A histochemical comparison of human corneal stromal glycoconjugates with eight other species. Distinct species-dictated differences in binding sites of Griffonia simplicifolia I

Sialyl Lewis X, Lewis X, and N-acetyllactosamine expression on normal and glaucomatous eyes

PURPOSE

Sialyl Lewis X (sLex), Lewis X (Lex), and N-acetyllactosamine are carbohydrate chains of neolactoglycoconjugates which are expressed by specific cell types and are important in the functioning of cells within an organism. This study attempts to determine the expression of these glycoconjugates on the conjunctiva, cornea, and trabecular meshwork (TM) of both normal and glaucomatous eyes.

METHODS

Frozen anterior segment sections of both normal and glaucomatous human cadaver eyes, as well as rabbit eyes, were stained with a panel of monoclonal antibodies (mAbs) to neolactoglycoconjugates using an Avidin Biotin Peroxidase Complex/Alkaline Phosphatase staining method.

RESULTS

SLex characteristically stained both human conjunctival and corneal epithelia in normal (n=5) and glaucomatous (n=5) sections. SLex stained corneal and conjunctival epithelia of glaucomatous eyes much more intensely than normal eyes. Rabbit cornea sections stained for sLex, Lex, and N-acetyllactosamine. However, human cornea only consistently stained with sLex. Normal and glaucomatous human TM sections did not stain for sLex, Lex, or N-acetyllactosamine.

CONCLUSIONS

The expression of glycoconjugates with sLex side chains appears to be upregulated in the conjunctival and corneal epithelia of glaucomatous eyes. Distinct species specific differences were noted in Lex and N-acetyllactosamine staining patterns in rabbit and human corneal epithelia.

Corneal glycoconjugates: an ultrastructural lectin-gold study

The oligosaccharide chains of cell surface and extracellular matrix glycoconjugates are essential for the biological properties of these molecules. We have, therefore, investigated carbohydrate residues in the rat cornea using biotinylated lectin-gold probes. Fixed corneas were removed and embedded in Lowicryl HM20 or LR White. Ultrathin sections were incubated in one of the lectins: Triticum vulgare (WGA), Canavalia ensiformis (Con A), Griffonia simplicifolia (GS-1), Limax flavus (LFA) and Allomyrina dichotoma (Allo A), followed by streptavidin-gold, or the sections were incubated in cationic colloidal gold. Semi-quantification of gold labelling was determined for corneal endothelium, Descemet's membrane, stroma and epithelium from electron micrographs. WGA and Con A binding sites were expressed either moderately or strongly throughout the cornea, suggesting a preponderance of alpha-mannose and N-acetylglucosamine residues. A particular concentration of these sugars was found in Descemet's membrane. In contrast, GS-1 (specific for alpha-galactose) and Allo A (specific for beta-galactose) labelled all regions weakly. Sialic acid residues, as defined by LFA labelling and the expression of neuraminidase-sensitive cationic colloidal gold binding sites, were sparsely distributed throughout the stroma, Descemet's membrane and endothelium. In contrast, sialoglycoconjugates were found in significant concentrations in the epithelium. Electron microscopy proved useful in providing new information on the cellular and subcellular localization of these lectin binding sites.

Localization of a corneal basement membrane glycoconjugate in bovine eye

Lectin histochemistry was used to analyze the ocular surface basement membrane in order to identify novel, tissue-specific glycoconjugates. Soybean agglutinin (SBA), a lectin marker for N-acetylgalactosamine residues, recognized a 130 kDa glycoconjugate in corneal but not conjunctival basement membrane. Following corneal epithelial cell wounding in vitro, the glycoconjugate was not expressed at the epithelial cell-stromal interface until 72-96 h in culture, much later than the expression of other basement membrane molecules. In epithelial cells maintained on various extracellular matrix substrates (laminin, collagen type IV, or a mixture of laminin, collagen type IV and heparan sulfate proteoglycan), the glycoconjugate localization patterns changed with time in culture from a diffuse cellular or perinuclear ring to an extensive extracellular filament network. The presence of SBA in the culture medium did not affect migration or adhesion in either the wound healing model or in culture cells. However, the presence of SBA did delay the deposition of laminin at the zone of basement membrane reformation in the wound healing model. The 130 kDa glycoconjugate is a candidate for molecules which impart a tissue-specificity to the corneal basement membrane, and which may be involved in cell-matrix interaction.

Pseudomonas aeruginosa infection of the cornea and asialo GM1

Extensive immunohistochemical and thin-layer chromatogram-immunostain analyses were carried out to establish whether asialo GM1, a glycolipid which contains binding sites for Pseudomonas aeruginosa, is present in corneal epithelium. The data suggest that rabbit corneal epithelium does not contain detectable levels of asialo GM1 even after corneas are scarified and incubated with trypsin, P. aeruginosa, or P. aeruginosa exoproducts to expose potential cryptic sites. Preliminary immunohistochemical analyses indicated that asialo GM1 is also not found in human corneas.

Corneal and ocular surface histochemistry

In summary, this review has provided information concerning the application of histochemical and cytochemical procedures used to detail the normal versus pathological cornea and ocular surface. Specifically, histochemical analysis has been used to study protein and peptide degradation in cornea, to analyze stromal non-collagenous and collagenous fibers and associated extracellular matrix. Cytochemistry of the ocular surface has been used to detail the morphology of corneal and conjunctival mucin. Use of small cationic probes as well as lectin-gold binding was advantageous to quantitatively demonstrate that ocular mucin contains sialylated residues and that the number of these residues significantly changes (increases) with age. These data are important in that the degree of sialylation has been shown to correlate with the ability of bacterial organisms to adhere to and infect the immature in contrast to the mature corneal surface. The use of lectin analysis of diseased ocular tissue also has shown that there are specific alterations in glycoconjugates which occur in the diseased versus normal human cornea. Wound healing in cornea is an important problem which has been studied at length using combined histochemical and biochemical approaches. Results support the hypothesis that apical cell surfaces of the leading edge of a migrating sheet differ from those of the normal epithelium. During wound healing, alpha 6 integrin expression by corneal epithelial cells has been demonstrated, but another protein, syndecan was only seen in non-migrating epithelium which had restratified. The association of immunoglobulins with the ocular surface epithelium of the cornea, their change with age and kinetics of appearance also has been demonstrated using a cytochemical approach. Histochemical procedures have been used to localize Class I and Class II molecules in cornea and conjunctiva. Class II antigen expression has been shown to be absent on corneal endothelium, but it can be induced by treatment with IFN-gamma. These data are of importance in corneal pathology such as that resulting in rejection of corneal transplants. Langerhans cells (Class II, Ia positive) also are not found in normal central cornea. They are localized in the peripheral cornea and are stained histochemically by ADPase, ATPase and by specific anti-Ia and other antisera. Increased numbers of LC have been demonstrated in cornea following various stimuli and in diseases of the cornea including both bacterial and viral induced keratitis.

Lectin-binding sites in quick-frozen, freeze-substituted extracellular matrix of rabbit corneal stroma: a comparative electron microscopic study with different embedding procedures

The efficiency of Epon, Lowicryl K4M and LR White resins for electron microscopy was tested on the reactivity of carbohydrate residues to lectins in quick-frozen, freeze-substituted extracellular matrix of rabbit corneal stroma, without the use of any chemical fixation. Three lectins conjugated to gold particles were used in a post-embedding method: Wheat germ agglutinin, Concanavalin A and Ricinus communis agglutinin 1. Best gold-labelling was obtained with Lowicryl K4M for Wheat germ agglutinin, and with both Lowicryl K4M and Epon for Concanavalin A and Ricinus communis agglutinin 1. Tissues embedded in LR White gave less intense labelling for the three lectins used in comparison with the two other resins. This study demonstrates the crucial role of resins in the optimal preservation of biological activity of carbohydrate residues in cryotechnically-processed cornea for electron microscopy.

New method of ultrastructural localization of WGA-binding sites in the rabbit cornea using quick-frozen specimens followed by freeze-substitution

Wheat germ agglutinin (WGA) conjugated with apo-horseradish peroxidase-gold was used as a probe for ultrastructural localization of N-acetyl-glucosamine and N-acetyl-neuraminic acid residues of glycoconjugates in posterior stroma and Descemet's membrane of rabbit corneas. Ultra-thin sections of quick-frozen, freeze-substituted, and Epon or Lowicryl K4M-embedded tissue were used. Numerous WGA-binding sites were detected in the stromal matrix and Descemet's membrane in both embedding procedures. Labelling was more intense with low-temperature embedding in Lowicryl K4M than with Epon. Gold particles preferentially labelled the major collagen fibrils of the stroma but otherwise appeared uniformly distributed over both extracellular matrices, except in some areas of the stromal matrix that were barely labelled. Frequently, the inter-particle distance of gold particles on collagen fibrils corresponded to the banding period of type I collagen fibrils, suggesting that they point out keratan sulphate proteoglycans which contain WGA-binding sites. The combination of cryotechniques and post-embedding procedures may be very effective for lectin histochemistry of the cornea and other eye tissues.

Selective neuronal glycoconjugate expression in sensory and autonomic ganglia: relation of lectin reactivity to peptide and enzyme markers

Several plant lectins were used to characterize the cell-surface carbohydrates expressed on sensory ganglion cells and their central terminals in the spinal cord dorsal horn. In the rat, galactose-terminal glycoconjugates on a large subpopulation of small neurons whose central axons project to the substantia gelatinosa were demonstrated with the alpha-D-galactose-specific Griffonia Simplicifolia I-B4 (GSA) lectin. This neuron subset was labelled by alternative D-galactose-, N-Acetylgalactosamine-, and beta Gal(1,3)NAcGal-binding lectins. Similar GSA lectin reactivity was also illustrated in selected peripheral autonomic, gustatory and visceral sensory and enteric neurons, and the accessory olfactory bulb. The sensory neuron-specific isoenzyme, fluoride-resistant acid phosphatase (FRAP) co-localized with the GSA lectin, as did the monoclonal antibody (MAb) 2C5, which is directed against a lactoseries carbohydrate constituting a backbone structure of ABH human blood group antigens. In contrast, calcitonin gene-related peptide-immunoreactivity (CGRP-IR), used as a representative marker of peptidergic neurons, exhibited limited co-localization with GSA. A polyclonal anti-rat red blood cell (RBC) antibody co-localized with GSA, suggesting that lectin-reactive carbohydrates on rat sensory neurons are related to rat RBC antigens. In the human spinal cord, the L-fucose-binding Ulex europaeus-I (UEA) lectin also labelled the substantia gelatinosa; in rabbit, a small sensory ganglion cell subset and the spinal cord substantial gelatinosa was co-labelled by both the GSA and UEA lectins. These studies illustrate significant lectin-reactive cell surface carbohydrate expression by non-peptidergic, FRAP(+) sensory ganglion cells in the rat, and provide a means for visualizing the extensive, non-peptidergic, small sensory ganglion cell subpopulations, probably including a substantial proportion of nociceptive and unmyelinated peripheral axons.