Suppression of Toll-like receptor-mediated innate immune responses at the ocular surface by the membrane-associated mucins MUC1 and MUC16

Immunopathology of Corneal Allograft Rejection and Donor-Specific Antibodies (DSAs) as Immunological Predictors of Corneal Transplant Failure

Despite tremendous developments in the field of laboratory testing in transplantation, the rules of eligibility for corneal transplantation still do not include typing of human leukocyte antigens (HLAs) in the donor and recipient or detection of donor-specific antibodies (DSAs) in the patient. The standard use of diagnostic algorithms is due to the cornea belonging to immunologically privileged tissues, which usually determines the success of transplantation of this tissue. A medical problem is posed by patients at high risk of transplant rejection, in whom the immune privilege of the eye is abolished and the risk of transplant failure increases. Critical to the success of transplantation in patients at high risk of corneal rejection may be the selection of an HLA-matched donor and recipient, and the detection of existing and/or de novo emerging DSAs in the patient. Incorporating the assessment of these parameters into routine diagnostics may contribute to establishing immune risk stratification for transplant rejection and effective personalized therapy for patients.

The role of sialoglycans in modulating dendritic cell function and tumour immunity

Dendritic cells (DCs) are crucial for initiating immune responses against tumours by presenting antigens to T cells. Glycosylation, particularly sialylation, plays a significant role in regulating cell functions, by modulating protein folding and signalling. This review aimed to provide a comprehensive overview of how sialic acids influence key aspects of DC biology, including maturation, migration, antigen presentation, and T cell interactions. Sialic acids influence DC endocytosis, affecting their ability to uptake and present antigens, while guiding their migration to lymph nodes and inflamed tissues. Removing sialic acids enhances DC-mediated antigen presentation to T cells, potentially boosting immune responses. Additionally, sialylated glycans on DCs modulate immune checkpoints, which can impact tumour immunity. Hypersialylation of tumour mucins further promotes immune evasion by interacting with DCs. Understanding the interplay between sialylation and DC functions offers promising avenues for enhancing cancer immunotherapy.

Construction and analysis of a reliable five-gene prognostic signature for colon adenocarcinoma associated with the wild-type allelic state of the COL6A6 gene

Background

Tumors emerge by acquiring a number of mutations over time. The first mutation provides a selective growth advantage compared to adjacent epithelial cells, allowing the cell to create a clone that can outgrow the cells that surround it. Subsequent mutations determine the risk of the tumor progressing to metastatic cancer. Some secondary mutations may inhibit the aggressiveness of the tumor while still increasing the survival of the clone. Meaningful mutations in genes may provide a strong molecular foundation for developing novel therapeutic strategies for cancer.

Methods

The somatic mutation and prognosis in colon adenocarcinoma (COAD) were analyzed. The copy number variation (CNV) and differentially expressed genes (DEGs) between the collagen type VI alpha 6 chain (COL6A6) mutation (COL6A6-MUT) and the COL6A6 wild-type (COL6A6-WT) subgroups were evaluated. The independent prognostic signatures based on COL6A6-allelic state were determined to construct a Cox model. The biological characteristics and the immune microenvironment between the two risk groups were compared.

Results

COL6A6 was found to be highly mutated in COAD at a frequency of 9%. Patients with COL6A6-MUT had a good overall survival (OS) compared to those with COL6A6-WT, who had a different CNV pattern. Significant differences in gene expression were established for 593 genes between the COL6A6-MUT and COL6A6-WT samples. Among them, MUC16, ASNSP1, PRR18, PEG10, and RPL26P8 were determined to be independent prognostic factors. The internally validated prognostic risk model, constructed using these five genes, demonstrated its value by revealing a significant difference in patient prognosis between the high-risk and low-risk groups. Specifically, patients in the high-risk group exhibited a considerably worse prognosis than did those in the low-risk group. The high-risk group had a significantly higher proportion of patients over 60 years of age and patients in stage III. Moreover, the tumor immune dysfunction and exclusion (TIDE) score and the expression of human leukocyte antigen (HLA) family genes were all higher in the high-risk group than that in the low-risk group.

Conclusions

The allelic state of COL6A6 and the five associated DEGs were identified as novel biomarkers for the diagnosis and prognosis of COAD and may be therapeutic targets in COAD.

MUC16: clinical targets with great potential

Mucin 16 (MUC16) is a membrane-bound mucin that is abnormally expressed or mutated in a variety of diseases, especially tumors, while being expressed in normal body epithelium. MUC16 and its extracellular components are often important cancer-related biomarkers. Abnormal expression of MUC16 promotes tumor progression through mesenchymal protein, PI3K/AKT pathway, JAK2/STAT3 pathway, ERK/FBW7/c-Myc, and other mechanisms, and plays an important role in the occurrence and development of tumors. In addition, MUC16 also helps tumor immune escape by inhibiting T cells and NK cells. Many drugs and trials targeting MUC16 have been developed, and MUC16 may be a new direction for future treatments. In this paper, the mechanism of action of MUC16 in the development of cancer, especially in the immune escape of tumor, is introduced in detail, indicating the potential of MUC16 in clinical treatment.

Selective Upregulation of Interleukin 1 Receptor Antagonist and Interleukin-8 in Fuchs' Endothelial Corneal Dystrophy with Accompanying Cataract

(1) Background: Patients with Fuchs' endothelial corneal dystrophy (FECD) may have coexisting cataracts and, therefore, may require a cataract surgery, which poses challenges due to potential endothelial cell damage. FECD is a degenerative eye disease of unclear etiology, with inflammatory cytokines maybe playing an important role in its development and progression. The present study aimed to investigate the cytokine profile in the aqueous humor of FECD eyes with cataract. (2) Methods: Fifty-two patients were included in the study, 26 with FECD + cataract and 26 with cataract as a control group. Samples of the aqueous humor were analyzed for pro- and anti-inflammatory cytokines using a Bio-Plex 200 system. (3) Results: Interleukin 1 receptor antagonist (IL-1Ra) and interleukin IL-8 levels were significantly higher in the aqueous humor of FECD + cataract patients compared to the control/cataract group. Moreover, the levels of anti-inflammatory IL-10 showed a strong trend to be higher in the FECD + cataract group compared to the control group. In contrast, there were no statistically significant differences in IL-1β, IL-6, IL-4, IL-10, IL-13, IL-17A, and tumor necrosis factor TNF-α between the groups. (4) Conclusions: Presented research contributes to a better understanding of FECD pathogenesis. Elevated levels of IL-1Ra and IL-8 may serve as a defense mechanism in people with FECD and coexisting cataract.

MUC1 and MUC16: critical for immune modulation in cancer therapeutics

The Mucin (MUC) family, a range of highly glycosylated macromolecules, is ubiquitously expressed in mammalian epithelial cells. Such molecules are pivotal in establishing protective mucosal barriers, serving as defenses against pathogenic assaults. Intriguingly, the aberrant expression of specific MUC proteins, notably Mucin 1 (MUC1) and Mucin 16 (MUC16), within tumor cells, is intimately associated with oncogenesis, proliferation, and metastasis. This association involves various mechanisms, including cellular proliferation, viability, apoptosis resistance, chemotherapeutic resilience, metabolic shifts, and immune surveillance evasion. Due to their distinctive biological roles and structural features in oncology, MUC proteins have attracted considerable attention as prospective targets and biomarkers in cancer therapy. The current review offers an exhaustive exploration of the roles of MUC1 and MUC16 in the context of cancer biomarkers, elucidating their critical contributions to the mechanisms of cellular signal transduction, regulation of immune responses, and the modulation of the tumor microenvironment. Additionally, the article evaluates the latest advances in therapeutic strategies targeting these mucins, focusing on innovations in immunotherapies and targeted drugs, aiming to enhance customization and accuracy in cancer treatments.

Animal Models in Eye Research: Focus on Corneal Pathologies

The eye is a complex sensory organ that enables visual perception of the world. The dysfunction of any of these tissues can impair vision. Conduction studies on laboratory animals are essential to ensure the safety of therapeutic products directly applied or injected into the eye to treat ocular diseases before eventually proceeding to clinical trials. Among these tissues, the cornea has unique homeostatic and regenerative mechanisms for maintaining transparency and refraction of external light, which are essential for vision. However, being the outermost tissue of the eye and directly exposed to the external environment, the cornea is particularly susceptible to injury and diseases. This review highlights the evidence for selecting appropriate animals to better understand and treat corneal diseases, which rank as the fifth leading cause of blindness worldwide. The development of reliable and human-relevant animal models is, therefore, a valuable research tool for understanding and translating fundamental mechanistic findings, as well as for assessing therapeutic potential in humans. First, this review emphasizes the unique characteristics of animal models used in ocular research. Subsequently, it discusses current animal models associated with human corneal pathologies, their utility in understanding ocular disease mechanisms, and their role as translational models for patients.

Roles unveiled for membrane-associated mucins at the ocular surface using a Muc4 knockout mouse model

Membrane-associated mucins (MAMs) are proposed to play critical roles at the ocular surface; however, in vivo evidence has been lacking. Here we investigate these roles by phenotyping of a Muc4 KO mouse. Histochemical analysis for expression of the beta-galactosidase transgene replacing Muc4 revealed a spiraling ribbon pattern across the corneal epithelium, consistent with centripetal cell migration from the limbus. Depletion of Muc4 compromised transcellular barrier function, as evidenced by an increase in rose bengal staining. In addition, the corneal surface was less smooth, consistent with disruption of tear film stability. While surface cells presented with well-developed microprojections, an increase in the number of cells with fewer microprojections was observed. Moreover, an increase in skin-type keratin K10 and a decrease in transcription factor Pax6 was observed, suggesting an incipient transdifferentiation. Despite this, no evidence of inflammatory dry eye disease was apparent. In addition, Muc4 had no effect on signaling by toll-like receptor Tlr4, unlike reports for MUC1 and MUC16. Results of this study provide the first in vivo evidence for the role of MAMs in transcellular barrier function, tear film stability, apical epithelial cell architecture, and epithelial mucosal differentiation at the ocular surface.

Epithelial barrier dysfunction in ocular allergy

The epithelial barrier is the first line of defense that forms a protective barrier against pathogens, pollutants, and allergens. Epithelial barrier dysfunction has been recently implicated in the development of allergic diseases such as asthma, atopic dermatitis, food allergy, and rhinitis. However, there is limited knowledge on epithelial barrier dysfunction in ocular allergy (OA). Since the ocular surface is directly exposed to the environment, it is important to understand the role of ocular epithelia and their dysfunction in OA. Impaired epithelial barrier enhances allergen uptake, which lead to activation of immune responses and development of chronic inflammation as seen in allergies. Abnormal expression of tight junction proteins that helps to maintain epithelial integrity has been reported in OA but sufficient data not available in chronic atopic (AKC) and vernal keratoconjunctivitis (VKC), the pathophysiology of which is not just complex, but also the current treatments are not completely effective. This review provides an overview of studies, which indicates the role of barrier dysfunction in OA, and highlights how ocular barrier dysfunction possibly contributes to the disease pathogenesis. The review also explores the potential of ocular epithelial barrier repair strategies as preventive and therapeutic approach.

The Epithelial Cell Glycocalyx in Ocular Surface Infection

The glycocalyx is the main component of the transcellular barrier located at the interface between the ocular surface epithelia and the external environment. This barrier extends up to 500 nm from the plasma membrane and projects into the tear fluid bathing the surface of the eye. Under homeostatic conditions, defense molecules in the glycocalyx, such as transmembrane mucins, resist infection. However, many pathogenic microorganisms have evolved to exploit components of the glycocalyx in order to gain access to epithelial cells and consequently exert deleterious effects. This manuscript reviews the implications of the ocular surface epithelial glycocalyx to bacterial, viral, fungal and parasitic infection. Moreover, it presents some ongoing controversies surrounding the functional relevance of the epithelial glycocalyx to ocular infectious disease.

A MUCINs expression signature impacts overall survival in patients with clear cell renal cell carcinoma

Background

Kidney cancer, especially clear cell renal cell carcinoma (ccRCC), is one of the most common cancers in the urinary system. Previous studies suggested that certain members of MUCINs could serve as independent predictors for the survival of ccRCC patients. None of them, however, is robust enough to predict prognosis accurately.

Objective

To analyze the correlation of MUCINs alterations and their expression levels with the prognosis of ccRCC patients and develop a prognosis‐related predictor.

Methods

We applied whole‐exome sequencing in samples from 22 Chinese ccRCC patients to identify genetic alterations in MUCIN genes and analyzed their genetic alterations, expression, and correlation with survival using the TCGA, GSE73731, and GSE29069 datasets.

Result

Genetic alternations in MUCINs were identified in 91% and 51% of ccRCC patients in our cohort and the TCGA database, respectively. No correlation with survival was found for the genetic alterations. Using unsupervised clustering analysis of gene expression, we identified two major clusters of MUCIN expression patterns. Cluster 1 was characterized by a global overexpression of MUC1, MUC12, MUC13, MUC16, and OVGP1; and cluster 2 was characterized by a global overexpression of MUC4, MUC5B, MUC6, MUC20, EMCN, and MCAM. Patients with cluster 1 expression pattern had significantly shorter overall survival time and worse clinical features, including higher tumor grades and metastasis. Meanwhile, they had a higher level of mutation counts and more infiltrated immune cells, but lower enrichment in angiogenesis signature genes. A five‐MUCINs expression signature was constructed from cluster 1, and notably, it was demonstrated to be associated with shorter overall survival. A similar worse clinical feature, lower angiogenesis but the more immune signature, was identified in samples presented with signature 1. In the validation data set GSE29069, patients with signature 1 were also associated with a trend of poor survival outcomes.

Conclusion

We established a five‐MUCINs expression signature as a new prognostic marker for ccRCC. The distinct tumor microenvironment feature between the two signatures may further affect ccRCC patients’ clinical management.

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.

The ocular surface immune system through the eyes of aging

Since the last century, advances in healthcare, housing, and education have led to an increase in life expectancy. Longevity is accompanied by a higher prevalence of age-related diseases, such as cancer, autoimmunity, diabetes, and infection, and part of this increase in disease incidence relates to the significant changes that aging brings about in the immune system. The eye is not spared by aging either, presenting with age-related disorders of its own, and interestingly, many of these diseases have immune pathophysiology. Being delicate organs that must be exposed to the environment in order to capture light, the eyes are endowed with a mucosal environment that protects them, the so-called ocular surface. As in other mucosal sites, immune responses at the ocular surface need to be swift and potent to eliminate threats but are at the same time tightly controlled to prevent excessive inflammation and bystander damage. This review will detail how aging affects the mucosal immune response of the ocular surface as a whole and how this process relates to the higher incidence of ocular surface disease in the elderly.

MUC1 Mitigates Renal Injury and Inflammation in Endotoxin Induced Acute Kidney Injury by Inhibiting the TLR4-MD2 Axis and Reducing Pro-Inflammatory Macrophages Infiltration

ABSTRACT

Sepsis is the leading cause of acute kidney injury (AKI) in critical care patients. A cornerstone of sepsis-associated AKI is dysregulated inflammation driven by excessive activation of Toll-like receptor 4 (TLR4) pathway. MUC1, a membrane bound mucin expressed in both epithelial tubular cells and renal macrophages, has been shown to be involved in the regulation of TLRs. Therefore we hypothesized that MUC1 could mitigate the renal inflammatory response to TLR4 activation. To test this hypothesis, we used a murine model of endotoxin-induced AKI by intraperitoneal injection of lipopolysaccharide (LPS). We showed that Muc1-/- mice have a more severe renal dysfunction, an increased activation of the tissular NF-kB pathway and secreted more pro inflammatory cytokines compare to Muc1+/+ mice. By flow cytometry, we observed that the proportion of M1 (pro-inflammatory) macrophages in the kidneys of Muc1-/- mice was significantly increased. In human and murine primary macrophages, we showed that MUC1 is only induced in M1 type macrophages and that macrophages derived from Muc1-/- mice secreted more pro-inflammatory cytokines. Eventually, in HEK293 cells, we showed that (i) MUC1 cytosolic domain (CT) seems necessary for the negative regulation of TLR4 (ii) by proximity ligation assay, MUC1-CT is in close relationship with TLR4 and acts as a competitive inhibitor of the recruitment of MYD88. Overall our results support that in the context of endotoxin-induced AKI, MUC1 plays a significant role in controlling disease severity by regulating negatively the TLR4-MD2 axis.

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.

Apolipoprotein E and Periostin Are Potential Biomarkers of Nasal Mucosal Inflammation. A Parallel Approach of In Vitro and In Vivo Secretomes

No previously suggested biomarkers of nasal mucosal inflammation have been practically applied in clinical fields, and nasal epithelium-derived secreted proteins as biomarkers have not specifically been investigated. The goal of this study was to identify secreted proteins that dynamically change during the differentiation from basal cells to fully differentiated cells and examine whether nasal epithelium-derived proteins can be used as biomarkers of nasal mucosal inflammation, such as chronic rhinosinusitis. To achieve this goal, we analyzed two secretomes using the isobaric tag for relative and absolute quantification technique. From in vitro secretomes, we identified the proteins altered in apical secretions of primary human nasal epithelial cells according to the degree of differentiation; from in vivo secretomes, we identified the increased proteins in nasal lavage fluids obtained from patients 2 weeks after endoscopic sinus surgery for chronic sinusitis. We then used a parallel approach to identify specific biomarkers of nasal mucosal inflammation; first, we selected apolipoprotein E as a nasal epithelial cell-derived biomarker through screening proteins that were upregulated in both in vitro and in vivo secretomes, and verified highly secreted apolipoprotein E in nasal lavage fluids of the patients by Western blotting. Next, we selected periostin as an inflammatory mediator-inducible biomarker from in vivo secretomes, the secretion of which was not induced under in vitro culture conditions. We demonstrated that those two nasal epithelium-derived proteins are possible biomarkers of nasal mucosal inflammation.

Microbe-MUC1 Crosstalk in Cancer-Associated Infections

Infection-associated cancers account for ∼20% of all malignancies. Understanding the molecular mechanisms underlying infection-associated malignancies may help in developing diagnostic biomarkers and preventative vaccines against malignancy. During infection, invading microbes interact with host mucins lining the glandular epithelial cells and trigger inflammation. MUC1 is a transmembrane mucin glycoprotein that is present on the surface of almost all epithelial cells, and is known to interact with invading microbes. This interaction can trigger pro- or anti-inflammatory responses depending on the microbe and the cell type. In this review we summarize the mechanisms of microbe and MUC1 interactions, and highlight how MUC1 plays contrasting roles in different cells. We also share perspectives on future research that may support clinical advances in infection-associated cancers.

Cancer-associated mucins: role in immune modulation and metastasis

Mucins (MUC) protect epithelial barriers from environmental insult to maintain homeostasis. However, their aberrant overexpression and glycosylation in various malignancies facilitate oncogenic events from inception to metastasis. Mucin-associated sialyl-Tn (sTn) antigens bind to various receptors present on the dendritic cells (DCs), macrophages, and natural killer (NK) cells, resulting in overall immunosuppression by either receptor masking or inhibition of cytolytic activity. MUC1-mediated interaction of tumor cells with innate immune cells hampers cross-presentation of processed antigens on MHC class I molecules. MUC1 and MUC16 bind siglecs and mask Toll-like receptors (TLRs), respectively, on DCs promoting an immature DC phenotype that in turn reduces T cell effector functions. Mucins, such as MUC1, MUC2, MUC4, and MUC16, interact with or form aggregates with neutrophils, macrophages, and platelets, conferring protection to cancer cells during hematological dissemination and facilitate their spread and colonization to the metastatic sites. On the contrary, poor glycosylation of MUC1 and MUC4 at the tandem repeat region (TR) generates cancer-specific immunodominant epitopes. The presence of MUC16 neo-antigen-specific T cell clones and anti-MUC1 antibodies in cancer patients suggests that mucins can serve as potential targets for developing cancer therapeutics. The present review summarizes the molecular events involved in mucin-mediated immunomodulation, and metastasis, as well as the utility of mucins as targets for cancer immunotherapy and radioimmunotherapy.

Occupational effect of sugarcane biomass burning on the conjunctival mucin profile of harvest workers and residents of an adjacent town - A Brazilian panel study

Pre-harvest burning of sugarcane fields produces large amounts of air pollutants which are known to cause health problems, including ocular surface abnormalities. In this study, we evaluated the effect of biomass burning on mucus quality and mucin gene expression (MUC1, MUC5AC, MUC16) in the conjunctiva of sugarcane workers (SWs) and residents of an adjacent town (RTs). Impression cytology samples of the inferior tarsal and bulbar conjunctiva of 78 SWs and 32 RTs were collected before (T1) and immediately after (T2) a 6-month harvest period. The neutral, acid and total mucus content of goblet cells was determined by PAS and AB staining. The levels of MUC5AC, MUC1 and MUC16 mRNA in the conjunctiva were measured by real-time PCR. Compared to RTs, SWs had higher levels of bulbar acid mucus and MUC16 mRNA and tarsal MUC5AC mRNA at T2 and lower levels of neutral mucus at T1 and T2. In the SW group, MUC1 mRNA levels were higher at T2 than at T1, but the levels of neutral and acid mucus were similar. In the RT group, acid mucus decreased and neutral mucus increased in the bulbar and tarsal conjunctiva at T2. In conclusion, our findings show that sugarcane harvesting is associated with abnormalities in mucus quality and content and changes in mucin mRNA levels on the ocular surface. This may help explain the ocular inflammatory signs and symptoms observed in subjects exposed to air pollutants and high temperatures from sugarcane biomass burning.

Membrane-associated mucins of the ocular surface: New genes, new protein functions and new biological roles in human and mouse

The mucosal glycocalyx of the ocular surface constitutes the point of interaction between the tear film and the apical epithelial cells. Membrane-associated mucins (MAMs) are the defining molecules of the glycocalyx in all mucosal epithelia. Long recognized for their biophysical properties of hydration, lubrication, anti-adhesion and repulsion, MAMs maintain the wet ocular surface, lubricate the blink, stabilize the tear film and create a physical barrier to the outside world. However, it is increasingly appreciated that MAMs also function as cell surface receptors that transduce information from the outside to the inside of the cell. A number of excellent review articles have provided perspective on the field as it has progressed since 1987, when molecular cloning of the first MAM was reported. The current article provides an update for the ocular surface, placing it into the broad context of findings made in other organ systems, and including new genes, new protein functions and new biological roles. We discuss the epithelial tissue-equivalent with mucosal differentiation, the key model system making these advances possible. In addition, we make the first systematic comparison of MAMs in human and mouse, establishing the basis for using knockout mice for investigations with the complexity of an in vivo system. Lastly, we discuss findings from human genetics/genomics, which are providing clues to new MAM roles previously unimagined. Taken together, this information allows us to generate hypotheses for the next stage of investigation to expand our knowledge of MAM function in intracellular signaling and roles unique to the ocular surface.

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.

The Ocular Surface Glycocalyx and its Alteration in Dry Eye Disease: A Review

Many studies have revealed that transmembrane mucins, large glycoproteins with heavily glycosylated glycans, are essential for maintaining ocular surface epithelium lubrication and wettability. Recent reports indicate that transmembrane mucins and galectin-3, a chimera type of galectin that binds β-galactoside in the glycan, play a crucial role in maintaining the epithelial glycocalyx barrier. This review summarizes current evidence regarding the role of galectin-3, the role of the three major transmembrane mucins (i.e., MUC1, MUC4, and MUC16), in the maintenance of ocular surface wettability and transcellular barrier. Pathological mechanisms of glycocalyx barrier disruption and epithelial surface wettability decreases in dry eye disease are also summarized. Lastly, new ophthalmic drugs that target transmembrane mucin are described.

MUC16 suppresses human and murine innate immune responses

OBJECTIVE

MUC16, the mucin that contains the CA125 epitopes, suppresses the cytolytic responses of human NK cells and inhibits the efficacy of therapeutic antibodies. Here, we provide further evidence of the regulatory role of MUC16 on human and murine NK cells and macrophages.

METHODS

Target cell cytolysis and doublet formation assays were performed to assess effects of MUC16 on human NK cells. The effect of MUC16 on ovarian tumor growth was determined in a mouse model by monitoring survival and ascites formation. Innate immune cells from spleens and peritoneal cavities of mice were isolated and stimulated in vitro with anti-CD40 antibody, lipopolysaccharide and IFN-γ and their ability to cytolyse MUC16 expressing and non-expressing cells was determined.

RESULTS

We confirm that MUC16 inhibits cytolysis by human NK cells as well as the formation of NK-tumor conjugates. Mice implanted with MUC16-knockdown OVCAR-3 show >2-fold increase in survival compared to controls. Murine NK cells and macrophages are more efficient at lysing MUC16-knockdown cells. In vitro cytotoxicity assays with NK cells and macrophages isolated from mice stimulated with anti-CD40 antibody showed 2-3-fold increased activity against the MUC16-knockdown cells as compared to matching target cells expressing this mucin. Finally, knockdown of MUC16 increased the susceptibility of cancer cells to ADCC by murine splenocytes.

CONCLUSIONS

For the first time, we demonstrate the immunoregulatory effects of MUC16 on murine NK cells and macrophages. Our study implies that the immunoregulatory role of MUC16 on murine NK cells and macrophages should be considered when examining the biology of MUC16 in mouse models.

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.

Glycosylation pathways of human corneal and conjunctival epithelial cell mucins

Mucin glycoproteins on the ocular surface are rich in O-glycans and have important roles in the protection from physical, chemical and microbial impact. In this work, we have cultured human corneal and conjunctival epithelial cells to examine the glycosyltransferase activities that synthesize the O-glycans of mucins. The results indicate that ocular surface epithelial cells have active enzymes that synthesize O-glycans with sialylated core 1, Galβ1-3GalNAcα, and core 2, GlcNAcβ1-6(Galβ1-3)GalNAcα structures which corresponds to previous structural studies. Eye cells also have enzymes that synthesize complex N-glycans that are found on mucins. Results from treatment of eye cells with TNFα suggest that epithelial O-glycosylation changes in a dynamic fashion during inflammatory stimuli of the eye surface.

Corneal Epithelial Cells Exhibit Myeloid Characteristics and Present Antigen via MHC Class II

Purpose

To explore the impact of ocular surface insults on the immunomodulatory capacity and phenotype of corneal epithelial cells (CECs) with a focus on epithelial-mesenchymal transition (EMT).

Methods

Corneas were harvested from mice 6 days following scratch injury, ragweed pollen-induced allergy, or herpes simplex virus type 1 (HSV-1) infection and compared to healthy tissue controls. Corneas were enzymatically digested and CECs phenotypically characterized using flow cytometry. CECs were defined as epithelial cell adhesion molecule (EpCAM)-positive CD45-negative cells. CECs were assessed by PCR to evaluate EMT-associated transcripts. Recombinant HSV-1 and transgenic mice were utilized to investigate the role of vascular endothelial growth factor A (VEGFA) on the phenotype observed. The immunomodulatory potential of CECs was assessed in coculture assays with ovalbumin-specific CD4 T cells.

Results

Ectopic expression of classic "myeloid" antigens Ly6G, CCR2, and CX3CR1 was identified in CEC subsets from all groups with evidence supporting an underlying partial EMT event resulting from loss of cell-cell contacts. Corneal HSV-1 infection induced Ly6C expression and major histocompatibility complex (MHC)-II upregulation in CECs through a VEGFA-linked mechanism. These Ly6C+ MHC-II+ CECs were found to function as amateur antigen-presenting cells and induced CD4 T cell proliferation in vitro.

Conclusions

This study characterizes a novel immunomodulatory CEC phenotype with possible implications for immune privilege, chronic inflammation, and tissue fibrosis. Moreover, the identification of CECs masquerading with multiple "myeloid" antigens warrants careful evaluation of flow cytometry data involving corneal digests.

C-terminus of MUC16 activates Wnt signaling pathway through its interaction with β-catenin to promote tumorigenesis and metastasis

MUC16/CA125 has been identified as a prominent cancer biomarker, especially for epithelial ovarian cancers, in clinical test for over three decades. Due to its huge mass, limited knowledge of MUC16 was acquired previously. By utilizing a well characterized self-made MUC16 monoclonal antibody, we identified the endogenous interaction between a C-terminal fragment of MUC16 (MUC16C) and β-catenin for the first time, and further elucidated that trans-activation domain of β-catenin is required for this interaction. Such interaction could activate the Wnt/β-catenin signaling pathway by facilitating cytosol-nucleus transportation of β-catenin, consequently induce cell proliferation and the migration, eventually lead to tumorigenesis and metastasis in nude mice. Consistently, knockdown of MUC16 significantly weakened the capabilities of cells for proliferation and migration. Based on our discovery, we suggest that MUC16 appears as an attractive target for the development of effective anticancer drugs.

MUC1: The First Respiratory Mucin with an Anti-Inflammatory Function

MUC1 is a membrane-bound mucin expressed on the apical surfaces of most mucosal epithelial cells. In normal lung epithelia, MUC1 is a binding site for Pseudomonas aeruginosa, an opportunistic human pathogen of great clinical importance. It has now been established that MUC1 also serves an anti-inflammatory role in the airways that is initiated late in the course of a bacterial infection and is mediated through inhibition of Toll-like receptor (TLR) signaling. MUC1 expression was initially shown to interfere with TLR5 signaling in response to P. aeruginosa flagellin, but has since been extended to other TLRs. These new findings point to an immunomodulatory role for MUC1 during P. aeruginosa lung infection, particularly during the resolution phase of inflammation. This review briefly summarizes the recent characterization of MUC1’s anti-inflammatory properties in both the respiratory tract and extrapulmonary tissues.

Pseudomonas aeruginosa increases MUC1 expression in macrophages through the TLR4-p38 pathway

Alveolar macrophages (AMs) play a critical role in the clearance of Pseudomonas aeruginosa (Pa) from the airways. However, hyper-activation of macrophages can impair bacterial clearance and contribute to morbidity and mortality. MUC1 mucin is a membrane-tethered, high molecular mass glycoprotein expressed on the apical surface of mucosal epithelial cells and some hematopoietic cells, including macrophages, where it counter-regulates inflammation. We recently reported that Pa up-regulates the expression of MUC1 in primary human AMs and THP-1 macrophages, and that increased MUC1 expression in these cells prevents hyper-activation of macrophages that appears to be important for host defense against severe pathology of Pa lung infection. The aims of this study were to elucidate the mechanism by which Pa increases MUC1 expression in macrophages. The results showed that: (a) Pa stimulation of THP-1 macrophages increased MUC1 expression both at transcriptional and protein levels in a dose-dependent manner; (b) Both Pa- and LPS-induced MUC1 expression in THP-1 cells were significantly diminished by an inhibitory peptide of TLR4; and (c) LPS-stimulated MUC1 expression was diminished at both the mRNA and protein levels by an inhibitor of the p38 mitogen-activated protein kinase, but not by inhibitors of ERK1/2, JNK, or IKK. We conclude that Pa-stimulated MUC1 expression in THP-1 macrophages is regulated mainly through the TLR4-p38 signaling pathway.

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.

Mucosal immune tolerance at the ocular surface in health and disease

The ocular surface is constantly exposed to environmental irritants, allergens and pathogens, against which it can mount a prompt immune response to preserve its integrity. But to avoid unnecessary inflammation, the ocular surface's mucosal immune system must also discriminate between harmless and potentially dangerous antigens, a seemingly complicated task. Despite its unique features, the ocular surface is a mucosal lining, and as such, it shares some homeostatic and pathophysiological mechanisms with other mucosal surfaces. The purpose of this review is to explore the mucosal homeostatic immune function of the ocular surface in both the healthy and diseased states, with a special focus on mucosal immunology concepts. The information discussed in this review has been retrieved by PubMed searches for literature published from January 1981 to October 2016.

Transmembrane Mucins: Signaling Receptors at the Intersection of Inflammation and Cancer

Mucosal surfaces line our body cavities and provide the interaction surface between commensal and pathogenic microbiota and the host. The barrier function of the mucosal layer is largely maintained by gel-forming mucin proteins that are secreted by goblet cells. In addition, mucosal epithelial cells express cell-bound mucins that have both barrier and signaling functions. The family of transmembrane mucins consists of diverse members that share a few characteristics. The highly glycosylated extracellular mucin domains inhibit invasion by pathogenic bacteria and can form a tight mesh structure that protects cells in harmful conditions. The intracellular tails of transmembrane mucins can be phosphorylated and connect to signaling pathways that regulate inflammation, cell-cell interactions, differentiation, and apoptosis. Transmembrane mucins play important roles in preventing infection at mucosal surfaces, but are also renowned for their contributions to the development, progression, and metastasis of adenocarcinomas. In general, transmembrane mucins seem to have evolved to monitor and repair damaged epithelia, but these functions can be highjacked by cancer cells to yield a survival advantage. This review presents an overview of the current knowledge of the functions of transmembrane mucins in inflammatory processes and carcinogenesis in order to better understand the diverse functions of these multifunctional proteins.

Functional Consequences of Differential O-glycosylation of MUC1, MUC4, and MUC16 (Downstream Effects on Signaling)

Glycosylation is one of the most abundant post-translational modifications that occur within the cell. Under normal physiological conditions, O-linked glycosylation of extracellular proteins is critical for both structure and function. During the progression of cancer, however, the expression of aberrant and truncated glycans is commonly observed. Mucins are high molecular weight glycoproteins that contain numerous sites of O-glycosylation within their extracellular domains. Transmembrane mucins also play a functional role in monitoring the surrounding microenvironment and transducing these signals into the cell. In cancer, these mucins often take on an oncogenic role and promote a number of pro-tumorigenic effects, including pro-survival, migratory, and invasive behaviors. Within this review, we highlight both the processes involved in the expression of aberrant glycan structures on mucins, as well as the potential downstream impacts on cellular signaling.

Epidemic Keratoconjunctivitis-Causing Adenoviruses Induce MUC16 Ectodomain Release To Infect Ocular Surface Epithelial Cells

Human adenoviruses (HAdVs) are double-stranded DNA viruses that cause infections across all mucosal tissues in the body. At the ocular surface, HAdVs cause keratoconjunctivitis (E. Ford, K. E. Nelson, and D. Warren, Epidemiol Rev 9:244–261, 1987, and C. M. Robinson, D. Seto, M. S. Jones, D. W. Dyer, and J. Chodosh, Infect Genet Evol 11:1208–1217, 2011, doi:10.1016/j.meegid.2011.04.031)—a highly contagious infection that accounts for nearly 60% of conjunctivitis cases in the United States (R. P. Sambursky, N. Fram, and E. J. Cohen, Optometry 78:236–239, 2007, doi:10.1016/j.optm.2006.11.012, and A. M. Pihos, J Optom 6:69–74, 2013, doi:10.1016/j.optom.2012.08.003). The infection begins with HAdV entry within ocular surface epithelial cells; however, the mechanisms used by HAdVs to transit the otherwise protective mucosal barrier of ocular surface epithelial cells prior to entry remain unknown. Here, we report that the highly virulent keratoconjunctivitis-causing HAdV-D37 induces release of the extracellular domain (ectodomain) of MUC16, a major component of the mucosal barrier of ocular surface epithelial cells, prior to infecting underlying cells. Currently, there is no specific treatment for controlling this infection. Understanding the early steps involved in the pathogenesis of keratoconjunctivitis and using this information to intercept adenoviral entry within cells may guide the development of novel strategies for controlling the infection.

Human adenoviruses (HAdV), species D in particular (HAdV-D), are frequently associated with epidemic keratoconjunctivitis (EKC). Although the infection originates at the ocular surface epithelium, the mechanisms by which HAdV-Ds bypass the membrane-associated mucin (MAM)-rich glycocalyx of the ocular surface epithelium to trigger infection and inflammation remain unknown. Here, we report that an EKC-causing adenovirus (HAdV-D37), but not a non-EKC-causing one (HAdV-D19p), induces ectodomain release of MUC16—a MAM with barrier functions at the ocular surface—from cultured human corneal and conjunctival epithelial cells. HAdV-D37, but not HAdV-D19p, is also found to decrease the glycocalyx barrier function of corneal epithelial cells, as determined by rose bengal dye penetrance assays. Furthermore, results from quantitative PCR (qPCR) amplification of viral genomic DNA using primers specific to a conserved region of the E1B gene show that, in comparison to infection by HAdV-D19p, infection by HAdV-D37 is significantly increased in corneal epithelial cells. Collectively, these results point to a MUC16 ectodomain release-dependent mechanism utilized by the EKC-causing HAdV-D37 to initiate infection at the ocular surface. These findings are important in terms of understanding the pathogenesis of adenoviral keratoconjunctivitis. Similar MAM ectodomain release mechanisms may be prevalent across other mucosal epithelia in the body (e.g., the airway epithelium) that are prone to adenoviral infection.

IMPORTANCE Human adenoviruses (HAdVs) are double-stranded DNA viruses that cause infections across all mucosal tissues in the body. At the ocular surface, HAdVs cause keratoconjunctivitis (E. Ford, K. E. Nelson, and D. Warren, Epidemiol Rev 9:244–261, 1987, and C. M. Robinson, D. Seto, M. S. Jones, D. W. Dyer, and J. Chodosh, Infect Genet Evol 11:1208–1217, 2011, doi:10.1016/j.meegid.2011.04.031)—a highly contagious infection that accounts for nearly 60% of conjunctivitis cases in the United States (R. P. Sambursky, N. Fram, and E. J. Cohen, Optometry 78:236–239, 2007, doi:10.1016/j.optm.2006.11.012, and A. M. Pihos, J Optom 6:69–74, 2013, doi:10.1016/j.optom.2012.08.003). The infection begins with HAdV entry within ocular surface epithelial cells; however, the mechanisms used by HAdVs to transit the otherwise protective mucosal barrier of ocular surface epithelial cells prior to entry remain unknown. Here, we report that the highly virulent keratoconjunctivitis-causing HAdV-D37 induces release of the extracellular domain (ectodomain) of MUC16, a major component of the mucosal barrier of ocular surface epithelial cells, prior to infecting underlying cells. Currently, there is no specific treatment for controlling this infection. Understanding the early steps involved in the pathogenesis of keratoconjunctivitis and using this information to intercept adenoviral entry within cells may guide the development of novel strategies for controlling the infection.

Pseudomonas aeruginosa stimulates tyrosine phosphorylation of and TLR5 association with the MUC1 cytoplasmic tail through EGFR activation

BACKGROUND

MUC1 is a membrane-tethered mucin expressed on the surface of epithelial and hematopoietic cells. Previous studies have established that MUC1 attenuates airway inflammation in response to Pseudomonas aeruginosa (Pa) through suppression of Toll-like receptor (TLR) signaling. Here, we elucidate the mechanism through which the MUC1 cytoplasmic tail (CT) inhibits TLR5 signaling in response to Pa and its flagellin in primary normal human bronchial epithelial (NHBE) cells.

METHODS

NHBE and human and mouse macrophages were stimulated with Pa or flagellin and transforming growth factor-α (TGF-α) and tumor necrosis factor-α (TNF-α) levels in cell culture supernatants were measured by ELISA. NHBE cells were stimulated with Pa, flagellin, or TNF-α and MUC1-CT, and epidermal growth factor receptor (EGFR) levels were measured by immunoblotting. NHBE cells were stimulated with Pa and MUC1-CT/TLR5 and MUC1-CT/EGFR association were detected by co-immunoprecipitation.

RESULTS

Stimulation of NHBE cells with Pa and flagellin each increased release of the EGFR ligand, TGF-α, from NHBE cells. Both stimuli also activated EGFR tyrosine phosphorylation in these same cells. By contrast, stimulation of NHBE cells with Pa failed to induce TNF-α release, whereas stimulation of human or mouse macrophages with Pa promoted TNF-α release. Stimulation of NHBE cells with recombinant TNF-α increased both MUC1 and EGFR protein levels, and stimulation of these cells with Pa enhanced MUC1-CT tyrosine phosphorylation and increased MUC1-CT/TLR5 and MUC1-CT/EGFR protein association, in an EGFR-dependent manner.

CONCLUSIONS

These results indicate that in response to Pa or flagellin, EGFR associates with and tyrosine phosphorylates MUC1-CT in primary NHBE cells, leading to increased MUC1-CT association with TLR5. Based on prior studies in tumor cells, increased MUC1-CT/TLR5 association in NHBE cells is predicted to competitively inhibit Pa/flagellin-stimulated TLR5 activation, reduce TLR5-dependent cell signaling, and down-regulate airway inflammation. Given that MUC1 is a universal suppressor of TLR signaling, the results from this study suggest that abnormal interactions between MUC1 and EGFR or TLRs may lead to the development of chronic inflammatory diseases. Thus, this is an important finding from the clinical point of view.