Supra-molecular assembly of a lumican-derived peptide amphiphile enhances its collagen-stimulating activity

C16-YEALRVANEVTLN, a peptide amphiphile (PA) incorporating a biologically active amino acid sequence found in lumican, has been examined for its influence upon collagen synthesis by human corneal fibroblasts in vitro, and the roles of supra-molecular assembly and activin receptor-like kinase ALK receptor signaling in this effect were assessed. Cell viability was monitored using the Alamar blue assay, and collagen synthesis was assessed using Sirius red. The role of ALK signaling was studied by receptor inhibition. Cultured human corneal fibroblasts synthesized significantly greater amounts of collagen in the presence of the PA over both 7-day and 21-day periods. The aggregation of the PA to form nanotapes resulted in a notable enhancement in this activity, with an approximately two-fold increase in collagen production per cell. This increase was reduced by the addition of an ALK inhibitor. The data presented reveal a stimulatory effect upon collagen synthesis by the primary cells of the corneal stroma, and demonstrate a direct influence of supra-molecular assembly of the PA upon the cellular response observed. The effects of PA upon fibroblasts were dependent upon ALK receptor function. These findings elucidate the role of self-assembled nanostructures in the biological activity of peptide amphiphiles, and support the potential use of a self-assembling lumican derived PA as a novel biomaterial, intended to promote collagen deposition for wound repair and tissue engineering purposes.

Size-Dependent Diffusion of Dextrans in Excised Porcine Corneal Stroma

Delivery of therapeutic agents to the eye requires efficient transport through cellular and extracellular barriers. We evaluated the rate of diffusive transport in excised porcine corneal stroma using fluorescently labeled dextran molecules with hydrodynamic radii ranging from 1.3 to 34 nm. Fluorescence correlation spectroscopy (FCS) was used to measure diffusion coefficients of dextran molecules in the excised porcine corneal stroma. The preferential sensitivity of FCS to diffusion along two dimensions was used to differentially probe diffusion along the directions parallel to and perpendicular to the collagen lamellae of the corneal stroma. In order to develop an understanding of how size affects diffusion in cornea, diffusion coefficients in cornea were compared to diffusion coefficients measured in a simple buffer solution. Dextran molecules diffuse more slowly in cornea as compared to buffer solution. The reduction in diffusion coefficient is modest however (67% smaller), and is uniform over the range of sizes that we measured. This indicates that, for dextrans in the 1.3 to 34 nm range, the diffusion landscape of corneal stroma can be represented as a simple liquid with a viscosity approximately 1.5 times that of water. Diffusion coefficients measured parallel vs. perpendicular to the collagen lamellae were indistinguishable. This indicates that diffusion in the corneal stroma is not highly anisotropic. Our results support the notion that the corneal stroma is highly permeable and isotropic to transport of hydrophilic molecules and particles with hydrodynamic radii up to at least 34 nm.

An association between corneal inflammation and corneal lymphangiogenesis after keratoplasty

PURPOSE

To examine the relationship between corneal inflammation and corneal lymphangiogenesis after keratoplasty.

METHODS

Rat corneal lymphangiogenesis was examined by lymphatic vessel endothelial receptor (LYVE-1) immunohistochemistry and whole mount immunofluorescence at 1, 3, 7, 10, and 14 days after corneal transplantation. Corneal inflammation was evaluated by inflammation index (IF) grading and NF-κB immunohistochemistry at the same time points. The association between lymphatic vessel counting (LVC) and the IF scores was then examined.

RESULTS

LYVE-1 positive lymphatic vessels occurred in the corneal stroma on day 3, developed throughout days 7 and 10, and peaked in number at day 14 after keratoplasty. Corneal inflammation was strong on day 3, and then resolved gradually, but increased again from days 7 to 14 after the transplantation. LVC was strongly and positively correlated with IF after keratoplasty (r = 0.41; P < 0.05). However, changes in IF scores and LVC were not parallel.

CONCLUSION

A close, but not parallel, relationship was found between corneal lymphangiogenesis and corneal inflammation after corneal transplantation.

Quantification of collagen ultrastructure after penetrating keratoplasty - implications for corneal biomechanics

PURPOSE

To quantify long-term changes in stromal collagen ultrastructure following penetrating keratoplasty (PK), and evaluate their possible implications for corneal biomechanics.

METHODS

A pair of 16 mm post-mortem corneo-scleral buttons was obtained from a patient receiving bilateral penetrating keratoplasty 12 (left)/28 (right) years previously. Small-angle x-ray scattering quantified collagen fibril spacing, diameter and spatial order at 0.5 mm or 0.25 mm intervals along linear scans across the graft margin. Corresponding control data was collected from two corneo-scleral buttons with no history of refractive surgery. Wide-angle x-ray scattering quantified collagen fibril orientation at 0.25 mm (horizontal)×0.25 mm (vertical) intervals across both PK specimens. Quantification of orientation changes in the graft margin were verified by equivalent analysis of data from a 13 year post-operative right PK specimen obtained from a second patient in a previous study, and comparison made with new and published data from normal corneas.

RESULTS

Marked changes to normal fibril alignment, in favour of tangentially oriented collagen, were observed around the entire graft margin in all PK specimens. The total number of meridional fibrils in the wound margin was observed to decrease by up to 40%, with the number of tangentially oriented fibrils increasing by up to 46%. As a result, in some locations the number of fibrils aligned parallel to the wound outnumbered those spanning it by up to five times. Localised increases in fibril spacing and diameter, with an accompanying reduction in matrix order, were also evident.

CONCLUSIONS

Abnormal collagen fibril size and spatial order within the PK graft margin are indicative of incomplete stromal wound remodelling and the long term persistence of fibrotic scar tissue. Lasting changes in collagen fibril orientation in and around PK wounds may alter corneal biomechanics and compromise the integrity of the graft-host interface in the long term.

Rapid thermal equilibration of differentially heated protein and water in bovine corneal stroma

We measure and simulate the thermal response of bovine corneal stroma to a picosecond IR heating pulse. A thermal diffusion model is developed for this tissue based on the spatial distribution and properties of protein and water constituents in the stroma. In this idealized model, differentially heated protein and water constituents thermally equilibrate with a thermalization time of 515 ps. Using transient absorption spectroscopy for picosecond protein thermometry, a significantly faster thermalization time of 165 ps is measured. The implications of this faster than expected thermalization for the energy-partition model of short-pulse mid-IR tissue ablation are discussed.

[Immunohistochemical characterization of collagen IV in control corneas and in corneas obtained from patients suffering from posterior polymorphous corneal dystrophy]

The expression of all six chains of collagen IV was studied using the indirect fluorescent immunohistochemistry in seven control corneas and seven corneas obtained from patients suffering from the posterior polymorphous corneal dystrophy. Heterogeneous staining, especially in the epithelial basement membrane and Descemet's membrane, was observed in the control corneas. An increase of the staining intensity for the alpha1 and alpha2 chains was observed, especially in the Descemet's membrane and the corneal stroma in samples obtained from the patients compared to the control tissues.

Picosecond Optical Thermometry of Protein in H2O

We demonstrate the use of transient IR absorption measurements for picosecond thermometry of protein in an aqueous environment. For small temperature changes, measured transient absorption changes are shown to be in excellent agreement with the "static" temperature dependence of the protein and water constituents of the sample as measured by FTIR spectroscopy. The thermally induced changes in IR absorption reach equilibrium within a few picoseconds, making this technique an excellent tool for picosecond thermometry.

Noninvasive corneal stromal collagen imaging using two-photon-generated second-harmonic signals

PURPOSE

To investigate the feasibility of using femtosecond-pulse lasers to produce second-harmonic generated (SHG) signals to noninvasively assess corneal stromal collagen organization.

SETTING

The Eye Institute, University of California, Irvine, California, USA.

METHODS

Mouse, rabbit, and human corneas were examined by two-photon confocal microscopy using a variable-wavelength femtosecond lasers to produce SHG signals. Two types were detected: forward scattered and backward scattered. Wavelength dependence of the SHG signal was confirmed by spectral separation using the 510 Meta (Zeiss). To verify the spatial relation between SHG signals and corneal cells, staining of cytoskeletons and nuclei was performed.

RESULTS

Second-harmonic-generated signal intensity was strongest with an excitation wavelength of 800 nm for all 3 species. Second-harmonic-generated forward signals showed a distinct fibrillar pattern organized into bands suggesting lamellae, while backscattered SHG signals appeared more diffuse and indistinct. Reconstruction of SHG signals showed two patterns of lamellar organization: highly interwoven in the anterior stroma and orthogonally arranged in the posterior stroma. Unique to the human cornea was the presence of transverse, sutural lamellae that inserted into Bowman's layer, suggesting an anchoring function.

CONCLUSIONS

Using two-photon confocal microscopy to generate SHG signals from the corneal collagen provides a powerful new approach to noninvasively study corneal structure. Human corneas had a unique organizational pattern with sutural lamellae to provide important biomechanical support that was not present in mouse or rabbit corneas.

Comparison of topical steroids for acute anterior uveitis

This study objectively compares efficacy of dexamethasone Na phosphate 0.1%, fluorometholone 0.1% (FML), loteprednol etabonate 0.5% (Lotemax [LE]; Bausch & Lomb Pharmaceuticals, Inc., Tampa, FL), prednisolone acetate 1% (Pred Forte [PRED F]; Allergan Pharmaceuticals, Irvine, CA), and generic prednisolone acetate 1% (PRED A). These steroids were administered for 24 hours or 72 hours to New Zealand white rabbits with endotoxin-induced uveitis. Intraocular pressure (IOP), slit-lamp examination, and confocal microscopy were performed daily. Internalization of the glucocorticoid receptor (GC) was assayed in iris tissue by Western blot, and protein in aqueous humor by Bradford assay. Only LE and PRED F treatments significantly internalized GC receptor after 72 hours of treatment. Only LE and PRED A reduced protein concentration between 24 hours and 72 hours of treatment. All drugs improved clinical signs after 24 hours of treatment. None of the steroids promoted return of the inflammation-induced corneal thickness to baseline. While none returned IOP to baseline, LE was most effective. Confocal microscopy indicated that only treatment with LE reverted the abnormal endothelial-cell shape to normal. In conclusion, all steroid treatments reduced uveitis to some degree but LE was consistently effective. A longer observation period may be required to document the return of IOP and corneal thickness to baseline values.

Transgenic studies on the role of optineurin in the mouse eye

Mutations in the OPTN gene encoding for optineurin have been associated with primary open-angle glaucoma. The functional role(s) of optineurin in the normal and glaucomatous eye are unclear. As optineurin interferes with TNF-alpha mediated cell death in vitro, an involvement of optineurin in the regulatory pathways leading to apoptosis of retinal ganglion cells has been suggested. The goal of the present study was to study the molecular properties of optineurin and its capabilities to prevent apoptosis in vivo in the eyes of transgenic mice. The chicken betaB1-crystallin promoter was used to overexpress ectopic optineurin in the lenses of transgenic mice. The expression of transgenic mRNA was monitored by northern blot analysis. The localization of transgenic optineurin was investigated by one- and two-dimensional western blot analysis and by immunohistochemistry, and compared with that of endogenous optineurin. To assess effects of transgenic optineurin on apoptosis, betaB1-crystallin-OPTN mice were crossbred with betaB1-crystallin-TGFbeta1 mice that undergo substantial TGF-beta1-induced apoptotic cell death in the lens. Two independent betaB1-crystallin-OPTN transgenic lines were established, in which transgenic optineurin was expressed strictly lens-specific as assessed by Northern and Western blotting, and by immunohistochemistry. In contrast, endogenous optineurin was preferentially expressed in the retina, where retinal ganglion cells showed strong labeling. Immunostaining for endogenous optineurin in the anterior eye was considerably weaker than in the posterior eye and was seen in iris, ciliary epithelium, cells of corneal stroma and endothelium, and in the trabecular meshwork. Neither transgenic nor endogenous optineurin was found in the aqueous humor. Transgenic overexpression of optineurin did not have measurable effects on TGFbeta1-induced apoptosis in mixed betaB1-crystallin-OPTN/betaB1-crystallin-TGFbeta1 transgenic mice. Our results show that optineurin is a cytoplasmatic rather than a secretory protein that is preferentially expressed in retinal ganglion cells, and argue against a major role of optineurin for the modulation of apoptosis in vivo.

An x-ray diffraction study of corneal structure in mimecan-deficient mice

PURPOSE

Keratan sulfate proteoglycans (KSPGs) in the corneal stroma are believed to influence collagen fibrillar arrangement. This study was performed to investigate the fibrillar architecture of the corneal stroma in mice homozygous for a null mutation in the corneal KSPG, mimecan.

METHODS

Wild-type (n = 9) and mimecan-deficient (n = 10) mouse corneas were investigated by low-angle synchrotron x-ray diffraction to establish the average collagen fibrillar spacing, average collagen fibril diameter, and level of fibrillar organization in the stromal array.

RESULTS

The mean collagen fibril diameter in the corneas of mimecan-null mice, as an average throughout the whole thickness of the tissue, was not appreciably different from normal (35.6 +/- 1.1 nm vs. 35.9 +/- 1.0 nm). Average center-to-center collagen fibrillar spacing in the mutant corneas measured 52.6 +/- 2.6 nm, similar to the 53.3 +/- 4.0 nm found in wild-type mice. The degree of local order in the collagen fibrillar array, as indicated by the height-width (H:W) ratio of the background-subtracted interfibrillar x-ray reflection, was also not significantly changed in mimecan-null corneas (23.4 +/- 5.6), when compared with the corneas of wild-types (28.2 +/- 4.8).

CONCLUSIONS

On average, throughout the whole depth of the corneal stroma, collagen fibrils in mimecan-null mice, unlike collagen fibrils in lumican-null mice and keratocan-null mice, are of a normal diameter and are normally spaced and arranged. This indicates that, compared with lumican and keratocan, mimecan has a lesser role in the control of stromal architecture in mouse cornea.

Proliferation and apoptosis in the corneal stroma in longterm follow-up after photorefractive keratectomy

This study aimed at investigating the proliferation and apoptosis of corneal cells following photorefractive keratectomy (PRK) treatment. PRK (-6.0 D correction) was performed with the Asclepion-Meditec MEL70 G-scan excimer laser on the right eye of each of 33 rabbits under combined local and general anaesthesia. Animals were sacrificed at 4 h, 1, 4, 7, 14, 28, 56, and 112 days postoperatively, and corneal samples from these eight groups were examined histologically. Stromal cell proliferation was evaluated by immunocytochemical analysis of Ki67. Apoptosis was detected using the terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick-end labeling (TUNEL) assay method. The untreated left eyes served as controls. Ki67 positivity was detected in the upper stroma on day 1, 4, 7, and 14, and keratocyte apoptosis on day 1, 4, 7, and 14 after PRK, but not at an earlier or later time. Neither Ki67 positivity nor apoptotic activity was observed in the controls (untreated corneas). PRK was found to trigger proliferation and apoptosis of corneal keratocytes. The frequency and spatial distribution of keratocyte proliferation and apoptosis are likely to be important determinants of the corneal wound healing process, but the detailed regulatory mechanisms have not yet been characterized.

Changes in collagen orientation and distribution in keratoconus corneas

PURPOSE

To map the collagen orientation and relative distribution of collagen fibrillar mass in keratoconus corneal buttons.

METHODS

Structural analysis was performed by obtaining synchrotron x-ray scattering patterns across the samples at 0.25-mm intervals. The patterns were analyzed to produce two-dimensional maps of the orientation of the lamellae and of the distribution of total and preferentially aligned lamellae.

RESULTS

Compared with normal corneas, in keratoconus the gross organization of the stromal lamellae was dramatically changed, and the collagen fibrillar mass was unevenly distributed, particularly around the presumed apex of the cone.

CONCLUSIONS

The development of keratoconus involves a high degree of inter- and probably intralamellar displacement and slippage that leads to thinning of the central cornea and associated changes in corneal curvature. This slippage may be promoted by a loss of cohesive forces and mechanical failure in regions where lamellae bifurcate.

Atypical Composition and Ultrastructure of Proteoglycans in the Mouse Corneal Stroma

PURPOSE

Recently, gene-targeted strains of mice with null mutations for specific proteoglycans (PGs) have been used for investigations of the functional role of these molecules. In the present study, the corneal stroma of the mouse was examined to provide some baseline PG morphologies in this species.

METHODS

Monoclonal antibodies to specific glycosaminoglycan (GAG) chain sulfation patterns were used to characterize PG composition in corneal extracts by SDS-PAGE and Western blot analysis and to identify their tissue distribution by immunofluorescence microscopy. PGs were also visualized by transmission electron microscopy after contrast enhancement with cationic dye fixation.

RESULTS

Western blot analysis of pooled corneal extracts and immunofluorescence of tissue sections identified 4-sulfated, but not 6-sulfated, chondroitin sulfate/dermatan sulfate (CS/DS). Keratan sulfate (KS) was present only as a low-sulfated moiety. Electron microscopic histochemistry disclosed a complex array of corneal PGs present as (1) fine filaments radiating from collagen fibrils, and (2) elongate, straplike structures, running either along the fibril axis or weaving across the primary fibril orientation. These large structures were digested by chondroitinase ABC, but not by keratanase.

CONCLUSIONS

KS in the mouse is predominantly undersulfated and generates an immunostaining pattern that differs from that observed in corneas of other mammalian species thus far investigated. The mouse cornea resembles other mammalian corneas in the presence of filamentous arrays of small, collagen-associated stromal PGs visualized by cationic dye staining. However, large dye-positive structures with a CS/DS component are also present and appear to be unique to the cornea of this species.

The organization of collagen in the corneal stroma

The cornea has evolved to fulfil the dual functions of enclosing and protecting the inner contents of the eye, and focussing light onto the retina with minimum scatter and optical degradation. It does this by means of the arrangement of the constituent collagen fibrils, an arrangement that is unique in connective tissues. This article reviews our current knowledge about the detailed organization of collagen in the corneal stroma, and presents new data suggesting that a significant proportion of collagen fibrils running across the cornea, change direction near the limbus and fuse with the circumferential limbal collagen.

A new three-dimensional model of the organization of proteoglycans and collagen fibrils in the human corneal stroma

The purpose of the present study was to re-evaluate the three-dimensional organization of collagen fibrils and proteoglycans (PGs) in the human corneal stroma using an improved ultrastructural approach. After a short aldehyde prefixation, one half of seven fresh corneal buttons was stained for PGs with Quinolinic Phtalocyanin (QP) or Cupromeronic Blue (CB). Strips of 1 mm width were cut, subsequently treated with aqueous phosphotungstic acid (PTA) and further processed for light and electron microscopy. The other half of the corneas served as control and was routinely processed with OsO4. Embedding was as such that ultrathin sections could be cut precisely parallel (frontal sections) or perpendicular (cross sections) to the corneal surface. The mutual connections between collagen fibrils and PGs were studied and the length of PGs and their mutual distance were measured manually at a calibrated final magnification of 70,000 x. Prefixed fresh corneal tissue treated with QP and CB shows no signs of swelling and exhibits well contrasted PGs. In cross sections PGs form a repeating network of ring-like structures (approximately 45 nm) around the collagen fibrils. In frontal sections PGs are aligned orthogonal to the collagen fibrils, are equidistant (approximately 42 nm) attached to the collagen fibrils along their full length and have a thickness of approximately 11 nm and a length of approximately 54 nm. The observed maximal length of the PGs and the occurrence of ring-like structures enwrapping the collagen fibrils urged us to revisit the prevailing model of maurice (1962) on the organization of the corneal stroma. In the new model hexagonal arranged collagen fibrils are interconnected at regular distances with their next-nearest neighbours by groups of six PGs, attached orthogonal to the circumference of the fibrils. In this way a regular meshwork of ring-like structures enwrapping the collagen fibrils is formed. It is discussed that this new model more convincingly explains corneal resistance to compression and stretching and further rationalizes corneal transparency because of the low refractive index difference between the regularly arranged collagen fibrils and their inter-space filled with PGs.

Changes in the refractive index of the stroma and its extrafibrillar matrix when the cornea swells

The transparency of the corneal stroma is critically dependent on the hydration of the tissue; if the cornea swells, light scattering increases. Although this scattering has been ascribed to the disruption caused to the arrangement of the collagen fibrils, theory predicts that light scattering could increase if there is an increased mismatch in the refractive indices of the collagen fibrils and the material between them. The purpose of this article is to use Gladstone and Dale's law of mixtures to calculate volume fractions for a number of different constituents in the stroma, and use these to show how the refractive indices of the stroma and its constituent extrafibrillar material would be expected to change as more solvent enters the tissue. Our calculations predict that solvent entering the extrafibrillar space causes a reduction in its refractive index, and hence a reduction in the overall refractive index of the bovine stroma according to the equation n'(s) = 1.335 + 0.04/(0.22 + 0.24 H'), where n'(s) is the refractive index and H' is the hydration of the swollen stroma. This expression is in reasonable agreement with our experimental measurements of refractive index versus hydration in bovine corneas. When the hydration of the stroma increases from H = 3.2 to H = 8.0, we predict that the ratio of the refractive index of the collagen fibrils to that of the material between them increases from 1.041 to 1.052. This change would be expected to make only a small contribution to the large increase in light scattering observed when the cornea swells to H = 8.

Advantage of the Mark-III FEL for biophysical research and biomedical applications

Although 6.45 micro m is not the strongest absorption band of biological tissues in the mid-infrared, a Mark-III free-electron laser (FEL) tuned to this wavelength can efficiently ablate tissue while minimizing collateral damage. A model has previously been presented that explains this wavelength dependence as a competition between two dynamic processes--explosive vaporization of saline and denaturation of structural proteins. Here it is shown that this model predicts a 'sweet-spot' for each wavelength, i.e. a region of parameter space (incident intensity and pulse width) in which explosive vaporization is preceded by substantial protein denaturation. This sweet-spot is much larger for wavelengths where protein is the dominant chromophore. At other wavelengths, collateral damage may be minimized within the sweet-spot, but the maximum intensities and pulse widths in these regions are insufficient to remove tissue at surgically relevant rates.

Collagen fibrils appear more closely packed in the prepupillary cornea: optical and biomechanical implications

PURPOSE

The size and organization of stromal collagen fibrils influence the biomechanical and optical properties of the cornea and hence its function. How fibrillar structure varies with position across the cornea has not been fully characterized. The present study was designed to quantify the collagen fibril spacing and diameter across the normal human cornea and to relate this to the properties of the tissue.

METHODS

Small-angle x-ray diffraction was used to map in detail the variation in fibril spacing and fibril diameter along orthogonal medial-lateral and inferior-superior meridians of five normal human corneoscleral discs.

RESULTS

Mean fibril diameters remained constant across all corneas up to the limbus, whereupon a sharp increase was observed. However, mean fibril spacing across the central 4 x 3 mm (prepupillary) cornea measured 5% to 7% lower than in the peripheral cornea.

CONCLUSIONS

Collagen fibrils in the prepupillary cornea appear to be more closely packed than in the peripheral cornea. Anisotropy in fibril packing across the cornea has potential implications for the transparency and refractive index of the tissue. Biomechanically, it is possible that the higher packing density of stress-bearing collagen fibrils in the prepupillary cornea is necessary for maintaining corneal strength, and hence curvature, in a region of reduced tissue thickness. By inference, these results could have important implications for the development of corneal models for refractive surgery.

Persistent Haze and Disorganization of Anterior Stromal Collagen Appear Unrelated Following Phototherapeutic Keratectomy

PURPOSE

The theoretical effects on corneal transparency induced by changes in collagen fibril packing following phototherapeutic keratectomy were compared to changes in objective measurements of haze.

METHODS

Phototherapeutic keratectomy was performed on the right eyes of four young rabbits; left eyes were used as controls. Postoperative slit-lamp measurements of haze were taken at regular intervals up to 19 months. Wounded stromas were studied by synchrotron x-ray diffraction to calculate the average interfibrillar spacing of the collagen fibrils. These data were combined with transmission electron microscope measurements, and the summation of scattered fields method was used to predict the transmission of visible light.

RESULTS

Objective measurements of haze were higher than the baseline control throughout the study. Electron micrographs of anterior stroma in 8-month-old wounds displayed irregularly spaced and poorly organized fibrils and x-ray diffraction indicated larger mean interfibrillar spacing compared to the controls. However, the predicted transmission of visible light through the anterior stromal scar tissue was not significantly different than normal.

CONCLUSIONS

Following phototherapeutic keratectomy, anterior corneal collagen fibrils were more widely spaced and unevenly organized than in the normal rabbit cornea. However, this did not cause a significant loss of transparency and was therefore unlikely to contribute to haze.

Neutron and X-ray scattering by ox corneal stroma differentially loaded with bound anions

Ox corneas at near physiological hydration were subjected to two variables: the amount of chloride ions bound to them and exposure of various mixtures of H(2)O/D(2)O as solvent. The preparations were then exposed to a neutron beam and the contrast match points, at which the collagen fibrils of the corneal stroma most nearly matched the scattering density of the various H(2)O/D(2)O mixtures, were measured. In both cases of high and low bound chloride, the contrast match points of the collagen fibril were equal, indicating that there were no significant changes in the water of electrostriction at the fibril surface when chloride ions bind to the stroma. The data suggest that the ligands which bind anions to corneal stroma are not located at the collagen fibril surface. When the chloride binding ligands were extracted from the corneal stroma there were significant changes in the structure of the fibrils. We suggest that the chloride binding ligands may be located within the collagen fibril.

Extraction and purification of decorin from corneal stroma retain structure and biological activity

We developed a method to purify decorin core protein from tissue with the goal of preserving its native structure and biological function. Currently, most procedures rely on the use of denaturing reagents potentially altering the biological activity. Decorin was purified from corneal stromas without the use of detergents or chaotropic reagents. Proteoglycans isolated using anion exchange chromatography on Q-Sepharose were treated with chondroitinase ABC. Decorin was isolated by a second Q-Sepharose chromatography with affinity chromatographies on heparin-Sepharose and concanavalin A-Sepharose. SDS-PAGE revealed a 98.4% pure 44kDa protein identified as decorin with a yield of 35mg per 100 bovine corneas. Identification was confirmed by NanoESI and MALDI qTOF. The novel inclusion of 20% propylene glycol in extraction and column buffers resulted in recoveries of proteoglycans comparable with those observed with detergents and urea. Purified decorin did alter the rate of fibrillogenesis of type I collagen and inhibited the lateral fusion of collagen fibrils. It also bound to [125I]TGF-beta1 with an apparent K(d) of 40nM. Circular dichroism spectroscopy of decorin displayed the spectra of alpha-helices and beta-pleated sheets consistent with those obtained from recombinant decorin. Urea-induced unfolding was cooperative and reversible while thermal denaturation caused irreversible unfolding. Native decorin can be purified from tissue in quantity and quality for biophysical, biochemical, and biological assays.

[A primary study on soluble protein and soluble antigen of corneal stroma in Mooren ' s ulcer patients]

OBJECTIVE

To investigate changes of soluble protein and soluble antigen of corneal stroma in Mooren's ulcer patients and autoantibody in the patients' sera against the soluble antigen.

METHODS

The corneal stromal specimens taken from 15 Mooren's ulcer patients who received lamellar keratoplasty were studied. Soluble protein and soluble antigen of the corneal stroma were extracted. The soluble protein contents were measured. The soluble corneal stromal extracts were tested by using sodium dodecyl sulfate polyacrylamide gel electrophoresis technique (SDS-PAGE). The protein strips of electrophoresis were analyzed using thin layer scanning system. Peripheral vein blood of 15 patients with Mooren's ulcer was taken. Indirect enzyme linked immunosorbent assay (ELISA) was used to test the level of autoantibody in the patients' sera against the soluble antigen of the corneal stroma.

RESULTS

Soluble protein contents of corneal stroma extracts were 0.435 mg/L in the patient group and 0.645 mg/L in the normal control group. SDS-PAGE results of the affected corneal stromal extracts showed that a protein about 12 000 of molecular weight appeared at 0.92 of Rf. 12 000 protein was not found in the control group. The content of 12 000 protein was 22.1% of the total soluble protein of the involved corneal stroma. The level of autoantibody in the patients' sera against the soluble antigen was much higher than that of normal control (t = 15.15,P < 0.001).

CONCLUSIONS

The appearance of 12 000 protein in the soluble protein of the involved corneal stroma may have a connection with the soluble corneal stromal antigen. The level of the autoantibody in the patients' sera against the soluble antigen is significantly higher than that of the control. It represents that Mooren's ulcer patients have an autoimmune reaction to the corneal stroma.

Regional and zonal variations in the sulfation patterns of chondroitin sulfate in normal equine corneal stroma

OBJECTIVE

To determine regional and zonal variation in sulfation patterns of chondroitin sulfate in normal equine corneal stroma.

SAMPLE POPULATION

22 normal eyes from 11 horses.

PROCEDURE

Corneas were collected within 24 hours of death from equine necropsy specimens. After papain-chondroitinase digestion of corneal tissue, disaccharides deltaDi4S and deltaDi6S were quantified by use of capillary zone electrophoresis in the superficial, middle, and deep zones of central and peripheral regions of the cornea.

RESULTS

For the 2 regions combined, deltaDi6S/deltaDi4S values were significantly lower in the deep and middle zones, compared with that of the superficial zone. In the central region, deep and middle zones had significantly lower deltaDi6S/deltaDi4S values than the superficial zone did. In the peripheral region, the deep zone had significantly lower deltaDi6S/deltaDi4S values, compared with superficial and middle zones. In the deep zone, the peripheral region had significantly lower deltaDi6S/deltaDi4S values than the central region did.

CONCLUSIONS AND CLINICAL RELEVANCE

Distribution of deltaDi6S/deltaDi4S values follows a gradient across the healthy equine cornea, being smallest in the deep and middle zones of the central region and the deep zone of the peripheral region. Regional and zonal differences in the distribution of stromal deltaDi6S and deltaDi4S may influence the role of glycosaminoglycans in health, disease, and wound repair of the equine cornea.

An x-ray diffraction investigation of corneal structure in lumican-deficient mice

PURPOSE

The corneas of mice homozygous for a null mutation in lumican, a keratan sulfate-containing proteoglycan, are not as clear as normal. In the present study, mutant corneas were examined by synchrotron x-ray diffraction to see what structural changes might lie behind the loss of transparency.

METHODS

X-ray diffraction patterns were obtained from the corneas of 6-month-old and 2-month-old lumican-null and wild-type mice. Measured in each cornea were the average collagen fibril diameter, average collagen fibril spacing, and the level of order in the collagen array.

RESULTS

The x-ray reflection arising from regularly packed collagen was well-defined on all x-ray patterns from 6-month-old wild-type corneas. Patterns from 6-month-old lumican-deficient corneas, however, contained interfibrillar reflections that were measurably more diffuse, a fact that points to a widespread alteration in the way the collagen fibrils are configured. The same distinction between mutant and wild-type corneas was also noted at 2-months of age. Average collagen fibril spacing was marginally higher in corneas of 6-month-old lumican-null mice than in corneas of normal animals. Unlike x-ray patterns from wild-type corneas, patterns from lumican-deficient corneas of both ages registered no measurable subsidiary x-ray reflection, evidence of a wider than normal range of fibril diameters.

CONCLUSIONS

The spatial arrangement of stromal collagen in the corneas of lumican-deficient mice is in disarray. There is also a considerable variation in the diameter of the hydrated collagen fibrils. These abnormalities, seen at 2 months as well as 6 months of age, probably contribute to the reduced transparency.

[Studies on the change of extracellular matrix in the corneal tissue after photorefractive keratectomy in rabbits]

OBJECTIVE

To investigate the relationship between the haze formation and the change of extracellular matrix (ECM) in the corneal tissue after photorefractive keratectomy (PRK).

METHODS

Twenty-four adult New Zealand White rabbits received PRK in the right eye for minus sign10.00 D ablation. According to the time of rabbits sacrifice, they were divided into 8 groups, and 3 rabbits were in each group. They were immediate, 24 hour, 1 week, 2 week, 1 month, 3 month, 6 month and 12 month groups. After surgery, the corneas were examined periodically by slip-lamp microscopy for haze formation. The right eyes were enucleated, and the corneas were studied with immunohistochemical evaluation for extracellular matrix, including collagen type I, type III, type VI, cellular fibronectin, tenascin and laminin.

RESULTS

The incidence of haze was 100% after PRK. The formation of haze had a tendency from mild to severe, and along with the time prolongation, it gradually decreased and disappeared. The newly synthesized collagen type I, type III, type VI, cellular fibronectin, tenascin and laminin were deposited in the anterior corneal stroma.

CONCLUSION

The formation of haze is closely related to the deposition of ECM elements in the anterior corneal stroma in the surgical area.

Hyaluronidase Treatment, Collagen Fibril Packing, and Normal Transparency in Rabbit Corneas

PURPOSE

Hyaluronidase treatment is the initial step of corneaplasty, a treatment under development that induces stromal softening and involves the application of a custom designed forming lens to achieve modification of refractive error. The purpose of this investigation was to examine changes in the arrangement of stromal collagen fibrils after hyaluronidase treatment.

METHODS

Rabbit corneas were evaluated by slit-lamp microscopy at 0, 2 and 7 days after treatment and haze was assessed by subjective observation. Molecular and interfibrillar Bragg spacing of corneal collagen were measured from synchrotron x-ray scattering patterns. Transmission electron microscopy and digital image analysis were used to calculate radial distribution functions from the positions of collagen fibrils. The calculated fibril sizes and positions were also used to predict the transmission of visible light through these corneas.

RESULTS

Hyaluronidase-treated corneas were shown to have a decreased interfibrillar Bragg spacing of 15% to 21%. Fibril hydration did not change. Transparency of these corneas remained unaltered.

CONCLUSIONS

Hyaluronidase reduced the hydration of the corneal stroma, which led to a more compacted collagen fibril arrangement. This compression was predicted to cause a small reduction in the transmission of visible light through the cornea but not to a point likely to cause visual impairment.

Distribution of ascorbate in the anterior bovine eye

PURPOSE

To analyze the ascorbate distribution in the anterior eye wall to better understand the functional significance of this compound in the eye.

METHOD

Ascorbic acid was determined by high-performance liquid chromatography using an LC-10 system (Shimadzu, Kyoto, Japan). Bovine eye samples were used.

RESULTS

The highest ascorbate concentration was observed in the corneal epithelium, with significantly higher values in the central (1.56 mg/g) than in the peripheral (1.39 mg/g) area. The ascorbate content was similar in the corneal stroma (0.22 mg/g), the Descemet's membrane (DM)/endothelium (0.22 mg/g), and the aqueous humor (0.21 mg/ml). By comparison, the sclera (0.15 mg/g) and the conjunctiva (0.11 mg/g) showed lower values, as did the lacrimal gland (0.09 mg/g) and the serum (0.0008 mg/ml).

CONCLUSIONS

(1) Peak ascorbate concentration was observed in the central corneal epithelium covering the pupillary area. This is compatible with the idea that the ascorbate may act as an UV filter shielding internal eye structures from radiation damage. (2) The ascorbate concentration in the corneal stroma and DM/endothelium was as high as in the aqueous humor, and it is suggested that the aqueous humor plays a key role in the distribution of ascorbate to the anterior eye wall.

Assessment of the effects of cetylpyridium chloride on water content of the collagen-keratocyte matrix of the mammalian corneal stroma ex vivo

The effects of cationic surfactants on the time-dependent increases in hydration of the corneal stroma were investigated to assess if the contribution of the proteoglycans could be titrated and how it might relate to the maximum and minimum swelling properties of the corneal stroma. From recent post-mortem eyes from adult sheep, square (8 x 8 mm) samples of corneal stroma were prepared and incubated in isotonic neutral pH mixed salts solution with added glucose, or pure water, at 37 degrees C. The time-dependent changes in wet mass were assessed over 24 h in the absence or presence of 0. 001-2% w/v cetylpyridium chloride (CPC) or benzalkonium chloride (BAC). The rate and magnitude of stromal swelling was reduced in a concentration-dependent fashion by the surfactants. In mixed salts solution, 100% inhibition of swelling could be achieved at 2% CPC and BAC. In pure water, the relative swelling was much more substantial and could only be attenuated by CPC.

Organization of fibrillar collagen in the human and bovine cornea: collagen types V and III

The localization and fibrillar organization of collagen types V and III in the human and bovine corneal stromas were studied. In the chicken cornea, type V co-assembles with type I collagen as heterotypic fibrils and this interaction is involved in the regulation of fibril diameter necessary for corneal transparency. To determine whether this is a regulatory mechanism common to the corneas of different species the human and bovine corneal stroma were studied. Collagen type V was found in the epithelium and Bowman's membrane in the untreated adult human and bovine cornea using immunofluorescence microscopy. In the absence of any treatment, there was no type V reactivity within the stroma. However, type V collagen was detected homogeneously throughout the corneal stroma after treatments that partially disrupt fibril structure. The reactivity was strongest in the cornea, weaker in the limbus and weakest in the sclera. Fetal corneas showed similar reactivity for type V collagen, but unlike the adult, the stroma was slightly reactive. Immunoelectron microscopy demonstrated that type V collagen was associated with disrupted, but not with intact, fibrils in both human and bovine corneal stroma. Type III collagen reactivity was not detected in the cornea, but was present subepithelially in the limbus and in the scleral stroma. These data indicate that type V collagen is a component of striated collagen fibrils throughout the human and bovine corneal stromas. The interaction of type I and V collagen as heterotypic fibrils masks the helical epitope recognized by the monoclonal antibody against type V collagen. The heterotypic interactions of collagen type V indicate a role in the regulation of fibril diameter analogous to that described in the avian cornea.

Collagen fibrils in the human corneal stroma: structure and aging

PURPOSE

Transparency and biomechanical properties of the cornea depend on the structure and organization of collagen fibrils. The authors determined diameter, axial period, and lateral molecular spacing of collagen fibrils in human corneal stroma as a function of age.

METHODS

Seventeen normal human corneas were investigated in their native state by means of small-angle and wide-angle x-ray scattering.

RESULTS

The mean radius of collagen fibrils, the axial period of collagen fibrils, and the lateral intermolecular Bragg spacing were found to be age dependent. The authors determined fibril radii of 16.1 +/- 0.5 nm in persons older than 65 years of age (n = 10) and 15.4 +/- 0.5 nm (mean +/- SD) in persons younger than 65 years (n = 7) (P < 0.022). The related age-dependent values were 66.4 +/- 0.7 nm (> 65 years) and 65.2 +/- 0.8 nm (< 65 years) for the axial period (P < 0.006) and 1.515 +/- 0.010 nm (> 65 years) and 1.499 +/- 0.013 nm (< 65 years) for the intermolecular Bragg spacing (P < 0.022).

CONCLUSIONS

Aging is related to a three-dimensional growth of collagen fibrils in the human corneal stroma. The age-related growth of the fibril diameter was mostly a result of an increased number of collagen molecules and, in addition, to some expansion of the intermolecular Bragg spacing probably resulting from glycation-induced cross-linking. The observed expansion of the fibrils in an axial direction may result from reduction of the molecular tilting angle within collagen fibrils. The observed alterations of the collagen framework may have implications for refractive surgery and ocular tonometry achieved through related changes in the biomechanical properties of the cornea.

The corneal stroma: an inhomogeneous structure

PURPOSE

This study was conducted to determine the elemental composition of the human cornea. Special attention was paid to corneal stroma inhomogeneity.

METHODS

Seventy human corneas were examined by means of energy-dispersive X-ray analysis. Epithelium, subepithelium, middle stroma, sub-Descemet layer, Descemet's membrane and endothelium were subjected to repeated measurements.

RESULTS

In the cellular layers the phosphorus concentrations were high [0.35 mol/kg dry weight (dw) in the epithelium and 0.403 mol/kg dw in the endothelium]. Similar concentrations were found for sulphur (0.38 mol/kg dw in the epithelium). Stromal layers showed high contents of sulphur: 0.26 mol/kg dw. The phosphorus concentration was found to be higher in the subepithelium than in the middle stroma. Sulphur concentrations were highest in Descemet's membrane, followed by the subepithelium and the middle stroma.

DISCUSSION

Nucleic acids and energy-containing phosphates explain the high levels of phosphorus in the cellular layers. The high sulphur concentrations may be related to the phosphoadenosinphosphosulfate and protein turnover in the epithelium. We interpret the inhomogeneous distribution of phosphorus in the stroma as a function of the density of keratocytes. An evaluation of all known sulphur-containing biochemical components of the stroma (0.217 mol sulphur/kg dw) corresponds to our measurements. In contrast to former results we find the corneal stroma to be an inhomogeneous structure.

Extrusion of abnormal endothelium into the posterior corneal stroma in a patient with posterior polymorphous dystrophy

PURPOSE

To report the presence of abnormal endothelium that extruded into the posterior corneal stroma in a patient with posterior polymorphous dystrophy.

METHODS

The corneal button of a man who underwent penetrating keratoplasty for posterior polymorphous dystrophy was examined by light and electron microscopy. Immunoperoxidase staining for cytokeratins, vimentin, and the endothelial antigen recognized by monoclonal antibody 2B4.14.1 antigen was performed. Two-color immunofluorescence staining for simultaneous detection of cytokeratins and 2B4.14.1 antigen was also done.

RESULTS

Much of the endothelium had characteristic features of epithelium-like cells, and abnormalities in Descemet's membrane were present. Curious oval and slit-like spaces in the posterior stroma were lined by epithelium-like endothelial cells and were continuous with the anterior chamber through defects in Descemet's membrane.

CONCLUSION

These abnormalities in the posterior stroma have not previously been described in histopathologic reports of posterior polymorphous corneal dystrophy and are likely an unusual variation in the spectrum of this hereditary disorder.

X-ray diffraction and transmission electron microscopy of Morquio syndrome type A cornea: a structural analysis

PURPOSE

This case report describes the structural characterization of the corneal stroma from a patient with Morquio syndrome type A.

METHODS

A left penetrating keratoplasty was performed, and the cornea was examined using transmission electron microscopy and synchrotron x-ray diffraction. The interfibrillar proteoglycans were visualized in the electron microscope by using cuprolinic blue.

RESULTS

Stromal collagen fibrils showed a bimodal distribution of diameters: 70% had a distribution comparable to that in normal tissue (20-30 nm) and 30% contained larger fibrils (30-42 nm) as seen by electron microscopy. Both electron microscopy and x-ray diffraction showed that the bulk numeric density of fibrils per unit area in cross-section (number density) was higher than normal in the Morquio syndrome cornea. The arrangement of proteoglycans throughout most of the Morquio syndrome cornea appeared normal, but many of the filaments were twice their normal length. In the anterior stroma, very large proteoglycan filaments (< or = 400 nm long) were found. Other ultrastructural differences also were noted, including abnormal keratocytes and long spacing collagen.

CONCLUSION

The variation in fibril diameter and number density were modeled to account for only a 5% decrease in light scattering compared with the normal cornea. The extensive corneal clouding seen in the Morquio syndrome cornea cannot therefore be attributed to the variation in fibril diameters; collagen-free areas and expanded cells seem to be the most likely cause.

Proteoglycans contain a 4.6-A repeat in corneas with macular dystrophy: II. Histochemical evidence

PURPOSE

Synchrotron x-ray diffraction experiments indicate that corneas with macular corneal dystrophy (MCD) contain unusual 4.6-A periodic repeats thought to reside in proteoglycans or glycosaminoglycans. Recently the 4.6-A x-ray reflection was found to be significantly diminished after incubation of MCD specimens in buffer containing chondroitinase ABC or N-glycanase. We examined the sulfated proteoglycans in these glycosidase-digested MCD corneas.

METHODS

Transmission electron microscopy was used in conjunction with cuprolinic blue-staining for sulfated proteoglycans.

RESULTS

Incubation of an MCD specimen in enzyme buffer left both small and large proteoglycan filaments in the stromal matrix, whereas incubation in the presence of chondroitinase ABC removed these molecules from the tissue. Incubation in buffer containing N-glycanase, on the other hand, removed the large proteoglycan filaments from the MCD stroma but left unaffected the small collagen-associated proteoglycans.

CONCLUSION

These results are consistent with the interpretation that 4.6-A periodic repeats in MCD corneas reside in large sulfated proteoglycan filaments (or aggregates thereof) that may contain chondroitin/dermatan sulfate and keratan sulfate or keratan components.

A comparative study of human corneal keratocyte and endothelial cell density during aging

PURPOSE

To investigate whether the human corneal keratocyte density changes during aging.

METHODS

Comparative data on keratocyte and endothelial cell density (ECD) were obtained from 178 normal corneas (89 persons ranging in age from 30 weeks of gestation to 90 years). Keratocyte density was quantified by using biochemical measurements of the stromal DNA/ mass content within the central 7-mm diameter zone (sDNA) (1 U equals 1 microgram DNA per milligram of dry tissue weight), whereas central ECD was assessed after alizarin red staining.

RESULTS

In the first decade of life, there was a mean sDNA of 1.64 +/- 0.29 U, corresponding to 6.22 +/- 1.1 x 10(4) keratocytes per mm3. A direct correlation between keratocyte density and donor age was found (r = -0.49; p < 0.0001) with a physiologic decline of 0.3% per year throughout life (density = 6.30 x 10(4) keratocytes per mm3-190 x age). A similar decrease of 0.3% per year was observed in the ECD during adulthood, whereas the annual decline was 2.9% during infancy and childhood. The interindividual variation in keratocyte density was of the same magnitude as that seen in ECD. Moreover, keratocyte density was positively correlated with ECD (r = 0.23; p < 0.001); however, after correcting for age by multiple regression analysis, this correlation disappeared, indicating that keratocytes and endothelial cells form distinct cell populations.

CONCLUSIONS

The study demonstrates a linear loss of human corneal keratocytes as a function of age, a loss that parallels the well established decline in ECD.

Organization of Collagen in the Lyophilized Cornea

PURPOSE

To investigate the organization of collagen fibrils in the lyophilized cornea.

METHODS

Freshly harvested porcine corneas (n = 10) were lyophilized and examined by synchrotron x-ray diffraction and transmission electron microscopy.

RESULTS

Collagen fibrils are highly compacted in lyophilized corneas. They become more widely spaced when the tissue is rehydrated, however, the distribution of imbibed water throughout the stroma is not necessarily homogeneous within an individual cornea, nor is it always similar in specimens that have been rehydrated to similar levels. In lyophilized corneas, the mean center-to-center interfibrilar spacing of the regularly arranged collagen reaches levels found in freshly thawed porcine corneas (between 74% and 78% water by weight) when between 74.3% and 81.6% of the rehydrated lyophilized cornea's weight is water.

CONCLUSION

Regularly arranged collagen fibrils are able to reapproximate their original spacings if lyophilized corneal tissue is rehydrated, although the manner in which imbibed water is distributed is somewhat unpredictable.

Collagen fibril orientation in the human corneal stroma and its implication in keratoconus

PURPOSE

The kind and the degree of preferred collagen fibril orientation in normal human corneal stroma were investigated as important qualities of the cornea with respect to its mechanical properties and, hence, to refractive surgery. To determine whether this information is relevant to corneal disease, the authors investigated collagen fibril orientation in several corneas with keratoconus.

METHODS

By means of low-angle x-ray scattering, 17 normal human corneas and four corneas of eyes with keratoconus were investigated.

RESULTS

Collagen fibrils in the normal human corneal stroma showed two preferred orientations orthogonal to each other. These were the horizontal and the vertical directions. The authors defined a degree of orientation gamma, determined to be gamma = 0.49 +/- 0.10 (mean +/- SD). This means that the excess of the preferentially oriented fibrils in relation to the total number of fibrils was approximately 49%. It follows from this value that approximately two thirds of the fibrils (66%) were within in a 45 degrees sector (+/-22.5 degrees) around the horizontal and vertical meridians, whereas approximately one third (34%) is oriented in the oblique sectors in between. No statistically significant variation of gamma within a central 7 mm zone could be detected in normal corneas. The orthogonal arrangement of the collagen fibrils was, however, profoundly altered in keratoconus, in which nonorthogonal orientations were found inside the apical scar.

CONCLUSIONS

The normal human corneal stroma shows a considerable degree of structural anisotropy. It is characterized by two preferred collagen fibril orientations orthogonal to each other. Alteration of the regular orthogonal arrangement of the fibrils in keratoconus may be related to the biomechanical instability of the tissue.

[Lectin-binding histochemical study on developing corneal epithelium and stroma of normal human being]

PURPOSE

To study glycoconjugates in developing cornea of normal human.

METHODS

Lectin-binding histochemical method with five biotin-coupled lectins (conA, LCA, PNA, WGA, RCA).

RESULTS

1. WGA mainly stained the membrane of epithelial cells, especially in surface cells. It also stained anterior stroma of embryo. 2. All the layers of epithelium could be stained by conA, uneven stain was seen in stroma of embryo corneas. 3. RCA, LCA receptor distributed in base of cornea epithelium. 4. PNA can only stain cornea epithelium of 16 embryo.

CONCLUSIONS

1. Glycoconjugates of binding conA, WGA, PNA, LCA, RCA are in the developing corneas of human. 2. The distribution of these lectin receptors may change with corneal developing. It may be related to the maturity of corneas.

Maturation of collagen fibrils in the corneal stroma results in masking of tyrosine-rich region of type V procollagen

PURPOSE

To determine the molecular form of type V procollagen in collagen fibrils in mammalian corneal stromas.

METHODS

The presence of the tyrosine-rich region in the NH2-propeptide of type V procollagen in collagen fibrils was examined in human, bovine, and mouse corneas and human corneal fibroblast cultures by immunofluorescence microscopy and immunoblot analysis using a polyclonal antibody specific for this region. The antibody was generated using a glutathione S-transferase-fusion peptide.

RESULTS

The tyrosine-rich region was detected readily in frozen sections of 5- to 6-month-old mouse corneal stromas without the need for any unmasking techniques, indicating that this domain is exposed on the surface of striated collagen fibrils. In contrast, frozen sections of adult human and bovine corneas did not label with the polyclonal sera to the tyrosine-rich region. Immunoblot analysis of bacterial collagenase digests of human and bovine corneas, however, indicated that peptide fragments containing the tyrosine-rich region of type V procollagen and of the expected molecular weight of 70 to 85 kDa were present. Further immunofluorescence microscopic studies and immunoblot analysis of mouse corneas at different ages and of collagen fibrils formed in human corneal fibroblast cultures over time indicated that, initially, the tyrosine-rich region of type V procollagen could be detected in all these collagen fibrils; however, as the age of the mouse and the culture increased, the ability to detect this region decreased.

CONCLUSIONS

These results suggest that, in vivo, the tyrosine-rich region of type V procollagen is retained on type V procollagen molecules within mammalian collagen fibrils from corneal stromas and that this region becomes masked as collagen fibrils mature or the species ages.

Amino acid sequence of an immunogenic corneal stromal protein

PURPOSE

A unique cornea-associated antigen (CO-Ag) has been purified previously from stromal extracts. The protein is the target for autoantibodies in patients with Mooren's ulcer. In this study, the amino acid sequence of CO-Ag was analyzed and the structure-function properties of CO-Ag was determined.

METHODS

Purified CO-Ag was subjected to N-terminal sequencing by automated Edman degradation. Binding of calcium (Ca2+) to CO-Ag was measured by a direct (45)Ca2+ -binding assay.

RESULTS

The complete amino acid sequence of CO-Ag has been determined. The protein contains 70 amino acids in a single chain and lacks cysteine, tryptophan, and methionine residues. A computerized data base search of protein and nucleic acid sequences revealed strong homology to the Ca2+ -binding proteins of the S-100 family. The sequence of CO-Ag shows a high homology with calgranulin C (CaG-C) previously purified from pig granulocytes. The functional Ca2+ -binding sites of CO-Ag and CaG-C were different based on homology with known Ca2+ -binding domains and their Ca2+ -binding properties. There are three amino acid substitutions in the N-terminal Ca2+ -binding domain. Differences were functionally conserved and compatible, with minimum single-base changes in the codon structures. The greatest difference was located in the C-terminal Ca2+ -binding domain. Five consecutive amino acid changes from D63-K-K-G-A67 in CO-Ag to M63-Q-D-E-Q67 occurred in CaG-C. These differences alter the structure of CO-Ag, which no longer can bind Ca2+ ions. The existence of this nonfunctional Ca2+ -binding site was corroborated by its Ca2+ -binding properties. The number of Ca2+ -binding sites for the CO-Ag sub-unit is approximately half that of the CaG-C monomer, although these two proteins have a similar low binding constant of approximately 2 x 10(-4) M.

CONCLUSIONS

These results suggest that CO-Ag is a new member of the Ca2+ -binding protein of the S-100 family heretofore undescribed in the cornea. Sequence data provide an important framework to search for sequence similarity with microbial proteins as possible substrates for molecular mimicry and for the identification of possible pathogenic epitopes in CO-Ag.

Mass spectrometry analysis of the by-products of intrastromal photorefractive keratectomy

BACKGROUND AND OBJECTIVE

To determine the nature and chemical composition of the cavitation bubbles generated by intrastromal photorefractive keratectomy (IPRK) with the Neodymium: Yttrium Lithium Fluoride (Nd:YLF) picosecond laser.

MATERIALS AND METHODS

IPRK was performed on the corneas of three fresh human cadaver eyes with the Nd:YLF picosecond laser. High energy levels of 200 mJ/pulse to 300 mJ/pulse were used to produce the maximum amount of gas within the corneal stroma. The cavitation bubbles produced in the stroma were aspirated using a gas-tight syringe connected to a valve, and the specimens were analyzed with a mass spectrometer.

RESULTS

The experiment was performed on three different occasions using one cadaver eye in each experiment. The analysis consistently revealed a mixture of carbon dioxide, carbon monoxide, water vapor, oxygen, and nitrogen. Atmospheric control samples also revealed oxygen and nitrogen. No other components were detected.

CONCLUSION

The gaseous by-products of IPRK with the picosecond laser result from the simple breakdown of stromal tissue and contain no toxic or unexpected components. We found that these gaseous products are capable of rapid elimination by absorption or diffusion out of the intact cornea.

Autoimmunity to a cornea-associated stromal antigen in patients with Mooren's ulcer

PURPOSE

To purify and characterize a cornea-associated antigen (CO-Ag) and to determine antibody levels to CO-Ag in patients with Mooren's ulcer.

METHOD

Standard ion exchange and gel filtration chromatographies were used to isolate and purify CO-Ag from crude bovine stromal extracts. The serum of a patient with Mooren's ulcer, containing a high level of antibodies directed against CO-Ag, was used to monitor isolation procedures. Using this newly purified CO-Ag, an enzyme-linked immunoabsorbent assay was used to detect the presence of antibodies to CO-Ag in the sera of other patients with Mooren's ulcer.

RESULTS

CO-Ag was purified to apparent homogeneity from bovine corneal stromal extracts by a series of ion exchange chromatographies and gel filtration. Polyacrylamide gel electrophoresis showed that CO-Ag was a tetramer with a molecular weight of 30,000 d that may dissociate under denaturing conditions into a monomer of 7000 d. Strong indirect immunofluorescent staining was demonstrated of the stroma by guinea pig anti-CO-Ag antibody. A statistically significant difference in the level of specific antibodies to CO-Ag between patients with Mooren's ulcer and controls was found (P < 0.001). The antibody level was elevated in patients with Mooren's ulcer (mean antibody level, 0.58 +/- 0.13) compared with the controls (mean antibody level, 0.22 +/- 0.04).

CONCLUSION

These results suggest that an autoantigen exists in the corneal stroma that reacts with serum antibodies from patients with Mooren's ulcer. The availability of a purified corneal antigen could facilitate the diagnosis and define the pathogenetic mechanisms in Mooren's ulcer.

Quantitative analysis of immunogold labellings of collagen types I, III, IV and VI in healthy and pathological human corneas

BACKGROUND

We studied the distribution of collagen types I, III, IV and VI in one healthy human cornea and in seven pathological human corneas, in which the disorders were three cases of pseudophakic bullous keratopathy (two severe, one moderate) and one case each of stage IV keratoconus, chronic ulcer, vascularized cornea and disciform keratitis.

METHODS

Transmission electron microscopy examinations were performed on post-embedding immunogold-labelled sections. The staining was evaluated by gold particle count in the different tissues. The presence or absence of a given antigen was determined by statistical analysis, using a d-value test.

RESULTS

Our results on healthy corneal tissues corroborate the data available from previous studies, except for collagen type VI, which we found to be absent in Bowman's layer. In pathological corneas with a collagenous layer posterior to Descemet's membrane, collagen types I, III and especially IV were detected in this collagenous layer. Collagen types I, III and VI were detected in the anterior healed stroma of other pathological corneas, except for the keratoconus cornea, in which intense collagen III staining was observed.

CONCLUSION

The presence of collagen types I and III in the posterior collagenous layer of our pseudophakic bullous keratopathy corneas suggests that this layer corresponds to scar tissue secreted by stimulated endothelial cells.

Alpha 1-antichymotrypsin is present in and synthesized by the cornea

The proteinase inhibitor alpha 1-antichymotrypsin is present in the epithelial, stromal and endothelial layers of the human cornea. This was determined by immunolocalization in corneal sections and by Western blot analysis of extracts from the three separated layers. The inhibitor was quantified in the extracts by immunodot blot analysis. The levels observed were 1.3 +/- 0.3 microgram/cornea for the epithelial layer, 22.8 +/- 3.8 micrograms/cornea for the stromal layer and an average of 0.17 micrograms/cornea for the endothelial layer. alpha 1-Antichymotrypsin is being synthesized by the cornea. Metabolically labeled inhibitor was immunoprecipitated from the three layers following organ culture of the intact cornea. Two major forms were detected. These were the native, mature 64 kDa form and a 50 kDa form which is either a degradation product or an incompletely glycosylated form. These results indicate that the cornea has the ability to locally control degradation through synthesis of this inhibitor. Local synthesis of this inhibitor releases the cornea from total dependance upon the vascular system for its supply of alpha 1-antichymotrypsin.

Remodelling of the corneal stroma after lamellar keratoplasty. A synchrotron x-ray diffraction study

Patients healing from lamellar keratorefractive surgeries invariably experience postoperative corneal haze. In our lamellar keratoplasty (LKP) rabbit model, visual recovery is concomitant with the invasion of the grafted tissue by viable keratocytes from adjacent host tissue, and the return of corneal clarity is attributed to the remodelling of the stromal tissue by these keratocytes. We used synchrotron x-ray diffraction techniques to elucidate the ultrastructure of the stromal collagen fibrils in rabbit corneas at various time points in the 3 months after LKP surgery. Lenticules were frozen in 50% of the cases. The average spacing of the collagen molecules, which constitute the stromal fibrils, remains unchanged by LKP in both frozen (1.69-1.73 nm) and nonfrozen (1.58-1.75 nm) cases. In the nonfrozen case, the collagen fibril diameters are initially slightly larger than normal (38.3-41.9 nm) but have receded by 2 months postoperatively [similar to the frozen cases (37.7-41.9 nm)]. The post-LKP spacing of the collagen fibrils in the nonfrozen corneas is unremarkable (56.2-69.2 nm). In contrast, the increased collagen interfibrillar spacing in the frozen case is considerable (56.7 to > 94.6 nm) and variable up to 21 days postoperatively. Because the changes in interfibrillar spacing did not always mirror pachymetry changes in the frozen cases, we suspect occasional graft malapposition or the formation of intrastromal, fluid-filled, collagen-free "lakes."

Ultrastructural distribution of hyaluronan in rat cornea

The ultrastructural distribution of hyaluronan (hyaluronic acid) in rat cornea was investigated using tissue processed by osmium tetroxide/microwave fixation and epoxy resin embedding or by glutaraldehyde/microwave fixation and lowicryl K4M embedding. Hyaluronan-binding proteins (HABP) and link proteins (LP) coupled to 15-20 nm gold particles were used as novel ultrastructural markers in a one-step post-embedding procedure. These probes labelled the intra- and extracellular aspects of corneal epithelial and endothelial plasma membranes, as well as those of stromal keratocytes. In the corneal epithelium, wing and superficial cell plasma membranes were intensely stained, whereas basal cells exhibited a weak reaction on their stromal-facing surfaces and an intermediate one on their lateroapical aspects. In the corneal endothelium labelling was associated primarily with the apical and lateral plasma membranes. The labelling intensity associated with keratocyte plasma membranes was weak.

Hyaluronic acid in the rabbit cornea after excimer laser superficial keratectomy

Hyaluronic acid (HA) is not normally found in the corneal stroma. Rabbit corneas were examined for the presence of stromal HA after excimer laser treatment. One eye in each of 28 rabbits received a 60 microns deep superficial keratectomy with the excimer laser. After 1, 8, 21, and 60 days, the corneas were analyzed by quantitative and histochemical methods specific for HA. A statistically significant increase in the HA concentration compared to the baseline amount in the untreated fellow eye was seen at 8, 21, and 60 days. HA was visualized histochemically in the anterior stroma of the excimer-treated eyes at all times tested. The presence of HA after excimer surgery may influence the hydration, thickness, and transparency of the cornea. The reactive production of HA in the stroma may represent a nonspecific corneal tissue response to injury.

Synchrotron x-ray diffraction studies of keratoconus corneal stroma

The aim of this study was to determine any differences in the collagen molecular and fibrillar packing, or the arrangement of the proteoglycans along the fibril axis, in the stroma of keratoconus and control corneas. High and low-angle x-ray diffraction patterns from the fibrillar and molecular packing of collagen in keratoconus and control corneas were obtained using a synchrotron radiation source. The results indicate no difference in interfibrillar spacing between keratoconus and control corneas at normal physiological hydration, or over a range of hydrations (H = 1-11). This unambiguously demonstrates that the thinning of the stroma that occurs in keratoconus is not a result of closer packing of the collagen fibrils in the stroma. Intermolecular spacings were shown to be significantly (P less than 0.001) lower in keratoconus corneas at normal physiological hydration and over a range of hydrations (H = 1-11). Meridional patterns from the axial distribution of electron density along the collagen fibrils were obtained from untreated control and keratoconus corneas and from the corneas after their proteoglycans were stained with cupromeronic blue. Analysis of the integrated intensities of the first nine orders of these reflections show there is a difference in the staining behavior of collagen-associated proteoglycans in control and keratoconus corneas. Determination of the electron density vectors along the collagen fibrils of cupromeronic blue-stained corneas by the use of Patterson functions indicates that the keratoconus corneal stroma has a specific, ordered proteoglycan that is present in lower numbers along the collagen fibrils, and that it stains less with cupromeronic blue or is in a more disordered arrangement than in the controls.