MUC19 expression in human ocular surface and lacrimal gland and its alteration in Sjögren syndrome patients

Advances in Sjögren's Syndrome Dry Eye Diagnostics: Biomarkers and Biomolecules beyond Clinical Symptoms

Sjögren's syndrome dry eye (SSDE) is a subset of Sjögren's syndrome marked by dry eye symptoms that is distinct from non-Sjögren's syndrome dry eye (NSSDE). As SSDE can lead to severe complications, its early detection is imperative. However, the differentiation between SSDE and NSSDE remains challenging due to overlapping clinical manifestations. This review endeavors to give a concise overview of the classification, pathophysiology, clinical features and presentation, ocular and systemic complications, clinical diagnosis, and management of SSDE. Despite advancements, limitations in current diagnostic methods underscore the need for novel diagnostic modalities. Thus, the current review examines various diagnostic biomarkers utilized for SSDE identification, encompassing serum, salivary, and tear analyses. Recent advancements in proteomic research and exosomal biomarkers offer promising diagnostic potential. Through a comprehensive literature review spanning from 2016 to 2023, we highlight molecular insights and advanced diagnostic modalities that have the potential to enhance our understanding and diagnosis of SSDE.

A Comprehensive Assessment of Tear-Film-Oriented Diagnosis (TFOD) in a Dacryoadenectomy Dry Eye Model

Tear film instability is a major cause of dry eye disease. In order to treat patients with short tear film breakup time (TBUT)-type dry eye, the development of tear film stabilizing agents is essential. However, the lack of an appropriate animal model of tear film instability has made drug development difficult. Although rabbit dry eye models have been reported in the past, there are only a few reports that focus on tear film instability. Herein, we assessed the tear film stability of a rabbit dry eye model induced by dacryoadenectomy. A clinical evaluation of the ocular surface, interferometry, and histological assessments of the cornea and conjunctiva were performed. Following the removal of the lacrimal glands, TBUT was shortened significantly, with dimple and random breakup patterns prominently observed. Furthermore, the blink rate in this model increased after dacryoadenectomy, suggesting that this model partially captured the phenotypes of human short TBUT-type dry eye and may be useful as an animal model for investigating potential drug candidates.

The MUC19 gene in Denisovans, Neanderthals, and Modern Humans: An Evolutionary History of Recurrent Introgression and Natural Selection

All humans carry a small fraction of archaic ancestry across the genome, the legacy of gene flow from Neanderthals, Denisovans, and other hominids into the ancestors of modern humans. While the effects of Neanderthal ancestry on human fitness and health have been explored more thoroughly, there are fewer examples of adaptive introgression of Denisovan variants. Here, we study the gene MUC19, for which some modern humans carry a Denisovan-like haplotype. MUC19 is a mucin, a glycoprotein that forms gels with various biological functions, from lubrication to immunity. We find the diagnostic variants for the Denisovan-like MUC19 haplotype at high frequencies in admixed Latin American individuals among global population, and at highest frequency in 23 ancient Indigenous American individuals, all predating population admixture with Europeans and Africans. We find that some Neanderthals--Vindija and Chagyrskaya--carry the Denisovan-like MUC19 haplotype, and that it was likely introgressed into human populations through Neanderthal introgression rather than Denisovan introgression. Finally, we find that the Denisovan-like MUC19 haplotype carries a higher copy number of a 30 base-pair variable number tandem repeat relative to the Human-like haplotype, and that copy numbers of this repeat are exceedingly high in American populations. Our results suggest that the Denisovan-like MUC19 haplotype served as the raw genetic material for positive selection as American populations adapted to novel environments during their movement from Beringia into North and then South America.

Tetraspanin-8 sequesters syntaxin-2 to control biphasic release propensity of mucin granules

Agonist-mediated stimulated pathway of mucin and insulin release are biphasic in which rapid fusion of pre-docked granules is followed by slow docking and fusion of granules from the reserve pool. Here, based on a cell-culture system, we show that plasma membrane-located tetraspanin-8 sequesters syntaxin-2 to control mucin release. Tetraspanin-8 affects fusion of granules during the second phase of stimulated mucin release. The tetraspanin-8/syntaxin-2 complex does not contain VAMP-8, which functions with syntaxin-2 to mediate granule fusion. We suggest that by sequestering syntaxin-2, tetraspanin-8 prevents docking of granules from the reserve pool. In the absence of tetraspanin-8, more syntaxin-2 is available for docking and fusion of granules and thus doubles the quantities of mucins secreted. This principle also applies to insulin release and we suggest a cell type specific Tetraspanin/Syntaxin combination is a general mechanism regulating the fusion of dense core granules.

Mucins as contrast agent targets for fluorescence-guided surgery of pancreatic cancer

Pancreatic cancer is difficult to resect due to its unique challenges, often leading to incomplete tumor resections. Fluorescence-guided surgery (FGS), also known as intraoperative molecular imaging and optical surgical navigation, is an intraoperative tool that can aid surgeons in complete tumor resection through an increased ability to detect the tumor. To target the tumor, FGS contrast agents rely on biomarkers aberrantly expressed in malignant tissue compared to normal tissue. These biomarkers allow clinicians to identify the tumor and its stage before surgical resection and provide a contrast agent target for intraoperative imaging. Mucins, a family of glycoproteins, are upregulated in malignant tissue compared to normal tissue. Therefore, these proteins may serve as biomarkers for surgical resection. Intraoperative imaging of mucin expression in pancreatic cancer can potentially increase the number of complete resections. While some mucins have been studied for FGS, the potential ability to function as a biomarker target extends to the entire mucin family. Therefore, mucins are attractive proteins to investigate more broadly as FGS biomarkers. This review summarizes the biomarker traits of mucins and their potential use in FGS for pancreatic cancer.

View from the Biological Property: Insight into the Functional Diversity and Complexity of the Gut Mucus

Due to mucin's important protective effect on epithelial tissue, it has garnered extensive attention. The role played by mucus in the digestive tract is undeniable. On the one hand, mucus forms "biofilm" structures that insulate harmful substances from direct contact with epithelial cells. On the other hand, a variety of immune molecules in mucus play a crucial role in the immune regulation of the digestive tract. Due to the enormous number of microorganisms in the gut, the biological properties of mucus and its protective actions are more complicated. Numerous pieces of research have hinted that the aberrant expression of intestinal mucus is closely related to impaired intestinal function. Therefore, this purposeful review aims to provide the highlights of the biological characteristics and functional categorization of mucus synthesis and secretion. In addition, we highlight a variety of the regulatory factors for mucus. Most importantly, we also summarize some of the changes and possible molecular mechanisms of mucus during certain disease processes. All these are beneficial to clinical practice, diagnosis, and treatment and can provide some potential theoretical bases. Admittedly, there are still some deficiencies or contradictory results in the current research on mucus, but none of this diminishes the importance of mucus in protective impacts.

An Overview of the Dry Eye Disease in Sjögren's Syndrome Using Our Current Molecular Understanding

Sjögren's syndrome is a chronic and insidious auto-immune disease characterized by lymphocyte infiltration of exocrine glands. The patients typically present with ocular surface diseases related to dry eye and other systemic manifestations. However, due to the high prevalence of dry eye disease and the lack of objective and clinically reliable diagnostic tools, discriminating Sjögren's syndrome dry eye (SSDE) from non-Sjögren's syndrome dry eye (NSSDE) remains a challenge for clinicians. Diagnosing SS is important to improve the quality of life of patients through timely referral for systemic workups, as SS is associated with serious systemic complications such as lymphoma and other autoimmune diseases. The purpose of this article is to describe the current molecular understanding of Sjögren's syndrome and its implications for novel diagnostic modalities on the horizon. A literature review of the pre-clinical and clinical studies published between 2016 and 2022 was conducted. The SSDE pathophysiology and immunology pathways have become better understood in recent years. Novel diagnostic modalities, such as tear and saliva proteomics as well as exosomal biomarkers, provide hope on the horizon.

Pathological Implications of Mucin Signaling in Metastasis

The dynamic mucosal layer provides a selective protective barrier for the epithelial cells lining the body cavities. Diverse human malignancies exploit their intrinsic role to protect and repair epithelia for promoting growth and survival. Aberrant expression of mucin has been known to be associated with poor prognosis of many cancers. However, the emergence of new paradigms in the study of metastasis recognizes the involvement of MUC1, MUC4, MUC5AC, MUC5B, and MUC16 during metastasis initiation and progression. Hence mucins can be used as an attractive target in future diagnostic and therapeutic strategies. In this review, we discuss in detail about mucin family and its domains and the role of different mucins in regulating cancer progression and metastasis. In addition, we briefly discuss insights into mucins as a therapeutic agent.

Transcriptome Analysis Reveals the Molecular Response to Salinity Challenge in Larvae of the Giant Freshwater Prawn Macrobrachium rosenbergii

Salinity is a crucial factor influencing the growth, development, immunity, and reproduction of aquatic organisms; however, little is known about the molecular mechanism of the response to salinity challenge in larvae of the giant freshwater prawn Macrobrachium rosenbergii. Herein, larvae cultured in three treatment groups with salinities of 10, 13, and 16‰ (S10, S13, and S16) were collected, and then transcriptome analysis was conducted by RNA-seq. A total of 6,473, 3,830 and 3,584 differentially expressed genes (DEGs) were identified in the S10 vs. S13 comparison, S10 vs. S16 comparison and S13 vs. S16 comparison, respectively. These genes are involved in osmoregulation, energy metabolism, molting, and the immune response. qPCR analysis was used to detect the expression patterns of 16 DEGs to verify the accuracy of the transcriptome data. Protein–protein interaction (PPI) analysis for DEGs and microsatellite marker screening were also conducted to reveal the molecular mechanism of salinity regulation. Together, our results will provide insight into the molecular genetic basis of adaptation to salinity challenge for larvae of M. rosenbergii.

LOC102724163 promotes breast cancer cell proliferation and invasion by stimulating MUC19 expression

Breast cancer (BC) is a malignant disease and the most commonly diagnosed cancer in women. Numerous studies have previously verified the important role of long non-coding RNAs in a number of biological processes in BC. In the present study, analysis of The Cancer Genome Atlas database and reverse transcription-quantitative PCR demonstrated that LOC102724163 expression levels were significantly upregulated in BC tissues compared to matched adjacent normal tissues and were associated with an unfavorable prognosis in patients with BC. Gain or loss of function assays indicated that overexpression of LOC102724163 significantly increased tumorgenicity in vivo and cell migration, proliferation and invasion in vitro. In the mechanistical aspect, LOC102724163 sponged microRNA (miR)-508-5p to elevate MUC19 expression. Additionally, rescue assays ascertained the function of the LOC102724163/miR-508-5p/MUC19 axis in the proliferation and invasion of BC cells. To the best of our knowledge, this is the first study to have demonstrated that LOC102724163 may act as a competing endogenous RNA to control MUC19 expression levels by competitively sponging miR-508-5p to modulate BC progression. Therefore, the present study has provided new insights into BC diagnosis and treatment.

Purified mucins in drug delivery research

One of the main challenges in the field of drug delivery remains the development of strategies to efficiently transport pharmaceuticals across mucus barriers, which regulate the passage and retention of molecules and particles in all luminal spaces of the body. A thorough understanding of the molecular mechanisms, which govern such selective permeability, is key for achieving efficient translocation of drugs and drug carriers. For this purpose, model systems based on purified mucins can contribute valuable information. In this review, we summarize advances that were made in the field of drug delivery research with such mucin-based model systems: First, we give an overview of mucin purification procedures and discuss the suitability of model systems reconstituted from purified mucins to mimic native mucus. Then, we summarize techniques to study mucin binding. Finally, we highlight approaches that made use of mucins as building blocks for drug delivery platforms or employ mucins as active compounds.

Autoimmune Epithelitis and Chronic Inflammation in Sjögren's Syndrome-Related Dry Eye Disease

Autoimmune epithelitis and chronic inflammation are one of the characteristic features of the immune pathogenesis of Sjögren’s syndrome (SS)-related dry eye disease. Autoimmune epithelitis can cause the dysfunction of the excretion of tear fluid and mucin from the lacrimal glands and conjunctival epithelia and meibum from the meibomian glands. The lacrimal gland and conjunctival epithelia express major histocompatibility complex class II or human leukocyte antigen-DR and costimulatory molecules, acting as nonprofessional antigen-presenting cells for T cell and B cell activation in SS. Ocular surface epithelium dysfunction can lead to dry eye disease in SS. Considering the mechanisms underlying SS-related dry eye disease, this review highlights autoimmune epithelitis of the ocular surface, chronic inflammation, and several other molecules in the tear film, cornea, conjunctiva, lacrimal glands, and meibomian glands that represent potential targets in the treatment of SS-related dry eye disease.

Membrane-associated mucins of the human ocular surface in health and disease

Mucins are a family of high molecular weight, heavily-glycosylated proteins produced by wet epithelial tissues, including the ocular surface epithelia. Densely-packed O-linked glycan chains added post-translationally confer the biophysical properties of hydration, lubrication, anti-adhesion and repulsion. Membrane-associated mucins (MAMs) are the distinguishing components of the mucosal glycocalyx. At the ocular surface, MAMs maintain wetness, lubricate the blink, stabilize the tear film, and create a physical barrier to the outside world. In addition, it is increasingly appreciated that MAMs function as cell surface receptors that transduce information from the outside to the inside of the cell. Recently, our team published a comprehensive review/perspectives article for molecular scientists on ocular surface MAMs, including previously unpublished data and analyses on two new genes MUC21 and MUC22, as well as new MAM functions and biological roles, comparing human and mouse (PMID: 31493487). The current article is a refocus for the audience of The Ocular Surface. First, we update the gene and protein information in a more concise form, and include a new section on glycosylation. Next, we discuss biological roles, with some new sections and further updating from our previous review. Finally, we provide a new chapter on MAM involvement in ocular surface disease. We end this with discussion of an emerging mechanism responsible for damage to the epithelia and their mucosal glycocalyces: the unfolded protein response (UPR). The UPR offers a novel target for therapeutic intervention.

Cevimeline-induced anti-inflammatory effect through upregulations of mucins in the ocular surface of a dry eye mouse model

This study aimed to investigate the effects of various concentrations of cevimelines (CVMs) and compare them with commercial drugs in a murine model of dry eye. The experimental mouse model used male and female NOD.B10.H2^b mice over 12 weeks of age. Desiccation stress was performed at 30-40% ambient humidity, and subcutaneous injection of 0.5 mg/0.2 mL scopolamine hydrobromide was performed four times a day for 10 days. The efficacy of various concentrations of CVMs (seven experimental groups) was first evaluated, and then 2% CVM was compared with commercial drugs, such as cyclosporine A (CsA), diquafosol (DQS), and rebamipide (REB) (seven experimental groups). The clinical changes, including tear production, corneal irregularity, and fluorescein staining, were measured after the instillation of various concentrations of CVMs and commercial drugs for 0, 3, 5, 7, and 10 days. Histological changes, such as corneal detachment, conjunctival goblet cell and mucin density staining, were assessed by staining the cornea or conjunctiva with hematoxylin-eosin, periodic acid-Schiff, and alcian blue. The expression of inflammatory markers and mucin factors was detected by immunohistochemistry and immunofluorescence in the lacrimal gland, cornea, and conjunctiva. Tear production was significantly increased in the 2% CVM group and was similar to that in the DQS and REB groups (P < 0.05). The corneal smoothness and fluorescein staining score were significantly improved in the 2% CVM group and were similar to those in the REB group (P < 0.05). Corneal epithelial cells were significantly decreased in the 2% CVM group, with similar observations made in the DQS and REB groups (P < 0.05). The conjunctival goblet cells and mucin density recovered in the 2% CVM group were similar to those in the CsA and REB groups (P < 0.05). The 2% CVM group showed suppressed expression of inflammatory factors in the lacrimal gland and was comparable to that seen in the CsA and REB groups. The expression of mucin factors was upregulated in the cornea and conjunctiva of the 2% CVM group and was similar to that of the CsA and REB groups. In conclusion, administration of CVM resulted in recovery or clinical and histological improvement of the murine dry eye model, and all the observed parameters were comparable to those with commercial drugs.

Association of MUC19 Mutation With Clinical Benefits of Anti-PD-1 Inhibitors in Non-small Cell Lung Cancer

Although anti-PD-1 inhibitors exhibit impressive clinical results in non-small cell lung cancer (NSCLC) cases, a substantial percentage of patients do not respond to this treatment. Moreover, the current recommended biomarkers are not perfect. Therefore, it is essential to discover novel molecular determinants of responses to anti-PD-1 inhibitors. We performed Whole Exome Sequencing (WES) in a cohort of 33 Chinese NSCLC patients. Patients were classified into the durable clinical benefit (DCB) and no durable benefit (NDB) groups. Infiltrating CD8⁺ cells in the tumor microenvironment (TME) were investigated by immunohistochemistry. We also used public datasets to validate our results. In our cohort, good clinical responses to anti-PD-1 inhibitors were more pronounced in younger patients with lower Eastern Cooperative Oncology Group (ECOG) scores and only extra-pulmonary metastasis. More importantly, we identified a novel MUC19 mutation, which was significantly enriched in DCB patients (P = 0.015), and MUC19-mutated patients had a longer progression-free survival (PFS) (hazard ratio = 0.3, 95% CI 0.1-0.9; P = 0.026). Immunohistochemistry results indicated that the MUC19 mutation was associated with increased infiltration by CD8⁺ T cells in the TME (P = 0.0313). When combining MUC19 mutation with ECOG scores and intra-pulmonary metastasis status, patients with more positive predictors had longer PFS (P = 0.003). Furthermore, MUC19 mutation was involved in immune responses and associated with a longer PFS in the Memorial Sloan-Kettering Cancer Center (MSKCC) cohort. Collectively, we identified that MUC19 mutations were involved in immune responses, and NSCLC tumors harboring mutated MUC19 exhibited good responses to anti-PD-1 inhibitors.

Clinical Implication of Patchy Pattern Corneal Staining in Dry Eye Disease

Corneal fluorescein staining in a form that is commonly called a “patchy pattern (PP)” is sometimes seen with or without superficial punctate keratopathy (SPK) in dry-eye diseases (DEDs). Here, we investigated the differences in the clinical features of DED patients with and without PP corneal staining (PPCS). This study involved 35 DEDs with PPCS (PPCS group) and 30 DEDs with SPK and without PPCS (non-PPCS group). The tear meniscus radius (TMR, mm), spread grade (SG) of the tear-film lipid layer (i.e., SG 1–5, 1 being best), noninvasive breakup time (NIBUT, seconds), fluorescein breakup time (FBUT, seconds), corneal epithelial damage (CED, 15 points maximum), conjunctival epithelial damage (CjED, six points maximum), the Schirmer’s 1 test (ST1, mm), and the prevalence of Sjögren’s syndrome (SS) were examined, and then compared between the two groups. Our findings revealed that between the groups (PPCS vs. non-PPCS), there was a statistically significant difference (p < 0.05) in CjED (3.1 ± 1.9 vs. 1.3 ± 1.6), ST1 (5.6 ± 7.4 vs. 14.8 ± 11.4), and the prevalence of SS (60.0% vs. 16.7%). Our findings suggest that DEDs and dry-eye patients with PPCS may indicate not only SS itself, but also the ophthalmological characteristics compatible with SS.

Sjögren's Syndrome Associated with Chikungunya Infection: A Case Report

Chikungunya virus (CHIKV) infection is caused by an arbovirus prevalent in various parts of the world. The virus can induce autoantibodies and rheumatic diseases, such as rheumatoid arthritis and spondylarthritis. However, until now, no case of Sjögren syndrome (SS) was described associated with CHIKV. In this article, we describe a 49-year-old female with polyarthralgia and a temporary rash on her trunk and arms. Her physical examination showed polyarthritis of her ankles and wrists. Serologies for CHIKV were interpreted as positive with IgM 6.5 (normal range < 0.8) and negative for IgG. Antinuclear antibodies were positive at a titer of 1:640 as well as anti-Ro/SS-A. The diagnosis of subacute CHIKV infection was determined. The Schirmer test, Rose Bengal, and salivary scintigraphy were positive and the diagnosis of SS was confirmed. She was treated with hydroxychloroquine, methotrexate, and a single dose of betamethasone depot. This is the first report on CHIKV associated with SS. Sequence analysis of the CHIKV proteome versus SS autoantigens showed an extensive peptide sharing between the virus and numerous SS autoantigens, thus supporting the hypothesis that autoimmune cross-reactivity might causally link CHIKV to SS.

The Role of Endogenous Antimicrobial Peptides in Modulating Innate Immunity of the Ocular Surface in Dry Eye Diseases

The ocular surface has the challenging responsibility of maintaining a clear moist refractive surface while protecting the eye from exogenous pathogens and the environment. Homeostasis of the ocular surface, including its innate immune components, is altered in ocular surface disease states. In this review, we focus on antimicrobial peptides and the role they play in the immune response of the ocular surface during healthy states and dry eye diseases. Antimicrobial peptides are of special interest to the study of the ocular surface because of their various roles that include microbial threat neutralization, wound healing, and immune modulation. This review explores current literature on antimicrobial peptides in ocular surface diseases and discusses their therapeutic potential in ocular surface diseases and dry eye.

Human Metapneumovirus Induces Mucin 19 Which Contributes to Viral Pathogenesis

Human Metapneumovirus (HMPV) remains one of the most common viral infections causing acute respiratory tract infections, especially in young children, elderly, and immunocompromised populations. Clinical symptoms can range from mild respiratory symptoms to severe bronchiolitis and pneumonia. The production of mucus is a common feature during HMPV infection, but its contribution to HMPV-induced pathogenesis and immune response is largely unknown. Mucins are a major component of mucus and they could have an impact on how the host responds to infections. Using an in vitro system and a mouse model of infection, we identified that Mucin 19 is predominantly expressed in the respiratory tract upon HMPV infection. Moreover, the lack of Muc19 led to an improved disease, lower lung viral titers and a decrease in the number of CD4+ T cells. These data indicate that mucin 19 contributes to the activation of the immune response to HMPV and to HMPV-induced pathogenesis.

Engineering drug delivery systems to overcome mucosal barriers for immunotherapy and vaccination

Mucosal surfaces protect our bodies from pathogens and external irritants using a system of biological barriers. Overcoming these barriers is a significant drug delivery challenge, particularly for immunotherapies that aim to modulate the local immune response. Reaching local lymphoid tissues and draining lymph nodes (LNs) requires crossing the mucus mesh, mucosal epithelium, and either targeting M cells covering lymphoid tissues or utilizing lymphatic transport that shuttles molecules and particulates from the periphery to the LN. We first highlight the barrier properties of mucus and mucosal epithelium, and the function of the mucosal immune system. We then dive into existing drug delivery technologies that have been engineered to overcome each of these barriers. We particularly focus on novel strategies for targeting lymphoid tissues, which has been shown to enhance immunotherapies and vaccinations, via directly targeting LNs, lymphatic vessels, and M cells that transport samples of mucosal content to the lymphoid tissues.

Mucins and Pathogenic Mucin-Like Molecules Are Immunomodulators During Infection and Targets for Diagnostics and Vaccines

Mucins and mucin-like molecules are highly O-glycosylated proteins present on the cell surface of mammals and other organisms. These glycoproteins are highly diverse in the apoprotein and glycan cores and play a central role in many biological processes and diseases. Mucins are the most abundant macromolecules in mucus and are responsible for its biochemical and biophysical properties. Mucin-like molecules cover various protozoan parasites, fungi and viruses. In humans, modifications in mucin glycosylation are associated with tumors in epithelial tissue. These modifications allow the distinction between normal and abnormal cell conditions and represent important targets for vaccine development against some cancers. Mucins and mucin-like molecules derived from pathogens are potential diagnostic markers and targets for therapeutic agents. In this review, we summarize the distribution, structure, role as immunomodulators, and the correlation of human mucins with diseases and perform a comparative analysis of mucins with mucin-like molecules present in human pathogens. Furthermore, we review the methods to produce pathogenic and human mucins using chemical synthesis and expression systems. Finally, we present applications of mucin-like molecules in diagnosis and prevention of relevant human diseases.

Autoimmune diseases in China

Autoimmune diseases, such as rheumatoid arthritis, systematic lupus erythematosus and Sjögren's syndrome, are a group of diseases characterized by the activation of immune cells and excessive production of autoantibodies. Although the pathogenesis of these diseases is still not completely understood, studies have shown that multiple factors including genetics, environment and immune responses play important roles in the development and progression of the diseases. In China, there are great achievements in the mechanisms of autoimmune diseases during the last decades. These studies provide new insight to understand the diseases and also shed light on the development of novel therapy.

Ocular mucins in dry eye disease

Dry eye disease is a common and multifactorial disease with a high prevalence worldwide. Water loss, reduced expression of glycocalyx mucins, and loss of goblet cells secreting gel-forming mucins are hallmarks of dry eye disease. Mucins are large and complex heavily glycosylated proteins. Their organization in the tear film remains unclear, but they play a key role to protect and maintain integrity of the ocular surface. Mice have been extremely valuable mammalian models with which to study ocular physiology and disease, and to evaluate eye therapies. Genetically modified mice and spontaneously occurring mutants with eye defects have proven to be powerful tools for the pharmaceutical industry, clinicians, and basic researchers investigating dry eye disease. However, ocular mucins remain relatively under-studied and inadequately characterized. This review aims to summarize current knowledge about mucin production at the ocular surface in healthy individuals and in dry eye disease, and to compile an overview of mouse models available for the study of mucins in dry eye disease.

Secreted Mucins on the Ocular Surface

Mucins, which play important roles on the ocular surface in wettability, lubrication, and barrier function, are classified into two categories: secreted mucins and membrane-associated mucins. The most important secreted mucin on the ocular surface is MUC5AC, which is secreted by the conjunctival goblet cells. In the human conjunctiva, goblet cells are present in higher concentrations in the fornix, inferior nasal bulbar, and the lid wiper on the lid margin. The number of conjunctival goblet cells and MUC5AC expression/secretion are decreased in a patient with dry eye. In Japan, drugs that stimulate mucin secretion or increase the number of conjunctival goblet cells are commercially available. A P2Y2 receptor, diquafosol, stimulates tear fluid secretion from conjunctival epithelial cells and promotes mucin secretion from conjunctival goblet cells. Rebamipide was marketed originally as an oral therapeutic drug to treat gastritis in Japan. Topical rebamipide increases numbers of goblet cells in the bulbar conjunctiva and the lid wiper area of palpebral conjunctiva. Many researchers have reported decreases in the ocular surface mucin expression including MUC5AC secreted by goblet cells in patients with dry eye. However, it is unknown whether changes in mucin expression on the ocular surface cause or result from dry eye. Further study is needed to determine the true mechanism of dry eye disease.

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.

Downregulation of hsa_circ_0007534 suppresses breast cancer cell proliferation and invasion by targeting miR-593/MUC19 signal pathway

Circular RNAs (circRNAs) are important non-coding RNAs that are reportedly involved in the progression of diverse human cancers through their action as a microRNA (miRNA) sponge. However, the exact roles of circRNAs in breast cancer (BC) remain largely unknown. The present data demonstrate the significantly upregulated expression of hsa_circ_0007534 circRNA in BC tissues and cell lines. In contrast, miR-593 expression was significantly downregulated. Knockdown of hsa_circ_0007534 inhibited BC cell proliferation, colony formation, and invasion, and promoted apoptosis in BC cells. Moreover, hsa_circ_0007534 was demonstrated to be a sponge of miR-593, and expression of miR-593 in BC cells was negatively correlated with hsa_circ_0007534. MUC19 expression was markedly increased in BC tissues and cell lines, and the 3'-UTR of MUC19 was targeted by miR-593. The expression of MUC19 was negatively regulated by miR-593 in BC cells. Our findings suggest an oncogenic role for hsa_circ_0007534 in BC by acting as a miR-593 sponge to promote MUC19 expression.

RGN-259 (thymosin β4) improves clinically important dry eye efficacies in comparison with prescription drugs in a dry eye model

This study evaluated the clinical activity of RGN-259 (thymosin β4) in comparison with cyclosporine A (CsA), diquafosol (DQS), and lifitegrast (LFA) in a murine model of dry eye. The model was NOD.B10-H2^b mice in a 30–40% humidified environment together with daily scopolamine hydrobromide injections for 10 days. After desiccation stress, all drugs were evaluated after 10 treatment days. RGN-259 increased tear production similar to that in the DQS- and LFA-treated mice while CsA was inactive. RGN-259 improved corneal smoothness and decreased fluorescein staining similar to that of LFA group while CsA and DQS were inactive. Corneal epithelial detachment was reduced by RGN-259, and DQS and LFA showed similar activity but the CsA was inactive. RGN-259 increased conjunctival goblet cells and mucin production comparable to that seen with CsA, while DQS and LFA were inactive. RGN-259 reduced the over-expression of inflammatory factors comparable to that with CsA and LFA, while DQS was inactive. RGN-259 increased mucin production comparable to that observed with CsA, while DQS and LFA were inactive. In conclusion, RGN-259 promoted recovery of mucins and goblet cells, improved corneal integrity, and reduced inflammation in a dry eye mouse model and was equal to or more effective than prescription treatments.

Transcriptomic and Proteomic Profiling Revealed High Proportions of Odorant Binding and Antimicrobial Defense Proteins in Olfactory Tissues of the House Mouse

Mammalian olfaction depends on chemosensory neurons of the main olfactory epithelia (MOE), and/or of the accessory olfactory epithelia in the vomeronasal organ (VNO). Thus, we have generated the VNO and MOE transcriptomes and the nasal cavity proteome of the house mouse, Mus musculus musculus. Both transcriptomes had low levels of sexual dimorphisms, while the soluble proteome of the nasal cavity revealed high levels of sexual dimorphism similar to that previously reported in tears and saliva. Due to low levels of sexual dimorphism in the olfactory receptors in MOE and VNO, the sex-specific sensing seems less likely to be dependent on receptor repertoires. However, olfaction may also depend on a continuous removal of background compounds from the sites of detection. Odorant binding proteins (OBPs) are thought to be involved in this process and in our study Obp transcripts were most expressed along other lipocalins (e.g., Lcn13, Lcn14) and antimicrobial proteins. At the level of proteome, OBPs were highly abundant with only few being sexually dimorphic. We have, however, detected the major urinary proteins MUP4 and MUP5 in males and females and the male-biased central/group-B MUPs that were thought to be abundant mainly in the urine. The exocrine gland-secreted peptides ESP1 and ESP22 were male-biased but not male-specific in the nose. For the first time, we demonstrate that the expression of nasal lipocalins correlates with antimicrobial proteins thus suggesting that their individual variation may be linked to evolvable mechanisms that regulate natural microbiota and pathogens that regularly enter the body along the ‘eyes-nose-oral cavity’ axis.

Gel-forming mucin interactome drives mucus viscoelasticity

Mucus is a hydrogel that constitutes the first innate defense in all mammals. The main organic component of mucus, gel-forming mucins, forms a complex network through both reversible and irreversible interactions that drive mucus gel formation. Significant advances in the understanding of irreversible gel-forming mucins assembly have been made using recombinant protein approaches. However, little is known about the reversible interactions that may finely modulate mucus viscoelasticity, which can be characterized using rheology. This approach can be used to investigate both the nature of gel-forming mucins interactions and factors that influence hydrogel formation. This knowledge is directly relevant to the development of new drugs to modulate mucus viscoelasticity and to restore normal mucus functions in diseases such as in cystic fibrosis. The aim of the present review is to summarize the current knowledge about the relationship between the mucus protein matrix and its functions, with emphasis on mucus viscoelasticity.

A slippery slope: On the origin, role and physiology of mucus

The mucosa of the gastrointestinal tract, eyes, nose, lungs, cervix and vagina is lined by epithelium interspersed with mucus-secreting goblet cells, all of which contribute to their unique functions. This mucus provides an integral defence to the epithelium against noxious agents and pathogens. However, it can equally act as a barrier to drugs and delivery systems targeting epithelial passive and active transport mechanisms. This review highlights the various mucins expressed at different mucosal surfaces on the human body, and their role in creating a mucoid architecture to protect epithelia with specialized functions. Various factors compromising the barrier properties of mucus have been discussed, with an emphasis on how disease states and microbiota can alter the physical properties of mucus. For instance, Akkermansia muciniphila, a bacterium found in higher levels in the gut of lean individuals induces the production of a thickened gut mucus layer. The aims of this article are to elucidate the different physiological, biochemical and physical properties of bodily mucus, a keen appreciation of which will help circumvent the slippery slope of challenges faced in achieving effective mucosal drug and gene delivery.

Meibomian gland features in a Norwegian cohort of patients with primary Sjögren´s syndrome

Purpose

To assess the tear film and meibomian gland (MG) features in a Norwegian cohort of patients with primary Sjögren´s syndrome (pSS) and in age- and gender-matched control subjects.

Methods

Thirty-four female patients with pSS (age 52.9±11.9 years) and 32 female control subjects (age 49.0±11.5 years) were recruited. After completion of Ocular Surface Disease Index (OSDI) questionnaire and McMonnies Dry Eye Questionaire, participants underwent measurements of tear osmolarity, tear break-up time (TBUT), ocular surface and corneal staining, Schirmer I test, corneal sensitivity, MG expressibility evaluations, and lid margin morphology examination using slitlamp microscopy. Non-contact infrared meibography images were assessed by computer-assisted analysis. The MG loss, calculated as (tarsal area-MG area)/tarsal area, was evaluated in both upper (UL) and lower lids (LL).

Results

Compared to the control group, pSS patients demonstrated higher MG loss in both UL (33.8±13.2% vs. 24.4±8.5%, p< 0.01) and LL (52.5±15.7% vs. 43.0±9.6%, p<0.05), as well as higher lid abnormality score (0.8±0.8 vs. 0.2±0.6, p< 0.01). Furthermore, pSS patients showed higher OSDI and McMonnies questionnaire scores, elevated osmolarity, shorter TBUT, shorter blink interval, less wetting in Schirmer I test, more ocular surface staining and more corneal staining. MG loss in UL correlated negatively with TBUT (r = -0.386, p = 0.029) in the pSS group, whereas MG loss in LL correlated negatively with TBUT (r = -0.380, p = 0.035) in the control group.

Conclusions

Significantly elevated dry eye symptoms and signs were found in the pSS group compared with the control group, which might be attributed to both decreased aqueous tear production and increased tear evaporation.

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.

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.

The properties of the mucus barrier, a unique gel--how can nanoparticles cross it?

The key criterion for a nanoparticle drug-delivery system is the ability to produce substantial bioavailability without damaging the physiological protective mechanisms. The main area for drug delivery is the aerodigestive tract. All epithelial surfaces have a membrane-bound layer and in the lung this layer is surmounted by a gel layer. In the gastrointestinal tract the membrane-bound mucin layer is covered by a mucus bilayer. The pore sizes of mucus gels are around 100 to 200 nm. Consequently, only nanoparticles in this size range could potentially penetrate without modification of these layers. To study nanoparticle permeation with results that pertain to in vivo conditions, native mucus mucin preparations must be used. Strategies to increase pores in mucus gels are discussed herein.

Searching the Evolutionary Origin of Epithelial Mucus Protein Components-Mucins and FCGBP

The gel-forming mucins are large glycosylated proteins that are essential components of the mucus layers covering epithelial cells. Using novel methods of identifying mucins based on profile hidden Markov models, we have found a large number of such proteins in Metazoa, aiding in their classification and allowing evolutionary studies. Most vertebrates have 5–6 gel-forming mucin genes and the genomic arrangement of these genes is well conserved throughout vertebrates. An exception is the frog Xenopus tropicalis with an expanded repertoire of at least 26 mucins of this type. Furthermore, we found that the ovomucin protein, originally identified in chicken, is characteristic of reptiles, birds, and amphibians. Muc6 is absent in teleost fish, but we now show that it is present in animals such as ghost sharks, demonstrating an early origin in vertebrate evolution. Public RNA-Seq data were analyzed with respect to mucins in zebrafish, frog, and chicken, thus allowing comparison in regard of tissue and developmental specificity. Analyses of invertebrate proteins reveal that gel-forming-mucin type of proteins is widely distributed also in this group. Their presence in Cnidaria, Porifera, and in Ctenophora (comb jellies) shows that these proteins were present early in metazoan evolution. Finally, we examined the evolution of the FCGBP protein, abundant in mucus and related to gel-forming mucins in terms of structure and localization. We demonstrate that FCGBP, ubiquitous in vertebrates, has a conserved N-terminal domain. Interestingly, this domain is also present as an N-terminal sequence in a number of bacterial proteins.

Effect of rebamipide ophthalmic suspension on signs and symptoms of keratoconjunctivitis sicca in Sjögren syndrome patients with or without punctal occlusions

PURPOSE

The aim of this study was to investigate the efficacy of 2% rebamipide suspension in treatment of keratoconjunctivitis sicca (KCS) in patients with Sjögren syndrome (SS) with or without punctal occlusions.

METHODS

Thirty patients with SS, diagnosed based on the presence of autoantibodies and/or focus score >1 on lip biopsies, with corneal fluorescein staining scores (FSS) >3, and conjunctival lissamine green-staining scores (LSS) >3, were treated 4 times daily for 4 weeks with 2% rebamipide ocular suspension. Ocular examinations were performed before treatment and 2 and 4 weeks after treatment to evaluate FSS (0-9), LSS (0-6), and tear film break-up time (BUT). Hyaluronate and/or artificial tears were not discontinued. The patients were interviewed regarding the 5 major KCS symptoms, foreign body sensation, dry eye sensation, photophobia, ocular pain, and blurred vision, with each graded from none (0) to very severe (4).

RESULTS

Of the 30 patients, 3 failed to attend all sessions, leaving 27 (25 females, 2 males, mean age 62.5 ± 10.8 years) to be studied. FSS and LSS showed improvement at week 2, but BUT showed improvement later, at week 4. All 5 symptoms improved significantly. When the patients were divided into 3 groups according to the presence of punctal occlusions, FSS and LSS were found to improve in all groups, but BUT improved only in patients with both puncta occluded at week 4.

CONCLUSIONS

Rebamipide ophthalmic suspension was effective in treating KCS of patients with SS, probably by increasing mucins and suppressing inflammatory cytokines. Punctal occlusions resulted in sufficient retention of tear fluid to enhance the activities of rebamipide and improve BUT.

Loss of NHE8 expression impairs ocular surface function in mice

Sodium/hydrogen exchanger (NHE) 8 is expressed at the apical membrane of the epithelial cells and plays important roles in neutral sodium absorption in the gastrointestinal tract and the kidney. It also has an important role in epithelial mucosal protection in the gastric gland and the intestine. Although NHE8 has broad tissue distribution, the precise location and the physiological role of NHE8 in the eye remain unknown. In the present study, we successfully detected the expression of NHE8 in the ocular surface by PCR and Western blot in human and mouse eyes. Immunohistochemistry staining located NHE8 protein at the plasma membrane of the epithelial cells in the conjunctiva, the cornea, and the lacrimal gland both in human and mouse. We also detected the expression of downregulated-in-adenoma (DRA, a Cl(-)/HCO3 (-) transporter) in the ocular surface epithelial cells. Using NHE8-/- mouse model, we found that loss of NHE8 function resulted in reduced tear production and increased corneal staining. These NHE8-/- mice also showed increased expression of TNF-α and matrix metalloproteinase 9 (MMP9) genes. The expression of epithelial keratinization marker genes, small proline-rich protein 2h (Sprr2h) and transglutaminase 1 (Tgm1), were also increased in NHE8-/- eyes. Furthermore, DRA expression in NHE8-/- mice was reduced in the conjunctiva, the cornea, and the lacrimal glands in association with a reduction in conjunctival mucosal pH. Altered ocular surface function and reduced epithelial DRA expression in NHE8-/- mice suggest that the role of NHE8 in ocular surface tissue involve in tear production and ocular epithelial protection. This study reveals a potential novel mechanism of dry eye condition involving abnormal NHE8 function.

The cellular mechanisms of dry eye: from pathogenesis to treatment

Dry eye is a complex disease characterized by changes in the ocular surface epithelia related to reduced quality and/or quantity of tears, inflammatory reaction, and impairment of ocular surface sensitivity. It has recently been proposed that increased tear osmolarity represents a main trigger to the altered cellular mechanisms leading to epithelial damage in dry eye. However, dry eye pathogenesis is multifactorial, with cytotoxic inflammatory mediators, altered lacrimal gland secretion and nerve function, squamous metaplasia of the conjunctival epithelium and decrease of goblet cells density, all playing a role in a detrimental loop that perpetuates and worsens damage to the corneal and conjunctival epithelia. Current topical treatments for dry eye patients include the use of lubricants and anti-inflammatory drugs. However, lubricants only improve symptoms temporarily, and chronic use of topical steroids is associated to severe ocular side effects such as cataract and glaucoma. The deeper understanding of the cellular mechanisms that are altered in dry eye is opening novel perspectives for patients and physicians, who are seeking treatments capable not only of improving symptoms but also of restoring the homeostasis of the ocular surface. In this review, we will focus on novel anti-inflammatory agents and on nerve growth factor, a neurotrophin that is altered in dry eye and has been suggested as a main player in the neuroimmune cross-talk of the ocular surface as well as in the stimulation of corneal sensitivity, epithelial proliferation and differentiation, and stimulation of mucin production by goblet cells. J. Cell. Physiol. 228: 2253-2256, 2013. © 2013 Wiley Periodicals, Inc.

Air exposure induced characteristics of dry eye in conjunctival tissue culture

There are several animal models illustrating dry eye pathophysiology. Current study would like to establish an ex vivo tissue culture model for characterizing dry eye. Human conjunctival explants were cultured under airlift or submerged conditions for up to 2 weeks, and only airlifted conjunctival cultures underwent increased epithelial stratification. Starting on day 4, the suprabasal cells displayed decreased K19 expression whereas K10 keratin became evident in airlift group. Pax6 nuclear expression attenuated already at 2 days, while its perinuclear and cytoplasmic expression gradually increased. MUC5AC and MUC19 expression dramatically decreased whereas the full thickness MUC4 and MUC16 expression pattern disappeared soon after initiating the airlift condition. Real time PCR showed K16, K10 and MUC16 gene up-regulated while K19, MUC5AC, MUC19 and MUC4 down-regulated on day 8 and day 14. On day 2 was the appearance of apoptotic epithelial and stromal cells appeared. The Wnt signaling pathway was transiently activated from day 2 to day 10. The inflammatory mediators IL-1β, TNF-α, and MMP-9 were detected in the conditioned media after 6 to 8 days. In conclusion, airlifted conjunctival tissue cultures demonstrated Wnt signaling pathway activation, coupled with squamous metaplasia, mucin pattern alteration, apoptosis and upregulation of proinflammatory cytokine expression. These changes mimic the pathohistological alterations described in dry eye. This correspondence suggests that insight into the pathophysiology of dry eye may be aided through the use of airlifted conjunctival tissue cultures.

Tear film mucins: front line defenders of the ocular surface; comparison with airway and gastrointestinal tract mucins

The ocular surface including the cornea and conjunctiva and its overlying tear film are the first tissues of the eye to interact with the external environment. The tear film is complex containing multiple layers secreted by different glands and tissues. Each layer contains specific molecules and proteins that not only maintain the health of the cells on the ocular surface by providing nourishment and removal of waste products but also protect these cells from environment. A major protective mechanism that the corneal and conjunctival cells have developed is secretion of the innermost layer of the tear film, the mucous layer. Both the cornea and conjunctiva express membrane spanning mucins, whereas the conjunctiva also produces soluble mucins. The mucins present in the tear film serve to maintain the hydration of the ocular surface and to provide lubrication and anti-adhesive properties between the cells of the ocular surface and conjunctiva during the blink. A third function is to contribute to the epithelial barrier to prevent pathogens from binding to the ocular surface. This review will focus on the different types of mucins produced by the corneal and conjunctival epithelia. Also included in this review will be a presentation of the structure of mucins, regulation of mucin production, role of mucins in ocular surface diseases, and the differences in mucin production by the ocular surface, airways and gastrointestinal tract.

Surfactant protein D contributes to ocular defense against Pseudomonas aeruginosa in a murine model of dry eye disease

Dry eye disease can cause ocular surface inflammation that disrupts the corneal epithelial barrier. While dry eye patients are known to have an increased risk of corneal infection, it is not known whether there is a direct causal relationship between these two conditions. Here, we tested the hypothesis that experimentally-induced dry eye (EDE) increases susceptibility to corneal infection using a mouse model. In doing so, we also examined the role of surfactant protein D (SP-D), which we have previously shown is involved in corneal defense against infection. Scopolamine injections and fan-driven air were used to cause EDE in C57BL/6 or Black Swiss mice (wild-type and SP-D gene-knockout). Controls received PBS injections and were housed normally. After 5 or 10 days, otherwise uninjured corneas were inoculated with 10(9) cfu of Pseudomonas aeruginosa strain PAO1. Anesthesia was maintained for 3 h post-inoculation. Viable bacteria were quantified in ocular surface washes and corneal homogenates 6 h post-inoculation. SP-D was measured by Western immunoblot, and corneal pathology assessed from 6 h to 4 days. EDE mice showed reduced tear volumes after 5 and 10 days (each by ∼75%, p<0.001) and showed fluorescein staining (i.e. epithelial disruption). Surprisingly, there was no significant difference in corneal pathology between EDE mice and controls (∼10-14% incidence). Before bacterial inoculation, EDE mice showed elevated SP-D in ocular washes. After inoculation, fewer bacteria were recovered from ocular washes of EDE mice (<2% of controls, p = 0.0004). Furthermore, SP-D knockout mice showed a significant increase in P. aeruginosa corneal colonization under EDE conditions. Taken together, these data suggest that SP-D contributes to corneal defense against P. aeruginosa colonization and infection in EDE despite the loss of barrier function to fluorescein.

Tissue distibution of murine Muc19/smgc gene products

The recently identified gene Muc19/Smgc encodes two diverse splice variants, Smgc (submandibular gland protein C) and Muc19 (mucin 19). Muc19 is a member of the large gel-forming mucin family and is an exocrine product of sublingual mucous salivary glands in mice. SMGC is a transiently expressed secretion product of developing rodent submandibular and sublingual glands. Little is known about the expression of Muc19/Smgc gene products in other murine salivary and non-salivary tissues containing the mucous cell phenotype. Muc19 expression was therefore initially assessed by RT-PCR and immunohistochemistry. As a complementary approach, we developed a knockin mouse model, Muc19-EGFP, in which mice express a fusion protein containing the first 69 residues of Muc19 followed by enhanced green fluorescent protein (EGFP) as a marker of Muc19 expression. Results from both approaches are consistent, with preferential Muc19 expression in salivary major and minor mucous glands as well as submucosal glands of the tracheolarynx and bulbourethral glands. Evidence also indicates that individual mucous cells of minor salivary and bulbourethral glands produce another gel-forming mucin in addition to Muc19. We further find tissue expression of full-length Smgc transcripts, which encode for SMGC, and are restricted to neonatal tracheolarynx and all salivary tissues.

Dry eye disease and microbial keratitis: is there a connection?

Dry eye is a common ocular surface disease of multifactorial etiology characterized by elevated tear osmolality and inflammation leading to a disrupted ocular surface. The latter is a risk factor for ocular surface infection, yet overt infection is not commonly seen clinically in the typical dry eye patient. This suggests that important innate mechanisms operate to protect the dry eye from invading pathogens. This article reviews the current literature on epidemiology of ocular surface infection in dry eye patients and laboratory-based studies on innate immune mechanisms operating at the ocular surface and their alterations in human dry eye and animal models. The review highlights current understanding of innate immunity in dry eye and identifies gaps in our knowledge to help direct future studies to further unravel the complexities of dry eye disease and its sequelae.

Mucin deficiency causes functional and structural changes of the ocular surface

MUC5AC is the most abundant gel-forming mucin in the ocular system. However, the specific function is unknown. In the present study, a Muc5ac knockout (KO) mouse model was subject to various physiological measurements as compared to its wide-type (WT) control. Interestingly, when KO mice were compared to WT mice, the mean tear break up time (TBUT) values were significantly lower and corneal fluorescein staining scores were significantly higher. But the tear volume was not changed. Despite the lack of Muc5ac expression in the conjunctiva of KO mice, Muc5b expression was significantly increased in these mice. Corneal opacification, varying in location and severity, was found in a few KO mice but not in WT mice. The present results suggest a significant difference in the quality, but not the quantity, of tear fluid in the KO mice compared to WT mice. Dry eye disease is multifactorial and therefore further evaluation of the varying components of the tear film, lacrimal unit and corneal structure of these KO mice may help elucidate the role of mucins in dry eye disease. Because Muc5ac knockout mice have clinical features of dry eye, this mouse model will be extremely useful for further studies regarding the pathophysiology of the ocular surface in dry eye in humans.

Regional differences in rat conjunctival ion transport activities

Active ion transport and coupled osmotic water flow are essential to maintain ocular surface health. We investigated regional differences in the ion transport activities of the rat conjunctivas and compared these activities with those of cornea and lacrimal gland. The epithelial sodium channel (ENaC), sodium/glucose cotransporter 1 (Slc5a1), transmembrane protein 16 (Tmem16a, b, f, and g), cystic fibrosis transmembrane conductance regulator (Cftr), and mucin (Muc4, 5ac, and 5b) mRNA expression was characterized by RT-PCR. ENaC proteins were measured by Western blot. Prespecified regions (palpebral, fornical, and bulbar) of freshly isolated conjunctival tissues and cell cultures were studied electrophysiologically with Ussing chambers. The transepithelial electrical potential difference (PD) of the ocular surface was also measured in vivo. The effect of amiloride and UTP on the tear volume was evaluated in lacrimal gland excised rats. All selected genes were detected but with different expression patterns. We detected αENaC protein in all tissues, βENaC in palpebral and fornical conjunctiva, and γENaC in all tissues except lacrimal glands. Electrophysiological studies of conjunctival tissues and cell cultures identified functional ENaC, SLC5A1, CFTR, and TMEM16. Fornical conjunctiva exhibited the most active ion transport under basal conditions amongst conjunctival regions. PD measurements confirmed functional ENaC-mediated Na(+) transport on the ocular surface. Amiloride and UTP increased tear volume in lacrimal gland excised rats. This study demonstrated that the different regions of the conjunctiva exhibited a spectrum of ion transport activities. Understanding the specific functions of distinct regions of the conjunctiva may foster a better understanding of the physiology maintaining hydration of the ocular surface.

Expression profiles of aquaporins in rat conjunctiva, cornea, lacrimal gland and Meibomian gland

The aim of the study was to elucidate aquaporin (AQP) family member mRNA expression and protein expression/localization in the rat lacrimal functional unit. The mRNA expression of all rat AQPs (AQP0-9, 11-12) in palpebral, fornical, and bulbar conjunctiva, cornea, lacrimal gland, and Meibomian gland was measured by Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) and real time RT-PCR. Antibodies against AQP1, 3, 4, 5, 9, and 11 were used in Western blotting and immunohistochemistry to determine protein expression and distribution. Our study demonstrated characteristic AQP expression profiles in rat ocular tissues. AQP1, 3, 4, 5, 8, 9, 11, and 12 mRNA were detected in conjunctiva. AQP0, 1, 2, 3, 4, 5, 6, 11, and 12 mRNA were expressed in cornea. AQP0, 1, 2, 3, 4, 5, 7, 8, and 11 mRNA were detected in lacrimal gland. AQP1, 3, 4, 5, 7, 8, 9, 11, and 12 mRNA were identified in Meibomian gland. By Western blot, AQP1, 3, 5, and 11 were detected in conjunctiva; AQP1, 3, 5, and 11 were identified in cornea; AQP1, 3, 4, 5, and 11 were detected in lacrimal gland; and AQP1, 3, 4, 5, 9, and 11 were present in Meibomian gland. Immunohistochemistry localized AQPs to distinct sites in the various tissues. This study rigorously analyzed AQPs expression and localization in rat conjunctiva, cornea, lacrimal gland, and Meibomian gland tissues. Our findings provide a comprehensive platform for further investigation into the physiological or pathophysiological relevance of AQPs in ocular surface.