Levels of mucin gene expression in normal human conjunctival epithelium in vivo

A comprehensive scoping review of methodological approaches and clinical applications of tear fluid biomarkers

Tear fluid is an emerging source of disease biomarkers, drawing attention due to its quick, inexpensive, and non-invasive collection. The advancements in detection techniques enable the measurement of ultra-low biomarker levels from small sample volumes typical of tear fluid. The lack of standardized protocols for collection, processing, and analysis of tear fluid remains a significant challenge. To address this, we convened the Tear Research Network Review Taskforce in 2022 to review protocols from the past three decades, providing a comprehensive overview of the methodologies used in tear fluid biomarker research. A total of 1484 articles published from January 1974 to May 2024 from two electronic databases, Embase and Ovid MEDLINE, were reviewed. An exponential increase in the number of articles on tear fluid biomarkers was observed from 2015 onwards. The two most commonly reported collection methods were; glass capillaries (45.2%), and Schirmer's strips (25%), with glass capillary tube collection remaining the most frequent method until 2019, when Schirmer's strips became the leading method. Most articles analyzed tear fluid proteins (65%) and focused on a single analyte (32.3%). In recent years, an increase was observed in the type and number of examined analytes. The differences in the reported methodologies and protocols underscore the need for standardization and harmonization within the field of tear fluid biomarkers to minimize methodological differences and reduce variability in clinical outcomes. Consistent and detailed reporting is essential for improving the reproducibility and validity of tear fluid studies, in order to advance their potential clinical applications.

Assessment of mucin-related gene alterations following treatment with rebamipide ophthalmic suspension in Sjögren's syndrome-associated dry eyes

Ocular surface mucins are thought to play vital roles in maintaining the homeostasis of the pre-ocular surface tear film. We performed ocular surface tests with impression cytology to assess the expression levels of mucin-related genes on the ocular surface in healthy eyes. In addition, we investigated alterations in mucin-related gene expression secondary to treatment with rebamipide ophthalmic suspension in patients with Sjögren’s syndrome-associated dry eyes (SS-DE). Thirty-three healthy individuals (control group) and 13 patients from our hospital with SS-DE were enrolled. Impression cytology was performed using Schirmer’s test paper for RNA sampling. The mRNA levels of SAM-pointed domain-containing ETS-like factor (SPDEF), mucin 5AC (MUC5AC), and mucin 16 (MUC16) were determined using a real-time reverse transcription-polymerase chain reaction. The ocular surface test was performed once for the control group, and at baseline as well as 2, 4, 8, and 12 weeks after treatment in the Sjögren’s syndrome-associated dry eyes group. mRNA levels of SPDEF, MUC5AC, and MUC16 were not significantly different between the control and SS-DE groups before rebamipide ophthalmic suspension treatment. SPDEF mRNA levels in control subjects were significantly correlated with levels of MUC5AC. Among SS-DE patients, SPDEF mRNA levels were significantly increased at 2, 4, and 8 weeks after treatment compared with baseline levels. MUC16 mRNA levels were significantly decreased from baseline levels at 4 and 8 weeks post-treatment. Ocular surface test using impression cytology is a clinically useful tool for assessing mucous conditions on the ocular surface and can be used to determine the effects of instillation treatment with eye drops that affect mucin production at the ocular surface.

Reconsidering the central role of mucins in dry eye and ocular surface diseases

Mucins are key actors in tear film quality and tear film stability. Alteration of membrane-bound mucin expression on corneal and conjunctival epithelial cells and/or gel-forming mucin secretion by goblet cells (GCs) promotes in ocular surface diseases and dry eye disease (DED). Changes in the mucin layer may lead to enhanced tear evaporation eventually contributing to tear hyperosmolarity which has been associated with ocular surface inflammation. Inflammatory mediators in turn may have a negative impact on GCs differentiation, proliferation, and mucin secretion. This sheds new light on the position of GCs in the vicious circle of DED. As contributor to ocular surface immune homeostasis, GC loss may contribute to impaired ocular surface immune tolerance observed in DED. In spite of this, there are no tools in routine clinical practice for exploring ocular surface mucin deficiency/dysregulation. Therefore, when selecting the most appropriate treatment options, there is a clear unmet need for a better understanding of the importance of mucins and options for their replacement. Here, we comprehensively revisited the current knowledge on ocular surface mucin biology, including functions, synthesis, and secretion as well as the available diagnostic tools and treatment options to improve mucin-associated homeostasis. In particular, we detailed the potential link between mucin dysfunction and inflammation as part of the uncontrolled chronic inflammation which perpetuates the vicious circle in DED.

Effects of environment pollution on the ocular surface

The twenty-first century is fraught with dangers like climate change and pollution, which impacts human health and mortality. As levels of pollution increase, respiratory illnesses and cardiovascular ailments become more prevalent. Less understood are the eye-related complaints, which are commonly associated with increasing pollution. Affected people may complain of irritation, redness, foreign body sensation, tearing, and blurring of vision. Sources of pollution are varied, ranging from gases (such as ozone and NO₂) and particulate matter produced from traffic, to some other hazards associated with indoor environments. Mechanisms causing ocular surface disease involve toxicity, oxidative stress, and inflammation. Homeostatic mechanisms of the ocular surface may adapt to certain chronic changes in the environment, so affected people may not always be symptomatic. However there are many challenges associated with assessing effects of air pollution on eyes, as pollution is large scale and difficult to control. Persons with chronic allergic or atopic tendencies may have a pre-existing state of heightened mucosal immune response, hence they may have less tolerance for further environmental antigenic stimulation. It is beneficial to identify vulnerable people whose quality of life will be significantly impaired by environmental changes and provide counter measures in the form of protection or treatment. Better technologies in monitoring of pollutants and assessment of the eye will facilitate progress in this field.

Physicochemical properties of mucus and their impact on transmucosal drug delivery

Mucus is a selective barrier to particles and molecules, preventing penetration to the epithelial surface of mucosal tissues. Significant advances in transmucosal drug delivery have recently been made and have emphasized that an understanding of the basic structure, viscoelastic properties, and interactions of mucus is of great value in the design of efficient drug delivery systems. Mucins, the primary non-aqueous component of mucus, are polymers carrying a complex and heterogeneous structure with domains that undergo a variety of molecular interactions, such as hydrophilic/hydrophobic, hydrogen bonds and electrostatic interactions. These properties are directly relevant to the numerous mucin-associated diseases, as well as delivering drugs across the mucus barrier. Therefore, in this review we discuss regional differences in mucus composition, mucus physicochemical properties, such as pore size, viscoelasticity, pH, and ionic strength. These factors are also discussed with respect to changes in mucus properties as a function of disease state. Collectively, the review seeks to provide a state of the art roadmap for researchers who must contend with this critical barrier to drug delivery.

Vascular-Associated Muc4/Vwf Co-Localization in Human Conjunctival Malignant Melanoma Specimens-Tumor Metastasis by Migration?

PURPOSE

To investigate whether vascular differentiation marker von Willebrand factor (vWf) and proliferation marker KI67 expression correlate with MUC4 localization around stromal tumor vascularization in human conjunctival malignant melanoma (CMM).

MATERIALS AND METHODS

For the purposes of this study, we analyzed samples from human CMMs (n = 4), conjunctival compound nevi (n = 7), and samples from healthy conjunctiva (n = 7) for MUC1, 4, and 16 by immunohistochemistry. To test CMM vessel association of MUC4, we investigated the co-localization of MUC4 with vWf or KI67 in human CMM specimens (n = 10) by immunohistochemistry. Also, we investigated the MUC4 localization around vessels of healthy conjunctiva (n = 10).

RESULTS

The immunohistochemical analysis demonstrated membrane-associated mucin expression in epithelia of CMM, nevi and healthy conjunctiva, whereas only MUC4 was localized perivascular in CMM tissue in this preliminary analysis. Co-staining analysis with vWf and KI67 demonstrated MUC4 localization around stromal vessels in human CMM specimens. In contrast, no MUC4 localization has been seen around healthy conjunctiva stroma vessels.

CONCLUSIONS

MUC4 was detected around vWf/KI67-positive CMM stromal vascular tissue, but not around healthy conjunctival stroma vessels. Therefore, we assume that MUC4 might play a role in tumor cell migration toward vessels inducing metastasis.

Effects of ambient levels of traffic-derived air pollution on the ocular surface: analysis of symptoms, conjunctival goblet cell count and mucin 5AC gene expression

PURPOSE

To quantify ocular symptoms, goblet cells (GC) and mucin 5AC (MUC5AC) gene expression on the conjunctiva of healthy subjects exposed to ambient levels of traffic-derived air pollution and to estimate its correlation with NO2 and particulate matter smaller than 2.5 μm (PM2.5) levels.

METHODS

Twenty-one taxi drivers or traffic controllers were assessed with the Ocular Surface Disease Index (OSDI) questionnaire and conjunctival impression cytology. MUC5AC mRNA levels were determined based on the cytology of the right eye, and GC density was assessed based on the cytology of the left eye. Mean individual levels of 24-h NO2 and PM2.5 exposure were assessed the day before examination. Possible associations between NO2 or PM2.5 levels, OSDI scores, GC densities and MUC5AC mRNA levels were verified.

RESULTS

The subjects were exposed to mean PM2.5 levels of 35±12 μg/m(3) and mean NO2 levels of 189±47 μg/m(3). OSDI scores were low (7.4±8) and GC densities were 521±257 and 782±322 cell/mm(2) on the bulbar and tarsal conjunctivas, respectively. The mean GC-derived MUC5AC mRNA expression was 14±7 fM/μg of total RNA. A significant and positive correlation was observed between MUC5AC mRNA levels and tarsal GC density (p=0.018). A trend toward association between PM2.5 levels and tarsal GC cell density (p=0.052) was found.

CONCLUSION

Exposure to ambient levels of air pollution impacts conjunctival GC density. An increase in MUC5AC mRNA levels may be part of an adaptive ocular surface response to long-term exposure to air pollution.

Chitosan-gelatin biopolymers as carrier substrata for limbal epithelial stem cells

The aim of this work was to evaluate semi-synthetic biopolymers based on chitosan (CH) and gelatin (G) as potential in vitro carrier substrata for human limbal epithelial cells (hLECs). To that end, human corneal epithelial cells (HCE) were cultured onto different CH-G membranes. None of the polymers were cytotoxic and cell proliferation was higher when CH was functionalized with G. Expression levels of corneal epithelial markers (K3, K12, E-caherin, desmoplakin, and zonula occludens (ZO)-1) were better maintained in HCE cells grown on CH-G 20:80 membranes than other proportions. Consequently, CH-G 20:80 was chosen for the subsequent expansion of hLECs. Cells derived from limbal explants were successfully expanded on CH-G 20:80 membranes using a culture medium lacking components of non-human animal origin. The expression levels found for corneal (K3 and K12) and limbal epithelial stem cells (K15) specific markers were similar to or higher than those found in limbal cells grown onto the control substratum. Our results demonstrate that CH-G 20:80 membranes are suitable for the expansion and maintenance of stem cells derived from the limbal niche. These results strongly support the use of polymers as alternative substrata for the transplantation of cultivated limbal cells onto the ocular surface.

MUC4 as a diagnostic marker in cancer

BACKGROUND

Mucins are high molecular mass glycoproteins whose role in diagnosis, prognosis and therapy is being increasingly recognized owing to their altered expression in a variety of carcinomas. MUC4, a membrane-bound mucin encoded by a gene located on chromosome locus 3q29, is aberrantly expressed in several cancers including those of the bile duct, breast, colon, esophagus, ovary, lung, prostate, stomach and pancreas.

OBJECTIVE

This review considers the potential use of the MUC4 expression pattern in the diagnosis and prognosis of various cancers.

RESULTS/CONCLUSION

MUC4 expression is a specific marker of epithelial tumors and its expression correlates positively with the degree of differentiation in several cancers. Importantly, MUC4 has emerged as a specific marker of dysplasia, being expressed in the earliest dysplastic lesions preceding several malignancies, including lethal pancreatic cancer. The presence of MUC4-specific antibodies in the serum and of the transcript in peripheral blood mononuclear cells of cancer patients raises the possibility of it emerging as a new diagnostic biomarker for bedside application in high-risk individuals and those with established cancer.

Effect of TGF-β on ocular surface epithelial cells

A role for transforming growth factor (TGF)-β in the pathogenesis of some ocular surface diseases has been proposed. We determined if secretion of TGF-β and expression of TGF-β receptors RI, RII, and RIII by human ocular surface epithelial cells were modified under inflammatory conditions. We also determined how these cells responded to TGF-β. A human corneal epithelial (HCE) cell line and a conjunctival epithelial cell line (IOBA-NHC) were exposed to TGF-β1 and -β2 and to proinflammatory cytokines. TGF-β receptor mRNAs were analyzed by real time reverse transcription polymerase chain reaction (RT-PCR) in both cell lines, and in conjunctival, limbal, and corneal epithelial cells from post-mortem human specimens. Expression of TGF-β receptors and pSMAD2/SMAD2 were determined by Western blot and immunofluorescence assays. Secretion of TGF-β isoforms, cytokine/chemokine, and metalloproteinases (MMPs) were analyzed in cell supernatants by immunobead-based assays. Secretory leukocyte proteinase inhibitor (SLPI) secretion was analyzed by enzyme-linked immunosorbent assay. TGF-β isoform and receptor gene expression was determined by RT-PCR in conjunctival epithelium of dry eye (DE) patients and healthy subjects. Our results showed that TGF-β RI expression was down-regulated with IL-4 exposure, whereas TGF-β RII and TGF-β2 were upregulated by TNF-α in HCE cells. TGF-β RIII receptor expression was upregulated in IOBA-NHC cells by TNF-α and IFN-γ. SMAD2 phosphorylation occurred in HCE and IOBA-NHC cells after TGF-β treatment. TGF-β significantly up- and down-regulated secretion of several cytokines/chemokines by both cell lines and MMP by HCE cells. TGF-β2 and TGF-β3 were upregulated and TGF-β RIII mRNA was down-regulated in DE conjunctival epithelium. These results show that TGF-β plays an important role in directing local inflammatory responses in ocular surface epithelial cells.

Conjunctival reconstruction with progenitor cell-derived autologous epidermal sheets in rhesus monkey

Severe ocular surface diseases are some of the most challenging problems that the clinician faces today. Conventional management is generally unsatisfactory, and the long-term ocular consequences of these conditions are devastating. It is significantly important to find a substitute for conjunctival epithelial cells. This study was to explore the possibility of progenitor cell-derived epidermal sheets on denuded amniotic membrane to reconstruct ocular surface of conjunctiva damaged monkeys. We isolated epidermal progenitor cells of rhesus monkeys by type IV collagen adhesion, and then expanded progenitor cell-derived epidermal sheets on denuded amniotic membrane ex vivo. At 3 weeks after the conjunctiva injury, the damaged ocular surface of four monkeys was surgically reconstructed by transplanting the autologous cultivated epidermal progenitor cells. At 2 weeks after surgery, transplants were removed and examined with Hematoxylin-eosin staining, Periodic acid Schiff staining, immunofluorescent staining, scanning and transmission electron microscopy. Histological examination of transplanted sheets revealed that the cell sheets were healthy alive, adhered well to the denuded amniotic membrane, and had several layers of epithelial cells. Electron microscopy showed that the epithelial cells were very similar in appearance to those of normal conjunctival epithelium, even without goblet cell detected. Epithelial cells of transplants had numerous desmosomal junctions and were attached to the amniotic membrane with hemidesmosomes. Immunohistochemistry confirmed the presence of the conjunctival specific markers, mucin 4 and keratin 4, in the transplanted epidermal progenitor cells. In conclusion, our present study successfully reconstructed conjunctiva with autologous transplantation of progenitor cell-derived epidermal sheets on denuded AM in conjunctival damaged monkeys, which is the first step toward assessing the use of autologous transplantation of progenitor cells of nonocular surface origin. Epidermal progenitor cells could be provided as a new substitute for conjunctival epithelial cells to overcome the problems of autologous conjunctiva shortage.

Monoclonal antibodies recognizing the non-tandem repeat regions of the human mucin MUC4 in pancreatic cancer

The MUC4 mucin is a high molecular weight, membrane-bound, and highly glycosylated protein. It is a multi-domain protein that is putatively cleaved into a large mucin-like subunit (MUC4α) and a C-terminal growth-factor like subunit (MUC4β). MUC4 plays critical roles in physiological and pathological conditions and is aberrantly overexpressed in several cancers, including those of the pancreas, cervix, breast and lung. It is also a potential biomarker for the diagnosis, prognosis and progression of several malignancies. Further, MUC4 plays diverse functional roles in cancer initiation and progression as evident from its involvement in oncogenic transformation, proliferation, inhibition of apoptosis, motility and invasion, and resistance to chemotherapy in human cancer cells. We have previously generated a monoclonal antibody 8G7, which is directed against the TR region of MUC4, and has been extensively used to study the expression of MUC4 in several malignancies. Here, we describe the generation of anti-MUC4 antibodies directed against the non-TR regions of MUC4. Recombinant glutathione-S-transferase (GST)-fused MUC4α fragments, both upstream (MUC4α-N-Ter) and downstream (MUC4α-C-Ter) of the TR domain, were used as immunogens to immunize BALB/c mice. Following cell fusion, hybridomas were screened using the aforementioned recombinant proteins ad lysates from human pancreatic cell lines. Three anti MUC4α-N-Ter and one anti-MUC4α-C-Ter antibodies were characterized by several inmmunoassays including enzyme-linked immunosorbent assay (ELISA), immunoblotting, immunofluorescene, flow cytometry and immunoprecipitation using MUC4 expressing human pancreatic cancer cell lines. The antibodies also reacted with the MUC4 in human pancreatic tumor sections in immunohistochemical analysis. The new domain-specific anti-MUC4 antibodies will serve as important reagents to study the structure-function relationship of MUC4 domains and for the development of MUC4-based diagnostics and therapeutics.

Entrapment of conjunctival goblet cells by desiccation-induced cornification

PURPOSE

To evaluate the effects of desiccating stress on conjunctival goblet cell density and morphology and the expression of cornified envelope precursors by the ocular surface epithelia.

METHODS

Experimental dry eye (EDE) was created in C57BL/6 mice. Real-time PCR evaluated the expression of cornified envelope (CE) precursor proteins (involucrin and small proline-rich [Sprr] -1a, -1b, -2a, -2b, -2f, and -2g proteins), the cross-linking transglutaminase 1 enzyme (Tg-1) and Muc5AC mRNA transcripts by the ocular surface epithelia. Laser scanning confocal microscopy evaluated the expression of the CE precursor proteins Tg-1 and Muc5AC in cryosections. Tg-1 activity was measured by a fluorescein cadaverine assay. Muc5AC concentration was measured by ELISA.

RESULTS

Levels of involucrin; Sprr-1a, -1b, -2a, -2b, -2f, and -2g; and Tg1-1 mRNA transcripts in ocular surface tissues increased in response to desiccating stress. Expression and activity of Tg in the conjunctiva markedly increased after EDE. Desiccating stress caused progressive loss of mucin-filled goblet cells. The apical portion of the remaining conjunctival goblet cells became entrapped by adjacent stratified apical epithelia expressing increased levels of cornified envelope precursors.

CONCLUSIONS

Exposure to desiccating stress stimulates ocular surface epithelia to produce cornified envelope precursors and the tissue transglutaminase enzyme that cross-links them. This effect is accompanied by loss of mucin-filled goblet cells and entrapment of mucin contents in the remaining ones by cornifying cells that block the egress of mucin contents to the ocular surface. This mechanism may contribute to the conjunctival mucin deficiency that develops in dry eye.

Conjunctival mucin mRNA expression in contact lens wear

PURPOSE

To investigate the influence of the water content in non-ionic hydrogel contact lenses (HCL) on the mRNA levels of human conjunctival mucin genes (MUCs).

METHODS

Sixteen healthy subjects with no history of contact lenses wear were selected and randomized into two equal groups. Group 1 subjects wore low water content (38%, Soflens 38) non-ionic HCLs. Group 2 wore high water content (66%, Soflens 66) non-ionic HCLs. Conjunctival impression cytology was applied to the superior bulbar conjunctiva of both eyes before, 6 months, and 1 year after HCL fitting, and 15 days after discontinuation of wearing. Total RNA was isolated, retrotranscribed, and amplified by conventional polymerase chain reaction (PCR) and by quantitative real time PCR to study the mRNA levels of MUCs and to analyze variations during the study period. Time- and HCL-dependent variations in mRNA expression were analyzed using Student's test.

RESULTS

From the known MUCs, transcripts from MUC1, MUC2, MUC4, MUC5AC, MUC7, MUC13, MUC15, MUC16, and MUC17 genes were detected in all subjects before HCL fitting. Except for MUC2, the expression of some MUC genes significantly increased whereas others significantly decreased at either the 6- and 12-month period. Statistically significant differences between both HCL groups (p < 0.001) were found in the MUC4, MUC13, and MUC15 mRNA expression after 1 year of wear and after the 15 days without HCL wear. However, these differences were not clearly related to the water content of the lenses.

CONCLUSIONS

Low and high water content non-ionic HCLs induced different changes in the mRNA levels of several MUCs, but the water content was not related to the changes. Recovery to basal levels of conjunctival MUC mRNA expression after wearing HCL lenses for a year takes longer than 15 days for some MUCs.

Structure, evolution, and biology of the MUC4 mucin

Mucins are high-molecular-weight glycoproteins and are implicated in diverse biological functions. MUC4, a member of transmembrane mucin family, is expressed in airway epithelial cells and body fluids like saliva, tear film, ear fluid, and breast milk. In addition to its normal expression, an aberrant expression of MUC4 has been reported in a variety of carcinomas. Among various potential domains of MUC4, epidermal growth factor (EGF) -like domains are hypothesized to interact with and activate the ErbB2 receptors, suggesting an intramembrane-growth factor function for MUC4. The heavily glycosylated tandem repeat domain provides the structural rigidity to the extended extracellular region. MUC4, by virtue of its extended structure, serves as a barrier for some cell-cell and cell-extracellular matrix interactions and as a potential reservoir for certain growth factors. An intricate relationship between MUC4 and growth factor signaling is also reflected in the transcriptional regulation of MUC4. The MUC4 promoter has binding sites for different transcription factors, which are responsible for the regulation of its expression in different tissues. The interferon-gamma, retinoic acid, and transforming growth factor-beta signaling pathways regulate MUC4 expression in a partially interdependent manner. Taken together, all of these features of MUC4 strongly support its role as a potential candidate for diagnostic and therapeutic applications in cancer and other diseases.

Strain-related cytokine profiles on the murine ocular surface in response to desiccating stress

PURPOSE

To evaluate the effects of desiccating ocular surface stress on levels of inflammatory cytokines in the corneal epithelium, conjunctiva, and tear fluid of BALB/c and C57BL/6 mice.

METHODS

Experimental dry eye (EDE) was created in BALB/c and C57BL/6 mice by cholinergic blockade and exposure to a desiccating environment. Real-time polymerase chain reaction was performed to measure levels of cytokine transcripts. A multiplex immunobead assay was used to measure concentrations of these cytokines in tears.

RESULTS

Experimental dryness significantly increased the expression of interleukin (IL)-1alpha, IL-6, and tumor necrosis factor (TNF)-alpha transcripts in the corneal epithelium and conjunctiva of C57BL/6 mice. Strain-specific changes in tear cytokine profiles were observed. C57BL/6 mice had significantly greater tear concentrations of IL-1alpha and TNF-alpha and the Th-1 cytokines IL-2, IL-12, and interferon-gamma in response to desiccating stress than BALB/c mice. The Th-2 cytokines IL-4 and IL-10 were significantly greater in BALB/c tears.

CONCLUSIONS

This study indicates that desiccating stress increases levels of certain cytokines in the corneal epithelium and conjunctiva in a strain-dependent fashion and that C57BL/6 mice had greater levels of Th-1 cytokines in their tears, whereas BALB/c mice had a greater increase in Th-2 cytokines.

Expression in human ocular surface tissues of the GalNAc-transferases that initiate mucin-type O-glycosylation

PURPOSE

Mucins are highly glycosylated proteins that act as lubricants, protectants, and mediators of signal transduction. The UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGaNTase) family members initiate mucin-type O-glycosylation. Because O-glycosylation provides mucins with the viscoelastic properties required for proper mucin function, ppGaNTase expression is vital to the maintenance of healthy epithelial surfaces including the ocular surface. Differences of expression of ppGaNTase isoforms might be factors influencing susceptibility to dry eye disease. Therefore, we determined the expression of the ppGaNTase isoforms in normal human ocular surface tissues and the conjunctival epithelium from patients with aqueous-deficient dry eye.

METHODS

Expression of ppGaNTase isoforms was quantitated by real-time reverse transcriptase polymerase chain rection (RT-PCR).

RESULTS

Conjunctiva and cornea expressed multiple ppGaNTase isoforms, with isoforms T12 and T4 being the most strongly expressed in conjunctiva and T12 and T3 in the cornea. In contrast, lacrimal gland expressed fewer isoforms and had lower total ppGaNTase expression. Brush cytology was found to be superior to impression cytology for harvesting conjunctival epithelium in terms of ease of use, safety, and reproducibility of results. Similar to whole conjunctiva, the strongest isoforms in conjunctival epithelial cells were T12 and T4, followed by T3, T1, T5, and T2. No significant differences of ppGaNTase expression were found between the conjunctival epithelium of dry eye and normal control groups.

CONCLUSION

Human ocular surface tissues express multiple ppGaNTase isoforms, suggesting a requirement for glycosylating diverse mucin-type substrates. However, there is no evidence to date to suggest that differences of ppGaNTase expression levels might contribute to susceptibility to dry eye disease.

Mucins and TFF peptides of the tear film and lacrimal apparatus

The three-dimensional organization of the tear film, which is produced and drained by the different structures of the ocular adnexa, is essential for maintainance and protection of the ocular surface. This is facilitated by a class of large, highly glycosylated, hydrophilic glycoproteins, the mucins, which are usually expressed in association with a class of peptides having a well-defined, structurally conserved trefoil domain, the mammalian trefoil factor family (TFF) peptides. In this review, the latest information regarding mucin and TFF peptide function and regulation in the human lacrimal system, the tear film and the ocular surface is summarized with regard to mucous epithelia integrity, rheological and antimicrobial properties of the tear film and tear outflow, age-related changes and certain disease states such as dry eye, dacryostenosis and dacryolith formation.

Expression and regulation of cornified envelope proteins in human corneal epithelium

PURPOSE

Stratified squamous epithelial cells assemble a specialized protective barrier structure on their periphery, termed the cornified envelope. The purpose of this study was to evaluate the presence and distribution of cornified envelope precursors in human corneal epithelium, their expression in human corneal epithelial cell cultures, and the effect of ultraviolet radiation (UVB) and transglutaminase (TG) inhibition on their expression.

METHODS

Tissue distribution of small proline-rich proteins (SPRRs) and filaggrin and involucrin was studied in human cornea sections by immunofluorescence staining. Primary human corneal epithelial cells (HCECs) from limbal explants were used in cell culture experiments. A single dose of UVB at 20 mJ/cm2 was used to stimulate these cells, in the presence or absence of mono-dansyl cadaverine (MDC), a TG inhibitor. SPRR2 and involucrin protein levels were studied by immunofluorescence staining and Western blot analysis. Gene expression of 12 proteins was investigated by semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

In human cornea tissue, SPRR1, SPRR2, filaggrin, and involucrin protein expression were detected in the central and peripheral corneal and limbal epithelium. In HCECs, SPRR2 and involucrin proteins were detected in the cytosolic fraction, and involucrin levels increased after UVB. Both SPRR2 and involucrin levels accumulated in the presence of MDC. Nine genes including involucrin, SPRR (types 1A, 1B, 2A, 2B, and 3), late envelope protein (LEP) 1 and 16, and filaggrin were expressed by HCECs. SPRR 4, loricrin, and LEP 6 transcripts were not detected. UVB downregulated SPRR (2A, 2B) and LEP 1 transcripts.

CONCLUSIONS

Various envelope precursors are expressed in human corneal epithelium and in HCECs, acute UVB stress differentially alters their expression in HCECs. The expression of envelope precursors and their rapid modulation by UVB supports the role of these proteins in the regulation of ocular surface stress. TG function may be relevant in the regulation of soluble precursors in UVB-stimulated corneal epithelium.

MUC1 splice variants in human ocular surface tissues: possible differences between dry eye patients and normal controls

Mucins are highly glycosylated proteins that are vital to the maintenance of healthy epithelial surfaces including the ocular surface. Mucins act as lubricants, protectants, and mediators of signal transduction. The majority of the O-glycosylation sites on the transmembrane mucin MUC1 are found in a highly polymorphic core region containing a variable number of tandem repeats (VNTR). MUC1 alleles can be divided into size classes that contain small (30-45) or large (60-90) numbers of repeats. Although at least 12 splice variants of MUC1 have been found in other tissues, no splice variants have been reported in human ocular surface tissues. We have used RT-PCR to identify MUC1 splice variants that were then confirmed by sequencing. We here report the presence in some samples of human cornea, conjunctiva, and lacrimal gland of MUC1/B which features canonical splicing between exons 1 and 2 and MUC1/A, a transcript that retains 27bp from the 3' end of intron 1 and is predicted to add 9 amino acids to the MUC1 sequence upstream of the tandem repeat region. Cornea and conjunctiva both contain the MUC1/SEC splice variant that lacks the transmembrane domain and, therefore, results in a soluble, secreted form of MUC1. Cornea and conjunctiva also contain MUC1/Y and MUC1/Z(X) variants that lack the tandem repeat region. Cornea, conjunctiva, and lacrimal gland also contain a previously undescribed MUC1 variant transcript, termed MUC1/YI, that retains 99bp from the 5' end and 27bp from the 3' end of the first intron, resulting in a frame shift and premature stop codon. This transcript is predicted to produce a novel 27 amino acid peptide after signal peptidase cleavage. Analysis of brush cytology samples revealed that the percentage of dry eye patients expressing the MUC1/A variant in the conjunctival epithelium is lower than in normal control donors. Western blotting confirmed that MUC1/A is associated with alleles containing the large size class of tandem repeats. Therefore, we propose that one factor in susceptibility to dry eye disease may be the lengths of the MUC1 VNTR in conjunctival epithelium based on the rationale that longer VNTR provide better lubrication and greater protection of the ocular surface against inflammation.

Laboratory findings in tear fluid analysis

The tear film, composed of the lipid, aqueous and mucin layers, has many functions including defending the ocular surface. The tear film covering the ocular surface presents a mechanical and antimicrobial barrier and ensures an optical refractive surface. The lipid component originates from the meibomian glands of the tarsus and forms the superficial layer of the tear film. The aqueous component contains electrolytes, water, and a large variety of proteins, peptides and glycoproteins, and is primarily secreted by the lacrimal gland. Mucins are glycoproteins expressed by epithelial tissues of mucous surfaces. They protect tissues by functioning as antioxidants, providing lubrication, and inhibiting bacterial adherence. Quantitatively and qualitatively, its composition must be maintained within the fairly narrow limits to maintain a healthy and functional visual system. Abnormalities of the tear film, affecting the constituents or the volume, can rapidly result in serious dysfunction of the eyelids and conjunctiva and ultimately affect the transparency of the cornea. Many ocular surface tests have been developed for the clinical diagnosis of dry eye syndromes. This paper provides an overview on laboratory methods for the analysis of the tear film. Understanding the components of the tear film will aid in the treatment of dry eye syndromes and the ocular surface diseases.

Subconjunctival cyst-like formations following impression cytology

PURPOSE

To report a case of an unusual adverse event subsequent to undertaking Conjunctival Impression Cytology (CIC).

METHODS

CIC was undertaken on a 54-year-old healthy Caucasian female, using a 10 mm Millipore membrane on the bulbar conjunctiva. Prior to the CIC procedure, two drops of topical proparacaine (Alcaine were instilled and following the procedure two drops of artificial tears (GenTeal were administered.

RESULTS

The subject reported excessive bulbar conjunctival hyperaemia in the left eye 5 min postprocedure, with no marked pain or discomfort. Slitlamp biomicroscopic evaluation revealed several "bubbles" or cystic formations trapped underneath the conjunctival tissue, in association with moderate to severe bulbar conjunctival redness. These cysts began to regress fairly rapidly and completely disappeared within 1h.

CONCLUSION

CIC is a useful tool for studying the ocular surface; however, care should be taken while performing this mildly invasive procedure. This is the first report of bubble formation in the conjunctiva following CIC. Subjects should be advised about the short term redness and discomfort that could occur following CIC.

Impression cytology of the ocular surface: a review

PURPOSE

To historically review the technique of impression cytology as a minimally invasive diagnostic tool for ocular surface pathology.

METHODS

A comprehensive review of published literature cited in PubMed since the first description of impression cytology in 1977 up to date has been undertaken.

RESULTS

A wide range of processing methods have been adapted to the technique of impression cytology (from conjunctiva, cornea or limbus): regular light microscopy with different stainings, transmission and scanning electron microscopy, immunofluorescence, immunocytochemistry, polymerase chain reaction analysis, immunoblotting analyses, or flow cytometry. At present, it is widely used as a non-invasive alternative to the full-thickness biopsy for the obtention of epithelial cells from the ocular surface.

CONCLUSIONS

Impression cytology represents a non- or minimally invasive biopsy of the ocular surface epithelium with no side effects or contraindications. It has demonstrated to be a useful diagnostic aid for a wide variety of processes involving the ocular surface. In addition, and mainly during the last decade, its use as a research tool has experienced an enormous growth and has greatly contributed to the understanding of ocular surface pathology.

Character of ocular surface mucins and their alteration in dry eye disease

At the ocular surface, three types of mucins are present. The large gel-forming mucin MUC5AC is expressed by conjunctival goblet cells. Some cells of the lacrimal gland acini express the small soluble mucin MUC7. The corneal and conjunctival epithelia express the membrane-associated mucins MUCs 1, 4, and 16. With the characterization of the mucin gene repertoire of the ocular surface epithelia, studies of the function of specific mucins, their gene regulation, and their alteration in ocular surface disease have begun. Current information suggests that all the mucins are hydrophilic and play a role in maintenance of water on the surface of the eye. The large secreted mucins represent the "janitorial service" that moves over the surface of the eye to wrap up and remove debris. The membrane-associated mucins form the glycocalyx, which provides a continuous barrier across the surface of the eye that prevents pathogen penetrance and has signaling capabilities that influence epithelial activity. Factors regulating mucin gene expression include retinoic acid, serum, and dexamethasone. Alteration in both secreted and membrane-associated mucins occur in drying ocular surface diseases. In Sjogren syndrome, MUC5AC expression is reduced, and in non-Sjogren dry eye, glycosylation of MUC16 appears to be altered. The pattern of expression of enzymes that glycosylate mucins is altered in ocular cicatricial pemphigoid. Therapies being evaluated for dry eye, including cyclosporine A, P2Y2 agonists, gefarnate, 15-(S)-HETE, and corticosteroids, may be efficacious due to their effect on mucin gene expression and secretion.