Keratocan-deficient mice display alterations in corneal structure

Ultrastructural changes in the retinopathy, globe enlarged (rge) chick cornea

In the cornea, the precise organisation of fibrillar collagen and associated proteoglycans comprising the stromal extracellular matrix plays a major role in governing tissue form and function. Recently, abnormal collagen alignment was noted in the misshapen corneas of mature chickens affected by the retinopathy, globe enlarged (rge) mutation. Here we further characterize corneal ultrastructural changes as the rge eye develops post-hatch. Wide-angle X-ray scattering disclosed alteration to dominant collagen lamellae directions in the rge chick cornea, compared to age-matched controls. These changes accompanied eye globe enlargement and corneal flattening in affected birds, manifesting as a progressive loss of circumferential collagen alignment in the peripheral cornea and limbus in birds older than 1 month. Collagen intermolecular separation was unchanged in rge. However, small-angle X-ray scattering results suggest collagen fibril separation and diameter increase more rapidly towards the corneal periphery in rge at 3 months post-hatch compared to controls, although central collagen fibril diameter was unchanged. By transmission electron microscopy utilising cuprolinic blue stain, the morphology and distribution of stromal proteoglycans were unaltered in rge corneas otherwise demonstrating abnormal collagen fibril organisation. From a numerical simulation of tissue mechanics, progressive remodelling of stromal collagen in rge during globe enlargement post-hatch appears to be related to the corneal morphometric changes presented by the disease.

Genetic evidence for the coordinated regulation of collagen fibrillogenesis in the cornea by decorin and biglycan

Decorin and biglycan are class I small leucine-rich proteoglycans (SLRPs) involved in regulation of collagen fibril and matrix assembly. We hypothesize that tissue-specific matrix assembly, such as in the cornea, requires a coordinate regulation involving multiple SLRPs. To this end, we investigated the expression of decorin and biglycan in the cornea of mice deficient in either SLRP gene and in double-mutant mice. Decorin and biglycan exhibited overlapping spatial expression patterns throughout the corneal stroma with differential temporal expression. Whereas decorin was expressed at relatively high levels in all developmental stages, biglycan expression was high early, decreased during development, and was present at very low levels in the mature cornea. Ultrastructural analyses demonstrated comparable fibril structure in the decorin- and biglycan-null corneas compared with wild-type controls. We found a compensatory up-regulation of biglycan gene expression in the decorin-deficient mice, but not the reverse. Notably, the corneas of compound decorin/biglycan-null mice showed severe disruption in fibril structure and organization, especially affecting the posterior corneal regions, corroborating the idea that biglycan compensates for the loss of decorin. Fibrillogenesis assays using recombinant decorin and biglycan confirmed a functional compensation, with both having similar effects at high SLRP/collagen ratios. However, at low ratios decorin was a more efficient regulator. The use of proteoglycan or protein core yielded comparable results. These findings provide firm genetic evidence for an interaction of decorin and biglycan during corneal development and further suggest that decorin has a primary role in regulating fibril assembly, a function that can be fine-tuned by biglycan during early development.

Homologous sequence in lumican and fibromodulin leucine-rich repeat 5-7 competes for collagen binding

Lumican and fibromodulin compete for collagen type I binding in vitro, and fibromodulin-deficient mice have 4-fold more lumican in tendons. These observations indicate that homologous sequences in lumican and fibromodulin bind to collagen type I. Here, we demonstrate that lumican binding to collagen type I is mediated mainly by Asp-213 in leucine-rich repeat (LRR) 7. The mutation D213N in lumican impairs interaction with collagen, and the lumican fragment spanning LRRs 5-7 is an efficient inhibitor of collagen binding. Also, the lumican LRR 7 sequence-based synthetic peptide CYLDNNKC inhibits the binding to collagen. Homologous collagen-binding site in fibromodulin, located in LRRs 5-7, inhibits the binding of lumican to collagen, and the mutation E251Q in this fibromodulin fragment does not inhibit the lumican-collagen binding. Lumican, but not the D213N mutation, lowers the melting point and affects the packing of collagen fibrils.

Increased stromal extracellular matrix synthesis and assembly by insulin activated bovine keratocytes cultured under agarose

Previously, pharmacological levels of insulin have been shown to stimulate the synthesis of normal corneal stromal collagen and proteoglycans by bovine keratocytes in culture. Here we compared insulin to physiological levels of IGF-I and found that IGF-I also stimulated the synthesis of these extracellular matrix components, but less than that of insulin. Keratocytes in monolayer culture secreted most of the collagen synthesized into the media in the form of procollagen, a precursor of collagen. We found that an overlay of 3% agarose on the keratocytes in culture enhanced the conversion of procollagen to collagen and increased the deposition of collagen and proteoglycans into the cell layer. The extracellular matrix associated with the keratocytes cultured under agarose exhibited a corneal stromal-like architecture. These results suggest that enhancing the conversion of procollagen to collagen is a key step in the formation of extracellular matrix by keratocytes in vitro. Agarose overlay of insulin activated keratocytes in culture is a useful model for studying corneal stromal extracellular matrix assembly in vitro.

Fragmentation of decorin, biglycan, lumican and keratocan is elevated in degenerate human meniscus, knee and hip articular cartilages compared with age-matched macroscopically normal and control tissues

Introduction

The small leucine-rich proteoglycans (SLRPs) modulate tissue organization, cellular proliferation, matrix adhesion, growth factor and cytokine responses, and sterically protect the surface of collagen type I and II fibrils from proteolysis. Catabolism of SLRPs has important consequences for the integrity of articular cartilage and meniscus by interfering with their tissue homeostatic functions.

Methods

SLRPs were dissociatively extracted from articular cartilage from total knee and hip replacements, menisci from total knee replacements, macroscopically normal and fibrillated knee articular cartilage from mature age-matched donors, and normal young articular cartilage. The tissue extracts were digested with chondroitinase ABC and keratanase-I before identification of SLRP core protein species by Western blotting using antibodies to the carboxyl-termini of the SLRPs.

Results

Multiple core-protein species were detected for all of the SLRPs (except fibromodulin) in the degenerate osteoarthritic articular cartilage and menisci. Fibromodulin had markedly less fragments detected with the carboxyl-terminal antibody compared with other SLRPs. There were fewer SLRP catabolites in osteoarthritic hip than in knee articular cartilage. Fragmentation of all SLRPs in normal age-matched, nonfibrillated knee articular cartilage was less than in fibrillated articular cartilage from the same knee joint or total knee replacement articular cartilage specimens of similar age. There was little fragmentation of SLRPs in normal control knee articular cartilage. Only decorin exhibited a consistent increase in fragmentation in menisci in association with osteoarthritis. There were no fragments of decorin, biglycan, lumican, or keratocan that were unique to any tissue. A single fibromodulin fragment was detected in osteoarthritic articular cartilage but not meniscus. All SLRPs showed a modest age-related increase in fragmentation in knee articular and meniscal cartilage but not in other tissues.

Conclusion

Enhanced fragmentation of SLRPs is evident in degenerate articular cartilage and meniscus. Specific decorin and fibromodulin core protein fragments in degenerate meniscus and/or human articular cartilage may be of value as biomarkers of disease. Once the enzymes responsible for their generation have been identified, further research may identify them as therapeutic targets.

Morphological differences between the trabecular meshworks of zebrafish and mammals

PURPOSE

The zebrafish has been used as an animal model to study ocular development and diseases, including glaucoma. However, there are still many concerns about the morphological differences between zebrafish and mammals. Before using the zebrafish for glaucoma studies, we should understand the morphological differences in the trabecular meshworks (TMs) of zebrafish and other animal models. This study investigated and compared the histological morphologies and compositions of the extracellular matrices of the TMs of the zebrafish and some commonly used animal models, including the mouse, rat, rabbit, and cow.

METHODS

Sections of the angular portions from the studied species (mouse, rat, rabbit, cow, zebrafish, and human) were prepared for immunohistochemical and electron microscopic analyses. Antibodies directed against cytoskeletal and extracellular matrix components (AE1/AE3, vimentin, alpha-smooth muscle actin, keratocan, and lumican) were used for immunolocalization. Reverse transcription polymerase chain reaction (RT-PCR) for keratocan and lumican was also performed.

RESULTS

The TMs of the mouse, rat, and human consist of extracellular matrix organized into a network of beams covered in trabecular endothelial cells. However, no lamellate meshwork exists in the TMs of the rabbit, cow, or zebrafish. Instead, a reticular meshwork (rabbit and cow) and an annular ligament (zebrafish) develop. Immunohistological analysis revealed that vimentin is expressed in the TMs of the rat, rabbit, and human, and alpha-smooth muscle actin is expressed in the TMs of the mouse, rat, rabbit, and human. Only the annular ligament of the zebrafish stained positively with anti-AE1/AE3 antibody. The annular ligament of the zebrafish also expresses keratocan and lumican. The human TM showed weakly positive staining of lumican. A prominent distribution of mitochondria and intracellular vacuoles is observed in the trabecular cells of the mouse, rat, rabbit, and cow, but not the zebrafish. The analysis of RT-PCR shows the keratocan and lumican mRNAs are expressed in the annular ligament of zebrafish, but not in mouse, rat, rabbit, and cow.

CONCLUSIONS

We conclude that the zebrafish expresses different extracellular matrix proteins and has a distinctive ultrastructure in the TM. Therefore, zebrafish should be used with caution for glaucoma studies.

Excess FGF-7 in corneal epithelium causes corneal intraepithelial neoplasia in young mice and epithelium hyperplasia in adult mice

We hypothesized that human ocular surface squamous neoplasia (OSSN) may result from the continuous growth stimulation of corneal epithelial progenitor cells. In the present study, we analyzed the effects of excess fibroblast growth factor-7 (FGF-7) on both the proliferation and differentiation of corneal epithelium in a novel Krt12-rtTA/tet-O-FGF-7 double transgenic mouse model in which cornea-specific FGF-7 overexpression is achieved by doxycycline (Dox) treatment. When such adult mice were exposed to Dox, they exhibited epithelial hyperplasia with increases in phospho-extracellular signal-regulated kinase 1/2-, nuclear beta-catenin-, and 5-bromo-2'-deoxyuridine-labeled cells and altered keratin (K) 14 (K14) expression pattern, a normal K12 expression pattern, and the normal absence of K10. Hyperplasia of the adult cornea was fully reversible 2 weeks after the removal of Dox from chow. In contrast, double transgenic embryos that were exposed to Dox from embryonic day 0.5 to postnatal day 21 developed papillomatous tumors in the cornea, resembling human OSSN, and ectopic gland-like structures in the limbus, accompanied by the down-regulation of K12 and the up-regulation of K14, Pax6, and p63. These epithelial anomalies observed in young experimental mice were not fully resolved after the termination of Dox induction. Taken together, Krt12-rtTA/tet-O-FGF-7 mice may be a suitable animal model for the study of the molecular and cellular mechanisms of human OSSN.

IGF-II is present in bovine corneal stroma and activates keratocytes to proliferate in vitro

Extracts of bovine corneal stroma have been shown to activate keratocytes in culture to proliferate. We fractionated stromal extract on a column of Sephacryl S-300 and tested the fractions for mitogenic activity using cell culture and for the presence of IGF-II and its binding protein IGFBP-2 by Western blot. We found that the mitogenic activity in the extract separated into major and minor peaks and that immunologically detectable IGF-II and IGFBP-2 co-eluted with the minor peak. We also compared the effects of 10 ng IGF-II/ml on keratocytes in culture to that of 2 ng TGF-beta/ml over a 7-day culture period. We found that IGF-II and TGF-beta, alone or combined, increased both (3)H-thymidine incorporation and DNA content of the cultures. The phenotype of the cells was determined by using antibodies to alpha-SM (smooth muscle) actin, fibronectin, SPARC, lumican and keratocan in Western blots of cell layers of media. Keratocytes cultured in IGF-II expressed no alpha-SM actin or fibronectin, low levels of SPARC and high levels of lumican and keratocan, indicating a native phenotype. Keratocytes in TGF-beta expressed alpha-SM actin, fibronectin, SPARC and lumican, and expressed no or low levels of keratocan, indicating a myofibroblast phenotype. Keratocytes cultured in IGF-II plus TGF-beta, however, expressed alpha-SM actin, fibronectin, SPARC, lumican, and keratocan by day 7 of culture. The results of this study show that IGF-II to be present in the corneal stroma, to stimulate keratocyte proliferation while maintaining native phenotype and to override the TGF-beta mediated down regulation of keratocan production. The IGF-II in the stroma may serve as a mechanism to immediately activate keratocytes upon wounding and to ameliorate the scarring effects of TGF-beta.

Molecular analysis and characterization of zebrafish keratocan (zKera) gene

Corneal small leucine-rich proteoglycans play a pivotal role in maintaining corneal transparency and function. In this study, we isolated and characterized the zebrafish (Danio rerio) keratocan (zKera) gene. The human keratocan sequence was used to search zebrafish homologues. The zKera full-length genomic DNA and cDNA were generated via PCR of zebrafish genomic DNA and reverse transcription-PCR of total zebrafish eye RNA, respectively. The zKera spanning 3.5 kilobase pairs consists of two exons and one intron and a TATA-less promoter. The zKera encodes 341 amino acids with 59% identity to its human counterpart and 57% identity to that of mouse keratocan. Like mouse and chick keratocan, zKera mRNA is selectively expressed in the adult cornea; however, during embryonic development, zKera mRNA is expressed in both the brain and the cornea. Interestingly, it is expressed mainly in corneal epithelium but also in the stroma. A pseudogene was proved by introducing a zKera promoter-driven enhanced green fluorescence protein reporter gene into fertilized zebrafish eggs. Using morpholino-antisense against zKera to knock down zKera resulted in a lethal phenotype due to massive caspase-dependent apoptosis, which was noted by a significant increase of active caspase-3 and caspase-8 in the developing forebrain area, including the eyes. This is different from mouse, for which keratocan-deficient mice are viable. Taken together, our data indicate that mammalian keratocan is conserved in zebrafish in terms of gene structure, expression pattern, and promoter function.

Keratocan and lumican regulate neutrophil infiltration and corneal clarity in lipopolysaccharide-induced keratitis by direct interaction with CXCL1

Keratocan and lumican are keratan-sulfate proteoglycans (KSPG), which have a critical role in maintaining corneal clarity. To determine whether these KSPGs have a role in corneal inflammation, we examined Kera(-/-) and Lum(-/-) mice in a model of lipopolysaccharide (LPS)-induced keratitis in which wild-type mice develop increased corneal thickness and haze due to neutrophil infiltration to the corneal stroma. Corneal thickness increases caused by LPS mice were significantly lower in Kera(-/-) and Lum(-/-) than wild-type mice. Further, LPS-injected Lum(-/-) mice had elevated corneal haze levels compared with that of Kera(-/-) and wild-type. At 24 h post-injection, total enhanced green fluorescent protein-positive bone marrow-derived inflammatory cells in chimeric mice was significantly lower in Kera(-/-) mice and Lum(-/-) mice compared with wild-type mice. Neutrophil infiltration was inhibited in Kera(-/-) and Lum(-/-) mice at 6 and 24 h post-stimulation, with Lum(-/-) corneas having the most profound defect in neutrophil migration. Reconstitution of keratocan and lumican expression in corneas of Kera(-/-) and Lum(-/-) mice using adeno-keratocan and adeno-lumican viral vectors, respectively, resulted in normal neutrophil infiltration in response to LPS. Immunoprecipitation/Western blot analysis showed that lumican and keratocan core proteins bind the CXC chemokine KC during a corneal inflammatory response, indicating that corneal KSPGs mediate neutrophil recruitment to the cornea by regulating chemokine gradient formation. Together, these data support a significant role for lumican and keratocan in a corneal inflammatory response with respect to edema, corneal clarity, and cellular infiltration.

Differential expression of the keratan sulphate proteoglycan, keratocan, during chick corneal embryogenesis

Keratan sulphate (KS) proteoglycans (PGs) are key molecules in the connective tissue matrix of the cornea of the eye, where they are believed to have functional roles in tissue organisation and transparency. Keratocan, is one of the three KS PGs expressed in cornea, and is the only one that is primarily cornea-specific. Work with the developing chick has shown that mRNA for keratocan is present in early corneal embryogenesis, but there is no evidence of protein synthesis and matrix deposition. Here, we investigate the tissue distribution of keratocan in the developing chick cornea as it becomes compacted and transparent in the later stages of development. Indirect immunofluorescence using a new monoclonal antibody (KER-1) which recognises a protein epitope on the keratocan core protein demonstrated that keratocan was present at all stages investigated (E10-E18), with distinct differences in localisation and organisation observed between early and later stages. Until E13, keratocan appeared both cell-associated and in the stromal extracellular matrix, and was particularly concentrated in superficial tissue regions. By E14 when the cornea begins to become transparent, keratocan was located in elongate arrays, presumably associated along collagen fibrils in the stroma. This fibrillar label was still concentrated in the anterior stroma, and persisted through E15-E18. Presumptive Bowman's layer was evident as an unlabelled subepithelial zone at all stages. Thus, in embryonic chick cornea, keratocan, in common with sulphated KS chains in the E12-E14 developmental period, exhibits a preferential distribution in the anterior stroma. It undergoes a striking reorganisation of structure and distribution consistent with a role in relation to stromal compaction and corneal transparency.

The molecular diversity of glycosaminoglycans shapes animal development

Proteoglycans (PGs), molecules in which glycosaminoglycans (GAGs) are covalently linked to a protein core, are components of the extracellular matrix of all multicellular organisms. Sugar moieties in GAGs are often extensively modified, which make these molecules enormously complex. We discuss here the role of PGs during animal development, emphasizing the in vivo significance of sugar modifications. We explore a model in which the modification patterns of GAG chains may provide a specific code that contributes to the correct development of a multicellular organism.

Ocular Surface Tissue Morphogenesis in Normal and Disease States Revealed by Genetically Modified Mice

Many transgenic and knockout mice exhibit pathogenic processes resembling human ocular surface diseases. Thus, the clinical manifestations of mouse lines can provide clues for identifying heritable human diseases of unknown etiology. However, mouse lines using conventional techniques of transgenesis and gene targeting often exhibit embryonic lethality and congenital defects, which preclude the use of such mouse models to study acquired ocular surface tissue diseases. These difficulties can be in part overcome by preparing mouse lines of inducible transgene expression, tissue-specific gene ablation, and inducible tissue-specific gene ablation. Conditional transgenic mouse lines live normally until administration of doxycycline and hormones that induce expression of the transgene and ablation of gene of interest. Toward this goal, we prepared 2 groups of genetically modified mouse lines: (1) transgenesis using keratocan promoter was used to create Kera-rtTA mice (doxycycline-inducible mice) and Cre-LoxP system (ie, Kera-Cre mice; conditional gene ablation in neural crest cell lineage and adult stromal keratocyte) and Kera-CrePR mice (RU-486 inducible); and (2) knock-in strategies were used to create Krt12-rtTA mice (doxycycline inducible), Krt12-Cre mice (conditional ablation in corneal epithelium), and Krt12rtTA-tet-O-Cre mice (doxycycline-inducible corneal epithelium-specific gene ablation). Using these mouse lines, we showed that transforming growth factor (TGF)-beta2 is essential for eye morphogenesis, TGF-alpha is a morphogen for eyelid formation, and lumican is a matrikine that has multiple regulatory functions on cell activities (eg, migration proliferation and gene expression) besides serving as a regulatory molecule of collagen fibrillogenesis. These mouse lines can also be used as models for development of therapeutic treatment regimens of ocular surface diseases using gene therapy and stem cell strategies.

Collagen fibril assembly during postnatal development and dysfunctional regulation in the lumican-deficient murine cornea

The transparent cornea is the outer barrier of the eye and is its major refractive surface. Development of a functional cornea requires a postnatal maturation phase involving development, growth and organization of the stromal extracellular matrix. Lumican, a leucine-rich proteoglycan, is implicated in regulating assembly of collagen fibrils and the highly organized extracellular matrix essential for corneal transparency. We investigated the regulatory role(s) of lumican in fibril assembly during postnatal corneal development using wild type (Lum+/+) and lumican-null (Lum-/-) mice. In Lum+/+ mice, a regular architecture of small-diameter fibrils is achieved in the anterior stroma by postnatal day 10 (P10), while the posterior stroma takes longer to reach this developmental maturity. Thus, the anterior and the posterior stroma follow distinct developmental timelines and may be under different regulatory mechanisms. In Lum-/- mice, it is the posterior stroma where abnormal lateral associations of fibrils and thicker fibrils with irregular contours are evident as early as P10. In contrast, the anterior stroma is minimally perturbed by the absence of lumican. In Lum+/+ mice, lumican is expressed throughout the developing stroma at P10, with strong expression limited to the posterior stroma in the adult. Therefore, the posterior stroma, which is most vulnerable to lumican-deficiency, demonstrates an early developmental defect in fibril structure and architecture in the Lum-/- mouse. These defects underlie the reported increased light scattering and opacity detectable in the adult. Our findings emphasize the early regulation of collagen structure by lumican during postnatal development of the cornea.

Corneal ectasia and hydrops in a patient with autosomal recessive cornea plana

PURPOSE

To report the development of corneal ectasia and hydrops in a patient with autosomal recessive cornea plana.

METHODS

Retrospective observational case report.

RESULTS

A 16-year-old male with a prior diagnosis of autosomal recessive cornea plana who complained of unilateral visual loss of one month's duration was found to have corneal edema consistent with resolving hydrops in the affected eye. The edema resolved over time, and keratometry revealed high astigmatism in both eyes despite documentation of no significant corneal astigmatism 11 years before. Slit-lamp examination confirmed corneal thinning in both eyes corresponding to the meridian of the astigmatism. The prior diagnosis of cornea plana was confirmed by molecular genetic testing.

CONCLUSIONS

Although not a characteristic finding of cornea plana, corneal ectasia can rarely occur and be associated with corneal hydrops.

Recessive cornea plana in the Kingdom of Saudi Arabia

OBJECTIVE

To characterize the molecular genetics of clinically diagnosed recessive cornea plana in the Kingdom of Saudi Arabia and establish the presence of common or limited founders (ancestors who originally harbored the disease-causing mutation) in the country's historically isolated population.

DESIGN

Prospective interventional case series.

PARTICIPANTS

Twelve affected patients from apparently unrelated Saudi Arabian nuclear families with clinically diagnosed recessive cornea plana.

METHODS

Clinical ophthalmic examination and venous blood sampling for DNA sequencing.

MAIN OUTCOME MEASURES

Age, gender, keratometry, best-corrected visual acuity, ocular alignment, cycloplegic refraction, significant findings of a complete ophthalmic examination, and keratocan gene (KERA) haplotype analysis.

RESULTS

All 12 individuals had classic phenotypic features of recessive cornea plana and were homozygous for 1 of 2 KERA mutations--a novel frameshift mutation (1634delC) or a previously reported nonsense mutation (R313X). Haplotype analysis was consistent with a separate distinct common founder effect for each instance. An additional Saudi KERA mutation (R279X) has been reported previously in one family.

CONCLUSION

Specific for mutation in KERA, the ophthalmic phenotype of recessive cornea plana does not significantly vary with different KERA mutations. The occurrence of a rare inherited disease in a historically isolated population is not always due to a single common founder effect; it may be explained by cultural preferences such as consanguinity (intrafamilial marriage) and endogamy (intratribal marriage), which enhance expression of recessively inherited diseases.

Matrix morphogenesis in cornea is mediated by the modification of keratan sulfate by GlcNAc 6-O-sulfotransferase

Matrix assembly and homeostasis in collagen-rich tissues are mediated by interactions with proteoglycans (PGs) substituted with sulfated glycosaminoglycans (GAGs). The major GAG in cornea is keratan sulfate (KS), which is N-linked to one of three PG core proteins. To ascertain the importance of the carbohydrate chain sulfation step in KS functionality, we generated a strain of mice with a targeted gene deletion in Chst5, which encodes an N-acetylglucosamine-6-O-sulfotransferase that is integral to the sulfation of KS chains. Corneas of homozygous mutants were significantly thinner than those of WT or heterozygous mice. They lacked high-sulfated KS, but contained the core protein of the major corneal KSPG, lumican. Histochemically stained KSPGs coassociated with fibrillar collagen in WT corneas, but were not identified in the Chst5-null tissue. Conversely, abnormally large chondroitin sulfate/dermatan sulfate PG complexes were abundant throughout the Chst5-deficient cornea, indicating an alteration of controlled PG production in the mutant cornea. The corneal stroma of the Chst5-null mouse exhibited widespread structural alterations in collagen fibrillar architecture, including decreased interfibrillar spacing and a more spatially disorganized collagen array. The enzymatic sulfation of KS GAG chains is thus identified as a key requirement for PG biosynthesis and collagen matrix organization.

The role of dermatopontin in the stromal organization of the cornea

PURPOSE

Dermatopontin (DPT) is an abundant component of the stromal extracellular matrix; however, its function in the cornea is poorly understood. This study was conducted to determine whether DPT has a direct role in corneal matrix organization by investigating the ultrastructure of Dpt-null (Dpt(-/-)) mouse corneas.

METHODS

Conventional light microscopy was used to compare the corneal thickness of Dpt(-/-) mice with that of the wild type. Collagen fibril distribution was studied using transmission electron microscopy and the datasets analyzed using image analysis software to determine fibrillar volume, fibril diameter, and spacing.

RESULTS

Light microscopy demonstrated that Dpt(-/-) corneas in 2-month-old mice showed a 24% reduction in average stromal thickness compared with wild type (P < 0.001). The epithelium and Descemet's membrane appeared normal. Examination of Dpt(-/-) stroma by transmission electron microscopy indicated significant disruption of fibril spacing within the posterior lamellae, whereas the mid and anterior regions appeared largely unaffected compared with wild type. The collagen fibrils in Dpt(-/-) stroma showed a lower fibril volume fraction and a pronounced change in posterior fibrillar organization. There was no apparent difference in fibril diameter between Dpt(-/-) and wild-type mice.

CONCLUSIONS

Collectively, these data suggest that DPT plays a key role in collagen fibril organization. The defects in collagen organization in Dpt(-/-) cornea appear to be most severe in the posterior stroma. It is possible that DPT interacts with corneal proteoglycans and that this interaction is involved in the maintenance of stromal architecture.

Clinical and molecular characterization of a patient with an interstitial deletion of chromosome 12q15-q23 and peripheral corneal abnormalities

PURPOSE

To describe the ocular features of a patient with an interstitial deletion of chromosome 12 and to determine the molecular boundaries of the deletion.

DESIGN

Observational case report and laboratory investigation.

METHODS

A patient with an interstitial deletion of chromosome 12 was clinically examined for ocular abnormalities. DNA samples were used for molecular studies to define the deletion boundaries.

RESULTS

Ocular examination showed abnormalities of the anterior segment consistent with a diagnosis of cornea plana. Molecular analyses showed the deletion included the KERA gene, the SLRP (small leucine repeat protein) gene cluster, the genetic loci for autosomal-dominant (CNA1) and autosomal-recessive (CNA2) cornea plana, and a portion of the mapped locus for high myopia (MYP3).

CONCLUSIONS

These results, combined with previous genetic linkage studies, identifies a 3-cM region located between microsatellite markers D12S82 and D12S351 that is likely to contain a gene responsible for CNA1.

Perlecan from human epithelial cells is a hybrid heparan/chondroitin/keratan sulfate proteoglycan

Perlecan is a multidomain proteoglycan, usually substituted with heparan sulphate (HS), and sometimes substituted with both HS and chondroitin sulphate (CS). In this paper, we describe perlecan purified from HEK-293 cells substituted with HS, CS and keratan sulphate (KS). KS substitution was confirmed by immunoreactivity with antibody 5D4, sensitivity to keratanase treatment, and fluorophore-assisted carbohydrate electrophoresis. HEK-293 perlecan failed to promote FGF-dependent cell growth in an in vitro assay. This study is the first to report perlecan containing KS, and makes perlecan one of only a very few proteoglycans substituted with three distinct types of glycosaminoglycan chains.

Differential gene expression in anatomical compartments of the human eye

DNA microarrays (representing approximately 30,000 human genes) were used to analyze gene expression in six different human eye compartments, revealing candidate genes for diseases affecting the cornea, lens and retina.

Background

The human eye is composed of multiple compartments, diverse in form, function, and embryologic origin, that work in concert to provide us with our sense of sight. We set out to systematically characterize the global gene expression patterns that specify the distinctive characteristics of the various eye compartments.

Results

We used DNA microarrays representing approximately 30,000 human genes to analyze gene expression in the cornea, lens, iris, ciliary body, retina, and optic nerve. The distinctive patterns of expression in each compartment could be interpreted in relation to the physiology and cellular composition of each tissue. Notably, the sets of genes selectively expressed in the retina and in the lens were particularly large and diverse. Genes with roles in immune defense, particularly complement components, were expressed at especially high levels in the anterior segment tissues. We also found consistent differences between the gene expression patterns of the macula and peripheral retina, paralleling the differences in cell layer densities between these regions. Based on the hypothesis that genes responsible for diseases that affect a particular eye compartment are likely to be selectively expressed in that compartment, we compared our gene expression signatures with genetic mapping studies to identify candidate genes for diseases affecting the cornea, lens, and retina.

Conclusion

Through genome-scale gene expression profiling, we were able to discover distinct gene expression 'signatures' for each eye compartment and identified candidate disease genes that can serve as a reference database for investigating the physiology and pathophysiology of the eye.

PAX6 expression identifies progenitor cells for corneal keratocytes

Keratocytes of the corneal stroma produce a transparent extracellular matrix required for vision. During wound-healing and in vitro, keratocytes proliferate, becoming fibroblastic, and lose biosynthesis of unique corneal matrix components. This study sought identification of cells in the corneal stroma capable of assuming a keratocyte phenotype after extensive proliferation. About 3% of freshly isolated bovine stromal cells exhibited clonal growth. In low-mitogen media, selected clonal cultures displayed dendritic morphology and expressed high levels of keratan sulfate, aldehyde dehydrogenase 3A1, and keratocan, molecular markers of keratocyte phenotype. In protein-free media, both primary keratocytes and selected clonal cells aggregated to form attachment-independent spheroids expressing elevated levels of those marker molecules. The selected clonal cells exhibited normal karyotype and underwent replicative senescence after 65-70 population doublings; however, they continued expression of keratocyte phenotypic markers throughout their replicative life span. The progenitor cells expressed elevated mRNA for several genes characteristic of stem cells and also for genes expressed during ocular development PAX6, Six2, and Six3. PAX6 protein was detected in the cultured progenitor cells and a small number of stromal cells in intact tissue but was absent in cultured keratocytes and fibroblasts. Cytometry demonstrated PAX6 protein in 4% of freshly isolated stromal cells. These results demonstrate the presence of a previously unrecognized population of PAX6-positive cells in adult corneal stroma that maintain the potential to assume a keratocyte phenotype even after extensive replication. The presence of such progenitor cells has implications for corneal biology and for cell-based therapies targeting corneal scarring.

Maintenance of the keratocyte phenotype during cell proliferation stimulated by insulin

Keratocytes normally express high levels of aldehyde dehydrogenase and keratocan. They proliferate and lose their keratocyte markers when they become fibroblastic during corneal wound healing. Keratocytes cultured in fetal bovine serum also become fibroblastic, proliferate, and lose these markers. In this report, we studied the effects of three serum growth factors, fibroblast growth factor-2, insulin, and platelet-derived growth factor-BB, on keratocyte proliferation and the maintenance of the keratocyte markers in 7-day cultures in cells plated at low (5,000 cells/cm2) and high (20,000 cells/cm2) density in serum-free medium. Keratocyte proliferation was measured by [3H]thymidine incorporation and by DNA content of the cultures. Cytosolic aldehyde dehydrogenase and keratocan accumulated in the medium were quantified by Western blot. The results showed that all the growth factors stimulated proliferation, but insulin stimulated proliferation more consistently. The keratocyte markers aldehyde dehydrogenase and keratocan were maintained after 7 days in culture in all growth factors, but keratocyte cell morphology was only maintained in medium containing insulin. Most of the proteoglycans were degraded in cultures of keratocytes plated at low density and cultured in the absence of growth factors. This degradation was prevented when keratocytes were cultured in the presence of the growth factors or when keratocytes were plated at high density. The results of this study show that insulin can expand keratocytes in vitro, maintain their phenotype, and prevent proteoglycan degradation.

Keratocan, a cornea-specific keratan sulfate proteoglycan, is regulated by lumican

Lumican is an extracellular matrix glycoprotein widely distributed in mammalian connective tissues. Corneal lumican modified with keratan sulfate constitutes one of the major proteoglycans of the stroma. Lumican-null mice exhibit altered collagen fibril organization and loss of corneal transparency. A closely related protein, keratocan, carries the remaining keratan sulfate of the cornea, but keratocan-null mice exhibit a less severe corneal phenotype. In the current study, we examined the effect of lumican overexpression in corneas of wild type mice. These mice showed no alteration in collagen organization or transparency but had increased keratocan expression at both protein and mRNA levels. Corneas of lumican-null mice showed decreased keratocan. This coupling of keratocan expression with lumican also was observed after intrastromal injection of a lumican expression minigene into the corneal stroma of Lum-/- mice. Small interfering RNA knockdown of lumican in vitro reduced keratocan expression, whereas co-injection of a lumican-expressing minigene with a beta-galactosidase reporter driven by the keratocan promoter demonstrated an increase of keratocan transcriptional activity in response to lumican expression in Lum-/- corneas in vivo. These observations demonstrate that lumican has a novel regulatory role in keratocan expression at the transcriptional level. Such results help provide an explanation for the differences in severity of corneal manifestation found in Lum-/- and Kera-/- mice. The results also suggest a critical level of small proteoglycans to be essential for collagen organization but that overabundance is not detrimental to extracellular matrix morphogenesis.

PITX2 gain-of-function in Rieger syndrome eye model

The human autosomal-dominant disorder Axenfeld-Rieger syndrome presents with defects in development of the eyes, teeth, and umbilicus. The eye manifests with iris ruptures, irido-corneal adhesions, cloudy corneas, and glaucoma. Transcription factors such as PITX2 and FOXC1 have been found to carry point mutations, causing the disorder. However, for approximately 40% of the cases, the pathogenesis is unknown. It has been reported that some mutations in PITX2 increase transactivation, whereas most mutations cause defects in DNA binding or transactivation. It is not known whether up-regulation of PITX2 activity can cause the disorder as well. Here we test this hypothesis directly by overexpressing PITX2A as a transgene in mouse corneal mesenchyme and iris, using keratocan-flanking sequences. The mice presented with corneal opacification, corneal hypertrophy, irido-corneal adhesions, and severely degenerated retina, resembling glaucoma. The corneal hypertrophy also resembles the corneal hypertrophy of Pitx2-/- mice. Control transgenic mice carrying point mutations T68P or K88E in PITX2A were normal. These findings indicate a novel pathogenetic mechanism in which excess corneal and iridal PITX2A cause glaucoma and anterior defects that closely resemble Axenfeld-Rieger syndrome.

Bone morphogenetic protein-1/tolloid-related metalloproteinases process osteoglycin and enhance its ability to regulate collagen fibrillogenesis

The mammalian bone morphogenetic protein-1 (BMP-1)/Tolloid-related metalloproteinases play key roles in regulating formation of the extracellular matrix (ECM) via biosynthetic processing of various precursor proteins into mature functional enzymes, structural proteins, and proteins involved in initiating the mineralization of hard tissue ECMs. They also have been shown to activate several members of the transforming growth factor-beta superfamily, and may serve to coordinate such activation with formation of the ECM in morphogenetic events. Osteoglycin (OGN), a small leucine-rich proteoglycan with unclear functions, is found in cornea, bone, and other tissues, and appears to undergo proteolytic processing in vivo. Here we have successfully generated recombinant OGN and have employed it to demonstrate that a pro-form of OGN is processed to varying extents by all four mammalian BMP-1/Tolloid-like proteinases, to generate a 27-kDa species that corresponds to the major form of OGN found in cornea. Moreover, whereas wild-type mouse embryo fibroblasts (MEFs) produce primarily the processed, mature form of OGN, MEFs homozygous null for genes encoding three of the four mammalian BMP-1/Tolloid-related proteinases produce only unprocessed pro-OGN. Thus, all detectable pro-OGN processing activity in MEFs is accounted for by products of these genes. We also demonstrate that both pro- and mature OGN can regulate type I collagen fibrillogenesis, and that processing of the prodomain by BMP-1 potentiates the ability of OGN to modulate the formation of collagen fibrils.

An X-ray scattering investigation of corneal structure in keratocan-deficient mice

The transparency of the cornea has been closely linked with the characteristic size and arrangement of its constituent collagen fibrils. This arrangement, in turn, is thought to depend on interactions with intervening matrix proteoglycans. The purpose of this investigation was to examine fibrillar collagen organisation in the corneas of mice homozygous for a null mutation in keratocan, a keratan sulfate-containing proteoglycan. Low-angle synchrotron X-ray scattering techniques were used. We found that keratocan-deficient mice had corneal collagen fibrils with significantly larger diameters than those in wild-type littermates. Furthermore, there was an increase in the centre-to-centre spacing of the collagen fibrils that was accompanied by a decrease in nearest-neighbour fibrillar order. We hypothesise that a lack of keratocan might lower the number of keratan sulfate proteoglycans that associate with collagen, leading to alterations in their diameters and spatial arrangements. Alternatively, it might change the osmotic balance between the inside and outside of fibrils, causing them to swell and move further apart.

The keratocan gene is expressed in both ocular and non-ocular tissues during early chick development

Extracellular matrix (ECM) keratan sulfate proteoglycans (KSPGs) are core proteins with sulfated polylactosamine side chains (KS). The KSPG core protein keratocan gene (Kera) is expressed almost exclusively in adult vertebrate cornea, but its embryonic expression is little known. Embryonic chick in situ hybridization reveals Kera mRNA expression in corneal endothelium from embryonic day (E) 4.5, Hamburger-Hamilton (HH) 25, in stromal keratocytes from E6.5, HH30, and in iris distal surface cells from E8, HH34. As highly sulfated, antibody I22-positive KS increases extracellularly from posterior to anterior across the stroma, nerves enter and populate only anterior stroma and epithelium. RT-PCR and in situ hybridization demonstrate that developmentally regulated Kera mRNA expression initiates in midbrain and dorsolateral mesenchyme at E1, HH7, then spreads caudally in hindbrain and cranial and trunk mesenchyme flanking the neural tube through E2, HH20. Cranial expression extends ventrally through the developing head, and concentrates in mesenchyme surrounding eye anterior regions and cranial ganglia, and in subepidermal pharyngeal arch mesenchyme by E3.5, HH22. Kera expression in the trunk at E3.5, HH22 and E4.5, HH25, is strong in dorsolateral subepidermal, sclerotomal and nephrogenic mesenchymes, but absent in neural tube, dorsal root ganglia, nerve outgrowths, notochord, heart and gut. Early limb buds express Kera mRNA throughout their mesenchyme, then in restricted proximal and distal mesenchymes. I22-positive KS appears only in notochord in E3.5, HH22 and E4.5, HH25, embryos. Results suggest the hypothesis that keratocan, or keratocan with minimally sulfated KS chains, may play a role in structuring ECM for early embryonic cell and neuronal migrations.