Tear fluid content of electrochemically active components including water soluble antioxidants

An Overview of Frog Skin-Derived Esc Peptides: Promising Multifunctional Weapons against Pseudomonas aeruginosa-Induced Pulmonary and Ocular Surface Infections

Antimicrobial resistance is a silent pandemic harming human health, and Pseudomonas aeruginosa is the most common bacterium responsible for chronic pulmonary and eye infections. Antimicrobial peptides (AMPs) represent promising alternatives to conventional antibiotics. In this review, the in vitro/in vivo activities of the frog skin-derived AMP Esc(1-21) are shown. Esc(1-21) rapidly kills both the planktonic and sessile forms of P. aeruginosa and stimulates migration of epithelial cells, likely favoring repair of damaged tissue. However, to undertake preclinical studies, some drawbacks of AMPs (cytotoxicity, poor biostability, and limited delivery to the target site) must be overcome. For this purpose, the stereochemistry of two amino acids of Esc(1-21) was changed to obtain the diastereomer Esc(1-21)-1c, which is more stable, less cytotoxic, and more efficient in treating P. aeruginosa-induced lung and cornea infections in mouse models. Incorporation of these peptides (Esc peptides) into nanoparticles or immobilization to a medical device (contact lens) was revealed to be an effective strategy to ameliorate and/or to prolong the peptides' antimicrobial efficacy. Overall, these data make Esc peptides encouraging candidates for novel multifunctional drugs to treat lung pathology especially in patients with cystic fibrosis and eye dysfunctions, characterized by both tissue injury and bacterial infection.

Oxidative stress in the eye and its role in the pathophysiology of ocular diseases

Oxidative stress occurs through an imbalance between the generation of reactive oxygen species (ROS) and the antioxidant defense mechanisms of cells. The eye is particularly exposed to oxidative stress because of its permanent exposure to light and due to several structures having high metabolic activities. The anterior part of the eye is highly exposed to ultraviolet (UV) radiation and possesses a complex antioxidant defense system to protect the retina from UV radiation. The posterior part of the eye exhibits high metabolic rates and oxygen consumption leading subsequently to a high production rate of ROS. Furthermore, inflammation, aging, genetic factors, and environmental pollution, are all elements promoting ROS generation and impairing antioxidant defense mechanisms and thereby representing risk factors leading to oxidative stress. An abnormal redox status was shown to be involved in the pathophysiology of various ocular diseases in the anterior and posterior segment of the eye. In this review, we aim to summarize the mechanisms of oxidative stress in ocular diseases to provide an updated understanding on the pathogenesis of common diseases affecting the ocular surface, the lens, the retina, and the optic nerve. Moreover, we discuss potential therapeutic approaches aimed at reducing oxidative stress in this context.

Oxidative Stress in the Protection and Injury of the Lacrimal Gland and the Ocular Surface: are There Perspectives for Therapeutics?

Oxidative stress (OS) is a major disruption in the physiology of the lacrimal functional unit (LFU). Antioxidant enzymes have dual protective activities: antioxidant and antimicrobial activities. Peroxidases have been indistinctly used as markers of the secretory activity of the LFU and implicated in the pathophysiology, diagnosis and treatment of dry eye disease (DED), even though they comprise a large family of enzymes that includes lactoperoxidase (LPO) and glutathione peroxidase (GPO), among others. Assays to measure and correlate OS with other local LFU phenomena have methodological limitations. Studies implicate molecules and reactions involved in OS as markers of homeostasis, and other studies identify them as part of the physiopathology of diseases. Despite these conflicting concepts and observations, it is clear that OS is influential in the development of DED. Moreover, many antioxidant strategies have been proposed for its treatment, including calorie restriction to nutritional supplementation. This review offers a critical analysis of the biological mechanisms, diagnostic outcomes, drug use, dietary supplements, and life habits that implicate the influence of OS on DED.

The Pathomechanism, Antioxidant Biomarkers, and Treatment of Oxidative Stress-Related Eye Diseases

Oxidative stress is an important pathomechanism found in numerous ocular degenerative diseases. To provide a better understanding of the mechanism and treatment of oxidant/antioxidant imbalance-induced ocular diseases, this article summarizes and provides updates on the relevant research. We review the oxidative damage (e.g., lipid peroxidation, DNA lesions, autophagy, and apoptosis) that occurs in different areas of the eye (e.g., cornea, anterior chamber, lens, retina, and optic nerve). We then introduce the antioxidant mechanisms present in the eye, as well as the ocular diseases that occur as a result of antioxidant imbalances (e.g., keratoconus, cataracts, age-related macular degeneration, and glaucoma), the relevant antioxidant biomarkers, and the potential of predictive diagnostics. Finally, we discuss natural antioxidant therapies for oxidative stress-related ocular diseases.

Metabolomics comparison of cord and peripheral blood-derived serum eye drops for the treatment of dry eye disease

Allogeneic peripheral blood-derived (PBS) serum eye drops have been largely used in the treatment of dry eye disease (DED). Recently, cord blood has emerged as an effective alternative serum source (cord blood serum, CBS), containing a higher amount of growth factors than PBS, it holds the promise of a better capability to stimulate corneal healing. However, the lack of a standardized method for preparation, dispensation, storage and a poor biochemical characterization still hamper the establishment of a clinical consensus. Here the metabolomes of the two different serum eye drop preparations were compared using proton nuclear magnetic resonance spectroscopy. We found that both PBS and CBS contained several organic compounds, the majority of them already detected in human tears and may be thereby considered lacrimal substitutes. Metabolites having in the multivariate statistical analysis Partial least squares discriminant analysis (PLS-DA) a VIP scores > 1.0 were considered to be significantly different. All the metabolites identified were found to have a p < 0.05 in the univariate analysis. CBS, in particular, showed the highest amount of choline, myo-inositol, glutamine, creatine and β-hydroxybutyrate. These evidences constitute relevant advances towards serum eye drops characterization and confirm that cord blood is a valid alternative source of serum eye drops.

Is ascorbate in human tears from corneal leakage or from lacrimal secretion?

BACKGROUND

This study investigated whether fresh main lacrimal gland secretion contains ascorbate, with a view to providing indirect evidence of an immediate source of this antioxidant in tears. Our hypothesis was that, if the source is corneal leakage, continuous tearing or rinsing of the eye will result in a marked decrease, by dilution, in ascorbate concentration in the reflex tears collected. Alternatively, the ascorbate concentration will be relatively constant if the main lacrimal gland secretion is the main immediate source.

METHODS

Five successive samples of yawn-induced reflex tears were collected from the same eye of each of 42 subjects. In 36 of these volunteers, the testing eye was then flushed with 10 ml of saline and a sixth tear sample ('post-flush') was collected immediately. Tear ascorbate concentrations were measured using a validated high performance liquid chromatography method.

RESULTS

The ascorbate concentration of the first sample (baseline) was slightly but significantly (P < 0.0001) lower (17.3 +/- 8.9 micro M) than the four subsequent samples in all subjects (average 21.4 microM). Ascorbate concentrations of post-flush samples were very similar to pre-flush values. Mean +/- SD ascorbate concentrations of pre- and post-flush samples were, respectively, 22.5 +/- 10.9 and 17.3 +/- 5.8 microM.

DISCUSSION

Results show that ascorbate is present in fresh reflex tears. Data do not support the view that the cornea is the source of tear ascorbate in healthy eyes. Rather, results indicate that ascorbate is present in main lacrimal gland secretion and that this antioxidant is depleted in basal tears. Measurement of tear ascorbate may offer useful information regarding antioxidant supply to and protection of the cornea.

Potential Protective and Therapeutic Roles of the Nrf2 Pathway in Ocular Diseases: An Update

Nuclear factor- (erythroid-derived 2-) like 2 (Nrf2) is a regulator of many processes of life, and it plays an important role in antioxidant, anti-inflammatory, and antifibrotic responses and in cancer. This review is focused on the potential mechanism of Nrf2 in the occurrence and development of ocular diseases. Also, several Nrf2 inducers, including noncoding RNAs and exogenous compounds, which control the expression of Nrf2 through different pathways, are discussed in ocular disease models and ocular cells, protecting them from dysfunctional changes. Therefore, Nrf2 might be a potential target of protecting ocular cells from various stresses and preventing ocular diseases.

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.

Effect of General Anesthesia Duration on Recovery of Secretion and Biochemical Properties of Tear Fluid in the Post-Anesthetic Period

Changes in the biochemical composition of the tear film is a critical risk factor for the development of chronic perioperative dry eye syndrome, because increasing the duration of general anesthesia did not affect the dynamics of tear secretion recovery, but slowed down normalization of its structure and antioxidant activity in the post-anesthetic period.

Ambient fine particulate matters induce cell death and inflammatory response by influencing mitochondria function in human corneal epithelial cells

Ambient fine particulate matter (AFP) is a main risk factor for the cornea as ultraviolet light. However, the mechanism of corneal damage following exposure to AFP has been poorly understood. In this study, we first confirmed that AFP can penetrate the cornea of mice, considering that two-dimensional cell culture systems are limited in reflecting the situation in vivo. Then, we investigated the toxic mechanism using human corneal epithelial (HCET) cells. At 24h after exposure, AFP located within the autophagosome-like vacuoles, and cell proliferation was clearly inhibited in all the tested concentration. Production of ROS and NO and secretion of pro-inflammatory cytokines were elevated in a dose-dependent manner. Additionally, conversion of LC3B from I-type to II-type and activation of caspase cascade which show autophagic- and apoptotic cell death, respectively, were observed in cells exposed to AFP. Furthermore, AFP decreased mitochondrial volume, inhibited ATP production, and altered the expression of metabolism-related genes. Taken together, we suggest that AFP induces cell death and inflammatory response by influencing mitochondrial function in HCET cells. In addition, we recommend that stringent air quality regulations are needed for eye health.

TFOS DEWS II Tear Film Report

The members of the Tear Film Subcommittee reviewed the role of the tear film in dry eye disease (DED). The Subcommittee reviewed biophysical and biochemical aspects of tears and how these change in DED. Clinically, DED is characterized by loss of tear volume, more rapid breakup of the tear film and increased evaporation of tears from the ocular surface. The tear film is composed of many substances including lipids, proteins, mucins and electrolytes. All of these contribute to the integrity of the tear film but exactly how they interact is still an area of active research. Tear film osmolarity increases in DED. Changes to other components such as proteins and mucins can be used as biomarkers for DED. The Subcommittee recommended areas for future research to advance our understanding of the tear film and how this changes with DED. The final report was written after review by all Subcommittee members and the entire TFOS DEWS II membership.

Real-Time Monitoring of Post-Surgical and Post-Traumatic Eye Injuries Using Multilayered Electrical Biosensor Chip

Lack of current techniques for the early monitoring of bleb leaks and other post-traumatic or post-surgical ocular injury has posed an unmet clinical need for the development of new techniques. Present evaluation techniques use either subjective or nonquantitative approaches. At present, there are no FDA approved ocular devices that can directly measure ascorbic acid (AA) concentration levels for both tear film (TF) and aqueous humor (AH) at point-of-care (POC) level. Toward this aim, we present a novel POC quantitative assay, called the ocular biosensor device, which can be used to evaluate the integrity of the anterior surface of the eye by measuring the concentration of AA in TF and AH. Herein, we utilize a novel scientific engineering approach for the development of a disposable paper based POC ocular biosensor strip. A grafted poly(styrene)-block-poly(acrylic acid) (PS-b-PAA) and graphene platelet composite with contour based μ-electrodes design (CBμE) exhibit a highly sensitive platform to perform electrochemical immunosensing technique to study clinical samples that have small volumes like tear fluid. Samples used in this study were collected clinically from subjects undergoing therapeutic anterior chamber paracentesis. The proposed biosensor reports the level of AA concentration on an electronic screen, making the results easy to read, efficient, and reliable.

Alterations in Tear Biochemistry Associated with Postanesthetic Chronic Dry Eye Syndrome

Perioperative dry eye syndrome (DES) is a common ocular complication of long-term general anesthesia. Chronic DES can lead to permanent damage to the cornea and disturbance of visual function, up to total loss of vision. Here, a relationship between the duration of general anesthesia and the risk of chronic DES in patients was demonstrated. Using an experimental model of perioperative corneal abrasions in rabbits, it was found that introduction of animals to 3-h general anesthesia resulted in clinically significant chronic damage to the cornea in 50% of cases. The development of the complication was not associated with irreversible or long-term impairment of tear secretion, but it was accompanied by a decrease in tear film stability and growth of the total protein content as well as decrease in total antioxidant activity of the tear induced by low molecular weight antioxidants. In addition, anesthesia-induced changes in activity of tear antioxidant enzymes including superoxide dismutase and enzymes providing homeostasis of reduced glutathione (glutathione peroxidase, glutathione-S-transferase, glutathione reductase) were observed. All these alterations were protracted (up to 1-2 weeks) and therefore might account for transition of the perioperative DES into the chronic form. These findings can be useful in the development of novel approaches for the prevention and treatment of chronic forms of DES in the postanesthetic period.

Highly sensitive colorimetric detection of glucose and uric acid in biological fluids using chitosan-modified paper microfluidic devices

This paper describes the modification of microfluidic paper-based analytical devices (μPADs) with chitosan to improve the analytical performance of colorimetric measurements associated with enzymatic bioassays. Chitosan is a natural biopolymer extensively used to modify biosensing surfaces due to its capability of providing a suitable microenvironment for the direct electron transfer between an enzyme and a reactive surface. This hypothesis was investigated using glucose and uric acid (UA) colorimetric assays as model systems. The best colorimetric sensitivity for glucose and UA was achieved using a chromogenic solution composed of 4-aminoantipyrine and sodium 3,5-dichloro-2-hydroxy-benzenesulfonate (4-AAP/DHBS), which provided a linear response for a concentration range between 0.1 and 1.0 mM. Glucose and UA were successfully determined in artificial serum samples with accuracies between 87 and 114%. The limits of detection (LODs) found for glucose and UA assays were 23 and 37 μM, respectively. The enhanced analytical performance of chitosan-modified μPADs allowed the colorimetric detection of glucose in tear samples from four nondiabetic patients. The achieved concentration levels ranged from 130 to 380 μM. The modified μPADs offered analytical reliability and accuracy as well as no statistical difference from the values achieved through a reference method. Based on the presented results, the proposed μPAD can be a powerful alternative tool for non-invasive glucose analysis.

Miniature biofuel cell as a potential power source for glucose-sensing contact lenses

A microscale membrane-less biofuel cell, capable of generating electrical energy from human lachrymal liquid, was developed by utilizing the ascorbate and oxygen naturally present in tears as fuel and oxidant. The biodevice is based on three-dimensional nanostructured gold electrodes covered with abiotic (conductive organic complex) and biological (redox enzyme) materials functioning as efficient anodic and cathodic catalysts, respectively. Three-dimensional nanostructured electrodes were fabricated by modifying 100 μm gold wires with 17 nm gold nanoparticles, which were further modified with tetrathiafulvalene-tetracyanoquinodimethane conducting complex to create the anode and with Myrothecium verrucaria bilirubin oxidase to create the biocathode. When operated in human tears, the biodevice exhibited the following characteristics: an open circuit voltage of 0.54 V, a maximal power density of 3.1 μW cm(-2) at 0.25 V and 0.72 μW cm(-2) at 0.4 V, with a stable current density output of over 0.55 μA cm(-2) at 0.4 V for 6 h of continuous operation. These findings support our proposition that an ascorbate/oxygen biofuel cell could be a suitable power source for glucose-sensing contact lenses to be used for continuous health monitoring by diabetes patients.

Flexible micro(bio)sensors for quantitative analysis of bioanalytes in a nanovolume of human lachrymal liquid

A flexible electrochemical micro(bio)sensor has been designed for determination of several biological compounds, specifically, ascorbate, dopamine, and glucose, in human lachrymal liquid (tears). The microsensor for simultaneous determination of ascorbate and dopamine concentrations was based on a gold microwire modified with the tetrathiafulvalen-7,7,8,8-tetracyanoquinodimethane complex as a catalyst. To monitor glucose concentration in tears, glucose dehydrogenase was immobilized on a gold microwire modified with carbon nanotubes and an osmium redox polymer. A capillary microcell was constructed for sampling tears. The cell had a working volume of 60-100 nL with a sampling deviation of 6.7%. To check if the microcell was properly filled with buffer or tear sample, a control electrode was introduced into the construction. The electrode was used to measure the electrical resistance of a fully filled nanovolume cell. The mechanical flexibility is one of the most important features of the prototype and allowed direct collection of tears with minimized risk of damage to the eye.

Environmental light and endogenous antioxidants as the main determinants of non-cancer ocular diseases

The human eye is constantly exposed to sunlight and artificial lighting. Exogenous sources of reactive oxygen species (ROS) such as UV light, visible light, ionizing radiation, chemotherapeutics, and environmental toxins contribute to oxidative damage in ocular tissues. Long-term exposure to these insults places the aging eye at considerable risk for pathological consequences of oxidative stress. Furthermore, in eye tissues, mitochondria are an important endogenous source of ROS. Over time, all ocular structures, from the tear film to the retina, undergo oxidative stress, and therefore, the antioxidant defenses of each tissue assume the role of a safeguard against degenerative ocular pathologies. The ocular surface and cornea protect the other ocular tissues and are significantly exposed to oxidative stress of environmental origin. Overwhelming of antioxidant defenses in these tissues clinically manifests as pathologies including pterygium, corneal dystrophies, and endothelial Fuch's dystrophy. The crystalline lens is highly susceptible to oxidative damage in aging because its cells and their intracellular proteins are not turned over or replaced, thus providing the basis for cataractogenesis. The trabecular meshwork, which is the anterior chamber tissue devoted to aqueous humor drainage, has a particular susceptibility to mitochondrial oxidative injury that affects its endothelium and leads to an intraocular pressure increase that marks the beginning of glaucoma. Photo-oxidative stress can cause acute or chronic retinal damage. The pathogenesis of age-related macular degeneration involves oxidative stress and death of the retinal pigment epithelium followed by death of the overlying photoreceptors. Accordingly, converging evidence indicates that mutagenic mechanisms of environmental and endogenous sources play a fundamental pathogenic role in degenerative eye diseases.

Tear analysis in ocular surface diseases

The thin layer of tears covering the ocular surface are a complex body fluid containing thousands of molecules of varied form and function of several origins. In this review, we have discussed some key issues in the analysis of tears in the context of understanding and diagnosing eye disease using current technologies of proteomics and metabolomics, and for their potential for clinical application. In the last several years, advances in proteomics/metabolomics/lipidomics technologies have greatly expanded our knowledge of the chemical composition of tear fluid. The quickened pace of studies has shown that tears as a complex extra-cellular fluid of the ocular surface contains a great deal of molecular information useful for the diagnosis, prognosis, and treatment of ocular surface diseases that has the ability to addresses the emphasis on personalized medicine and biomarkers of disease. Future research directions will likely include (1) standardize tear collection, storage, extraction, and sample preparation; (2) quantitative proteomic analysis of tear proteins using multiple reaction monitoring (MRM)-based mass spectrometry; (3) population based studies of human tear proteomics/metabolomics; (4) tear proteomics/metabolomics for systemic diseases; and (5) functional studies of tear proteins.

Free cysteine modulates the conformation of human C/EBP homologous protein

The C/EBP Homologous Protein (CHOP) is a nuclear protein that is integral to the unfolded protein response culminating from endoplasmic reticulum stress. Previously, CHOP was shown to comprise extensive disordered regions and to self-associate in solution. In the current study, the intrinsically disordered nature of this protein was characterized further by comprehensive in silico analyses. Using circular dichroism, differential scanning calorimetry and nuclear magnetic resonance, we investigated the global conformation and secondary structure of CHOP and demonstrated, for the first time, that conformational changes in this protein can be induced by the free amino acid L-cysteine. Addition of L-cysteine caused a significant dose-dependent decrease in the protein helicity--dropping from 69.1% to 23.8% in the presence of 1 mM of L-cysteine--and a sequential transition to a more disordered state, unlike that caused by thermal denaturation. Furthermore, the presence of small amounts of free amino acid (80 µM, an 8:1 cysteine∶CHOP ratio) during CHOP thermal denaturation altered the molecular mechanism of its melting process, leading to a complex, multi-step transition. On the other hand, high levels (4 mM) of free L-cysteine seemed to cause a complete loss of rigid cooperatively melting structure. These results suggested a potential regulatory function of L-cysteine which may lead to changes in global conformation of CHOP in response to the cellular redox state and/or endoplasmic reticulum stress.

Antioxidant defenses in the ocular surface

The human eye is subjected constantly to oxidative stress due to daily exposure to sunlight, high metabolic activities, and oxygen tension. Reactive oxygen species generated from environmental insults and pathological conditions render the human eye particularly vulnerable to oxidative damage. The ocular surface composed of the tear film, the cornea, and the aqueous humor forms the first physical and biochemical barrier of the eye and plays a pivotal role in combating free radicals. These ocular compartments are enriched in certain antioxidants in the form of metabolic enzymes or small molecules. Such an antioxidant defense system in the ocular surface is essential for the maintenance of redox homeostasis in the eye and protection against oxidative damage. Herein, we review the properties and functions of key constituent antioxidants of the ocular surface.

Ultraviolet radiation: cellular antioxidant response and the role of ocular aldehyde dehydrogenase enzymes

Solar ultraviolet radiation (UVR) exposes the human eye to near constant oxidative stress. Evidence suggests that UVR is the most important environmental insult leading to the development of a variety of ophthalmoheliosis disorders. UVR-induced reactive oxygen species (ROS) are highly reactive with DNA, proteins, and cellular membranes, resulting in cellular and tissue damage. Antioxidant defense systems present in ocular tissues function to combat ROS and protect the eye from oxidative damage. Important enzymatic antioxidants are the superoxide dismutases, catalase, glutathione peroxidases, glutathione reductase, and members of the aldehyde dehydrogenase (ALDH) superfamily. Glutathione, ascorbic and uric acids, α-tocopherol, nicotinamide-adenine dinucleotide phosphate, and ferritin serve as small molecule, nonenzymatic antioxidants. Ocular tissues have high levels of these antioxidants, which are essential for the maintenance of reduction-oxidation homeostasis in the eye and protection against oxidative damage. ALDH1A1 and ALDH3A1, present abundantly in the cornea and lens, have been shown to have unique roles in the defense against UVR and the downstream effects of oxidative stress. This review presents the properties and functions of ocular antioxidants that play critical roles in the cellular response to UVR exposure, including a focused discussion of the unique roles that the ALDH1A1 and ALDH3A1 enzymes have as multifunctional ocular antioxidants.

Characterization of the human tear metabolome by LC-MS/MS

The tear film overlying the epithelial cells of the eye's surface is vital to visual function, and its composition is reflective of ocular surface health. The ultrasmall volume of tears poses challenges in its analysis, contributing to the limited number of reports on the tear metabolome. In addition, using a standard clinical method of tear collection posed some confounding factors in metabonomic analysis. We sought to establish an analytical platform for the global characterization of human tear metabolites. Following information dependent acquisition (IDA) directed liquid chromatography-tandem mass spectrometry (LC-MS/MS), isotope pattern matched peak mining was performed using Extracted Ion Chromatogram (XIC) manager within the PeakView software. Sixty metabolites representing diverse compound classes were identified in human tears, most of which have not been previously reported. Selected metabolites were verified using pure standards. Unsupervised chemometric analysis showed good separation between tear samples and blanks (PC1 = 87%, R(2) = 0.91, Q(2) = 0.87). The results demonstrated the potential of our platform for untargeted metabonomic studies of eye diseases.

Pharmacotherapy of dry eye

INTRODUCTION

based on data from the largest studies of dry eye to date - the Women's Health Study (WHS) and the Physicians' Health Study (PHS) - and other studies, it has been estimated that about 3.23 million women and 1.68 million men, for a total of 4.91 million Americans aged ≥ 50 years, have dry eye. Tens of millions more have less severe symptoms and probably a more episodic manifestation of the disease that is notable only during contact with some adverse contributing factor(s), such as low humidity or contact lens wear. Dry eye disease is a common yet frequently under-recognized public health problem whose etiology and management challenge clinicians and researchers involved in this field.

AREAS COVERED

advances in the understanding of the disease have been made over the past 10 years in areas of epidemiology, pathogenesis, clinical manifestation, and possible therapy. Historical aspects and recent information in relation to the use of artificial tear substitutes and anti-inflammatory agents in dry eye disease, including topical cyclosporin and corticosteroids, autologous serum, tetracyclines and systemic immunosuppressants, are covered in this review. The reader will gain insight into the recent views on the pharmacological menu of treatments for dry eyes following the recommendations of the 2007 International Dry Eye Workshop.

EXPERT OPINION

dry eye is a visually disabling disease, the treatment of which needs tailoring according to the type and severity of dry eye disease.

Tear analysis of ascorbic acid, uric acid and malondialdehyde with capillary electrophoresis

Tears have a significant role in antioxidant defense in ocular tissues and since their collection is quick and noninvasive, their analysis would facilitate monitoring of pathophysiological changes. However, their low volume and low content of antioxidants makes analysis difficult; methods of high sensitivity are needed. In this paper, we present a method for tear analysis of two antioxidant molecules (ascorbic and uric acid) and of a lipid peroxidation indicator (malondialdehyde) with capillary electrophoresis. Tears were collected with Schirmer strips, extracted with a low-pH phosphate buffer, centrifuged through membrane filters and an antioxidant was added. They were stable at -70 degrees C for 15 days. After pilot experiments, optimum electrophoretic separation was achieved in a 25 mM borate buffer, pH 10.0, containing 100 mM sodium dodecyl sulfate at 25 degrees C and 20 kV. The developed method has good repeatability (<5% RSD), precision (<15% relative error values) and high sensitivity (LLOQ values of 20, 2.3 and 2.5 microM for ascorbate, urate and malondialdehyde, respectively). It was applied to the analysis of tears from healthy individuals and the antioxidant levels are in agreement with those obtained with other techniques. This method might serve as a tool to clarify the role of endogenous antioxidants in the pathophysiology of ocular diseases.

Determination of uric acid concentrations in human tear fluid, aqueous humour and serum

PURPOSE

The aim of this investigation was to assess and compare levels of uric acid in human tear fluid, aqueous humour and serum in cataract patients in order to obtain information about uric acid-dependent antioxidative status.

METHODS

Reflex tear fluid, aqueous humour and serum were obtained from 103 cataract patients (52 women, 51 men). Uric acid was determined by a reversed phase chromatographic method with electrochemical detection. The Randox test was used to measure the contribution of uric acid to the total antioxidative status (TAS) of these body fluids.

RESULTS

Mean concentrations of uric acid were 20 +/- 8 microg/ml in tear fluid, 18 +/- 6 microg/ml in aqueous humour and 58 +/- 15 microg/ml in serum. Significantly less uric acid was found in tear fluid (p < 0.01) and aqueous humour (p < 0.01) in female cataract patients compared with male cataract patients. There was a significant positive correlation between uric acid content in tear fluid, aqueous humour and serum, respectively (p < 0.01). The contribution of uric acid to TAS (Randox test) amounted to 38% in tear fluid, 10% in aqueous humour and 37% in serum.

CONCLUSIONS

Uric acid was found in comparable amounts in tear fluid and aqueous humour, but these were two- to threefold lower than in serum. Female cataract patients had lower uric acid values in tear fluid and aqueous humour compared with male cataract patients. Uric acid is an important antioxidant, as proven by its contribution to TAS as determined by the Randox test. The precise role of uric acid in the antioxidative reactions in tear fluid and aqueous humour is still under debate.

Oxidative stress in diseases of the human cornea

Intense exposure to light, robust metabolic activity, and high oxygen tension render the human eye particularly vulnerable to oxidative damage and the list of ophthalmological disorders implicating reactive oxygen and nitrogen species is rapidly expanding. Here, we review the roles of oxidative stress in the etiopathogeneses and pathophysiology of diseases of the human cornea including pterygium, keratoconus, trauma and chemical injury, and a host of inflammatory, metabolic, degenerative, and iatrogenic conditions. Data from animal and tissue culture experimentation germane to these conditions are also adduced.

Glutathione and its transporters in ocular surface defense

Glutathione (GSH) is an abundant antioxidant ubiquitous in nearly all cell types. Deficiency of GSH has been linked to ocular disease and viral infection. Other established vital roles of GSH include detoxification and immunoprotection. Endogenous GSH plays a protagonist's role in safeguarding active transport processes compartmentalized at the interface between conjunctival mucosa and the tear film. Optimal electrokinetic transport across the conjunctival epithelium requires the mucosal presence of GSH. Glutathione is the most abundant known endogenous antioxidant molecule in tear fluid, mainly derived from conjunctival secretion. Conjunctival GSH transport, a major kinetic component of GSH turnover, occurs through multiple functionally distinct mechanisms. Cell membrane potential regulates conjunctival GSH efflux, while conjunctival GSH uptake requires extracellular Na(+). Significant modulation of GSH, its constituent amino acids, and functions of associated transporters occurs in the conjunctival epithelium with viral inflammatory disease. Topical conjunctival delivery of GSH, its metabolic precursors, or pharmacologic stimulation of endogenous conjunctival GSH secretion carry potential in alleviating viral-inflammatory conjunctivitis.

The role of corneal crystallins in the cellular defense mechanisms against oxidative stress

The refracton hypothesis describes the lens and cornea together as a functional unit that provides the proper ocular transparent and refractive properties for the basis of normal vision. Similarities between the lens and corneal crystallins also suggest that both elements of the refracton may also contribute to the antioxidant defenses of the entire eye. The cornea is the primary physical barrier against environmental assault to the eye and functions as a dominant filter of UV radiation. It is routinely exposed to reactive oxygen species (ROS)-generating UV light and molecular O(2) making it a target vulnerable to UV-induced damage. The cornea is equipped with several defensive mechanisms to counteract the deleterious effects of UV-induced oxidative damage. These comprise both non-enzymatic elements that include proteins and low molecular weight compounds (ferritin, glutathione, NAD(P)H, ascorbate and alpha-tocopherol) as well as various enzymes (catalase, glucose-6-phosphate dehydrogenase, glutathione peroxidase, glutathione reductase, and superoxide dismutase). Several proteins accumulate in the cornea at unusually high concentrations and have been classified as corneal crystallins based on the analogy of these proteins with the abundant taxon-specific lens crystallins. In addition to performing a structural role related to ocular transparency, corneal crystallins may also contribute to the corneal antioxidant systems through a variety of mechanisms including the direct scavenging of free radicals, the production of NAD(P)H, the metabolism and/or detoxification of toxic compounds (i.e. reactive aldehydes), and the direct absorption of UV radiation. In this review, we extend the discussion of the antioxidant defenses of the cornea to include these highly expressed corneal crystallins and address their specific capacities to minimize oxidative damage.

Iodide iontophoresis as a treatment for dry eye syndrome

BACKGROUND/AIMS

Among the causes related to the development or perpetuation and aggravation of dry eye disease, oxidative reactions may have a role in the pathogenesis of this disorder. Antioxidants, such as iodide, have shown a strong effect in preventing the oxidative damage to constituents of the anterior part of the eye. In this clinical trial the effectiveness of iodide iontophoresis and iodide application without current in moderate to severe dry eye patients was compared.

METHODS

16 patients were treated with iodide iontophoresis and 12 patients with iodide application without current for 10 days. Subjective improvement, frequency of artificial tear application, tear function parameters (break up time, Schirmer test without local anaesthesia), vital staining (fluorescein and rose bengal staining) as well as impression cytology of the bulbar conjunctiva were evaluated before treatment, 1 week, 1 month, and 3 months after treatment.

RESULTS

A reduction in subjective symptoms, frequency of artificial tear substitute application, and an improvement in certain tear film and ocular surface factors could be observed in both groups. A stronger positive influence was seen after application of iodide with current (iontophoresis), as observed in a distinct improvement in break up time, fluorescein and rose bengal staining, and in a longer duration of this effect compared with the non-current group. No significant change in Schirmer test results and impression cytology were observed in both groups.

CONCLUSIONS

Iodide iontophoresis has been demonstrated to be a safe and well tolerated method of improving subjective and objective dry eye factors in patients with ocular surface disease.

Lachrymal Determinations: Methods and Updates on Biopharmaceutical and Clinical Applications

This article displays different procedures used to collect lachrymal fluid and describes some of its applications. Sampling tears represents the main difficulty to produce precise and reproducible results. The direct sampling procedure consists in collecting tears with capillary tubes and has the drawback of demanding previous stimulation that induces major dilution. The indirect method does not require preliminary stimulation but has been held responsible for altering epithelium and promoting leakage from plasma. Schirmer strips and sponges are classically required. Applications are numerous in biopharmaceutical and clinical fields. The determination of endogenous components has great potentiality as a diagnostic tool, but the use of tear as a substitute of plasma does not present clinical relevance. Levels of drugs like immunosuppressive or antibiotic agents are determined in tears to verify that pharmacological concentrations are reached and frequency of administration is deduced from kinetic fitting.

Iodide protection from UVB irradiation-induced degradation of hyaluronate and against UVB-damage of human conjunctival fibroblasts

BACKGROUND

To determine whether iodide protects from UVB irradiation-induced destruction of hyaluronate and against UVB injury of cultured human conjunctival fibroblasts.

METHODS

Hyaluronate and primary cultured human conjunctival fibroblasts were incubated with various concentrations of iodide and then exposed to UV light irradiation of 312 nm. Hyaluronate destruction was determined by viscosity measurements. Cell viability was assessed with MTT assay.

RESULTS

Iodide protects hyaluronate from UVB light-induced degradation of this macromolecule in a concentration-dependent manner. Incubation of human conjunctival fibroblasts with iodide inhibited cells from damage by UVB light.

CONCLUSION

Iodide protects hyaluronate, a component of tear fluid and tissues of the anterior part of the eye, against UVB light-induced degradation. Also, injury of human conjunctival cells can be prevented by incubation with iodide before UVB irradiation. The mechanism of protection is likely to include an antioxidative reaction. To support the natural defence mechanisms of the eyes, the administration of an antioxidant such as iodide to artificial tears, for example, may help to prevent the damage of the eye provoked by oxidative stress.

Fourier transform IR attenuated total reflectance spectroscopy studies of cysteine-induced changes in secondary conformations of bovine serum albumin after UV-B irradiation

Fourier transform IR spectroscopy equipped with attenuated total reflection was used to investigate the cysteine-induced alteration of the protein secondary structure of bovine serum albumin (BSA) in aqueous solution before and after UV-B irradiation. Several amino acids were also studied. The results indicate the unchanged IR spectra of BSA coincubated with amino acids, except cysteine, did not change after 72-h UV-B irradiation. There was no difference in the IR spectrum of the unirradiated BSA coincubated with cysteine. A shoulder at 1620 cm(-1) attributed to the intermolecular beta-sheet structure was observed for the IR spectrum of BSA coincubated with cysteine after 72-h UV-B irradiation. Moreover, the peak intensity at 1303 cm(-1) that is due the alpha-helix structure was reduced, but the peak intensity at 1247 cm(-1) corresponding to beta-sheet structures was increased. Longer UV-B exposure for a BSA solution coincubated with cysteine changed the BSA solution from clear to viscous to gel form in which a transparent gel and another white gel were simultaneously observed. A gradual IR spectral alteration was found for BSA coincubated with cysteine and subjected to increased UV-B irradiation. The longer UV-B irradiation yielded increased intensity at 1620 cm(-1). The second-derivative IR peaks at 1655, 1631, and 1548 cm(-1) were shifted to 1650, 1620, and 1544 cm(-1), respectively, by the increase of UV-B irradiation, suggesting a progressive transformation from an alpha-helix to an intermolecular beta-sheet structure for BSA coincubated with cysteine. This strongly implies that longer UV-B exposure time for the BSA solution in the presence of cysteine did alter the protein secondary structures of BSA more, thus inducing gel formation by protein aggregation.

Reflex Tear Ascorbate in Hong Kong Chinese Subjects: Method Comparison and Biological Variation

BACKGROUND

Tear ascorbate is important for corneal health. A rapid and simple method for measurement of ascorbate in tears is needed, and adequate knowledge of physiological variation of tear ascorbate is important to facilitate comparative studies of the effect of, for example, contact lens wear and environmental conditions and stresses. However, there are currently no data on physiological variation of tear ascorbate. This study validated a simple and speedy method for tear ascorbate and investigated between-eye and between-day variation in tear ascorbate in healthy young adults.

METHODS

Yawn-induced reflex tears were collected from 32 healthy Hong Kong Chinese subjects and measured by both high-performance liquid chromatography (HPLC) and by an enzyme-linked colorimetric method known as FRASC (total ferric reducing (antioxidant) activity and ascorbate concentration measurement). For between-eye variation, yawn reflex tears were collected from each eye of the same 32 healthy subjects, and ascorbate was measured using HPLC; in a separate experiment for between-day variation, tears were collected on two separate days from 14 subjects, and ascorbate was measured by FRASC.

RESULTS

Both HPLC and FRASC showed high precision, and results obtained using FRASC were not statistically different from those using HPLC; mean +/- SD were, respectively, 18.5 +/- 4.4 microM and 18.5 +/- 4.8 microM for HPLC and FRASC methods (p = 0.943). No significant between-eye difference in tear ascorbate was found (p = 0.386), and no significant between-day variation was found overall: mean +/- SD ascorbate was 20.0 +/- 6.2 microM on day 1 and 19.3 +/- 6.8 microM on day 2 (p = 0.772). However, between-day variation was large in seven of 14 subjects.

CONCLUSION

FRASC is an acceptable alternative to HPLC for measurement of tear ascorbate. Tears for ascorbate investigation can be collected from either eye or, if necessary, from both eyes and pooled. However, tear ascorbate may vary widely from day to day in the same individual. The reasons for this variation require further study but may relate to differences in ascorbate supply or demand within the precorneal tear layer.