Enzymes of urea synthesis are expressed at the ocular surface, and decreased urea in the tear fluid is associated with dry-eye syndrome

Review of genes potentially related to hyposecretion in male non-obese diabetic (NOD) mice, a Sjögren's syndrome model

BACKGROUND

Sjögren's syndrome (SS) is known to cause dry eyes and mouth due to inflammation of the lacrimal and salivary glands. However, some reports imply that other factors trigger dry eyes and mouth. We previously investigated various factors using RNA-sequencing analysis of lacrimal glands from male non-obese diabetic (NOD) mice, an SS model. In this review, we described (1) the exocrine features of male and female NOD mice, (2) the up- and down-regulated genes in the lacrimal glands of male NOD mice as revealed by our RNA-sequencing data, and (3) comparisons between these genes and data in the Salivary Gland Gene Expression Atlas.

HIGHLIGHTS

Male NOD mice exhibit a steady worsening of lacrimal hyposecretion and dacryoadenitis, whereas females exhibit a complex pathophysiological condition that includes diabetic disease, salivary hyposecretion, and sialadenitis. Ctss, an up-regulated gene, is a potential inducer of lacrimal hyposecretion and is also expressed in salivary glands. Two other up-regulated genes, Ccl5 and Cxcl13, may worsen the inflammation of SS in both the lacrimal and salivary glands. The genes Esp23, Obp1a, and Spc25 were detected as down-regulated, but judging the relationship between these genes and hyposecretion is difficult as only limited information is available. Another down-regulated gene, Arg1, is involved in lacrimal hyposecretion, and it also has the potential to cause salivary hyposecretion in NOD mice.

CONCLUSION

In NOD mice, males may be better than females at evaluating the pathophysiology of SS. Some regulated genes revealed by our RNA-sequencing data might be potential therapeutic targets for SS.

Chronic canaliculitis with canaliculoliths due to Providencia stuartii infection

Canaliculitis, inflammation of the lacrimal canaliculi, can be caused by numerous pathogens, most commonly bacteria from the genera Actinomyces, Streptococcus, and Staphylococcus. Primary canaliculitis often requires surgical canaliculolith removal and appropriate antibiotic coverage. The authors report a case of a 77-year-old woman with a history of punctal plugs who presented with chronic canaliculitis with canaliculoliths that grew Providencia stuartii. P. stuartii has not previously been described as a cause of primary canaliculitis. This case highlights a new organism that causes canaliculitis with canaliculoliths and stresses the importance of speciation and antibiotic sensitivity testing following canaliculotomy and curettage. P. stuartii should be considered in the differential for bacterial canaliculitis with canaliculoliths, especially in patients with persistent symptoms on topical antibiotic therapy without canaliculotomy.

Urea and ocular surface: Synthesis, secretion and its role in tear film homeostasis

Urea has been detected in the tear film, aqueous humor, and vitreous of the eye. While most of the urea in the aqueous humor and vitreous is considered to be an ultrafiltrate from the blood vessels, the presence of urea transporters and urea-synthesizing enzymes in the lacrimal gland, meibomian glands, conjunctiva, and cornea suggests ureagenesis occurring at the ocular surface. This review summarizes the distribution and function of urea transporters, urea and its synthesizing enzymes at the ocular surface to analyze their role in the tear film homeostasis. Urea transporters (UT)-A- and UT-B-as well as the enzymes arginase I, II, and agmatinase are located at the ocular surface. Urea concentration on the ocular surface is influenced by blood urea concentration, the amount of urea released by the tear fluid, tear evaporation, and arginase concentration in the tears. There are conflicting reports on the relationship between tear and plasma urea levels though a linear correlation exists between their levels. Urea protects the ocular surface from osmotic stress and is thought to maintain a lipid-water interface in the lamellar phase of the tear film. The reduction of urea levels in the tears of patients with evaporative dry eye suggests its possible role in tear film stability. Other than mitigating osmotic stress, urea has hydrating properties as well. Animal studies have demonstrated the healing effects of urea on the corneal epithelium. Future studies examining the variations in urea content in tears from different ocular surfaces, at different times of day, and under different environmental conditions would further solidify the role of urea in tear film stability.

Immunohistochemical detection of urea transporter-A in the tear-producing part of the lacrimal system

OBJECTIVE

Urea constitutes a physiological and presumably well-regulated constituent of tear fluid. Its lacrimal concentration is significantly decreased in dry eye disease. Urea homeostasis within the tear fluid may also depend on the expression of urea transporters. The present study reports on the expression patterns of urea transporter A (UT-A) in the cells and tissues of the ocular surface and the lacrimal glands.

METHODS

UT-A immunohistochemistry was performed on 5 µm paraffin sections of paraformaldehyde-fixed human, porcine, and murine corneas, eyelids, and lacrimal glands (n = 5 each).

RESULTS

UT-A immunostaining was largely comparable in all three species. UT-A signals were detected in the corneal epithelium and endothelium, in the conjunctival epithelium, in the acinar cells and excretory ducts of the lacrimal gland, Meibomian gland, and in the glands of Moll and Zeis. The Meibomian glands and the glands of Zeis exhibited a marked UT-A-positive staining in the basal cells of the alveolar epithelia and in the ductal epithelia.

CONCLUSION

UT-A shows comparable expression patterns to UT-B (previous study) at the ocular surface and in the lacrimal glands, as determined by immunohistochemistry. The presence of both urea transporters in the lacrimal functional unit suggests that they are essential for the normal function of the lacrimal system and the integrity of the tear film. Potential alterations in urea transporter expression might be associated with the significant reduction of urea found in the tear fluid of dry eye patients. They may thus play an important role in the pathogenesis of dry eye disease.

Urea transporter-B expression on the ocular surface and in the lacrimal glands

OBJECTIVE

Urea is a component of tear fluid showing a significantly decreased concentration in dry eye disease. The urea content of tear fluid may depend on urea transporters. The purpose of this study was to examine the expression of urea transporter B (UT-B) at the ocular surface and in the lacrimal glands.

METHODS

UT-B protein and mRNA expression was investigated in human, porcine, and murine samples. Immunohistochemical staining for UT-B was performed on paraffin sections of human, porcine, and murine corneas, eyelids, and lacrimal glands (n = 5 each). Reverse transcriptase polymerase chain reaction was conducted to detect UT-B mRNA in human and murine cornea, conjunctiva, Meibomian gland, and lacrimal gland (n = 5 each).

RESULTS

UT-B protein expression was comparable in all three species. It was found in the corneal epithelium and endothelium, in the conjunctival epithelium, in the end pieces and excretory ducts of the lacrimal gland, Meibomian gland, and in the glands of Moll and Zeis. The glands of Zeis and the Meibomian glands showed intense UT-B signals in the basal layers of the alveolar epithelia and in the cells of the ductal epithelia. UT-B mRNA was detected in all samples analyzed.

CONCLUSION

UT-B is expressed by the cells and tissues of the ocular surface and in the lacrimal glands. Potential changes in urea transporter expression might have implications for the pathogenesis of dry eye disease. Since comparable results were obtained for all species investigated, the presented findings may open the door for DED-relevant experimentation on porcine and murine model systems.

Potential Ophthalmological Application of Extracts Obtained from Tuna Vitreous Humor Using Lactic Acid-Based Deep Eutectic Systems

A green technique was developed to extract hyaluronic acid (HA) from tuna vitreous humor (TVH) for its potential application in managing dry eye disease. Deep eutectic solvents (DES) were used to extract HA and were synthesized using natural compounds (lactic acid, fructose, and urea). The DES, the soluble fraction of TVH in DES (SF), and the precipitated extracts (PE) were evaluated for their potential use in dry eye disease treatment. In vitro experiments on human corneal epithelial cell lines and the effect on dry eye-associated microorganisms were performed. The influence of the samples on the HCE viability, their intracellular reactive oxygen species (ROS) scavenging capacity, inflammatory response, and antimicrobial properties were studied. According to the results, all samples displayed an antioxidant effect, which was significantly higher for PE in comparison to SF. Most of the tested samples did not induce an inflammatory response in cells, which confirmed the safety in ophthalmic formulations. In addition, the DES and SF proved to be efficient against the studied bacterial strains, while PE did not show an antimicrobial effect. Hence, both DES and SF at defined concentrations could be used as potential compounds in dry eye disease management.

Evaluation of Tear Film Urea Level as a Diagnostic Marker of Dry Eye Disease: A Pilot Study

Introduction Urea secreted in the sweat is important for skin moisture. Similarly, ocular surface moisture is maintained by the conjunctiva. Based on this, the level of urea in tear film can be used as a potential diagnostic test for dry eye disease (DED). One of the standard tests for DED is Schirmer's test (ST). The aim of this study was to compare tear film urea to values of ST.

Methods Fifty patients symptomatic for DED having ST ≤ 10 mm/5 min were enrolled in the study. Fifty age- and sex-matched asymptomatic subjects with ST > 10 mm/5 min were taken as controls. All patients were subjected to an estimation of tear film urea, collected using micropipettes, and analyzed by an Erba Chem 5 semi-autoanalyzer. Based on the ST reading as per the Dry Eye Workshop 2007 (DEWS) classification, dry eye was classified as dry eye (≤ 10 mm/5 min), severe dry eye (≤ 5 mm/5 min), and very severe dry eye (≤ 2 mm/5 min). Tear film urea values were compared with ST values that were considered standard. Statistical analysis was done using Medcalc version 19.7. A p-value ≤ 0.05 was considered significant.

Results The mean tear film urea levels in cases were (26.78 ± 5.70 mg/dL) significantly lower compared with controls (41.72 ± 6.86 mg/dL). The area under the receiver characteristic operator curve (AUC) for tear film urea in diagnosing DED was 0.936 (p < 0.0001) with a cutoff of ≤ 37.2 mg/dL, yielding a sensitivity of 96% and a specificity of 76%. For diagnosing severe DED, the AUC for tear film urea was 0.824 (p < 0.0001) with a cutoff value of ≤ 23.4 mg/dL, yielding a sensitivity of 60.8% and a specificity of 92.59%. For diagnosis of very severe DED, the AUC for tear film urea was 0.972 (p < 0.0001) with a cutoff value of ≤ 19.8 mg/dL, yielding a sensitivity of 100% and a specificity of 93.62%. On comparing ST values to tear film urea, the regression coefficient was 0.85 (p < 0.0001), suggesting a linear relationship between ST and tear film urea.

Conclusion The study demonstrates that tear film urea can be a potential diagnostic marker for DED. The study also indicates that tear film urea level is linearly related to Schirmer's test values and provides an approximate diagnostic cutoff level for the design of future large-scale studies.

[Results of the Dry Eye Research Grant Award 2016]

The Sicca-Förderpreis (Dry Eye Award) supports the development of scientific research on the pathogenesis, diagnostics, and treatment of dry eye and ocular surface diseases. It is awarded after a limited call for proposals in German-speaking countries, written application and selection of the award winner after evaluation by a jury of ophthalmologists working in basic and clinical science. In this article examples of the results of funded projects of the Sicca-Förderpreis 2016 are cursorily described, which were presented at the Ophthalmological Academy of Germany 2019 (Augenärztliche Akademie Deutschland 2019) and therefore provide an insight into current scientific developments. The role of muscarinic receptors and those of urea in the pathogenesis of dry eye as well as the (missing) correlation of tear film stability, viscosity and surface tension are highlighted. A project on the early detection of ocular involvement in graft versus host disease and the idea of treating meibomian gland dysfunction with eyelid surgery techniques are also groundbreaking. The outlined projects represent the potential for further substantial developments in the understanding, diagnostics and treatment of dry eye; however, their long-term clinical relevance still needs to be established.

Arginase 1 is involved in lacrimal hyposecretion in male NOD mice, a model of Sjögren's syndrome, regardless of dacryoadenitis status

Key points

Few reports have explored the possibility of involvement of non‐inflammatory factors in lacrimal hyposecretion in Sjögren's syndrome (SS).

RNA‐sequencing analysis revealed that only four genes, including arginase 1, were downregulated in the lacrimal gland of SS model male mice (NOD mice) after onset of lacrimal hyposecretion and dacryoadenitis.

Even in non‐dacryoadenitis‐type NOD mice, tear secretion and arginase 1 expression remained low.

An arginase 1 inhibitor reduced tear secretion and partially reduced saliva secretion in BALB/c mice.

The results indicate that a non‐inflammatory factor, arginase 1, is involved in lacrimal hyposecretion in male NOD mice, regardless of dacryoadenitis status.

Abstract

Lacrimal fluid (tears) is important for preservation of the ocular surface, and thus lacrimal hyposecretion in Sjögren's syndrome (SS) leads to reduced quality of life. However, the cause(s) of lacrimal hyposecretion remains unknown, even though many studies have been conducted from the perspective of inflammation. Here, we hypothesized that a non‐inflammatory factor induces lacrimal hyposecretion in SS pathology, and to elucidate such a factor, we conducted transcriptome analysis of the lacrimal glands in male non‐obese diabetic (NOD) mice as an SS model. The NOD mice showed inflammatory cell infiltration and decreased pilocarpine‐induced tear secretion at and after 6 weeks of age compared to age‐matched BALB/c mice. RNA‐sequencing analysis revealed that only four genes, including arginase 1, were downregulated, whereas many genes relating to inflammation were upregulated, in the lacrimal glands of male NOD mice after onset of lacrimal hyposecretion and dacryoadenitis (lacrimal gland inflammation). Changes in the level of arginase 1 expression were confirmed by real‐time RT‐PCR and western blot analysis. Furthermore, non‐dacryoadenitis‐type NOD mice were used to investigate the relationships among arginase 1 expression, lacrimal hyposecretion and dacryoadenitis. Interestingly, these NOD mice retained the phenotype of dacryoadenitis with regard to tear secretion and arginase 1 expression level. An arginase 1 inhibitor reduced tear secretion and partially reduced saliva secretion in BALB/c mice. In conclusion, a non‐inflammatory factor, arginase 1, is involved in lacrimal hyposecretion in male NOD mice, regardless of dacryoadenitis status. These results shed light on the pathophysiological role of arginase 1 in SS (dry eye).

Few reports have explored the possibility of involvement of non‐inflammatory factors in lacrimal hyposecretion in Sjögren's syndrome (SS).

RNA‐sequencing analysis revealed that only four genes, including arginase 1, were downregulated in the lacrimal gland of SS model male mice (NOD mice) after onset of lacrimal hyposecretion and dacryoadenitis.

Even in non‐dacryoadenitis‐type NOD mice, tear secretion and arginase 1 expression remained low.

An arginase 1 inhibitor reduced tear secretion and partially reduced saliva secretion in BALB/c mice.

The results indicate that a non‐inflammatory factor, arginase 1, is involved in lacrimal hyposecretion in male NOD mice, regardless of dacryoadenitis status.