Ординатура / Офтальмология / Английские материалы / Ocular Periphery and Disorders_Dartt, Bex, Amore_2011

Ocular Mucins

M Berry, Bristol Eye Hospital, Bristol, UK

ã 2010 Elsevier Ltd. All rights reserved.

Glossary

Alternative splicing – A variation mechanism in which linear combinations of exons are translated, resulting in a variety of mature products encoded by a single gene.

Atomic force microscopy (AFM) – A technique of high-resolution imaging through the measurement of forces between atoms in the sample and those on the instrument tip.

Glycan – The oligosaccharide portion of a glycoconjugate.

Glycocalyx – An outer, carbohydrate-rich coating on the surface of cells.

Glycoforms – Variations in the amount or compositions of oligosaccharides decorating the same peptide core.

Meibomian glands – The special sebaceous glands at the rim of the eyelids that supply the lipid layer of the tear film.

Mucins – A family of large, heavily glycosylated molecules, with most glycan chains O-linked to the peptide core.

Persistence length – A measure of polymer stiffness; it is the length over which correlations in the direction of the tangent are lost.

Reptation – Movement of a long polymer parallel to itself, similar to the movement of a snake.

Tandem repeats – The adjacent repetition of a pattern of two or more nucleotides.

Worm-like model – A model for the behavior of semiflexible polymers, considered continuously flexible.

Young’s modulus – A measure of elasticity, defined as the ratio of stress to strain.

Introduction

In addition to the well-defined anatomical blind sac formed by the cornea and conjunctiva, the meibomian and lacrimal glands, the ocular surface comprises a mucosal immune system, rich neural and endocrine loops, as well as the blink reflex. As with other mucosal systems the ocular surface is further integrated into the adaptive immunity of the organism, and into the microbial richness

of the outer environment. An additional and specific requirement of the ocular surface is the maintenance of transparency that applies to the cornea as well as to the preocular fluid.

Bathing the exposed part of the outer eye is a complex fluid whose elements are secreted by the wet epithelia, and the lacrimal and meibomian glands. Mucins are the main component of a mucus gel and responsible for its viscoelastic properties. They form a dynamic matrix wetted by a plasma dialysate enriched by secretions from the lacrimal glands and topped by a layer of waxes and lipids originating in the meibomian glands. This fluid is periodically sheared and mixed by the movement of the lids during blinking. Underneath the mucous gel, the epithelial glycocalyx anchors the tear film to the ocular surface: mucins and glycoproteins are the major components of this layer.

Mucin Architecture

A very rich glycosylation, with most sugar chains O-linked through N-acetylgalactosamine (GalNAc) to serine or threonine in the peptide core, is diagnostic of mucins. Sugar chains tend to be clustered in discrete regions resulting in concentrations of negative charges. In these high-charge regions the peptide core is rich in serine, threonine, and proline (PTS domains), and repeated sequences of aminoacids (variable number of tandem repeats, hence VNTR domains) are present, specific to the encoding gene. Other regions are less richly glycosylated and contain relatively more N-linked glycan chains, the latter necessary for mucin transit through intracellular microtubules during synthesis. In humans, ocular mucin regions of tens of nanometers seem to be almost naked: the molecular diameter measured in liquid with atomic force microscopy (AFM) is not significantly higher than that of aminoacids in a helix (Figure 1(a)), giving mucin polymers the appearance of strings of small beads, small beads of dense bottlebrushes.

Toward the N- and C-termini of mucins, the arrangement of moduli is similar to those found in proteins involved in coagulation (von Willebrand factor, VWF domains), cysteine knots, or SEA (sea urchin sperm protein, enterokinase, and agrin) domains. These are useful in tracing the evolution of mucin genes. Genes encoding for PTS domains and multiple VWF domains (D1–D2–D3 PTS, as in secreted mucins) can be found early in the evolution of metazoa, preceding hemostasis or coagulation

(Figure 1(b)). SEA domain mucins appear in vertebrates, while the auto-catalytically cleaved SEA domain is restricted to mammals (Figure 1(c)).

Regions where the peptide core is extended by the insertion of the first sugar (GalNAc) and repulsion between negatively charged sugar chains alternate with more flexible polypeptide chain stretches, suggesting that mucins behave in solution like stiff random coils. Mucins occupy a large volume which indicates interpenetration of molecular domains at relatively low polymer concentrations. Using the worm-like model, calculations indicate that human ocular mucins are more flexible than DNA molecules of similar length; mucin persistence length is 35 nm and that of DNA is 50 nm. Polymer conformation and stiffness, for example, of human ocular MUC5AC glycoforms, are greatly influenced by the degree and nature of post-translational glycosylation.

Mucin architecture is determinant of mucin role and function at the mucosal surface and in the gel. The organ, developmental state, and physiological status, in turn, affect mucin expression and details of glycosylation. Ocular mucins have short oligosaccharide chains: in humans they are mostly less than six sugars long, negatively charged and terminated in sialic acid, with fucosylation representing less than one-fifth of the sugars (Figures 2 and 3). In dogs and rabbits, glycans are mainly neutral and terminated in fucose and/or GalNAc. The short oligosaccharides might be related to transparency and (relatively fast) turnover of mucins, and terminal sugars to environmental microbiota.

Mucin Families

Mucin genes appeared through a combination of moduli existing in other proteins. The close connection between

mucin structure and function gives rise to a classification necessarily reflecting both.

Surface-associated mucins

Some mucins spend part of their life anchored into the apical cell membrane before they are shed into the luminal, that is, tear, fluid. Formerly known as membranebound mucins, they are now called cell-surface-associated mucins. These are heterodimers, with a large mucin subunit outside the cell, a (mostly hydrophobic) membranespanning region and an intracellular tail. A number of subfamilies are represented at the ocular surface: the mammalian-specific MUC1 with its SEA domain; the MUC16 that contains multiple SEA domains, not all of which are cleaved; and the MUC4 that has VWD but neither cysteine-rich domains nor SEA. Shedding of these mucins is thought to cause changes in the neighboring membrane domains, potentially transferring information to the cell interior. A further possibility is that information is conveyed through the cytoplasmic tail to the cytoskeleton. An important result of surface mucin release in the tear fluid is the renewal of the glycocalyx and the tear fluid itself.

This group of mucins is heterogeneous and most genes also encode splice variants that are secreted: MUC4, a cell-surface mucin in normal cornea and conjunctiva, is a goblet cell mucin in some pterygia (Figure 4). MUC1/SEC, a splice variant of MUC1 that lacks the transmembrane domain and, therefore, results in a soluble, secreted form of MUC1, is present in human cornea and conjunctiva.

Secreted mucins

Milliseconds after the secretion of mucins stored in granules, often in specialized epithelial cells, their volume

2AB

Disialyl core 1

2AB

1 core -6sialyl 2 Mono

2AB

Mono 2-3 sialyl core 1

increases 100-fold in the extracellular fluid as a result of hydration. These secreted mucins have a well-defined VWF-D2-D3 PTS architecture, and Cys-rich domains or Cys knots. VWF domains and Cys are involved in the concatenation of mucin subunits (Figure 5). The secreted mucins – MUC2, MUC5AC, MUC5B, MUC7, MUC19, and MUC23 – have been found at the ocular surface, in cells, impressions, and tears. MUC20 messenger RNA (mRNA) has also been identified in the conjunctiva. With the notable exception of MUC7, secreted mucins form linear polymers of subunits linked by disulfide bonds and form gels, and are thus called secreted gel-forming mucins. Polymers extracted from cells (and protected from proteolysis) may be few microns long; in the secretion, submicron lengths dominate. MUC7 oligomers are like spokes of a wheel around a central, yet to be fully described, entity. Though MUC7 is not gel forming, it is found in gels, for example, saliva, from which it has been originally described.

Biosynthesis and Turnover

Synthetic pathways

Mucin genes account for nearly 4% of genes expressed in the normal conjunctiva, with a further 29% dedicated to glycosyltransferases. During synthesis, molecular species with varying properties are transported to the location corresponding to their stage in this complex process. MUC5AC might take over 2 h from initiation to storage

Figure 4 MUC4 alternative splicing in pterygium. In normal conjunctival tissue MUC4 is mainly a surface-associated mucin. In pterygia, triangular growths of the conjunctiva over the cornea, it can be present either as surface associated (red arrows) or secreted (yellow arrows) within prominent goblet cells. Where pterygial MUC4 is surface associated, goblet cells do not light up with anti-MUC4 antibodies (green arrows). MUC4 was visualized with antibody 4F12 (DSHB, Yowa, USA); counterstain: hemalum. Images: courtesy of Friedrich Paulsen, Halle University, Germany.

in the secretory granule. The making of secreted mucins starts with the synthesis of the peptide core in the endoplasmic reticulum, where N-linked sugars are also added. Following folding of the C- and N-termini, the peptides dimerize through S–S bonds between cysteine knot

Mucin domains

Figure 5 Schematic of secreted mucin structure. Secreted mucins have a well-conserved architecture. Within mucin domains, repeated sequence(s) called tandem repeats are gene specific. The concatenation of mucin subunits gives rise to a polymer with concentrated regions of negative charge, interspersed with less charged, less glycosylated regions. vWF, Von Willebrand factor domains; Cys, cysteine.

domains at the C-termini. O-glycosylation is initiated and elaborated in the cis- and medial Golgi. Further polymerization, S–S bonds between VWD3 domains at the N-terminal, occurs in the trans-Golgi and the secretory granule. Cell-surface mucins too acquire their N-linked sugars in the endoplasmic reticulum, where the peptide core is cleaved into the two domains, and further glycosylated as these mucins progress through the Golgi. A signal peptide in the N domain localizes cell-surface- associated mucins at the apical membrane; the cytoplasmic domain starts with a signal sequence involved in retention at the plasma membrane and mucin recycling.

Glycosylation

Glycosyltransferases

Mucin glycans are synthesized in the Golgi apparatus. The presence, activity, and localization of glycosyltransferases along the Golgi cisterns are the primary determinants of glycan chain density and sequence. The initial transfer of GalNAc from UDP-GalNAc to the hydroxyl group of Ser/Thr in the peptide backbone is catalyzed by uridine diphosphate GalNAc:polypeptide N-acetylgalactosaminyl transferases (ppGalNAcTs). Subsequent sequential sugar additions are catalyzed by glycosyltransferases (GalNAcTs). Nucleotide-sugar synthases and hydrolases, and genes encoding their transport proteins add further dimensions to the function and regulation of mucin molecules.

In other mucosae, for example, respiratory epithelium, epidermal growth factor (EGF), Th2 cytokines, and alltrans retinoic acid alter the expression of GalNAc-Ts, some of which are also downregulated in cancer. Molecular details of transcriptional regulation of glycosylating enzymes are not known for the eye. The final density of glycosylation and chain structure strongly depends on the availability of sugar nucleotides, and competition between enzymes for acceptor intermediates during

glycan elongation, incorporating a measure of randomness in the glycan population.

At the human ocular surface, ppGalNAcTs are distributed in an epithelial layer and in a cell-type-specific manner. Isoform –T3 was detected in all the epithelial layers, while –T2 in restricted to basal cells; –T4 is expressed in all apical cells. One member of the family, ppGalNAcT-6, was found in goblet cells only in normal subjects, but in apical cells of conjunctival epithelia in patients with ocular cicatricial pemphigoid, together with the –T2 isoform.

How elongation of oligosaccharide chains is regulated is not yet understood: in humans, rabbit, and dog, ocular mucins have short sugar chains. The same mucin gene product in the respiratory or gastrointestinal mucosa is decorated by chains many tens of sugars long.

The ensemble of human ocular surface enzymes is such that mucin oligosaccharides are negatively charged and mostly terminated in sialylated structures, including the histo-blood Lewis group antigens (sialyl Lex and sialyl Lea). The number of different structures is low, barely in double figures, compared with many hundreds in respiratory epithelia from one subject, for example. Histo-blood group antigens are involved in neutrophil adherence and activation. Because of the short chains, sugar epitopes that are cryptic in other mucosae are overt in human ocular mucins, for example, Tn or SialylTn that are richly present in the normal conjunctival epithelium. Which sugars or end groups are exposed in the glycan envelope of the tissue remains to be clarified for the ocular surface. Variation in this envelope is likely to modulate both immune effector cells and bacterial adhesion to the conjunctiva or cornea.

Turnover

Quantities, species, and glycoform composition of mucin populations on the ocular surface are a dynamic result of mucin production, secretion, and degradation. Not all the factors involved in this process are known: immune and infective agents modulate synthesis and secretion, as do neural stimuli; enzymes in the tears contribute to mucin degradation, while tearing and blinking are believed to remove spent mucus from the surface.

There are no data on average half-life of mucins in the preocular fluid. Extraction of large and glycosylated mucin polymers from contact lenses suggests that some escape degradation during waking hours.

Recycling

There is evidence that surface-associated mucins are reuptaken into the Golgi and re-glycosylated. In the cell lines where these experiments have been done, the final glycosylation of the mucin is different from its original, and, at least for MUC1, the uptake depends on mucin

palmitoylation. It is not known whether this also occurs at the surface of the eye.

Degradation

Bacteria are an essential player in mucin degradation: glycosidases and proteolytic enzymes contribute to mucus gel breakdown and renewal; adherent bacteria are probably wrapped in mucin epitopes (sacrificial epitopes) and removed from the surface. During sleep, neutrophil enzymes degrade the gel and cause or enhance the cleavage of surface-associated mucins, promoting recoating of epithelial surfaces with a mucus gel.

Control of Secretion

Mucins are a defensive secretion, augmented in response to pathogens and stimuli, including mechanical, thermal, and chemical insults (through nocioceptors in cornea and conjunctiva). Though the signaling pathway triggered by the external stimulus might be distinct, the involvement of protein tyrosine kinases, mitogen-activated protein kinases (MAPKs), and transcription factor NF-kappa B are relatively common.

In the conjunctiva, the eicosanoid 15-(S)-hydroxy- 5,8,11,13-eicosatetraenoic acid (15(S)-HETE) stimulates MUC1, but not MUC2, MUC4 release, suggesting mucinspecific control of surface-associated mucin liberation in the tears. Further evidence for gene-product-specific modulation is derived from studies of a human corneal limbal epithelial cell line where matrix metalloproteinase-7 and neutrophil elastase induced the release of MUC16, but not of MUC1 or MUC4.

Nucleotide agonists acting locally through P2Y2 purinoceptors (belonging to the major G-protein-coupled receptor (GPCR) family) on apical membranes of goblet cells provide a major regulatory system for mucin secretion. Cholinergic agonists are potent stimuli of mucin secretion. One of the pathways to secretion is through EGF receptor (EGFR) induction of MAPK. A fast Ca2+ sensor for the soluble NSF attachment protein receptor (SNARE) complex (the core machinery of membrane fusion) is essential for regulated secretion.

Individual Variation

It is not known whether the number of goblet cells varies in individuals. Their number, as assayed by impression cytology, represents those that are discharging in response to external stimuli, and depends on external factors, for example, wind, humidity, and spectacle wear. Goblet cell numbers decrease in severe, but not mild, dry eye disease.

Allelic variation, for example, in the number of tandem repeats, is but one source of individual variation. Additionally, variation in glycosyltransferases and in the availability of donor sugars in different physiological states gives rise to potentially large individual variation. However, the

size-to-charge ratio of mucin populations is surprisingly well conserved: migration patterns of mucins from conjunctivas of men and women of different ages are similar, suggesting a tight control on the molecular size/charge ratio of intracellular mucins. Conservation of population parameters is also encountered in the largest mucins extracted from tears or contact lenses as shown in Figures 6 and 7. Ratios of the different mucin species sampled from the ocular surface showed little variation in asymptomatic contact lens wearers. The variation must therefore be more subtle and is yet to be understood.

Mucin Function

Mucins are essential for the defense of the organ from external factors. Mucins are: (1) lubricants easing the movement of the lids over the surface of the eye; (2) a physical barrier; (3) a trap for microbes; and (4) a matrix where tear constituents among which enzymes, antimicrobial peptides, and other signal molecules fulfill their physiological role. The surface mucins that are richly represented on cornea and conjunctiva are expected to fulfill signaling functions, which are yet to be clearly understood.

Adequate production of mucins is controlled by neural, hormonal, and other signaling networks. In addition,

250 KDa

MUC5AC in individual tear samples

Figure 6 Electrophoresis of MUC5AC from individual tear samples. Very little variation was seen between the mobilities of the largest MUC5AC polymers in tears from nine individuals. Samples (not equalized for protein content) were run on 1% agarose electrophoresis, vacuum blotted, and visualized after incubation with antibody CLH2 to MUC5AC peptide core. The graphic underneath shows details of mobility for the color-coded respective lane.

a massive discharge of mucins occurs as a response to environmental stimuli, from mechanical stimulation to bacterial lipopolysaccharide.

Gel Formation

Mucins adhere to surfaces through either sugars or peptide sequences. In a physiological buffer, sequential adhesions of a mucin lowered onto mica were most often spaced similar to the atoms on this surface, indicating that no region of the molecule is barred from adhering to the substrate. The early events in adhesion are governed by random diffusion. Later (i.e., after the first polymers have adhered), cooperative sequential adsorption occurs, creating regions of high mucin density.

In dilute solutions, mucins reptate, that is, the polymer slides parallel to itself. A long mucin can thus extricate itself from entanglements, like a strand of spaghetti from a tangle.

Mucin polymers associate through long-range hydrogen bonds; they interact with like-charged moieties (e.g., sugars on another polymer) through bridging divalent cations, and form disulfide bonds between unpaired cysteins. Lectinlike site adhesions and hydrophobic domain interactions are also expected to contribute to gel formation, additional to the entanglement of these flexible polymers (Figure 8). The ocular surface is covered by a stable and dynamic mucous gel. In vitro, the elastic properties of the gel are conserved in time and recover after addition of purified ocular mucin – paralleling the discharge of granules from a goblet cell in the conjunctiva (Figure 9). The dynamic nature of the gel can be gleaned in changes in the roughness of its surface, and the temporary oscillations in elastic qualities after intervention. Calcium chelation weakens

Figure 8 Mucin adsorption and intermolecular bonds. Schematic of mucin adsorption and association (animation). Sugar or peptide-core moieties can adhere to a surface; intermolecular long-range bonds are established between charged epitopes, bridged by divalent cations in solutions, and through unpaired Cys moieties.

human preocular mucus gels: subsequent addition of Ca restores their initial elastic qualities. The gel collapses on mucin depolymerization. Shorter mucins (i.e., 500 nm long) are more mobile in a model gel than the longest polymers ( 1 mm long). It must be emphasized that the qualities of a gel depend not only on the mucins, but on all its constituents. Intracellular autocatalytic cleavage in the C-termini of MUC5AC is expected to generate reactive termini that form cross-links and enhance gel formation. The pattern of reactivities with antibodies to different parts of MUC5AC suggests that these cleavages occur in normal mucins, but not in those from dry eye patients.

Significant for mucin function is the fact that small entities (compared to pore size) gain more local mobility in a mucin gel than in viscous polymer solution, as

0 40 nm

(b)Mica

Figure 9 Purified mucin gels in vitro. (a) AFM topographical images during formation of mucin gels by repeated deposition of purified mucins on mica, in HEPES buffer (animation).

(b) Characteristics of ocular mucin gels in vitro. Correlates of Young’s modulus (elastic modulus, E*) of the gels were calculated from indenting the gel surface with an AFM tip. These elastic characteristics of the gel were calculated at different depths (of the order of nanometres) within the gel, and differences reflect the effect of the stiff substrate. Elasticity being conserved, as illustrated for three cycles of indentation separated by more than 1 h, indicates that the gel is a stable structure. Throughout this period the gel was kept hydrated in HEPES buffer.

relatively large, typically 200–400-nm, fluid-filled pores open up. Thus, the transport of biologically significant molecules is enhanced in the preocular gel, while micronsized pathogens are still prevented from penetrating.

Epithelium

Figure 10 Schematic of the tear film anchoring to the ocular surface. Collage of confocal image of conjunctival epithelium and AFM images of cell surface associated (gray) and secreted mucins (blue). Glycocalyx is schematically shown in red.

Anchoring to the Ocular Surface

Surface tension is responsible for maintaining the tear film onto the ocular surface. Apical epithelial cells of the cornea and conjunctiva express a rich glycocalyx that interfaces with the secreted phase of the fluid. Entanglement, hydrogen bonding, sugar–sugar interactions, and lectin–sugar interactions are all involved in the interpenetration of the sessile and the stirred layer of the preocular fluid (Figure 10). Surface-associated mucins are longer than most other glycocalyx components and are expected to penetrate and entangle with elements of the secreted fluid.

Physical and Chemical Barriers

Particles (e.g., from mascara or cigarette smoke) can be seen trapped in the tear film for a period of time, after which they are eliminated from the ocular surface. This mechanism might involve wrapping in mucin or mucin aggregates, and it is also believed to be part of mucin turnover eliminating spent mucins. Sacrificial glycosylated epitopes, to which bacteria or viruses adhere before being eliminated, or which accept reactive molecular species, are also involved in the protective function of mucins.

Mucins are resistant to proteolysis because the peptide core is shielded by sugars. Degradation occurs at specific sites giving rise to small mucin polymers that are very mobile in the mucin gel. The presence of cleavage sites outside the cell membrane to which cell-surface mucins are anchored underlies the turnover of the glycocalyx and tear film itself, for example, through the action of neutrophil enzymes. The same mechanism has a protective

value, when, in response to bacterial enzymes, the cleavage of mucins and portions of gel removes invading organisms from the surface of the eye.

Lubrication

Water lubricates sliding soft surfaces densely covered with bottlebrushes of oligosaccharide chains. Mucin polymers incorporate large quantities of aqueous tears that bridge between negatively charged oligosaccharides. This cushioning layer sustains the pressure of the moving lids, thus decreasing friction. Furthermore, tears lower their viscosity with shear, further easing the blink. In vitro, in a physiological buffer, human ocular mucin macromolecular aggregates slide past each other without forming adhesions.

Tear Breakup

Evaporation is the organizing event of tear breakup. The evaporation of water from the liquid trapped in the pores of the gel may cause an increase in salts, and importantly in divalent cations, which causes a local salting-out of mucins and other polyelectrolytes, in turn causing a local collapse of the scaffold, and thus a dry spot on the ocular surface.

Antimicrobial Activity

The ocular surface is not sterile; it hosts commensal bacteria that are part of the normal ocular surface physiology. Mucins are considered to have co-evolved with bacteria. Their mutual relationships result in continued and effective protection of the underlying tissues. Signaling between microflora and host mucosa impacts on local metabolism and immune function in ways which are a current focus of research.

Carbohydrates, including mucin glycans, can be used as an energy source for bacteria. At the ocular surface, mucin oligosaccharides are protected from bacterial degradation by the acetylation of their terminal sugars: most sialic acids are mono-O-acetylated, and a minority diacetylated. Acetylation prevents or restricts bacterial glycolytic enzymes. A proportion of mucin terminal sugars is sulfated, which also protects from cleavage of the glycan by bacterial enzymes.

For many bacteria, mucins are antiadhesive: denuding a cornea of mucins resulted in a large increase in the number of organisms attached to its surface. This effect can be achieved by mucin glycans interacting with bacterial adhesins and thus blocking them, and because bacteria coated in mucins do not bind to the mucosal surface. Mucins glide over those without any adhesions. In tears, Pseudomonas aeruginosa bind to specific oligosaccharide receptors containing sialic acids, and a substantial

proportion of glycans are potential receptors. Mucin glycans, however, contain 10 times fewer sialic acids in the a2–6 linkage – the receptor for this bacterial species– than a2–3-linked sialic acids that are not adhesive (Figure 3). This example illustrates the two-pronged defensive mechanism of binding and removing in the bulk of the preocular fluid by dissolved moieties, and increasing antiadhesion closer to the epithelial surface.

To degrade complex substrates such as mucins, bacteria need an impressive array of glycosidases and peptidases acting in sequence. These are rarely in the arsenal of a single species, though they can be – and are – expressed by a bacterial community. Commensal bacteria degrade mucins: they cleave sugar chains and peptide backbones, changing the physicochemical characteristic of the mucin molecule. It is thought that in this way the commensal flora contributes to the renewal of the mucus component of the tear film.

Immune Protection

In large epithelial tracts, a link between mucins innate mucosal immunity and mucosal inflammatory responses is provided by modulation of mucin expression by inflammatory cytokines such as interleukins (IL)-1b, IL-4, IL-6, IL-9, IL-13, interferons, tumor necrosis fac- tor-a, or nitric oxide. Neutrophils – the main patrolling cells of the closed eyes – can also stimulate increases in production of both gel-forming and cell-surface mucins through neutrophil elastase. In vitro, their degranulation and activation were shown to be different on normal and dry eye mucins.

Clinical Relevance and Pathology

Contact Lens Wear

If all species present in the preocular fluid also adhere to the contact lens without prejudice to its optical properties and gas permeability, a mucin coating should add cushioning, and provide added antimicrobial protection to the ocular surface.

Among mucins adhering to contact lenses there are very large and also (relatively) short mucin polymers, with both secreted and surface-associated mucins well represented. Mucins adherent to contact lenses might act as acceptors for reactive groups produced at the ocular surface. In these mucins, there exist subunits with much higher negative charge than observed in intracellular mucins. Naive wearers deposit more mucins on their lenses than experienced wearers, probably prior to habituation to the stimulus provided by the lens. Contact-lens- induced dry eye does not appear to alter the proportion of mucin species at the ocular surface.

Dry Eye Syndromes

Involvement of mucins in dry eye syndromes is suggested by a number of observations: (1) goblet cell numbers decrease in severe disease; (2) the quality of mucus can be manifestly affected with adherent plaques forming on the ocular surface; (3) ocular surface epithelia are (and feel) dry and even keratinized. The increase in squamous epithelial proliferation and relative (or absolute) decrease in goblet cells in dry eye syndromes highlight the different functions of surface-associated and secreted mucins in the preocular fluid.

Mucin genes are not altered in the conjunctiva of patients with moderate nonimmune (non-Sjo¨gren) dry eye compared to normal conjunctiva. However, the expression of fucosyland sialyl-transferases is decreased, consistent with the decrease in sialylation observed in mucins of dry eye patients. Differences between neutrophil adherence and activation (dependent at least in part on fucosylated epitopes) on fields of normal mucins and mucins in Sjo¨gren patients suggest that these differences become more severe with the severity of disease.

See also: Adaptive Immune System and the Eye: Mucosal Immunity; Conjunctival Goblet Cells; Defense Mechanisms of Tears and Ocular Surface; Imaging of the Cornea; Immunopathogenesis of Pseudomonas Keratitis; Innate Immune System and the Eye; Overview of Electrolyte and Fluid Transport Across the Conjunctiva.

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Further Reading

Argueso, P. (2003). The cell-layer- and cell-type-specific distribution of GalNAc-transferases in the ocular surface epithelia is altered during keratinization. Investigative Ophthalmology and Visual Science 44:

86–92.

Basbaum, C. (1999). Control of mucin transcription by diverse injuryinduced signaling pathways. American Journal of Respiratory and Critical Care Medicine 160: S44–S48.

Relevant Websites

http://www.functionalglycomics.org – Consortium for functional glycomics.

http://www.hugo-international.org – HUGO Gene Nomenclature Committee.

http://www.library.nhs.uk – National Library for Health Specialist Libraries.

http://www.genenames.org/genefamily/muc.php – The HGNC database in 2008: a resource for the human genome.