dynamic nature of the human tear film. To overcome this issue, we employ a minimalistic in vitro lipid model of TFLL. We study its biophysical characteristics by using a combination of the Langmuir trough with fluorescence microscopy. The model consists of two-component polar-nonpolar lipid films with a varying component ratio spread on the aqueous subphase at physiologically relevant temperature. We demonstrate that the model lipid mixture undergoes substantial structural reorganization as a function of lateral pressure and polar to nonpolar lipid ratio. In particular, the film is one-molecule-thick and homogenous under low lateral pressure. Upon compression, it transforms into a multilayer structure with inhomogeneities in the form of polar-nonpolar lipid assemblies. Based on this model, we hypothesize that TFLL in vivo has a duplex polar-nonpolar structure and it contains numerous mixed lipid aggregates formed because of film restructuring. These findings, despite the simplified character of the model, seem relevant for TFLL physiology as well as for understanding pathological conditions related to the lipids of the tear film.

Analysis of (O-acyl) alpha- and omega-hydroxy fatty acids in vernix caseosa by high-performance liquid chromatography-Orbitrap mass spectrometry

Fatty acid esters of long-chain hydroxy fatty acids or (O-acyl)-hydroxy fatty acids (OAHFAs) were identified for the first time in vernix caseosa and characterized using chromatography and mass spectrometry. OAHFAs were isolated from the total lipid extract by a two-step semipreparative TLC. The general structure of OAHFAs was established using high-resolution and tandem mass spectrometry of intact lipids and their transesterification and derivatization products. Two isomeric lipid classes were identified: O-acyl esters of ω-hydroxy fatty acids (ωOAHFA) and O-acyl esters of α-hydroxy fatty acids (αOAHFAs). To the best of our knowledge, αOAHFAs have never been detected in any biological sample before. Chromatographic separation and identification of OAHFAs species were achieved using non-aqueous reversed-phase HPLC coupled to electrospray ionization hybrid linear ion trap-Orbitrap mass spectrometry. The lipid species were detected as deprotonated molecules, and their structures were elucidated using data-dependent fragmentation in the negative ion mode. More than 400 OAHFAs were identified in this way. The most abundant ωOAHFAs species were 28:0/ω-18:2, 29:0/ω-18:2, 30:0/ω-18:2, 32:0/ω-18:2, and 30:0/ω-18:3, while αOAHFAs comprised saturated species 21:0/α-24:0, 22:0/α-24:0, 23:0/α-24:0, 24:0/α-24:0, and 26:0/α-24:0. OAHFAs were estimated to account for approximately 0.04% of vernix caseosa lipids. Graphical Abstract.

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.

Evaluation of use of essential fatty acids in topical ophthalmic preparations for dry eye

PURPOSE

Essential fatty acids (EFAs) as dietary supplements are used in treating dry-eye for reducing inflammation at the ocular surface. Their topical application in eye drops to deliver fatty acid (FA) directly to the ocular surface requires thorough investigation. Being lipids in nature EFAs can interact with tear lipids and affect tear stability. This study aimed at investigating the biophysical interactions of EFAs with Meibomian lipids.

METHODS

Rheology of mixtures of Human Meibomian lipids with EFAs (LA-linoleic acid, ALA-alpha-linolenic acid), OA (oleic acid), and GLA (gamma-linolenic acid) was studied using Langmuir trough technology on an artificial tear solution at the ocular surface temperature. Pressure-area profiles were used to determine compressibility and elasticity of the mixed films.

RESULTS

LA enhanced spreading of Meibomian lipids and increased their compressibility and elasticity which can be beneficial for tear stability. ALA condensed Meibomian lipids film with less elasticity deemed unfavourable for tear stability. OA expanded Meibomian lipids but decreased elasticity at high compressions making films less stable. GLA had little or no favourable effect on tear stability. Higher concentrations of FAs made films less stable.

CONCLUSIONS

EFAs or OA in topical ophthalmic preparations can affect spread and stability of the tear film lipid layer. Rheology of mixed films should be tested using Langmuir trough technology to determine suitable type and amount of a lipid additive for therapeutic eye drops. In topical applications, the omega-6 LA (not omega-3 FA) at low concentrations (20 mol%) can be beneficial for enhancing tear stability in dry eye patients.

Cyclic Change of Fatty Acid Composition in Meibum During the Menstrual Cycle

Purpose

To evaluate the fatty acid (FA) composition in the meibum of pre- and postmenopausal women and age-matched men.

Methods

This prospective study involved 24 healthy subjects; six premenopausal women in their 30s with a regular menstrual cycle (young-female [yF] group), six postmenopausal women in their 60s (elderly-female [eF] group), and 12 age-matched men (i.e., young-male [yM] and elderly-male [eM] groups, respectively). The menstrual cycle was divided into six phases (phase I–VI). Meibum was obtained from the meibomian gland orifices via a Daviel spoon, and its FA composition was then analyzed via gas chromatography mass spectrometry (GC-MS). Principal component analysis (PCA) was performed on the GC-MS results.

Results

The mean FA composition of all subjects was 40% saturated FAs (SFA) and 60% unsaturated FAs (UFAs). The PCA results of all groups indicated two categories (PC1 [77.5%] and PC2 [12.4%]); one consisting of yF-group samples of mainly phase II and III and the other consisting of the yF-group samples of the rest of the cycle, as well as from eF-group, yM-group, and eM-group samples. Each group had a distinctive nature. The FAs that most contributed to PC1 were C_14:0, C_16:0, and C_18:0 in a positive correlation, and C_18:1n9 in a negative correlation.

Conclusions

FA composition noticeably changes during the menstrual cycle and is somewhat affected by sex and age. The ratio of SFAs (C_16:0, C_18:0) to mono-UFAs (C_18:1n9) in the FA composition might have an impact on the lipid quality of meibum, thus suggesting alteration of its melting temperature and viscosity.

Comparative Transcriptomic and Lipidomic Analyses of Human Male and Female Meibomian Glands Reveal Common Signature Genes of Meibogenesis

Meibum is a lipid secretion that is produced by holocrine Meibomian glands (MGs). MGs are a specialized type of sebaceous glands that are embedded in the human eyelids. Chemically, meibum and sebum are different. A detailed characterization of lipidome and transcriptome of MG is required to deconvolute a complex and poorly characterized array of biosynthetic reactions (termed meibogenesis) that lead to formation of meibum. Changes in the composition and quality of meibum have been linked to various ocular disorders, some of which are more prevalent in males, while others in females. To establish the role of gender in meibogenesis in humans, we characterized MG transcriptomes and lipidomes of females and males, and identified signature genes of meibogenesis in both genders. Specimens of MG tissues were subjected to mRNA microarray analyses. Chemical composition of meibum samples was assessed chromatographically and mass spectrometrically. Both targeted and untargeted approaches were used. About 290 signature genes of meibogenesis were identified. The analyses of their expression patterns demonstrated no major differences between the genders. Lipid profiling of major classes of meibomian lipids, such as wax esters, cholesteryl esters, free cholesterol, (O)-acylated omega-hydroxy fatty acids (OAHFA), cholesteryl esters of OAHFA, and triacylglycerols, also demonstrated only minor (and random) differences in these lipids. The results of transcriptomic analyses correlated well with lipidomic data. Taken together, our data imply that in males and females, meibogenesis proceeds in a similar fashion, yielding secretions with similar, highly conserved, compositions. This finding is important for designing novel, gender-independent diagnostic and therapeutic approaches to various MG-related diseases and pathological conditions.

Tear Metabolomics in Dry Eye Disease: A Review

Dry eye disease (DED) is a multifactorial syndrome that can be caused by alteration in the quality or quantity of the precorneal tear film. It is considered one of the most common ocular conditions leading patients to seek eye care. The current method for diagnostic evaluations and follow-up examinations of DED is a combination of clinical signs and symptoms determined by clinical tests and questionnaires, respectively. The application of powerful omics technologies has opened new avenues toward analysis of subjects in health and disease. Metabolomics is a new emerging and complementary research discipline to all modern omics in the comprehensive analysis of biological systems. The identification of distinct metabolites and integrated metabolic profiles in patients can potentially inform clinicians at an early stage or during monitoring of disease progression, enhancing diagnosis, prognosis, and the choice of therapy. In ophthalmology, metabolomics has gained considerable attention over the past decade but very limited such studies have been reported on DED. This paper aims to review the application of tear metabolomics in DED.

Dyslipidemia and Meibomian Gland Dysfunction: Utility of Lipidomics and Experimental Prospects with a Diet-Induced Obesity Mouse Model

Meibomian gland dysfunction (MGD) is the leading cause of dry eye disease and loss of ocular surface homeostasis. Increasingly, several observational clinical studies suggest that dyslipidemia (elevated blood cholesterol, triglyceride or lipoprotein levels) can initiate the development of MGD. However, conclusive evidence is lacking, and an experimental approach using a suitable model is necessary to interrogate the relationship between dyslipidemia and MGD. This systematic review discusses current knowledge on the associations between dyslipidemia and MGD. We briefly introduce a diet-induced obesity model where mice develop dyslipidemia, which can serve as a potential tool for investigating the effects of dyslipidemia on the meibomian gland. Finally, the utility of lipidomics to examine the link between dyslipidemia and MGD is considered.

A comparative study of the quality of non-stimulated and stimulated tears in normal eye male subjects using the tear ferning test

PURPOSE

To evaluate and compare the tear ferning patterns of non-stimulated and stimulated tears in normal eye male subjects using the tear ferning test.

METHODS

Thirty-five male subjects aged 20-38 years with normal eyes were enrolled in the study. Subjects with dry eye or with a risk factor for eye dryness (eg, recent ocular surgery, contact-lens wearers, smokers, anemic, diabetes, and thyroid gland problems) were excluded from the study. Dry eye subjects were excluded based on the ocular surface diseases index scores, non-invasive tear break-up time, and tear meniscus height measurements from the right and left eyes. A tear sample was collected from both eyes in each subject to perform the tear ferning test for non-stimulated tears. After 10 mins, the tears were stimulated using a Schirmer strip, a tear sample was collected from both eyes in each participant, the tear ferning test was performed, and the ferns were graded based on the five-point tear ferning grading scale.

RESULTS

The average tear ferning grade [median (interquartile range)] for non-stimulated tears from the right and left eyes was 0.60 (1.25) and 0.50 (1.31), respectively. For stimulated tears, the average tear ferning grade from the right and left eyes was 0.50 (1.05) and 0.75 (1.25), respectively. There was no significant difference in the tear ferning grades obtained from non-stimulated and stimulated tears (Wilcoxon test, P>0.05).

CONCLUSION

This study found no significant difference in the tear ferning patterns obtained from non-stimulated and stimulated tears, or between the tear ferning patterns in the tears collected from the right and left eyes before and after stimulating tears. It seems that there were no changes in the chemical composition of tears before and after stimulation, possibly due to the steady concentration of electrolytes and lipids within the tear film for both non-stimulated and stimulated tears.

Investigating the Role of Specific Tear Film Lipids Connected to Dry Eye Syndrome: A Study on O-Acyl-ω-hydroxy Fatty Acids and Diesters

Dry eye syndrome (DES) is a prevalent disease in which the tear film homeostasis is compromised. One of the main causes of DES is thought to be an alteration in the composition of the outermost layer of the tear film, the tear film lipid layer (TFLL), resulting in an increased evaporation of water from the tear film and subsequent drying of the ocular surface. Recent studies have suggested that the specific TFLL lipids, namely, O-acyl-ω-hydroxy fatty acids (OAHFAs) and diesters (DiEs), may play a role in the development of DES. However, their specific connection to DES has remained largely unknown until now because of the lack of information on their biophysical properties and their role in the TFLL. Herein, we have addressed this issue by studying the biophysical properties and evaporation resistance of a library containing 10 synthetic analogues of TFLL OAHFAs and DiEs. Our results show how the variations of chain length and polar groups affect the phase behavior of these lipids at the tear film surface. In addition, the results revealed that the OAHFAs exhibiting a liquid-expanded to solid phase transition formed films with high evaporation resistance, whereas the DiEs were found to have no evaporation resistance. Altogether, our results shed new light on the role of the OAHFAs and DiEs in the TFLL and their connection to DES, suggesting that OAHFAs are likely a key lipid class in maintaining the TFLL evaporation resistance.

Prevalence of dry eye disease and its association with dyslipidemia

BACKGROUND

Dry eye disease (DED) is a common ocular surface disease significantly affecting the quality of life of patients. The aim of our study is to focus on the prevalence of DED and to determine the relationship between dyslipidemia and DED.

METHODS

The study was performed with the age group of 25-70 years, who attended the ophthalmology outpatient department at Sri Lakshmi Narayana Institute of Medical Sciences with complaints of dry eye. A standard questionnaire was taken, and tear film tests were performed to diagnose dry eye. Further eyelid margin was examined to detect meibomian gland dysfunction. Based on the tests and examination, patients were grouped as men with and without DED and women with and without DED. Fasting lipid profile was investigated for these groups.

RESULTS

The study showed the prevalence of DED mainly in women and found significant association between DED and dyslipidemia. There is a significant relationship between total cholesterol and DED groups especially in women (p<0.001). We also found the association between triglycerides, high density lipoprotein cholesterol, low density lipoprotein cholesterol and DED particularly in women as compared to men.

CONCLUSIONS

Based on the findings, we emphasize that there is a strong relationship between dyslipidemia and the progression of DED particularly in women. Ophthalmologists may increase their role to educate themselves to diagnose dyslipidemia and ensure comprehensive eye care to prevent blindness and cardiovascular disease. Recent treatment modalities could be aimed to improve the quality of life of women and elderly patients suffering from DED.

Efficacy of intra-meibomian gland injection of the anti-VEGF agent bevacizumab for the treatment of meibomian gland dysfunction with lid-margin vascularity

Purpose

To investigate the efficacy of a novel treatment - intra-meibomian gland (MG) injection of the anti-VEGF agent bevacizumab - for MG dysfunction (MGD) with eyelid-margin vascularity.

Methods

A total of 26 eyes from 13 patients diagnosed with MGD and eyelid-margin vascularity were included in our study. Patients received intra-meibomian gland injections of bevacizumab (150 μL, 2.5 mg/0.1 mL) at multiple sites with a 29 G needle where telangiectasia was severe. The Ocular Surface Disease Index (OSDI), tear film, tear-breakup time (TBUT), eyelid-margin features, MG features, conjunctiva, and corneal staining were assessed at 1 day before injection and 1 week, 1 month, and 3 months after injection. Blood pressure, best-corrected visual acuity, intraocular pressure, and slit lamp examinations were performed to assure the safety of patients at 1 day before and 1 day, 1 week, 1 month, and 3 months after injection.

Results

Lid-margin vascularity, conjunctival injection, expressed secretion quality, expressivity of the MG, TBUT, corneal staining, and OSDI were significantly improved 1 week, 1 month, and 3 months after injection compared to baseline values. Lid-margin vascularity, conjunctival injection, meibomian gland expressivity, TBUT, and OSDI continued to improve; the greatest improvements were observed at 1 month and sustained for 3 months. Spearman's correlation analysis indicated that age and sex significantly influenced TBUT improvement. Females and older patients tended to have shorter baseline TBUT that followed a different trend from that of males and younger patients during postinjection visits, revealed by subgroup analysis. No local or systemic side effects were observed at follow-up visits.

Conclusion

This study is the first to explore a novel therapy for MGD - intra-MG injection of the anti-VEGF agent bevacizumab - and it demonstrates that the treatment is effective and safe in eliminating eyelid-margin vascularity, improving MG function and relieving clinical signs and symptoms of MGD.

Glycosylation pathways at the ocular surface

Glycosylation is a major form of enzymatic modification of organic molecules responsible for multiple biological processes in an organism. The biosynthesis of glycans is controlled by a series of glycosyltransferases, glycosidases and glycan-modifying enzymes that collectively assemble and process monosaccharide moieties into a diverse array of structures. Many studies have provided insight into various pathways of glycosylation at the ocular surface, such as those related to the biosynthesis of mucin-type O-glycans and N-glycans on proteins, but many others still remain largely unknown. This review provides an overview of the different classes of glycans described at the ocular surface focusing on their biosynthetic pathways and biological relevance. A precise understanding of these pathways under physiological and pathological conditions could help identify biomarkers and novel targets for therapeutic intervention.

Lipidomic Analysis: From Archaea to Mammals

Lipids are among the most important organic compounds found in all living cells, from primitive archaebacteria to flowering plants or mammalian cells. They form part of cell walls and constitute cell storage material. Their biosynthesis and metabolism play key roles in faraway topics such as biofuel production (third-generation biofuels produced by microorganisms, e.g. algae) and human diseases such as adrenoleukodystrophy, Zellweger syndrome, or Refsum disease. Current lipidomic analysis requires fast and accurate processing of samples and especially their characterization. Because the number of possible lipids and, more specifically, molecular species of lipids is of the order of hundreds to thousands, it is necessary to process huge amounts of data in a short time. There are two basic approaches to lipidomic analysis: shotgun and liquid chromatography-mass spectometry. Both methods have their pros and cons. This review deals with lipidomics not according to the type of ionization or the lipid classes analyzed but according to the types of samples (organisms) under study. Thus, it is divided into lipidomic analysis of archaebacteria, bacteria, yeast, fungi, algae, plants, and animals.

Meibomian glands, meibum, and meibogenesis

Meibum is a lipid-rich secretion that is produced by fully differentiated meibocytes in the holocrine Meibomian glands (MG) of humans and most mammals. The secretion is a part of a defense mechanism that protects the ocular surface from hazardous environmental factors, and from desiccation. Meibomian lipids that have been identified in meibum are very diverse and unique in nature. The lipid composition of meibum is different from virtually any other lipid pool found in the human body. In fact, meibum is quite different from sebum, which is the closest secretion that is produced by anatomically, physiologically, and biochemically related sebaceous glands. However, meibum of mice have been shown to closely resemble that of humans, implying similar biosynthetic mechanisms in MG of both species. By analyzing available genomic, immunohistochemical, and lipidomic data, we have envisioned a unifying network of enzymatic reactions that are responsible for biosynthesis of meibum, which we call meibogenesis. Our current theory is based on an assumption that most of the biosynthetic reactions of meibogenesis are catalyzed by known enzymes. However, the main features that make meibum unique - the ratio of identified classes of lipids, the extreme length of its components, extensive ω-hydroxylation of fatty acids and alcohols, iso- and anteiso-branching of meibomian lipids (e.g. waxes), and the presence of rather unique complex lipids with several ester bonds - make it possible that either the activity of known enzymes is altered in MG, or some unknown enzymes contribute to the processes of meibogenesis, or both. Studies are in progress to elucidate meibogenesis on molecular level.

Structure-function relationship of tear film lipid layer: A contemporary perspective

Tear film lipid layer (TFLL) stabilizes the air/tear surface of the human eye. Meibomian gland dysfunction (MGD) resulting in quantitative and qualitative modifications of TFLL major (>93%) component, the oily secretion of meibomian lipids (MGS), is the world leading cause of dry eye syndrome (DES) with up to 86% of all DES patients showing signs of MGD. Caused by intrinsic factors (aging, ocular and general diseases) and by extrinsic everyday influences like contact lens wear and extended periods in front of a computer screen, DES (resulting in TF instability, visual disturbances and chronic ocular discomfort) is the major ophthalmic public health disease of the present time affecting the quality of life of 10-30% of the human population worldwide. Therefore there is a pressing need to summarize the present knowledge, contradictions and open questions to be resolved in the field of TFLL composition/structure/functions relationship. The following major aspects are covered by the review: (i) Do we have a reliable mimic for TFLL: MGS vs contact lens lipid extracts (CLLE) vs lipid extracts from whole tears. Does TFLL truly consist of lipids only or it is important to keep in mind the TF proteins as well?; (ii) Structural properties of TFLL and of its mimics in health and disease in vitro and in vivo. How the TFLL uniformity and thickness ensures the functionality of the lipid layer (barrier to evaporation, surface properties, TF stability etc.); (iii) What are the main functions of the TFLL? In this aspect an effort is done to emphasize that there is no single main function of TFLL but instead it simultaneously fulfills plethora of functions: suppresses the evaporation (alone or probably in cooperation with other TF constituents) of the aqueous tears; stabilizes (due to its surface properties) the air/tear surface at eye opening and during the interblink interval; and even acts as a first line of defense against bacterial invasion due to its detergency action on the bacterial membranes. An effort is done to highlight how the concept on the importance of TFLL and TF viscoelasticity transpires from old and new studies and what are its clinical implications. An attempt is made to outline the future hot directions of research into the field ranging from quest for molecules that can significantly alter TFLL properties to addressing open questions on the contribution of TFLL to the overall performance of the TF.

Meibomian glands, meibum, and meibogenesis

Meibum is a lipid-rich secretion that is produced by fully differentiated meibocytes in the holocrine Meibomian glands (MG) of humans and most mammals. The secretion is a part of a defense mechanism that protects the ocular surface from hazardous environmental factors, and from desiccation. Meibomian lipids that have been identified in meibum are very diverse and unique in nature. The lipid composition of meibum is different from virtually any other lipid pool found in the human body. In fact, meibum is quite different from sebum, which is the closest secretion that is produced by anatomically, physiologically, and biochemically related sebaceous glands. However, meibum of mice have been shown to closely resemble that of humans, implying similar biosynthetic mechanisms in MG of both species. By analyzing available genomic, immunohistochemical, and lipidomic data, we have envisioned a unifying network of enzymatic reactions that are responsible for biosynthesis of meibum, which we call meibogenesis. Our current theory is based on an assumption that most of the biosynthetic reactions of meibogenesis are catalyzed by known enzymes. However, the main features that make meibum unique - the ratio of identified classes of lipids, the extreme length of its components, extensive ω-hydroxylation of fatty acids and alcohols, iso- and anteiso-branching of meibomian lipids (e.g. waxes), and the presence of rather unique complex lipids with several ester bonds - make it possible that either the activity of known enzymes is altered in MG, or some unknown enzymes contribute to the processes of meibogenesis, or both. Studies are in progress to elucidate meibogenesis on molecular level.