Dry eye disease in astronauts: a narrative review.

Long-duration spaceflight can have adverse effects on human health. One of the most common ocular conditions experienced by astronauts is dry eye disease (DED). Symptoms of DED include feelings of eye irritation, eye strain, foreign body sensation and blurred vision. Over 30% of International Space Station expedition crew members reported irritation and foreign body sensation. We reviewed the current literature on the prevalence and mechanisms of DED in astronauts and its potential implications for long-duration spaceflight, including the influence of environmental factors, such as microgravity and fluid shift on tear film physiology in space. DED has negative effects on astronaut performance, which is why there is a need for further research into the pathophysiology and countermeasures. As an in-flight countermeasure, neurostimulation seems to be among the most promising options.

Effects on the Human Tear Film of Applying Skin Lipids to the Ocular Surface.

PURPOSE: The effect of skin lipids on the formation and stability of the human tear film was investigated. METHODS: Skin swab substances (SSSs) were applied to the eyes of volunteers and studied using fluorescein or with TearView, which records infrared emissivity showing tear film integrity in real time. Results were compared with similar experiments using castor oil, freshly collected meibum, or acetic acid, which simulated the low pH of the skin. RESULTS: Fluorescein and TearView results were comparable. TearView showed the natural unaltered tear film over the whole eye, instant changes to the tear film, and meibomian gland activity. Minimal amounts of SSS destroyed the integrity of the film and caused pain. Corneal epithelial damage could be detected. TearView showed that SSS stimulated meibomian gland secretion if applied directly to the posterior eyelid margin. Excess meibum had no effect on the tear film spread or integrity. Castor oil formed floating lenses on the tear film which were spread by a blink but then condensed back toward themselves. There was no pain or surface damage with these oils. CONCLUSIONS: SSS contamination of the ocular surface disrupts the tear film, causes stinging, and fluorescein staining of the corneal epithelial cells after a blink. SSS stimulates meibomian gland activity. It is possible that various ocular conditions associated with dry eye, such as blepharitis and ocular rosacea, may compromise a meibomian lipid barrier of the eye lid margin. Skin lipids would then have access to the ocular surface and cause dry eye symptoms.

Novel Nano-Liposome Formulation for Dry Eyes with Components Similar to the Preocular Tear Film.

Dry eye is commonly treated with artificial tears; however, developing artificial tears similar to natural tears is difficult due to the complex nature of tears. We characterized and evaluated a novel artificial tear formulation with components similar to the lipid and aqueous constituents of natural tears. Nano-liposomes, composed in part of phosphatidylcholine, were dispersed in an aqueous solution of bioadhesive sodium hyaluronate. Liposome size, zeta potential, and physicochemical properties of the fresh and stored (4 °C) liposomal formulation were analyzed. In vitro tolerance was tested using human corneal and conjunctival cell lines by exposures of 15 min to 4 h. The tolerance of the liposomal formulation was evaluated in animals (rabbits). The average liposome size was 186.3 ± 7.0 nm, and the zeta potential was negative. The osmolarity of the formulation was 198.6 ± 1.7 mOsm, with a surface tension of 36.5 ± 0.4 mN/m and viscosity of 3.05 ± 0.02 mPa·s. Viability values in the human corneal and conjunctival cell lines were always >80%, even after liposomal formulation storage for 8 weeks. Discomfort and clinical signs after instillation in rabbit eyes were absent. The new formulation, based on phosphatidylcholine-liposomes dispersed in sodium hyaluronate has suitable components and characteristics, including high in vitro cell viability and good in vivo tolerance, to serve as a tear substitute.

An Experimental Model to Study the Impact of Lipid Oxidation on Contact Lens Deposition In Vitro.

PURPOSE: This study was to establish a controlled in vitro test system to study the effect of lipid oxidation on lipid deposition on contact lenses. METHODS: Fatty acids with varying degree of unsaturation were oxidized using the Fenton reaction. The degree of lipid oxidation and the lipid moieties formed during the oxidation were identified and estimated by various lipid staining techniques following separation with thin-layer chromatography, and by measuring thiobarbituric acid reactive substances or peroxides in solution. Two different silicone hydrogel-based contact lenses (Balafilcon A and Senofilcon A) were incubated with fatty acids laced with radioactive tracer oxidized to varying degrees, and the amount of lipid deposition was measured using unoxidized lipid samples as controls. RESULTS: The Fenton reaction together with the analytical methods to analyze the lipid oxidation can be used to control oxidation of lipids to a desired amount. In general, saturated fatty acids are not oxidized, the monounsaturated oleic acid produced peroxides while poly-unsaturated lipids initially produced peroxides and then fragmented into reactive aldehydes. Incubation with mildly oxidized lipids (most likely lipid peroxides) resulted in increased lipid deposition on Balafilcon A lenses compared to unoxidized lipids, but this was not observed for Senofilcon A lenses. Further oxidation of the lipids (carbon chain breakup) on the other hand resulted in diminished lipid deposition for both contact lens types. CONCLUSIONS: This study provides a method for inducing and controlling lipid oxidation so that the effect of lipid oxidation on contact lens binding can be compared. It could be shown that the degree of lipid oxidation has different effects on the lipid deposition on different contact lens types.

Negative Ions in Space.

Until a decade ago, the only anion observed to play a prominent role in astrophysics was H-. The bound-free transitions in H- dominate the visible opacity in stars with photospheric temperatures less than 7000 K, including the Sun. The H- anion is also believed to have been critical to the formation of molecular hydrogen in the very early evolution of the Universe. Once H2 formed, about 500 000 years after the Big Bang, the expanding gas was able to lose internal gravitational energy and collapse to form stellar objects and "protogalaxies", allowing the creation of heavier elements such as C, N, and O through nucleosynthesis. Although astronomers had considered some processes through which anions might form in interstellar clouds and circumstellar envelopes, including the important role that polycyclic aromatic hydrocarbons might play in this, it was the detection in 2006 of rotational line emission from C6H- that galvanized a systematic study of the abundance, distribution, and chemistry of anions in the interstellar medium. In 2007, the Cassini mission reported the unexpected detection of anions with mass-to-charge ratios of up to ∼10 000 in the upper atmosphere of Titan; this observation likewise instigated the study of fundamental chemical processes involving negative ions among planetary scientists. In this article, we review the observations of anions in interstellar clouds, circumstellar envelopes, Titan, and cometary comae. We then discuss a number of processes by which anions can be created and destroyed in these environments. The derivation of accurate rate coefficients for these processes is an essential input for the chemical kinetic modeling that is necessary to fully extract physics from the observational data. We discuss such models, along with their successes and failings, and finish with an outlook on the future.

Using Biotinylated Proteins to Demonstrate Immunodetection of Antigens via Western Blotting, Dot Blots, and Immunohistochemistry.

Using biotinylated targets for detection by enzyme-linked avidin allows immunodetection methods to become more economic in cost and time as it negates the need for a specific primary antibody. Methods are described to use exogenously added biotin to complex biological samples to demonstrate western blotting, dot blots, and immunohistochemistry. These methods can be used in biological science tertiary teaching laboratories to allow novices to gain skills in a risk-free environment to promote student motivation and engagement.

The real reason for having a meibomian lipid layer covering the outer surface of the tear film - A review.

This review critically evaluates a broad range of literature in order to show the relationship between meibum, tear lipids and the tear film lipid layer (TFLL). The relationship of meibum composition to dry eye syndrome is briefly discussed. The review also explores the interactions between aqueous and the TFLL by examining the correlations between meibomian lipids and lipids extracted from whole tears, and by considering protein adsorption to the TFLL from the aqueous. Although it is clear to the authors that a normal tear film resists evaporation, an emerging idea from the literature is that the main purpose of the TFLL is to allow the spread of the tear film and to prevent its collapse onto the ocular surface, rather than to be an evaporative blanket. Current models on the possible structure of the TFLL are also examined.

The tachykinin peptide neurokinin B binds copper(I) and silver(I) and undergoes quasi-reversible electrochemistry: towards a new function for the peptide in the brain.

The tachykinin neuropeptide family, which includes substance P and neurokinin B, is involved in a wide array of biological functions. Among these is the ability to protect against the neurotoxic processes in Alzheimer's Disease, but the mechanisms driving neuroprotection remain unclear. Dysregulation of metal ions, particularly copper, iron and zinc is a common feature of Alzheimer's Disease, and other amyloidogenic disorders. Copper is known to be released from neurons and recent work has shown that some tachykinins can bind Cu(II) ions, and that neurokinin B can inhibit copper uptake into astrocytes. We have now examined whether neurokinin B is capable of binding Cu(I), which is predicted to be available in the synapse. Using a combination of spectroscopic techniques including cyclic voltammetry and magnetic resonance we show that neurokinin B can bind Cu(I) either directly from added CuCl or by reduction of Cu(II)-bound neurokinin B. The results showed that the Cu(I) binding site differs greatly to that of Cu(II) and involves thioether coordination via Met2 and Met10 and an imidazole nitrogen ligand from His3. The Cu(I) coordination is also different to the site adopted by Ag(I). During changes in oxidation state, copper remains bound to neurokinin B despite large changes to the inner coordination sphere. We predict that neurokinin B may be involved in synaptic copper homeostasis.

Tear film stability: a review.

Tear film stability can be assessed via a number of tools designed for clinical as well as research purposes. These techniques can give us insights into the tear film, and allow assessment of conditions that can lead to dry eye symptoms, and in severe cases, to significant ocular surface damage and deterioration of vision. Understanding what drives tear film instability and its assessment is also crucial for evaluating existing and new therapies. This review examines various techniques that are used to assess tear film instability: evaluation of tear break-up time and non-invasive break-time; topographic and interferometric techniques; confocal microscopic methods; aberrometry; and visual function tests. It also describes possible contributions of different tear film components; namely meibomian lipids, ocular mucins and proteins, and factors such as age, contact lens wear, ocular surgery and environmental stimuli, that may influence tear film instability.

An investigation of the likely role of (O-acyl) ω-hydroxy fatty acids in meibomian lipid films using (O-oleyl) ω-hydroxy palmitic acid as a model.

(O-acyl) ω-hydroxy fatty acids (OAHFAs) are a recently found group of polar lipids in meibum. Since these lipids can potentially serve as a surfactant in the tear film lipid layer, the surface properties of a molecule of this lipid class was investigated and compared with a structurally related wax ester and a fatty acid. (O-oleyl) ω-hydroxy palmitic acid was synthesized and used as the model OAHFA. It was spread either alone or mixed with human meibum on an artificial tear buffer in a Langmuir trough, and pressure-area isocycle profiles were recorded at different temperatures and compared with those of palmityl oleate and oleic acid. These measurements were accompanied by fluorescence microscopy of meibum mixed films during pressure-area isocycles. The pressure area curves indicated that pure films of the model OAHFA are as surface active as oleic acid films, cover a much larger surface area than either palmityl oleate or oleic acid and show a distinct biphasic pressure-area isocycle profile. The OAHFAs appeared to remain on the aqueous surface and show only a minor re-arrangement into multi-layered structures during repetitive pressure area isocycles. All these properties can be explained by OAHFAs binding weakly to the aqueous surface via an ester group and strongly via a carboxyl group. By contrast, the pressure area profiles of palmityl oleate films indicate that they form multi-layers and oleic acid presumably forms micelles and desorbs into the subphase. When mixed with meibum, similar features as for pure films were observed. In addition, meibum-OAHFA films appeared very homogeneous; a feature not seen with other mixtures. In conclusion these data support the notion that the tested OAHFA is a very potent surfactant which is important in spreading and stabilising meibomian lipid films.

Effects of keratin and lung surfactant proteins on the surface activity of meibomian lipids.

PURPOSE: In vitro studies indicate that surface tension and surface viscosity of the tear film lipid layer (TFLL) are governed by interactions between meibomian lipids and proteins from the aqueous layer. The role of minor tear proteins with strong lipophilic properties or those correlated with pathological states is still unknown. The discovery of lung surfactant proteins (SPs) in tears and keratin in normal and abnormal meibomian gland excretions warrants investigation into their effects on the surface activity of meibomian lipid films. METHODS: Commercial keratin and bovine lung SPs were used in vitro to assess the surface pressure of meibomian lipid films using a Langmuir trough. RESULTS: The pressure-area profiles of meibomian lipid films seeded with SPs (2.5 µL; 0.1 µg) demonstrated hybrid characteristics between meibomian lipid films alone and SPs alone but reached much higher maximum surface pressures (approximately 30 vs. 24 mN/m). Microscopically, the appearance of meibomian lipid films was not altered by SPs. Maximum surface pressure of meibomian films premixed with keratin was much higher than meibum alone. The pressure-area isocycles appeared more like those of meibomian lipids with a low concentration of protein and more like pure keratin films at a high concentration. CONCLUSIONS: The data strongly indicate that SPs and keratin likely interact with the TFLL. SPs are likely to act as strong surfactants and to reduce the surface tension of the lipid layer. Excess concentrations of keratin as identified in patients with meibomian gland dysfunction could disrupt the normal structure of the meibomian lipid film.

Interfacial dilatational viscoelasticity of human meibomian lipid films.

PURPOSE/AIM: The meibomian lipid layer is able to withstand the enormous stresses and deformations that occur during blinking due to the combination of its elastic and viscous properties. The purpose of this study was to measure the dilatational viscoelasticity of in vitro meibomian lipid films and compare how these properties differ between room temperature and physiological temperatures. Viscoelasticity was also compared with meibomian lipid films seeded with cholesterol or ß-carotene (the levels of these lipid species change in disease states). MATERIALS AND METHODS: Dilatational viscoelasticity (E) was measured using an oscillating pendant drop method. Measurements were carried out on spread films at the air-water interface as a function of frequency (0.1256-12.56 rad/s) at various temperatures between 18-43 °C. RESULTS: Generally, E gradually decreased as the overall temperature was increased. At both 37 and 20 °C, films demonstrated that the elastic modulus (E') was more dominant than the viscous modulus (E″), indicating films were more solid-like than fluid-like, regardless of temperature. E' and E″ were also dependant on frequency, indicating some molecular rearrangements of the lipid molecules as films were compressed and expanded. Films seeded with cholesterol or ß-carotene showed a modest increase in the moduli. CONCLUSIONS: These results are consistent with previous findings which have predicted and indicated that the meibomian lipid layer is a viscoelastic film at the air-liquid interface. These properties are integral to how the tear film lipid layer is able to maintain its structure, and hence integrity of the ocular surface.

Lipid component contributions to the surface activity of meibomian lipids.

PURPOSE: Meibomian lipid films have very complex physical properties that enable them to be compressed and expanded without collapsing. These properties can be attributed to the self assembly of the individual components, mainly wax and cholesteryl esters (WE and CE). Here, the surface pressure properties of WEs and CEs films have been compared to evaluate their contributions to meibomian lipid films. METHODS: Films of different WEs and CEs were spread on a Langmuir trough and their surface pressure area profiles were compared with a particular emphasis on the effects caused by the degree of saturation of the alkyl/alkene chains. RESULTS: Fully saturated WEs and CEs formed unstable films that collapsed upon compression. Very unsaturated waxes and CEs tended to have two distinct phases, one that reflects interaction with the aqueous subphase, while the second appeared to be with the multilayered bulk of the lipid film. With aging of the films, the WEs tended to move off the surface into the bulk. When meibomian lipid films were seeded with large amounts of WEs, only minor changes could be seen unless the WE was very unsaturated. CONCLUSIONS: These data are consistent with meibomian lipid films having a surfactant layer with a complex bulk layer external to this. It is speculated that the bulk layer contains thermotropic smectic chiral liquid crystals of CEs that are interacting with the WEs. This structure would tend to prevent collapse of the meibomian lipids onto the ocular surface and be very tolerant of lipophilic contaminants.

The use of biotin to demonstrate immunohistochemistry, Western blotting, and dot blots in university practical classes.

Immunological detection of proteins is an essential method to demonstrate to undergraduate biology students, however, is often difficult in resource and time poor student laboratory sessions. This method describes a failsafe method to rapidly and economically demonstrate this technique using biotinylated proteins or biotin itself as targets for detection. This negates the need for a specific primary antibody, saving cost and time. In addition, the easily available and safe reagents allow the methods to be readily adopted without specialist technical expertise. As a result, staff can confidently transfer ownership of the task to the student so as to also develop scientific inquiry skills which promotes student motivation and engagement.

Analysis of Comparison of Human Meibomian Lipid Films and Mixtures with Cholesteryl Esters In Vitro Films using High Resolution Color Microscopy.

PURPOSE: The lipid layer of the tears has been studied in vivo using high resolution color microscopy (HRCM). The purpose of these experiments was to gain further insight into the structure of the lipid layer by applying HRCM to in vitro meibomian lipid films. METHODS: Films of human meibomian lipids, cholesteryl nervonate, cholesteryl palmitate, or their mixtures, were spread on a Langmuir trough. Changes to the films were monitored using HRCM as the films were compressed to different surface pressures. The penetration of albumin into a meibomian lipid film also was studied. RESULTS: Small amounts of meibomian lipids at low pressures formed very thin films estimated to be 5.2 nm thick. Compression caused spots to appear in the films. At higher concentrations, micro lenses were a feature of the film. Cholesteryl nervonate formed a multilayered oil slick that did not change with surface pressure. Cholesteryl palmitate formed a stiff film that collapsed at high compression. Mixtures of cholesteryl nervonate and meibomian lipids showed that they mixed to increase surface pressures above that of the individual components. HRCM also allowed albumin to be seen penetrating the meibomian lipid film. CONCLUSIONS: HRCM combined with in vitro surface pressure measurements using a Langmuir trough is useful for modeling meibomian lipid films. The films often resemble the appearance of the lipid layer of in vivo films. The data indicate that the lipid layer might be modeled best as a duplex film containing an array of liquid crystals.

Divalent cations in tears, and their influence on tear film stability in humans and rabbits.

PURPOSE: Reduced tear film stability is reported to contribute to dry eye. Rabbits are known to have a more stable tear film than humans. Thus, we sought to examine the tears of rabbits and humans for metal cations, and to test how they influence tear film stability. METHODS: Tears were collected from 10 healthy humans and 6 rabbits. Tear osmolality was measured by vapor pressure osmometer, and metals analyzed using inductively coupled plasma (ICP) mass spectrometry or ICP atomic emission spectroscopy. The influence of divalent cations on tears was analyzed by measuring surface tension using the Langmuir trough in vitro, using different concentrations of cations in the subphase, and grading the tear break-up in rabbits in vivo after instillation of chelating agents. RESULTS: Rabbit tears had a higher osmolality compared to humans. Major metals did not differ between species; however, rabbits had higher levels of Mg(2+) (1.13 vs. 0.39 mM) and Ca(2+) (0.75 vs. 0.36 mM). In rabbit tears in vitro, diminishing divalent cations resulted in a decrease in the maximum surface pressure from 37 to 30 mN/m. In vivo, an increase in the amount of tear film that was broken-up was found. In contrast, when changing divalent cation concentrations in human tears, the maximum surface pressure remained at 26 mN/m. CONCLUSIONS: The normal osmolality of rabbit tears is significantly higher than that in humans. While divalent cations had little influence on human tears, they appear to have an important role in maintaining tear film stability in rabbits.

Barriers to clinical uptake of tear osmolarity measurements.

AIM: The aim of the study was to examine the possibilities of measuring tear osmolarity in a general clinical setting, and to identify the barriers preventing the uptake of new methodologies for its measurement. METHODS: Five non-contact-lens wearers were recruited to evaluate the diagnostic capability of the TearLab. Three osmolarity measurements were taken at 1 min intervals in the morning at 09:00, midday between 12:00 and 13:00 and afternoon at 16:00 for two consecutive days. Forty more osmolarity measurements were carried out at different times on one subject with low and one subject with high tear osmolarity over 4 months. The osmolarity of a standard solution, 290 mOsm/l, was measured 19 times alternatively with the TearLab by two examiners. RESULTS: Consecutive tear osmolarity readings in an individual varied up to 35 mOsm/l, but an average over three readings was found to be a reliable indicator of tear osmolarity at 95% confidence level. For population studies, a power analysis based on the variability of the data showed that three repeat measurements would be required to obtain reliable data for a study with <50 subjects, whereas one measurement would suffice for 490 or more subjects. There were no interobserver or interinstrumental differences, but readings obtained for the standard solution varied up to 89 mOsm/l. CONCLUSION: Three consecutive readings are required with the TearLab to obtain a reliable measure of tear osmolarity. The variation in recorded tear osmolarity makes it difficult to use the technique for the diagnosis of mild dry eye.