[Acid mucopolysaccharides and glycoproteins in the human cornea in relationship to age and keratoconus]

A review of collagen cross-linking in cornea and sclera

Riboflavin/UVA cross-linking is a technique introduced in the past decades for the treatment of keratoconus, keratectasia, and infectious keratitis. Its efficacy and safety have been investigated with clinical and laboratory studies since its first clinical application by Wollensak for the treatment of keratoconus. Although its complications are encountered during clinical practice, such as infection inducing risk, minimal invasion merits a further investigation on its future application in clinical practice. Recently, collagen cross-linking in sclera shows a promising prospect. In present study, we summarized the representative studies describing the clinical and laboratory application of collagen cross-linking published in past decades and provided our opinion on the positive and negative results of cross-linking in the treatment of ophthalmic disorders.

The pathogenesis of keratoconus

Keratoconus (KC) is a common degenerative condition that frequently results in visual loss with an onset typically in early adulthood. It is the single most common reason for keratoplasty in the developed world. The cause and underlying pathological mechanism are unknown, but both environmental and genetic factors are thought to contribute to the development of the disease. Various strategies have been employed to address the gap in our understanding of this complex disease, with the expectation that over time more sophisticated therapies will be developed. In this review we summarise our current knowledge of the aetiology and risk factors associated with KC.

Traumatic Graft Dehiscence after Penetrating Keratoplasty

OBJECTIVE

To determine the incidence and to explore the risk factors for traumatic graft dehiscence after penetrating keratoplasty.

DESIGN

Retrospective case note review.

PARTICIPANTS

Five hundred seventy-two consecutive cases were included.

INTERVENTION

All subjects who underwent penetrating keratoplasty in 1 regional center between 1992 and 2004 inclusive.

MAIN OUTCOME MEASURES

Cases that experienced postoperative traumatic graft dehiscence were identified. Results from 12 other similar studies were pooled for comparison.

RESULTS

Fifteen eyes (2.6%) were treated for traumatic wound dehiscence after penetrating keratoplasty. The most striking feature of this series was the bimodal relationship of age and cause of graft dehiscence, with older patients involved in falls and younger patients in accidental or deliberate trauma. Factors that may influence the risk of traumatic graft dehiscence are discussed, in the light of the present findings and pooled data from previous series.

CONCLUSIONS

This case series indicates that there is long-term risk of traumatic wound dehiscence after penetrating keratoplasty. Younger patients, especially males, should be made aware that their eye, after keratoplasty, will always be vulnerable to injury. High-risk situations should be avoided if possible. Older patients at particular risk should have adequate risk reduction strategies, social support, and supervision, in particular to minimize the risk of falls.

The control of matrix metalloproteinase-2 expression in normal and keratoconic corneal keratocyte cultures

PURPOSE

Early phase keratoconic corneas and their cultured keratocytes abnormally produce the Mr 62,000 form of the matrix metalloproteinase-2 (MMP-2). It is known that platelet derived growth factor (PDGF) and transforming growth factor-beta (TGF-beta) are involved in the regulation of MMP activity and tissue inhibitor of metalloproteinase (TIMP) production in non-ocular tissues. The purpose of this enquiry was to determine whether these growth factors also play a role in the activity and/or production of corneal MMP-2 and TIMP, and whether their activity could account for the existence of the Mr 62,000 form of MMP-2 in keratoconic corneas.

METHODS

Confluent cultures of normal and early-phase keratoconic corneal keratocytes were established and incubated in serum-free media in the presence or absence of PDGF and TGF-beta. The proteins secreted by these cells over periods of 7 days were harvested for analysis. The total protein produced was determined spectrophotometrically. MMP-2 was visualised by SDS-gelatin polyacrylamide gel electrophoresis and assayed using radiolabelled type IV collagen as substrate. The enzyme inhibitors, TIMP-1 and TIMP-2, were quantified by dot blot immunoassay.

RESULTS

The addition of PDGF or TGF-beta to the culture medium of keratoconic corneal keratocytes had no significant effect on overall protein production, MMP-2 activity or on the amounts of TIMP- 1 and TIMP-2 secreted. These observations also applied to normal corneal keratocytes, with the exception that PDGF induced expression of the Mr 62,000 species of MMP-2.

CONCLUSIONS

PDGF may be involved in the production of the Mr 62,000 species of MMP-2 that is abnormally produced by early-phase keratoconic corneal keratocytes.

Traumatic wound dehiscence after penetrating keratoplasty

BACKGROUND

Traumatic wound dehiscence after penetrating keratoplasty is probably underestimated.

PATIENTS

From the files of the University Eye Hospital Tübingen (1981-1993), 15 patients with a traumatic wound dehiscence after penetrating keratoplasty were investigated.

RESULTS

Latency between corneal grafting and wound rupture ranged from 1 month to 25 years (mean 6.2 years). Wound dehiscence was quite evenly distributed over the circle. Intraocular tissue was lost to a variable extent. Three eyes were primarily or secondarily enucleated. Seven eyes (47%) with an originally reduced but useful vision became blind or were removed. Most (67%) of the resutured grafts lost transparency.

CONCLUSIONS

Traumatic wound dehiscence is a serious and not very rare complication after penetrating keratoplasty. Functional results are poor.

Age-related changes of scleral hydration and sulfated glycosaminoglycans

Human scleras were analyzed to determine if tissue hydration changed in an age-related fashion, and whether this change could be related to decreased levels of glycosaminoglycans. The anterior half of normal human scleras were examined with donor ages ranging from newborn to 99 years (n = 40). Tissue hydration was found to decrease by 1.06% per decade (P = 0.0012). Glycosaminoglycans were extracted from scleras with guanidine hydrochloride, purified by anion exchange chromatography and quantitated using dimethylene blue and selective enzymatic digestion. The glycosaminoglycans of the sclera were primarily dermatan sulfate and chondroitin sulfate with small quantities of hyaluronic acid. While no loss of chondroitin sulfate with age was detected, a significant age-related loss of dermatan sulfate was found (P = 0.0006, n = 14). An examination of the relationship between glycosaminoglycan concentration and scleral hydration showed that increased levels of sulfated glycosaminoglycans were associated with increased levels of scleral hydration (P < 0.02, n = 14). No improvement in the correlation could be achieved by considering either dermatan sulfate or chondroitin sulfate individually. Our results suggest that, as in other connective tissues, aging is associated with a loss of glycosaminoglycans, and their loss is coincident with decreased tissue hydration.

Age-related changes of sulfated proteoglycans in the normal human trabecular meshwork

Ultrastructural changes in sulfated proteoglycans were studied in 12 pairs of normal eye-bank eyes (aged 1 day to 92 years), using the cationic dye, cuprolinic blue, in a 'critical electrolyte concentration'. Pretreatment of trabecular meshwork sections with various glycosaminoglycanases and nitrous acid served to characterize these proteoglycans. Three sizes of proteoglycan-cuprolinic blue (PG-CB) complexes were found in association with different extracellular matrix components. Small, thin PG-CB complexes were closely associated with collagen fibrils. Large, thick PG-CB complexes, although located close to collagen fibrils in a variety of places, were most commonly seen between the boundaries of the collagen bundles, where they were associated with fine filaments. Both types of collagen-associated PG-CB complexes contained chondroitin sulfate and dermatan sulfate, with dermatan sulfate predominant. Basal lamina-associated PG-CB complexes contained heparan sulfate. An age-related, progressive coalescence of collagen was found in normal trabecular meshwork in a statistically significant fashion; the regions of collagen coalescence were associated with a decrease of small, collagen-associated PG-CB complexes and an increase of a previously unrecognized matrix material. The measurement of areas of coalescence of collagen was used as an indirect indicator of small, collagen-associated PG-CB complex loss with age. Large collagen-associated PG-CB complexes and basal lamina-associated PG-CB complexes decreased from infant to young adult; no additional loss with age was found. Further studies will be needed to determine whether loss of sulfated PGs plays a role in increased aqueous outflow resistance that characterizes glaucoma.

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.

Phosphatic metabolites in keratoconus

Sixteen corneal buttons were obtained from 16 patients with keratoconus at the time of penetrating keratoplasty (mean age, 34 +/- 3.2 yr) and prepared for phosphorus-31 nuclear magnetic resonance spectroscopy following perchloric acid extraction. The corneas were compared with age-matched corneas from fresh eye-bank eyes with normal slit-lamp biomicroscopic appearance. Higher levels (P less than 0.05) of nucleoside monophosphates and choline phosphate and lower levels (P less than 0.05) of adenosine diphosphate were detected in keratoconus than in eye-bank corneas. The level of an unidentified metabolite at 3.96 delta, present in eye-bank corneas, was ninefold higher in keratoconus corneas. Another unidentified metabolite at 3.31 delta, accounting for 1.5% of the total detected phosphatic metabolites, appeared in keratoconus but not in eye-bank cornea spectra. These findings of altered phosphatic metabolites are consistent with altered corneal metabolism. The unidentified, unknown metabolites in the spectral profile uniquely distinguish keratoconus from eye-bank corneas.

Keratoconus: I. Biochemical studies

The present study analyses collagenous and non-collagenous components from age-matched normal and keratoconus corneas. Intact keratoconus corneas showed decreased collagen, total protein, and hydroxylysine levels, with normal reducible collagen cross-linking. Non-collagenous fractions were isolated from corneas with a 4 M guanidine procedure. As demonstrated by PAGE-silver stain, the keratoconus cornea guanidine extracts had a 75 kDa band that was absent in normal cornea guanidine extracts. In addition, there was a markedly increased level of protein, uronic acid and neutral hexose in keratoconus extracts as compared with controls. Our Western blot studies showed increased affinity for the castor-bean agglutinin (RCA120, specific for terminal galactose) in the keratoconus extracts as compared with normals. These data suggest the presence of an abnormal noncollagenous component in keratoconus corneas.

Comparative studies of collagens in normal and keratoconus corneas

In this paper we present strong evidence that the aberrations in keratoconus corneas are not directly related to alterations in collagen composition and distribution. This conclusion is based on comparative studies of collagen types I, III, IV, V and the recently described collagen types VI and VII in keratoconus and normal corneas. The data are derived from biochemical analysis of collagen fractions sequentially extracted with pepsin and sodium-dodecylsulphate, from amino acid analysis of hydrolysates of entire corneal tissues as well as from immunoblotting of the extracted collagens with specific antibodies. These antibodies were also used to examine the distribution of the collagens in immunofluorescence experiments on corneal sections. The yields of the collagen extractions were demonstrated to be age dependent but were not altered in keratoconus samples. Apart from one case associated with osteogenesis imperfecta type I, comparative studies of keratoconus and normal corneas showed no differences in collagen composition of the extracts. This was confirmed by amino acid analysis of tissue-hydrolysates. The distributions of collagen types I, III, IV, V, VI and VII in corneal sections were in general unchanged in keratoconus corneas, the only differences being in scar tissues observed in the Bowman layer of some keratoconus samples.

Phase transitions in ocular tissue - NMR and temperature measurements

The human cornea, sclera and lens is studied by nuclear magnetic resonance (NMR) measurements and temperature curves. The bound water fractions of these tissues have been determined as 3% in the cornea, 20% in the sclera and 17% in the lens. In contrast to other investigations we found freezing intervals near -1 degree C.

Sensitization of topo-optical reactions in showing tissue glycosaminoglycans of low local concentration

The intensity and sensitivity of inversive topo-optical reactions of GAGs can be increased by acetylation prior to staining with cationic dyes. The combination renders possible the polarization optical analysis of connective tissue matrices containing small amount of GAGs.

Variation of corneal glycosaminoglycan values of hares in relation to environmental pollution by industrial emissions

In localities contaminated by industrial emissions a significant decrease in keratan sulfate and chondroitin 4-sulfate occurs in the cornea of the hare. The changes also occur in hares living near small emission sources with a local emission zone; therefore they should not be neglected, especially if the area is used for recreation, as in the case reported here. The decrease in glycosaminoglycan values is more pronounced in adults than in juveniles, because of the length of exposure. Relatively large amounts of keratan sulfate in the glycosaminoglycan fraction correspond to changes expected in a prematurely senescent tissue. Because of the painstaking laboratory work required, assessment of glycosaminoglycans could be used for special bioindication purposes but would not be suitable for routine monitoring.

[Proof of glycosaminoglycans in the corneal endothelium]

The corneal endothelium of three species (man, dog, and guinea pig) was examined for glycosaminoglycans by using TEM, REM, and EDAX (energy-dispersive analysis of X-rays). Since no clear results can be determined by using ruthenium red (RR) alone, a testicular hyaluronidase was used to digest the glycocalyx. After incubation with hyaluronidase, the dark RR borderline of the outer leaflet of the cell membrane became more fluffy when examined by using TEM and REM. When EDAX was applied, the Ru peak became smaller compared with the S peak. These impressive results had not been found when the material was fixed in glutaraldehyde for 24 h before incubation. Considering the individual results, it can be concluded that RR, even wouthout OsO4, marks the acid groups of glycosaminoglycans that might be in the cell membrane, as well as other acid groups. There was no difference in the results among the three species used in this experiment. Some pictures showed that RR/OsO4 penetrates through the intercellular space into the Descemet's membrane.

[Acute keratoconus in mongolism (author s transl)]

Ten cases of mongolism are reported in which acute keratoconus developed. The results according to age and sex distribution, onset of the disease, status of the fellow eye, and histology of the cornea are compared with former reports. Our theory of the etiopathogenesis of acute keratoconus in patients with trisomy 21 and other diseases with generalized weakness of the connective tissue is presented and discussed.

Corneal superficial unwettable defect. I. Clinical and histopathological study

In 11 patients, the clinical, histological and histochemical picture of an unusual corneal affection termed "corneal superficial unwettable defect", is reported. It develops during the healing of various corneal lesions. Clinically, it is described as a defect of the epithelium with a denuded and changed Bowman's membrane, the surface of which repells tears and cannot be stained with fluorescein. In the histological picture, the changed Bowman's membrane has fibrillar or foamy appearence. It stains strongly with colloidal iron and alcian blue, but PAS and toluidine blue reactions are weak. Histochemical reactions for neutral fats are strongly positive, but those for phospholipids are negative. The results of extractions speak for the presence of some lipid component, which is neither neutral fat nor phospholipid. Hence it is supposed that there may be some waxes-ointment bases, reacting with the pathologically changed Bowman's membrane. The possible mechanisms injuring or influencing the Bowman's membrane are also discussed.