Compositional Analysis of Wax Esters in Human Meibomian Gland Secretions by Direct Infusion Electrospray Ionization Mass Spectrometry

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.

Dynamic Changes in the Gene Expression Patterns and Lipid Profiles in the Developing and Maturing Meibomian Glands

Meibomian glands (MGs) and their holocrine secretion-meibum-play crucial roles in the physiology of the eye, providing protection from environmental factors and desiccation, among other functions. Importantly, aging was implicated in the deterioration of the morphology and functions of MGs, and the quantity and quality of meibum they produce, leading to a loss of its protective properties, while the meibum of young individuals and experimental animals provide ample protection to the eye. Currently, the molecular mechanisms of meibum biosynthesis (termed meibogenesis) are not fully understood. To characterize the physiological changes in developing and maturing MGs, we studied the lipidomes and transcriptomes of mouse MGs ranging from newborns to adults. The results revealed a gradual increase in the critical genes of meibogenesis (such as Elovl3, Elovl4, Awat2, and Soat1, among others) that positively correlated with the biosynthesis of their respective lipid products. The MG transcriptomes of young and adult mice were also analyzed using single-cell RNA sequencing. These experiments revealed the existence of multiple unique populations of MG cells (meibocytes, epithelial cells, and others) with specific combinations of genes that encode meibogenesis-related proteins, and identified clusters and subclusters of cells that were tentatively classified as meibocytes at different stages of differentiation/maturation, or their progenitor cells. A hypothesis was formulated that these cells may produce different types of lipids, and contribute differentially to the Meibomian lipidome.

Androgen and meibomian gland dysfunction: from basic molecular biology to clinical applications

Meibomian gland, the largest sebaceous gland of the body, is responsible for the biosynthesis of lipid layer of the tear film to prevent excessive evaporation. The loss of normal functions of meibomian gland, known as meibomian gland dysfunction (MGD), is a chronic disease and is the leading cause of dry eye symptoms in the clinics. Studies have found sex hormones, especially androgen, play vital roles in the regulation of the functions of meibomian gland. Recently, androgen has also been preliminarily applied in clinics for the treatment of MGD and showed promising results, especially in people with endogenous androgen deficiency. This review summarized the mechanisms of the function of androgen on meibomian gland based on molecular, animal, and clinical studies, and proposed evidence-based views about its potential applications for the treatment of 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.

Untargeted lipidomic analysis of human tears: A new approach for quantification of O-acyl-omega hydroxy fatty acids

PURPOSE

The purpose of this work was to optimize methodology to analyze the human tear film lipids by using untargeted, direct infusion electrospray ionization-mass spectrometry (ESI-MS) to establish the analytical approach for a large-scale clinical translational study of tear film lipids in ocular surface disease, particularly associated with the O-acyl-omega hydroxy fatty acids (OAHFAs).

METHODS

Meibum and tear samples were collected from both eyes of five normal subjects without ocular disease using two different microcapillary collection tubes, glass and polytetrafluoroethylene (PTFE). An untargeted lipidomics approach was used to analyze the lipids in human tear and meibum samples using direct infusion ESI-MS in positive and negative ion modes. Direct and indirect quantification methods were evaluated.

RESULTS

The amount of OAHFAs measured in tears using these techniques was approximately 0.7-0.8% of the total lipids. More phospholipids, including phosphatidylcholine and sphingomyelin, were detected in the tear samples associated with glass microcapillaries compared to PTFE.

CONCLUSIONS

Reliable assessment of lipids in small volumes of tear film is possible using high resolution, untargeted ESI-MS in positive and negative ion modes. Using this technique, the concentration of OAHFAs can be quantified, as can the presence of other polar lipids.

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

Lipids secreted from the meibomian glands form the outermost layer of the tear film and reduce its evaporation. Abnormal changes in the quantities or compositions of lipids present in meibomian gland secretions (meibum) are known to lead to dry eye disease, although the underlying mechanism is not yet well understood. The tree shrew is the non-primate mammal most closely related to humans. To assess the utility of the tree shrew as a model for the study of dry eye disease, we analyzed the lipid profile of tree shrew meibum using an untargeted ESI-MS and MS/MS^all shotgun approach. The resulting lipidome shared many similarities with human meibum, while displaying some interesting differences. For example, several classes of lipids, including wax esters, cholesteryl esters, diesters, and (O-acyl)-ω-hydroxy fatty acids, had relatively longer chain lengths in tree shrew meibum. These increases in length may promote more effective reduction of tear evaporation in the tree shrew, which likely underlies the much longer blinking interval of this mammal. Our results suggest that the tree shrew could be an effective model for the study of dry eye.

Comprehensive shotgun lipidomics of human meibomian gland secretions using MS/MSall with successive switching between acquisition polarity modes

The lipid composition of human meibomian gland secretions (meibum) has been analyzed using both targeted and untargeted mass spectrometric approaches, each of which has its advantages and disadvantages. Herein we report the results of shotgun lipidomic profiling of human meibum using a new approach that combines the advantages of targeted and untargeted analyses to yield highly sensitive and comprehensive profiles. Samples containing an estimated 7-13 µg (8-16 nL) of human meibum lipids were analyzed using MS/MS^all, an untargeted approach for MS/MS. Using MS/MS^all with ESI and successive polarity switching, we obtained tandem mass spectra in both modes at every 1 Da step for all ions in the m/z 200-1,200 range. In approximately 12 min, a total of 2 MS spectra and 2,000 MS/MS spectra were acquired for each sample, from which targeted analysis information was extracted. This approach allowed for the comprehensive and highly sensitive detection of meibum lipids, including species low in abundance. Altogether, more than 600 unique lipid molecular species were identified in meibum, 3 times more than previously reported in untargeted analyses of meibum samples. This untargeted MS and MS/MS^all approach may be extended to other biological systems for the detection of lipids with sensitivity comparable to targeted analysis.

Expression Profiling of Nonpolar Lipids in Meibum From Patients With Dry Eye: A Pilot Study

Purpose

The purpose of this investigation was to characterize differentially expressed lipids in meibum samples from patients with dry eye disease (DED) in order to better understand the underlying pathologic mechanisms.

Methods

Meibum samples were collected from postmenopausal women with DED (PW-DED; n = 5) and a control group of postmenopausal women without DED (n = 4). Lipid profiles were analyzed by direct infusion full-scan electrospray ionization mass spectrometry (ESI-MS). An initial analysis of 145 representative peaks from four classes of lipids in PW-DED samples revealed that additional manual corrections for peak overlap and isotopes only slightly affected the statistical analysis. Therefore, analysis of uncorrected data, which can be applied to a greater number of peaks, was used to compare more than 500 lipid peaks common to PW-DED and control samples. Statistical analysis of peak intensities identified several lipid species that differed significantly between the two groups. Data from contact lens wearers with DED (CL-DED; n = 5) were also analyzed.

Results

Many species of the two types of diesters (DE) and very long chain wax esters (WE) were decreased by ∼20% in PW-DED, whereas levels of triacylglycerols were increased by an average of 39% ± 3% in meibum from PW-DED compared to that in the control group. Approximately the same reduction (20%) of similar DE and WE was observed for CL-DED.

Conclusions

Statistical analysis of peak intensities from direct infusion ESI-MS results identified differentially expressed lipids in meibum from dry eye patients. Further studies are warranted to support these findings.