Synthesis of glycosaminoglycans in corneal organ cultures

Fibroblast growth factor-2 promotes keratan sulfate proteoglycan expression by keratocytes in vitro

Keratocytes of the corneal stroma produce a specialized extracellular matrix responsible for corneal transparency. Corneal keratan sulfate proteoglycans (KSPG) are unique products of keratocytes that are down-regulated in corneal wounds and in vitro. This study used cultures of primary bovine keratocytes to define factors affecting KSPG expression in vitro. KSPG metabolically labeled with [(35)S]sulfate decreased during the initial 2-4 days of culture in quiescent cultures with low serum concentrations (0.1%). Addition of fetal bovine serum, fibroblast growth factor-2 (FGF-2), transforming growth factor beta, or platelet derived growth factor all stimulated cell division, but only FGF-2 stimulated KSPG secretion. Combined with serum, FGF-2 also prevented serum-induced KSPG down-regulation. KSPG secretion was lost during serial subculture with or without FGF-2. Expression of KSPG core proteins (lumican, mimecan, and keratocan) was stimulated by FGF-2, and steady state mRNA pools for these proteins, particularly keratocan, were significantly increased by FGF-2 treatment. KSPG expression therefore is supported by exogenous FGF-2 and eliminated by subculture of the cells in presence of serum. FGF-2 stimulates KSPG core protein expression primarily through an increase in mRNA pools.

Effect of epithelial debridement on glycosaminoglycan synthesis by human corneal explants

The purpose of the present work was to investigate the effects of mechanical epithelial debridement upon glycosaminoglycan synthesis by human corneal explants. Corneal explants were maintained under tissue culture conditions for 2-72 days and the glycosaminoglycans synthesized in 24 h were metabolically labeled by addition of 35S-sulfate to the culture medium. These compounds were isolated from the tissue explants and analyzed by a combination of agarose gel electrophoresis and enzymatic degradation with specific mucopolysaccharidases. The glycosaminoglycans synthesized by isolated epithelial cells and by corneas previously submitted to epithelial cell debridement were compared to controls. Keratan sulfate (26 kDa) and dermatan sulfate (43 kDa) were the main corneal glycosaminoglycans, each one corresponding to about 50% of the total. Nevertheless, the main 35S-labeled glycosaminoglycan was 35S-dermatan sulfate (73%), with smaller amounts of 35S-keratan sulfate (15%) and 35S-heparan sulfate (12%), suggesting a lower synthesis rate for keratan sulfate. The main glycosaminoglycan synthesized by isolated epithelial cells was heparan sulfate. The removal of epithelial layer caused a decrease in heparan sulfate labeling and induced the synthesis of dermatan sulfate by stromal cells. This increased synthesis of dermatan sulfate suggests a relationship between epithelium and stroma and could be related to the corneal opacity that may appear after epithelial cell debridement.

Glycosaminoglycans in components of the rabbit eye: synthesis and characterization

PURPOSE

To trace the eye components involved in proteoglycan synthesis and to characterize the sulfated glycosaminoglycans which are associated to these macromolecules.

METHODS

Sodium [(35)S]-sulfate was injected intravitreally and the rabbits were killed at different time intervals after the injection. The glycosaminoglycans of choroid, ciliary body, cornea, iris, lens capsule, retina and sclera were extracted and processed for estimations of their specific activities, and for electrophoresis plus autoradiography with or without previous treatment with specific enzymes. In addition, methacrylate sections of the eyes were analysed by autoradiography.

RESULTS

The peak of specific activities of the glycosaminoglycans of all eye components occurred at 2 days after the intravitreal injection of [( 35)S]-sulfate. The autoradiography of the agarose gels revealed three types of glycosaminoglycans, namely, heparan-, chondroitin- and dermatan sulfate, only in the retina. The other eye components contained heparan sulfate and either chondroitin or dermatan sulfate. Tissue autoradiography together with the biochemical techniques contributed to unravel the origin of the glycosaminoglycans in the eye components.

CONCLUSIONS

The results of the present investigation have shown that heparan sulfate, contrasting to chondroitin sulfate and dermatan sulfate, is synthesized in all eye components studied and that the glycosaminoglycan composition differs according to the tissue of origin.

Synthesis of corneal keratan sulfate proteoglycans by bovine keratocytes in vitro

Keratan sulfate proteoglycans (KSPGs) are the major proteoglycans of the cornea and are secreted by keratocytes in the corneal stroma. Previous studies have been able to show only transient secretion of KSPG in cell culture. In this study, cultures of bovine keratocytes were found to secrete the three previously characterized KSPG proteins into culture medium. Reactivity with monoclonal antibody I22 demonstrated substitution of these proteins with keratan sulfate chains. KSPG constituted 15% of the proteoglycan metabolically labeled with [35S]sulfate in keratocyte culture medium. This labeled KSPG contained keratan sulfate chains of 4700 Da compared to 21,000 Da for bovine corneal keratan sulfate. Labeled keratan sulfate from cultures contained nonsulfated, monosulfated, and disulfated disaccharides that were released by digestion with endo-beta-galactosidase or keratanase II. Nonsulfated disaccharides were relatively more abundant in keratan sulfate from culture than in corneal keratan sulfate. These results show that cultured bovine keratocytes maintain the ability to express all three of the known KSPG proteins, modified with keratan sulfate chains and sulfated on both N-acetylglucosamine and galactose moieties. KSPG made in vitro differs from that found in vivo in the length and sulfation of its keratan sulfate chains. The availability of cell cultures secreting corneal keratan sulfate proteoglycans provides an opportunity to examine biosynthesis and control of this important class of molecules.

Cytological and immunocytochemical approaches to the study of corneal endothelial wound repair

The vertebrate corneal endothelium represents a unique model system for investigating many cellular aspects of wound repair within an organized tissue in situ. The tissue exists as a cell monolayer that resides upon its own natural basement membrane that can be prepared as a flat mount to observe the entire cell population. Thus, it readily avails itself to many cytological and immunocytochemical methods at both the light microscopic and ultrastructural levels. In addition, the tissue is easily explanted into organ culture where further investigations can be carried out. These techniques have enabled investigators to use many approaches to explore function and changes in response to injury. In vivo, the endothelium acts as a transport tissue to actively pump Na+ and bicarbonate ions from the corneal stroma into the aqueous humor to control corneal transparency. Physiological findings indicate that fluid diffuses back into the stroma, across the endothelium, and thus hydration is said to be controlled by a pump-leak mechanism. Ultrastructural investigations, some employing horseradish peroxidase and lanthanum, have established the morphological basis for this mechanism as apical focal junctions that are not the classical tight junctions and do not constitute a complete zona occludens. Along with these apical focal junctions are gap junctions that appear identical to their counterparts in other cell types. Cytochemical studies localized both Na+K(+)-ATPase and carbonic anhydrase, the main pump enzymes associated with corneal hydration, to the lateral plasma membranes. Corneal endothelial cells of noninjured tissue do not traverse the cell cycle and are considered to be in the "Go" phase of the cell cycle as determined by microfluorometric analysis with DNA binding dyes such as auramin O and pararosaniline-Feulgen. However, injury can initiate cell cycle transverse and histochemical and cytological methods have been used to understand the tissue's response. Classical histochemical studies revealed that increased staining was observed for metabolic (NADase and NADPase) and lysosomal enzymes in cells bordering the wound area. The use of radiolabelled agents has further lead to an understanding of the endothelial wound response. Autoradiographic analyses of 3H-actinomycin D incorporation indicated that injury initiates changes in chromatin leading to increased binding levels of the drug in cells surrounding the wound. This change suggests that those cells undergo heightened macromolecular synthesis and this was confirmed by examining 3H-uridine and 3H-thymidine incorporation. The major mechanism involved in corneal endothelial repair is cell migration. Cytochemical and immunocytochemical investigations have allowed investigators an opportunity to gain some insight into changes that occur during this cellular process.(ABSTRACT TRUNCATED AT 400 WORDS)

Ocular histopathology and ultrastructure of Morquio syndrome (systemic mucopolysaccharidosis IV A)

Morquio syndrome (mucopolysaccharidosis IV) is a hereditary lysosomal storage disease characterized by dwarfism, spondyloepiphyseal and dental abnormalities, corneal opacification, and normal intelligence. We report the light and electron microscopic features of two patients with mucopolysaccharidosis type IV A (MPS IV A). Variable degrees of mucopolysaccharide deposition were seen in tissue surveyed by light microscopy. Transmission electron microscopy demonstrated fibrillogranular and multimembranous membrane-bound inclusions distributed primarily in the cornea and trabecular meshwork, to a milder degree in the conjunctiva and sclera, and rarely in the retinal pigment epithelium. Our findings indicate that the accumulation of mucopolysaccharide in MPS IV A occurs primarily in the structures of the anterior segment.

Biochemical studies on human corneal proteoglycans--a comparison of normal and keratoconic eyes

Human corneas from normal (healthy) donors and patients with keratoconus were either metabolically labelled under organ culture conditions or investigated without preincubation. The sulfated proteoglycans were isolated from a 4 M guanidinium chloride/2% Triton X 100 extract. Two predominant proteoglycans were obtained from normal cornea after digestion of total sulfated proteoglycans with chondroitin ABC-lyase or endo-beta-galactosidase. One had an overall mass of 150 kDa, two dermatan sulfate chains (Mr approximately 50 kDa) with an iduronic acid content of 24%-28% and, after chondroitin ABC-lyase digestion, a core protein of 48 kDa. The other proteoglycan had an overall mass of 110 kDa, one keratan sulfate chain of approximately 60 kDa and, following endo-beta-galactosidase (keratanase) digestion, a core protein of 46 kDa. Each proteoglycan population was further fractionated into two subpopulations by chromatography on concanavalin A-Sepharose. The dermatan sulfate- and keratan sulfate-containing proteoglycans isolated from keratan and healthy cornea had comparable Mr values and core proteins with identical molecular weights, but the ratio of dermatan sulfate/keratan sulfate proteoglycan was increased in keratoconic cornea and the keratan sulfate chains of two keratan sulfate proteoglycans from keratoconic cornea were considerably shorter (Mr 44 and 33 kDa) than those from normal corneas.

Organ culture of rabbit cornea: morphological analyses

Tissues maintained in vitro often undergo changes in the pattern of protein synthesis that result in the deposition of macromolecules quantitatively or qualitatively unrelated to those normally synthesized. In these preliminary studies, we modified published techniques to maintain adult and neonate rabbit corneas in vitro for 24 to 48 h. Measurements of corneal wet weight, and histologic and ultrastructural analyses were made to determine the success of maintaining rabbit corneas in culture. The results show that rabbit corneas can maintain normal corneal hydration and tissue structure for at least 48 h when incubated in Coon's modification of Ham's F12, 5% fetal or newborn calf serum, 2 mM glutamine, and 2% chondroitin sulfate or 2% 50 kDa dextran sulfate at 37 degrees C in a 5% CO2/air atmosphere. In addition, we confirmed previous observations that corneal explants must have a 1 to 2 mm rim of limbal sclera, and the organ placed in the culture dish with the epithelial side down to guard against damage and insure endothelial functioning. Normal ultrastructure of neonate rabbit cornea is also maintained when organ-cultured with these procedures. Moreover, neonate corneas continue to synthesize collagen in culture.

Proteoglycans of rabbit cornea: labelling in organ culture and in vivo

Rabbit corneas maintained with radioactively-labelled precursors in organ culture for up to 42 hr produced labelled proteoglycans of the same kind as those that exist normally or that are produced by labelling in vivo. Whole corneas, including a narrow strip of sclera, were kept in culture in the presence of [3H]-glucosamine and [35S]-sulfate. The rate of incorporation of sulfate into extractable proteoglycans was linear over the time investigated, as was the rate of incorporation of glucosamine after a short lag. Three labelled proteoglycans were isolated and found to behave in ion-exchange chromatography and gel chromatography in the same way as they did in previous studies by chemical analysis. Their labelled glycosaminoglycans were primarily dermatan sulfate and keratan sulfate, with traces of hyaluronic acid and heparan sulfate. When labelled precursors were injected directly into the anterior chamber of rabbit eyes, the resulting labelled proteoglycans were similar to those obtained in organ culture. Both in vivo and during organ culture, the specific activity of hexosamine in the keratan sulfate proteoglycans was about one-half that in dermatan sulfate, probably because of different synthetic rates or different specific activities of immediate precursors.