Hyperosmolarité : effets intracellulaires et implication dans la sécheresse oculaire

Cellular Stress in Dry Eye Disease-Key Hub of the Vicious Circle

Disturbance or insufficiency of the tear film challenges the regulatory systems of the ocular surfaces. The reaction of the surfaces includes temporary mechanisms engaged in the preservation of homeostasis. However, strong or persisting challenges can lead to the potential exhaustion of the coping capacity. This again activates the vicious circle with chronic inflammation and autocatalytic deterioration. Hence, the factors challenging the homeostasis should be addressed in time. Amongst them are a varying osmolarity, constant presence of small lesions at the epithelium, acidification, attrition with mechanical irritation, and onset of pain and discomfort. Each of them and, especially when occurring simultaneously, impose stress on the coping mechanisms and lead to a stress response. Many stressors can culminate, leading to an exhaustion of the coping capacity, outrunning normal resilience. Reaching the limits of stress tolerance leads to the manifestation of a lubrication deficiency as the disease we refer to as dry eye disease (DED). To postpone its manifestation, the avoidance or amelioration of stress factors is one key option. In DED, this is the target of lubrication therapy, substituting the missing tear film or its components. The latter options include the management of secondary sequelae such as the inflammation and activation of reparative cascades. Preventive measures include the enhancement in resilience, recovery velocity, and recovery potential. The capacity to handle the external load factors is the key issue. The aim is to guard homeostasis and to prevent intercellular stress responses from being launched, triggering and invigorating the vicious circle. Considering the dilemma of the surface to have to cope with increased time of exposure to stress, with simultaneously decreasing time for cellular recovery, it illustrates the importance of the vicious circle as a hub for ocular surface stress. The resulting imbalance triggers a continuous deterioration of the ocular surface condition. After an initial phase of the reaction and adaption of the ocular surface to the surrounding challenges, the normal coping capacity will be exhausted. This is the time when the integrated stress response (ISR), a protector for cellular survival, will inevitably be activated, and cellular changes such as altered translation and ribosome pausing are initiated. Once activated, this will slow down any recovery, in a phase where apoptosis is imminent. Premature senescence of cells may also occur. The process of prematurization due to permanent stress exposures contributes to the risk for constant deterioration. The illustrated flow of events in the development of DED outlines that the ability to cope, and to recover, has limited resources in the cells at the ocular surface. The reduction in and amelioration of stress hence should be one of the key targets of therapy and begin early. Here, lubrication optimization as well as causal treatment such as the correction of anatomical anomalies (leading to anatomical dry eye) should be a prime intent of any therapy. The features of cellular stress as a key hub for the vicious circle will be outlined and discussed.

Expression of GPR-68 in Human Corneal and Conjunctival Epithelium. Possible indicator and mediator of attrition associated inflammation at the ocular surface

INTRODUCTION

Attrition and osmotic stress have been identified as major forces in the pathophysiology of dry eye. Impaired tolerance to mechano-transduction in the presence of insufficient lubrication has been associated with disturbances of ocular surface homeostasis and encouragement of inflammatory reactions, challenging the usual regulatory coping mechanisms. In spite of the probable link between enhanced attrition and secondary inflammation, the key mediators driving the vicious cycle of severe dry eye disease have not yet been identified. The goal of this study was therefore to investigate human corneal and conjunctival epithelium for the presence of the G protein-coupled receptor GPR-68. This protein had most recently been shown to be not only chemically activated but also mechanically, possibly through attrition.

METHODS

De-identified sections of human cornea and conjunctiva were stained for the presence of G protein-coupled receptor 68 with specific antibodies using immunohistochemical methods. Results Specific staining for G-protein-coupled receptor 68 (GPR68) was observed in all samples of the cornea throughout the epithelial layers of the corneal epithelium, most prominently in the area of the wing cells and the basement membrane. Even in the conjunctiva, specific staining for GPR-68 was found.

DISCUSSION

The detection of G protein-coupled receptor GPR-68 in human corneal and conjunctival epithelium raises the question of its function and purpose. The mechanical activation of GPR68 in situations with enhanced friction and attrition could modify various cellular functions and possibly jeopardize normal inflammatory homeostasis at the ocular surface. Accordingly, decreased lubrication in dry eye disease could result in activation of GPR-68. This could lead to secondary inflammation, initially in the epithelium and surrounding stroma. Continuous mechanical stress could result in chronic inflammation, also reaching deeper structures of the cornea, possibly making GPR-68 an important actor in the vicious cycle of dry eye disease.

CONCLUSION

G protein-coupled receptor GPR-68, sensitive to flow and mechanic stimulation, is present in the human corneal epithelium and conjunctiva. Decreased lubrication and increased attrition, accompanied by sensations typical for dry eye, might lead to local inflammation. It is possible that subtle signs of conjunctival, and later corneal, surface damage in the context of these sensations could be a better indicator of the need for and success of therapy than the clinical signs of dry eye disease alone, at least in the early stages of the disease. Inhibition of G protein-coupled receptor GPR-68 could represent a new strategy in the treatment of dry eye disease.

Assessment of tear film osmolarity using the IPen® Vet osmometer in Pug and Shih-Tzu dogs with and without keratoconjunctivitis sicca

OBJECTIVE

To establish tear film osmolarity (TFO) values in Pugs and Shih-Tzus, with and without keratoconjunctivitis sicca (KCS).

ANIMALS STUDIED

A total of 82 adult dogs were evaluated.

PROCEDURE

The inclusion criteria for the healthy group was a Schirmer tear test (STT-1) ≥15 mm/min with no clinical signs of KCS, whereas those with KCS had clinical signs and a STT-1 ≤10 mm/min. All animals underwent complete ophthalmological evaluation prior to STT-1 and TFO. Student's t tests were used to compare STT-1 and TFO in KCS and healthy eyes as well as possible differences in TFO between breeds. In addition, a linear regression to model the relationship between the two variables (STT-1 and TFO) was performed. A P-value ≤ 0.05 was considered statistically significant.

RESULTS

STT-1 results were significantly lower (p = 0.0001) in the KCS group (4.46 ± 1.74) compared with the control group (18.80 ± 2.02). Mean TFO results were significantly higher in the KCS group (353.02 ± 16.58 mOsm/L) (p < 0.0001) compared with the control group (315.27 ± 6.15 mOsm/L). The formula Y = 365.059-2.625 * X significantly predicts (p < 0.001) the value of the variable Y (TFO mOsm/L) as a function of the variable X (STT-1 mm/min), with a coefficient of determination of 0.71.

CONCLUSIONS

The results revealed differences in TFO and STT-1 between KCS and healthy dogs. Additionally, STT-1 and TFO values were correlated with the aim to use STT-1 values to predict TFO values in brachycephalic breeds.

Lipid Annotation by Combination of UHPLC-HRMS (MS), Molecular Networking, and Retention Time Prediction: Application to a Lipidomic Study of In Vitro Models of Dry Eye Disease

Annotation of lipids in untargeted lipidomic analysis remains challenging and a systematic approach needs to be developed to organize important datasets with the help of bioinformatic tools. For this purpose, we combined tandem mass spectrometry-based molecular networking with retention time (t_R) prediction to annotate phospholipid and sphingolipid species. Sixty-five standard compounds were used to establish the fragmentation rules of each lipid class studied and to define the parameters governing their chromatographic behavior. Molecular networks (MNs) were generated through the GNPS platform using a lipid standards mixture and applied to lipidomic study of an in vitro model of dry eye disease, i.e., human corneal epithelial (HCE) cells exposed to hyperosmolarity (HO). These MNs led to the annotation of more than 150 unique phospholipid and sphingolipid species in the HCE cells. This annotation was reinforced by comparing theoretical to experimental t_R values. This lipidomic study highlighted changes in 54 lipids following HO exposure of corneal cells, some of them being involved in inflammatory responses. The MN approach coupled to t_R prediction thus appears as a suitable and robust tool for the discovery of lipids involved in relevant biological processes.

[Tear film analysis and evaluation of optical quality: A review of the literature (French translation of the article)]

Dry eye is a complex multifactorial disease of the ocular surface and tears. It is associated with ocular surface symptoms and is one of the most common causes for ophthalmologic consultation. Despite their frequent use in clinical practice, the usual tests to evaluate dry eye and ocular surface disease-history of symptoms, tear break-up time (TBUT), Meibomian gland evaluation, corneal fluorescein staining, Schirmer test-have shown low reproducibility and reliability. In addition, subjective symptoms are often weakly or poorly correlated with objective signs. Since the tear film is the first system through which light must pass, the optical quality of the eye is highly dependent on the homogeneity of the tear film. Various investigative methods have been developed to evaluate both the structural and functional quality of the tear film, such as corneal topography, interferometry, tear meniscus measurement, evaporation rate, tear osmolarity and even aberrometry. Some are easily accessible to clinicians, while others remain in the field of clinical research. All of these tests provide a better understanding of the pathophysiology of the tear film. This review hopes to provide an overview of the existing tests and their role in evaluating the significance of the tear film in visual function.

Tear film analysis and evaluation of optical quality: A review of the literature

Dry eye is a complex multifactorial disease of the ocular surface and tears. It is associated with ocular surface symptoms and is one of the most common causes for ophthalmologic consultation. Despite their frequent use in clinical practice, the usual tests to evaluate dry eye and ocular surface disease-history of symptoms, tear break-up time (TBUT), Meibomian gland evaluation, corneal fluorescein staining, Schirmer test-have shown low reproducibility and reliability. In addition, subjective symptoms are often weakly or poorly correlated with objective signs. Since the tear film is the first system through which light must pass, the optical quality of the eye is highly dependent on the homogeneity of the tear film. Various investigative methods have been developed to evaluate both the structural and functional quality of the tear film, such as corneal topography, interferometry, tear meniscus measurement, evaporation rate, tear osmolarity and even aberrometry. Some are easily accessible to clinicians, while others remain in the field of clinical research. All of these tests provide a better understanding of the pathophysiology of the tear film. This review hopes to provide an overview of the existing tests and their role in evaluating the significance of the tear film in visual function.