Lipid Saturation and the Rheology of Human Tear Lipids

Integrated multi-omics and machine learning approach reveals lipid metabolic biomarkers and signaling in age-related meibomian gland dysfunction

Meibomian gland dysfunction (MGD) is a prevalent inflammatory disorder of the ocular surface that significantly impacts patients' vision and quality of life. The underlying mechanism of aging and MGD remains largely uncharacterized. The aim of this work is to investigate lipid metabolic alterations in age-related MGD (ARMGD) through integrated proteomics, lipidomics and machine learning (ML) approach. For this purpose, we collected samples of female mouse meibomian glands (MGs) dissected from eyelids at age two months (n = 9) and two years (n = 9) for proteomic and lipidomic profilings using the liquid chromatography with tandem mass spectrometry (LC-MS/MS) method. To further identify ARMGD-related lipid biomarkers, ML model was established using the least absolute shrinkage and selection operator (LASSO) algorithm. For proteomic profiling, 375 differentially expressed proteins were detected. Functional analyses indicated the leading role of cholesterol biosynthesis in the aging process of MGs. Several proteins were proposed as potential biomarkers, including lanosterol synthase (Lss), 24-dehydrocholesterol reductase (Dhcr24), and farnesyl diphosphate farnesyl transferase 1 (Fdft1). Concomitantly, lipidomic analysis unveiled 47 lipid species that were differentially expressed and clustered into four classes. The most notable age-related alterations involved a decline in cholesteryl esters (ChE) levels and an increase in triradylglycerols (TG) levels, accompanied by significant differences in their lipid unsaturation patterns. Through ML construction, it was confirmed that ChE(26:0), ChE(26:1), and ChE(30:1) represent the most promising diagnostic molecules. The present study identified essential proteins, lipids, and signaling pathways in age-related MGD (ARMGD), providing a reference landscape to facilitate novel strategies for the disease transformation.

Correlations between bulk and surface properties of meibomian lipids with alteration of wax-to-sterol esters content

In a recent study (Ewurum et al., 2021), wax (WE) and sterol esters (CE) from human meibum secretions (MGS) were separated and reconstituted with controlled WE/CE ratios (0%, 20%, 30%, 40%, 50%, 75% and 100% CE weight fractions). It was found that the alterations in the CE content of WE/CE mixtures modified the hydrocarbon chain conformation and packing of the mixture. A major question that emerges is whether the spectroscopic packing parameters determined for bulk meibum translate to a change in the performance of meibomian layers at the air/water interface, as it is the surface film functionality that is crucial for the performance of MGS at the ocular surface. The study of human meibum films with Langmuir surface balance was performed to access the surface properties at blink-like deformations of the film area. Surface pressure (π)-area (A) isocycles and stress relaxations were used to assess the layer's reorganization during area cycling and dilatational elasticity, respectively. The morphology of the films was monitored by Brewster angle microscopy. It was found that the increased order and chain melting temperature of the bulk samples correlated with a raise in the maximum surface pressure attained at minimal surface area and in the transient dilatational modulus of the meibomian layers. Such correlations may allow for development of an improved understanding between the bulk and surface properties of human meibum and of other natural and synthetic tear lipid films.

Fluid dynamics of droplet generation from corneal tear film during non-contact tonometry in the context of pathogen transmission

Noninvasive ocular diagnostics demonstrate a propensity for droplet generation and present a potential pathway of distribution for pathogens such as the severe acute respiratory syndrome coronavirus 2. High-speed images of the eye subjected to air puff tonometry (glaucoma detection) reveal three-dimensional, spatiotemporal interaction between the puff and tear film. The interaction finally leads to the rupture and breakup of the tear film culminating into sub-millimeter sized droplet projectiles traveling at speeds of 0.2 m/s. The calculated droplet spread radius ( ∼ 0.5 m) confirms the likelihood of the procedure to generate droplets that may disperse in air as well as splash on instruments, raising the potential of infection. We provide a detailed physical exposition of the entire procedure using high fidelity experiments and theoretical modeling. We conclude that air puff induced corneal deformation and subsequent capillary waves lead to flow instabilities (Rayleigh-Taylor, Rayleigh-Plateau) that lead to tear film ejection, expansion, stretching, and subsequent droplet formation.

Physiological fluid interfaces: Functional microenvironments, drug delivery targets, and first line of defense

Fluid interfaces, i.e. the boundary layer of two liquids or a liquid and a gas, play a vital role in physiological processes as diverse as visual perception, oral health and taste, lipid metabolism, and pulmonary breathing. These fluid interfaces exhibit a complex composition, structure, and rheology tailored to their individual physiological functions. Advances in interfacial thin film techniques have facilitated the analysis of such complex interfaces under physiologically relevant conditions. This allowed new insights on the origin of their physiological functionality, how deviations may cause disease, and has revealed new therapy strategies. Furthermore, the interactions of physiological fluid interfaces with exogenous substances is crucial for understanding certain disorders and exploiting drug delivery routes to or across fluid interfaces. Here, we provide an overview on fluid interfaces with physiological relevance, namely tear films, interfacial aspects of saliva, lipid droplet digestion and storage in the cell, and the functioning of lung surfactant. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe therapies and drug delivery approaches targeted at fluid interfaces. STATEMENT OF SIGNIFICANCE: Fluid interfaces are inherent to all living organisms and play a vital role in various physiological processes. Examples are the eye tear film, saliva, lipid digestion & storage in cells, and pulmonary breathing. These fluid interfaces exhibit complex interfacial compositions and structures to meet their specific physiological function. We provide an overview on physiological fluid interfaces with a focus on interfacial phenomena. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe novel therapies and drug delivery approaches targeted at fluid interfaces. This sets the scene for ocular, oral, or pulmonary surface engineering and drug delivery approaches.

Monolayers of Cholesterol and Cholesteryl Stearate at the Water/Vapor Interface: A Physico-Chemical Study of Components of the Meibum Layer

Langmuir monolayers containing different amounts of cholesterol and cholesteryl stearate were studied at two different temperatures (24 °C and 35 °C). The main goal was to contribute towards the understanding of how the variations in the chemical composition may affect the physico-chemical properties of these specific lipid monolayers. The model mixture was chosen considering that cholesteryl esters are present in cell membranes and some other biological systems, including human tear lipids. Therefore, an investigation into the effect of the lipid monolayer composition on their interfacial properties may elucidate some of the fundamental reasons for the deficiencies in cell membranes and tear film functioning in vivo. The experimental results have shown that the molar ratio of the mixture plays a crucial role in the modulation of the Langmuir film properties. The condensing effects of the cholesterol and the interactions between the lipids in the monolayer were the main factors altering the monolayer response to dilatational deformation. The modification of the mixture compositions leads to significant changes in the Langmuir films and the mechanical performance, altering the ability of the monolayer to reduce the surface tension and the viscoelastic properties of the monolayers. This suggests that subtle modifications of the biomembrane composition may significantly alter its physiological function.

Meibum Lipid Composition and Conformation in Parkinsonism

PURPOSE

Patients with Parkinson's disease (PD) exhibit unstable tear films. Tear film lipid composition and structure are related to tear film stability and dry eye and tear lipids have not been characterized in people with PD. The aim of this study is to characterize Meibum tear lipids in donors with PD using ¹H-NMR and infrared spectroscopy.

METHODS

Three cohorts were compared: meibum from donors with PD (Mp) n = 10, meibum from donors with PD and dry eye (Mpd) n = 3, meibum from donors without PD (Mn) n = 29.

RESULTS

There were no significant differences, P > 0.05, in hydrocarbon branching for Mp compared with Mn. Mn contained twice as much cholesteryl esters compared with Mp, P < 0.0001. The cooperativity of the phase transition was significantly 37% lower for Mp compared with Mn, P < 0.0001. Mpd was much more ordered (stiffer) with compared with Mp and Mn, P < 0.0001.

CONCLUSION

Changes in meibum lipid composition and structure could be a marker for and/or contribute to increase the susceptibility of dry eye in patients with PD. A less cooperative phase transition for Mp compared with Mn indicates that Mp was more heterogeneous and/or contained more contaminants than Mn. The data support the idea that more ordered lipid contributes to dry eye.

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.

Meibum lipid hydrocarbon chain branching and rheology after hematopoietic stem cell transplantation

Purpose

Meibum from donors who have had hematological stem cell transplantations (M_HSCT) are susceptible to severe dry eye symptoms and exhibit very high lipid order (stiffness) compared with meibum from donors without dry eye (M_n). Since lipid order could have functional consequences, we compared the rheology and composition of M_n and M_HSCT to measure meibum compositional, structural and functional relationships.

Methods

The rheology and composition was measured using Langmuir trough and ¹H NMR spectroscopy, respectively.

Results

M_HSCT and M_n was studied from 16 to 43 donors, respectively, using NMR spectroscopy. M_HSCT contained significantly 16% more straight chain and 24% less iso-chain hydrocarbons compared with M_n. The cholesteryl ester to wax ester molar ratio, and hydrocarbon chain unsaturation were not significantly different, for M_HSCT compared with M_n.

Surface pressure-area isotherms of meibum from 30 donors without dry-eye were grouped into 4 pools (PC) and meibum from 32 donors with dry eye who had hematopoietic stem cell transplantation (PT) were grouped into 3 pools. Above 15 years of age the П_max and (C_s⁻¹)_max increased with age for both the PC and the PT cohorts. (C_s⁻¹)_max values were higher for PT samples compared with age matched PC samples, indicating they had higher elasticity and stiffness. A more ordered lipid could contribute to the formation of a discontinuous patchy tear film lipid layer, which in turn results in deteriorated spreading, and decreased surface elasticity.

Conclusions

The composition and rheology of meibum from donors with dry eye and who have had HSCT support the idea from other studies that more ordered meibum may contribute to or be a marker of dry eye.

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More straight chain and less iso-chain hydrocarbons could contribute to HSCT dry eye.

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Meibum elasticity and stiffness increased with age.

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Meibum elasticity and stiffness increased with HSCT dry eye.

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Differences could contribute to a discontinuous patchy tear film lipid layer.

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Differences could result in deteriorated spreading, and decreased surface elasticity.

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.