SDS-gradient polyacrylamide gel electrophoresis of individual ocular mucus samples from patients with normal and diseased conjunctiva

Transmembrane Mucin 1 Blocks Fluorescein Ingress to Corneal Epithelium

Purpose

To determine the role of transmembrane mucins in blocking fluorescein ingress to the corneal epithelium and its deficiency in contributing to corneal fluorescein punctate staining.

Methods

A dry eye model was established by extirpating lacrimal and Harderian glands in rabbits to correlate the expression of mucins with fluorescein-stained areas on the corneal button using immunofluorescence. Expression of transmembrane mucins was promoted in human corneal epithelial cells (HCECs) by culturing with the mucin-promoting medium (MPM) or diquafosol treatment. Conversely, the expression of mucins was downregulated by knockdown with short hairpin RNA. The role of mucin1 extracellular domain in fluorescein ingress was further verified by overexpression of N-terminally truncated mucin1 in HCECs.

Results

In the rabbit dry eye model, the expression level of mucin1 was significantly decreased in superficial corneal epithelial cells where fluorescein punctate staining was observed. Upregulation of mucin1 and mucin16 in HCECs promoted by MPM or by diquafosol treatment impeded intracellular fluorescein ingress. Downregulation of mucin1 and mucin16 enhanced fluorescence ingress in HCECs after fluorescein staining. Overexpression of truncated mucin1 did not alter the fluorescein intensity of fluorescein-stained HCECs, supporting the notion that the ability of mucin1 to block fluorescein ingress was primarily mediated by its extracellular domain. Minimal inherent expression of mucin16 in the rabbit cornea limited the validation of its role in blocking fluorescein ingress in vivo.

Conclusion

Transmembrane mucin1 blocks fluorescein ingress in the corneal epithelium, explaining how fluorescein staining is positive when the level of transmembrane mucins is disturbed in dry eyes.

Blood group related antigens in ocular cicatricial pemphigoid

AIM

To study the MUC5AC and the blood group related antigen expression in ocular cicatricial pemphigoid (OCP) according to the distribution of Lewis and secretor phenotypes in OCP patients compared to normal subjects.

METHODS

Immunostaining was performed on conjunctival biopsy specimens from 22 consecutive patients suffering from OCP, using monoclonal antibodies (Mabs) directed against the peptidic core MUC5AC mucin (anti-M1/MUC5AC Mabs) and against the saccharide moieties (anti-blood group related antigens). These latter included anti-Le(a), anti-Le(b), anti-sialyl Le(a), and H type 2 Mabs, which immunoreact with Lewis positive and non-secretor (Le(a)), Lewis positive and secretor (Le(b)), Lewis positive (sialyl Le(a)), and secretor (H type 2) phenotypes respectively. Serological tests were also performed to confirm the phenotype of each patient. The immunohistopathological patterns and the distribution of Lewis and secretor phenotypes were compared with the results of a previous study in normal individuals.

RESULTS

(1) In OCP patients compared to the normal population, anti-M1 immunoreactivity of goblet cells was unchanged, whereas anti-Le(a), anti-Le(b), and anti-sialyl Le(a) immunoreactivities of epithelial and/or goblet cells were markedly decreased. (2) 41% of OCP patients had a non-secretor phenotype, which is statistically significantly more than the estimated incidence of the same phenotype in the French population (20%) (p approximately 0.04).

CONCLUSIONS

Mucins in OCP patients showed a decreased expression of blood group related antigens whereas the MUC5AC peptidic core detected by anti-M1 Mab remained unchanged. These results also seem to indicate that OCP may be associated with a non-secretor phenotype.

Biochemical analysis of ocular surface mucin abnormalities in dry eye: the canine model

This study examines the canine model of keratoconjunctivitis sicca (KCS, 'dry eye') in order to establish the biochemical basis of altered ocular mucin secretion in this condition. It follows a previous examination of ocular mucins in the normal dog. Mucus was collected by suction from the ocular surface of dogs with KCS, and dispersed in guanidine hydrochloride containing a cocktail of protease inhibitors. Caesium chloride density gradient centrifugation was used to separate floating 'rafts' of cell membranes from gradients containing secreted mucins. Gradient fractions were collected into pools on the basis of differential staining by Periodic Acid Schiff, Wheat Germ Agglutinin, and antibodies to MUC5AC peptide. High molecular weight glycoproteins were purified from the pooled material by gel filtration chromatography. Membrane-associated glycoproteins were also derived from the membrane rafts using octyl glucoside extraction and/or reduction and alkylation. Secreted mucins and membrane extracts from KCS samples were compared to equivalent material obtained from normal eyes. Density gradient staining profiles for normal and KCS mucus were similar over the buoyant density range typical for secreted mucins, enabling the collection of identical pools of gradient fractions for direct comparison. The following differences were observed in KCS secreted mucins compared to normal samples: an increase in the proportion of mucin with low buoyant density; a decrease in mannose content detected with Concanavalin A lectin; an increase in N-acetylglucosamine structures detected with Lycopersicon esculentum lectin; increased migration and lack of evidence for distinct subunit structure on agarose gels. In membrane extracts, the main difference was the presence of T antigen (Gal beta 1-3GalNAc) in KCS. These results demonstrate alterations in the subunit linkage of mucins in KCS, and suggest that glycosylation, core protein expression and/or post-synthetic modification of ocular surface mucins may also be changed.

The normal human tear glycoprotein profile detected with lectin probes

Tear samples were collected from 46 healthy volunteers evenly distributed according to sex and age (mean age 43.5 years). Samples were denatured in a Tris-HCl sample buffer containing 2-mercaptoethanol and SDS, and applied to a gradient SDS-polyacrylamide gel for electrophoresis. The proteinaceous material was transferred to nitrocellulose by a semi-dry blotting technique, and the glycoprotein content subsequently visualized by incubation with four lectins (WGA, PHA, PSA and SBA) and staining with avidin horseradish-peroxidase. Glycoprotein bands were generally found to be significantly less frequent in persons under the age of 30 years. Apart from this the technique gave a uniform picture of the glycoprotein profile, with only modest differences according to age and/or sex. The technique may therefore be suitable for the detection of differences in the glycoprotein composition indicative of disease.

Cicatricial pemphigoid and erythema multiforme

Cicatricial pemphigoid (CP) and erythema multiforme (EM) are bullous diseases that involve the skin and mucous membranes including the conjunctiva. Of all the bullous diseases, CP and EM not only involve the conjunctiva most frequently but also cause the most severe conjunctival disease. A chronic, progressive disease, CP is characterized by shrinkage of the conjunctiva, symblepharon, entropion, trichiasis, dry eye, and finally reduced vision from corneal opacification. It is primarily a disease of the elderly that affects more women than men and is characterized by blisters or bullae in a subepithelial location and immunoglobulins and complement bound to the basement membrane zone of skin and mucous membranes including the conjunctiva. Circulating antibodies to the basement membrane zone can be demonstrated occasionally. Treatment includes artificial tears, topical antibiotics, correction of entropion and trichiasis, therapeutic soft contact lenses, and systemic immunosuppressive therapy including corticosteroids. An acute, generally self-limited, inflammatory disorder of the skin and mucous membranes, EM occurs primarily in young, healthy individuals. The most frequent precipitating factors are (1) drugs and (2) infections caused by Mycoplasma pneumoniae and herpes simplex. Conjunctival involvement ranges from a mild catarrhal conjunctivitis which terminates without sequelae to membranous conjunctivitis which may heal leaving scarring, symblepharon, and even ankyloblepharon. Histopathologic findings include subepithelial bullae and perivascular mononuclear cell infiltrates. Patients with EM have circulating immune complexes and immunoreactant deposition in the blood vessel walls of the dermis. After the acute episode has subsided, they may require artificial tears, topical antibiotics, correction of entropion and trichiasis, therapeutic soft contact lenses, tarsorrhaphy, and mucous membrane grafts.

Corneal infection in mucosal scarring disorders and Sjögren's syndrome

We reviewed 69 episodes of microbial keratitis occurring over an 11-year period in 56 patients with a mucosal scarring disorder or Sjögren's syndrome. Gram-positive bacterial isolates were the most common cause of infection, and accounted for almost all cases in patients with Sjögren's syndrome. Trichiasis (cicatricial pemphigoid), topical corticosteroids, bandage contact lenses, and corneal surgery were the main predisposing factors in the development of the corneal infection. In patients with ocular cicatricial pemphigoid, infection was much less common after chemotherapeutic control had been achieved. Recurrent infections were relatively frequent. There was a high rate of major complications, particularly in microbial keratitis complicating Sjögren's syndrome.

Detection of ocular mucus in normal human conjunctiva and conjunctiva from patients with cicatricial pemphigoid using lectin probes and histochemical techniques

Conjunctival biopsies from patients with cicatricial pemphigoid affecting the conjunctiva and patients undergoing cataract surgery (normal conjunctiva) were snap-frozen, cryostat sectioned and incubated with fluorescein-conjugated lectins; peanut agglutinin (PNA), Helix pomatia agglutinin (HPA), soybean agglutinin (SBA), wheat germ agglutinin (WGA) and succinylated wheat germ agglutinin (S-WGA). Controls consisted of preincubating the lectins with the appropriate blocking sugars before applying the lectins to the sections. PNA and HPA stained the mucus granules contained in the conjunctival goblet cells but did not stain mucus or glycocalyx at the ocular surface distal to the goblet cells. Native WGA and S-WGA had high affinities for conjunctival goblet cells and the apical epithelial cell layers. Native WGA stained mucus and glycocalyx at the ocular surface. This staining of the ocular surface by WGA was confirmed at the transmission electron microscopic level using WGA conjugated to ferritin. Cicatricial pemphigoid patients in this study had reduced numbers of goblet cells; however, those goblet cells which were observed in cicatricial pemphigoid conjunctiva stained positively with HPA, PNA, WGA, and SWGA as did goblet cells in normal conjunctiva.

Scanning electron microscopy of conjunctival surfaces in patients with ocular cicatricial pemphigoid

The conjunctival surfaces of ten patients with active, ocular cicatricial pemphigoid, three patients with drug-controlled ocular cicatricial pemphigoid, and six patients with normal conjunctivas were studied using scanning electron microscopy. A homogeneous granular sheet of amorphous mucin-like material was observed covering extensive areas of the conjunctiva in eight of ten patients with active ocular cicatricial pemphigoid. This sheet of amorphous material was absent on drug-controlled ocular cicatricial pemphigoid and normal conjunctival specimens. Our study demonstrates that patients with active ocular cicatricial pemphigoid possess ocular surface mucus that appears thicker and more continuous than normal ocular mucus when observed with scanning electron microscopy. This observation is in agreement with clinical observations of thick mucus strands in the inferior fornix of patients with active ocular cicatricial pemphigoid.