Role In Ocular Development Research Articles

Glucocorticoid Receptor Signaling Is Critical for Mouse Corneal Development, Inhibition of Inflammatory Response, and Neovascularization of the Cornea

The cornea protects the interior of the eye from external agents such as bacteria, viruses, and debris. Synthetic glucocorticoids are widely prescribed in the treatment of ocular infections and disorders. The actions of glucocorticoids are mediated by the glucocorticoid receptor (GR); however, the molecular and physiological functions of GR signaling in the cornea are poorly understood. This study found that treatment of mice with glucocorticoid eye drops led to a profound regulation of the corneal transcriptome. These glucocorticoid-regulated genes were associated with multiple biological functions, including the immune response. To understand the direct role of GR signaling in the cornea, mice with conditional knockout of GRs in the corneal epithelium were generated. Mice lacking corneal GRs exhibited microphthalmia, loss of pupils, a deformed and opaque lens, and mislocalization of key structural proteins within the corneal epithelial layers. Global transcriptomic approaches revealed that loss of GR signaling in the cornea also resulted in the dysregulation of a large cohort of genes strongly associated with an enhanced inflammatory response. Finally, corneal GR signaling was required for preventing neovascularization of blood and lymphatic vessels and thereby immune cell infiltration of the cornea. These results reveal that corneal GR signaling plays a critical role in ocular development and in maintaining the homeostasis of the eye.

Characterization of neural damage and neuroinflammation in Pax6 small-eye mice

Aniridia is a panocular condition characterized by a partial or complete loss of the iris. It manifests various developmental deficits in both the anterior and posterior segments of the eye, leading to a progressive vision loss. The homeobox gene PAX6 plays an important role in ocular development and mutations of PAX6 have been the main causative factors for aniridia. In this study, we assessed how Pax6-haploinsufficiency affects retinal morphology and vision of Pax6Sey mice using in vivo and ex vivo metrics. We used mice of C57BL/6 and 129S1/Svlmj genetic backgrounds to examine the variable severity of symptoms as reflected in human aniridia patients. Elevated intraocular pressure (IOP) was observed in Pax6Sey mice starting from post-natal day 20 (P20). Correspondingly, visual acuity showed a steady age-dependent decline in Pax6Sey mice, though these phenotypes were less severe in the 129S1/Svlmj mice. Local retinal damage with layer disorganization was assessed at P30 and P80 in the Pax6Sey mice. Interestingly, we also observed a greater number of activated Iba1+ microglia and GFAP + astrocytes in the Pax6Sey mice than in littermate controls, suggesting a possible neuroinflammatory response to Pax6 deficiencies.

Diversity of clinical phenotypes in a cohort of Han Chinese patients with PAX6 variants.

The PAX6 gene plays an important role in ocular development. Mutations of the PAX6 gene may result in a series of ocular abnormalities, including congenital aniridia, anterior segment dysgenesis (ASD), progressive corneal opacification, glaucoma, and hypoplasia of the fovea and optic nerve, leading to reduced visual acuity and even blindness. This study aimed to describe the diversity of clinical features caused by PAX6 pathogenic variants in 45 Han Chinese patients from 23 unrelated families. All patients underwent detailed clinical assessment. Genetic testing was performed to identify pathogenic variations in the PAX6 gene by next-generation sequencing, minigene splicing assay, RT-qPCR, and long-range PCR. Twenty pathogenic variations were detected in the PAX6 gene from 12 pedigrees and 11 sporadic patients, of which 12 were previously reported and 8 were novel. The clinical phenotypes obtained as a result of the PAX6 gene mutations were complicated and vary among patients, even among those who carried the same variants. Genetic testing is helpful for differential diagnosis. Our genetic findings will expand the spectrum of pathogenic variations in the PAX6 gene. PAX6 pathogenic variants not only cause defects in ocular tissues, such as the iris and retina, but also lead to maldevelopment of the whole eye, resulting in microphthalmia.

Genetic analyses of human fetal retinal pigment epithelium gene expression suggest ocular disease mechanisms

The retinal pigment epithelium (RPE) serves vital roles in ocular development and retinal homeostasis but has limited representation in large-scale functional genomics datasets. Understanding how common human genetic variants affect RPE gene expression could elucidate the sources of phenotypic variability in selected monogenic ocular diseases and pinpoint causal genes at genome-wide association study (GWAS) loci. We interrogated the genetics of gene expression of cultured human fetal RPE (fRPE) cells under two metabolic conditions and discovered hundreds of shared or condition-specific expression or splice quantitative trait loci (e/sQTLs). Co-localizations of fRPE e/sQTLs with age-related macular degeneration (AMD) and myopia GWAS data suggest new candidate genes, and mechanisms by which a common RDH5 allele contributes to both increased AMD risk and decreased myopia risk. Our study highlights the unique transcriptomic characteristics of fRPE and provides a resource to connect e/sQTLs in a critical ocular cell type to monogenic and complex eye disorders.

Ciliary margin-derived BMP4 does not have a major role in ocular development.

Heterozygous Bmp4 mutations in humans and mice cause severe ocular anterior segment dysgenesis (ASD). Abnormalities include pupil displacement, corneal opacity, iridocorneal adhesions, and variable intraocular pressure, as well as some retinal and vascular defects. It is presently not known what source of BMP4 is responsible for these defects, as BMP4 is expressed in several developing ocular and surrounding tissues. In particular, BMP4 is expressed in the ciliary margins of the optic cup which give rise to anterior segment structures such as the ciliary body and iris, making it a good candidate for the required source of BMP4 for anterior segment development. Here, we test whether ciliary margin-derived BMP4 is required for ocular development using two different conditional knockout approaches. In addition, we compared the conditional deletion phenotypes with Bmp4 heterozygous null mice. Morphological, molecular, and functional assays were performed on adult mutant mice, including histology, immunohistochemistry, in vivo imaging, and intraocular pressure measurements. Surprisingly, in contrast to Bmp4 heterozygous mutants, our analyses revealed that the anterior and posterior segments of Bmp4 conditional knockouts developed normally. These results indicate that ciliary margin-derived BMP4 does not have a major role in ocular development, although subtle alterations could not be ruled out. Furthermore, we demonstrated that the anterior and posterior phenotypes observed in Bmp4 heterozygous animals showed a strong propensity to co-occur, suggesting a common, non-cell autonomous source for these defects.

Mutation update of transcription factor genes FOXE3, HSF4, MAF, and PITX3 causing cataracts and other developmental ocular defects.

Mutations in the transcription factor genes FOXE3, HSF4, MAF, and PITX3 cause congenital lens defects including cataracts that may be accompanied by defects in other components of the eye or in nonocular tissues. We comprehensively describe here all the variants in FOXE3, HSF4, MAF, and PITX3 genes linked to human developmental defects. A total of 52 variants for FOXE3, 18 variants for HSF4, 20 variants for MAF, and 19 variants for PITX3 identified so far in isolated cases or within families are documented. This effort reveals FOXE3, HSF4, MAF, and PITX3 to have 33, 16, 18, and 7 unique causal mutations, respectively. Loss-of-function mutant animals for these genes have served to model the pathobiology of the associated human defects, and we discuss the currently known molecular function of these genes, particularly with emphasis on their role in ocular development. Finally, we make the detailed FOXE3, HSF4, MAF, and PITX3 variant information available in the Leiden Online Variation Database (LOVD) platform at https://www.LOVD.nl/FOXE3, https://www.LOVD.nl/HSF4, https://www.LOVD.nl/MAF, and https://www.LOVD.nl/PITX3. Thus, this article informs on key variants in transcription factor genes linked to cataract, aphakia, corneal opacity, glaucoma, microcornea, microphthalmia, anterior segment mesenchymal dysgenesis, and Ayme-Gripp syndrome, and facilitates their access through Web-based databases.

Lutein and Zeaxanthin Isomers in Eye Health and Disease.

Current evidence suggests lutein and its isomers play important roles in ocular development in utero and throughout the life span, in vision performance in young and later adulthood, and in lowering risk for the development of common age-related eye diseases in older age. These xanthophyll (oxygen-containing) carotenoids are found in a wide variety of vegetables and fruits, and they are present in especially high concentrations in leafy green vegetables. Additionally, egg yolks and human milk appear to be bioavailable sources. The prevalence of lutein, zeaxanthin, and meso-zeaxanthin in supplements is increasing. Setting optimal and safe ranges of intake requires additional research, particularly in pregnant and lactating women. Accumulating evidence about variable interindividual response to dietary intake of these carotenoids, based on genetic or metabolic influences, suggests that there may be subgroups that benefit from higher levels of intake and/or alternate strategies to improve lutein and zeaxanthin status.

The Expression of Bone Morphogenetic Protein 2 and Matrix Metalloproteinase 2 through Retinoic Acid Receptor Beta Induced by All-Trans Retinoic Acid in Cultured ARPE-19 Cells.

PurposeAll-trans retinoic acid (ATRA) plays an important role in ocular development. Previous studies found that retinoic acid could influence the metabolism of scleral remodeling by promoting retinal pigment epithelium (RPE) cells to secrete secondary signaling factors. The purpose of this study was to investigate whether retinoic acid affected secretion of bone morphogenetic protein 2 (BMP-2) and matrix metalloproteinase 2 (MMP-2) and to explore the signaling pathway of retinoic acid in cultured acute retinal pigment epithelial 19 (ARPE-19) cells.MethodsThe effects of ATRA (concentrations from 10−9 to 10−5 mol/l) on the expression of retinoic acid receptors (RARs) in ARPE-19 cells were examined at the mRNA and protein levels using reverse transcription-polymerase chain reaction (RT-PCR) and western blot assay, respectively. The effects of treating ARPE-19 cells with ATRA concentrations ranging from 10−9 to 10−5 mol/l for 24 h and 48 h or with 10-6mol/l ATRA at different times ranging from 6h to 72h were assessed using real-time quantitative PCR (qPCR) and enzyme-linked immunosorbent assay (ELISA). The contribution of RARβ-induced activation of ARPE-19 cells was confirmed using LE135, an antagonist of RARβ.ResultsRARβ mRNA levels significantly increased in the ARPE-19 cells treated with ATRA for 24h and 48h. These increases in RARβ mRNA levels were dose dependent (at concentrations of 10−9 to 10−5 mol/l) with a maximum effect observed at 10−6 mol/l. There were no significant changes in the mRNA levels of RARα and RARγ. Western blot assay revealed that RARβ protein levels were increased significantly in a time-dependent manner in ARPE-19 cells treated with 10−6 mol/l ATRA from 12 h to 72 h, with a marked increase observed at 24 h and 48 h. The upregulation of RARβ and the ATRA-induced secretion in ARPE-19 cells could be inhibited by the RARβ antagonist LE135.ConclusionATRA induced upregulation of RARβ in ARPE-19 cells and stimulated these cells to secrete BMP-2 and MMP-2.

Expansion of ocular phenotypic features associated with mutations in ADAMTS18.

We describe novel ocular phenotypic features caused by mutations in ADAMTS18. The exact role of ADAMTS18 in ocular disease is unclear, and our work further contributes to the understanding of this gene and its protein. To expand the phenotypic characterization in patients with homozygous mutations in ADAMTS18 and report novel mutational data. A case series with genetic investigations was conducted at tertiary referral clinical and university settings. Three families participated. Phenotype and genotype description of 3 families. Four affected patients from 3 families with an unusual ocular phenotype had full ophthalmic and systemic examination. A single affected individual in the first family had bilateral microcornea, ectopic pupils, and cone-rod dystrophy. In a second family, 2 brothers showed bilateral microcornea, childhood cataract, ectopia lentis, rhegmatogenous retinal detachment, and cone-rod dystrophy. In the third family, a single affected individual had the same features as those in family 2, without ectopia lentis. Causative mutations were sought using homozygosity mapping, Sanger sequencing, and massively parallel sequencing of the whole exome. Novel homozygous mutations in ADAMTS18 were identified, consisting of c.1067T>A [p.L356*] in the first proband, c.2159G>C [p.C720S] in the 2 affected brothers, and c.1952G>A [p.R651Q] in the third proband. All 3 mutations are predicted to be pathogenic. Mutations in ADAMTS18 are associated with ocular developmental abnormalities including microcornea, ectopia lentis, and early onset of cone-rod dystrophy. This report provides further evidence that ADAMTS18 plays a key role in ocular development. Physicians should consider screening ADAMTS18 in patients with microcornea and cone-rod dystrophy.

Megalin-deficiency causes high myopia, retinal pigment epithelium-macromelanosomes and abnormal development of the ciliary body in mice.

In man, mutations of the megalin-encoding gene causes the rare Donnai-Barrow/Facio-Oculo-Acoustico-Renal Syndrome, which is partially characterized by high-grade myopia. Previous studies of renal megalin function have established that megalin is crucial for conservation of renal filtered nutrients including vitamin A; however, the role of megalin in ocular physiology and development is presently unknown. Therefore, we investigate ocular megalin expression and the ocular phenotype of megalin-deficient mice. Topographical and subcellular localization of megalin as well as the ocular phenotype of megalin-deficient mice were examined with immunological techniques using light, confocal and electron microscopy. We identified megalin in the retinal pigment epithelium (RPE) and non-pigmented ciliary body epithelium (NPCBE) in normal mouse eyes. Immunocytochemical investigations furthermore showed that megalin localizes to vesicular structures in the RPE and NPCBE cells. Histological investigations of ocular mouse tissue also identified a severe myopia phenotype as well as enlarged RPE melanosomes and abnormal ciliary body development in the megalin-deficient mice. In conclusion, the complex ocular phenotype observed in the megalin-deficient mice suggests that megalin-mediated developmental abnormalities may contribute to the high myopia phenotype observed in the Donnai-Barrow Syndrome patients and, thus, that megalin harbors important roles in ocular development and physiology. Finally, our data show that megalin-deficient mice may provide a valuable model for future studies of megalin in ocular physiology and pathology.

Discovery and functional annotation of SIX6 variants in primary open-angle glaucoma.

Glaucoma is a leading cause of blindness worldwide. Primary open-angle glaucoma (POAG) is the most common subtype and is a complex trait with multigenic inheritance. Genome-wide association studies have previously identified a significant association between POAG and the SIX6 locus (rs10483727, odds ratio (OR) = 1.32, p = 3.87×10−11). SIX6 plays a role in ocular development and has been associated with the morphology of the optic nerve. We sequenced the SIX6 coding and regulatory regions in 262 POAG cases and 256 controls and identified six nonsynonymous coding variants, including five rare and one common variant, Asn141His (rs33912345), which was associated significantly with POAG (OR = 1.27, p = 4.2×10−10) in the NEIGHBOR/GLAUGEN datasets. These variants were tested in an in vivo Danio rerio (zebrafish) complementation assay to evaluate ocular metrics such as eye size and optic nerve structure. Five variants, found primarily in POAG cases, were hypomorphic or null, while the sixth variant, found only in controls, was benign. One variant in the SIX6 enhancer increased expression of SIX6 and disrupted its regulation. Finally, to our knowledge for the first time, we have identified a clinical feature in POAG patients that appears to be dependent upon SIX6 genotype: patients who are homozygous for the SIX6 risk allele (His141) have a statistically thinner retinal nerve fiber layer than patients homozygous for the SIX6 non-risk allele (Asn141). Our results, in combination with previous SIX6 work, lead us to hypothesize that SIX6 risk variants disrupt the development of the neural retina, leading to a reduced number of retinal ganglion cells, thereby increasing the risk of glaucoma-associated vision loss.

Arl6ip1 Plays a Role in Proliferation during Zebrafish Retinogenesis

Objective: Arl6ip1 has been reported to play a role in ocular development, but its regulatory function as it relates to proliferation is unclear. Therefore, this study aimed to evaluate how Arl6ip1 may regulate the proliferative behavior of retinal progenitor cells during zebrafish embryogenesis. Method: Arl6ip1 was specifically knocked down by introducing morpholino nucleic acid oligomers. The DNA content of cells dissociated from morphant eyes was analyzed by fluorescence-activated cell sorting (FACS). Retinal cells in the S- and late G2/M-phase were detected by labeling with BrdU and then immunostaining with anti-BrdU antibody and by immunostaining with phospho-histone H3 antibody, respectively. We also examined the expressions of shh,p57kip2, and cyclin D1 in retinas of experimental animals. The bidirectional plasmid pGFP:HSE:p57kip2 was used to rescue the defect in cell cycle exit. Results: FACS analysis showed that the >2C DNA content per cell in the eyes of arl6ip1 morphants was 2-fold greater than that of the wild type. Following functional Arl6ip1 knockdown, anti-BrdU- and anti-phospho-histone H3-positive signals were higher and the retinal progenitors kept expressing cyclin D1 but not shh or p57kip2, suggesting that eye progenitor cells remained in an early progenitor state and could not exit the cell cycle to progress to differentiation. Interestingly, overexpression of p57kip2, which enables exit from the cell cycle, led to a reduction of anti-BrdU-positive signals in the retinas of arl6ip1 morphants. Conclusion: Arl6ip1 not only affects signals controlling eye development but also plays an important role in the proliferation of retinal progenitor cells.

Mouse Eye Enucleation for Remote High-throughput Phenotyping

The mouse eye is an important genetic model for the translational study of human ophthalmic disease. Blinding diseases in humans, such as macular degeneration, photoreceptor degeneration, cataract, glaucoma, retinoblastoma, and diabetic retinopathy have been recapitulated in transgenic mice.(1-5) Most transgenic and knockout mice have been generated by laboratories to study non-ophthalmic diseases, but genetic conservation between organ systems suggests that many of the same genes may also play a role in ocular development and disease. Hence, these mice represent an important resource for discovering new genotype-phenotype correlations in the eye. Because these mice are scattered across the globe, it is difficult to acquire, maintain, and phenotype them in an efficient, cost-effective manner. Thus, most high-throughput ophthalmic phenotyping screens are restricted to a few locations that require on-site, ophthalmic expertise to examine eyes in live mice. (6-9) An alternative approach developed by our laboratory is a method for remote tissue-acquisition that can be used in large or small-scale surveys of transgenic mouse eyes. Standardized procedures for video-based surgical skill transfer, tissue fixation, and shipping allow any lab to collect whole eyes from mutant animals and send them for molecular and morphological phenotyping. In this video article, we present techniques to enucleate and transfer both unfixed and perfusion fixed mouse eyes for remote phenotyping analyses.

Genetics of anterior segment dysgenesis disorders

Anterior segment dysgenesis (ASD) disorders encompass a spectrum of developmental conditions affecting the cornea, iris, and lens and are generally associated with an approximate 50% risk for glaucoma. These conditions are characterized by both autosomal dominant and recessive patterns of inheritance often with incomplete penetrance/variable expressivity. This article summarizes what is known about the genetics of ASD disorders and reviews recent developments. Mutations in Collagen type IV alpha-1 (COL4A1) and Beta-1,3-galactosyltransferase-like (B3GALTL) have been reported in ASD patients. Novel findings in other well known ocular genes are also presented, among which regulatory region deletions in PAX6 and PITX2 are most notable. Although a number of genetic causes have been identified, many ASD conditions are still awaiting genetic elucidation. The majority of characterized ASD genes encode transcription factors; several other genes represent extracellular matrix-related proteins. All of the involved genes play active roles in ocular development and demonstrate conserved functions across species. The use of novel technologies, such as whole genome sequencing/comparative genomic hybridization, is likely to broaden the mutation spectrums in known genes and assist in the identification of novel causative genes as well as modifiers explaining the phenotypic variability of ASD conditions.

The spectrum of ocular phenotypes caused by mutations in the BEST1 gene

Bestrophin-1 is an integral membrane protein, encoded by the BEST1 gene, which is located in the basolateral membrane of the retinal pigment epithelium. The bestrophin-1 protein forms a Ca(2+) activated Cl(-) channel and is involved in the regulation of voltage-dependent Ca(2+) channels. In addition, bestrophin-1 appears to play a role in ocular development. Over 120 different human BEST1 mutations have been described to date, associated with a broad range of ocular phenotypes. The purpose of this review is to describe this spectrum of phenotypes, which includes Best vitelliform macular dystrophy and adult-onset foveomacular vitelliform dystrophy, autosomal dominant vitreoretinochoroidopathy, the MRCS (microcornea, rod-cone dystrophy, cataract, posterior staphyloma) syndrome, and autosomal recessive bestrophinopathy. The genotype-phenotype correlations that are observed in association with BEST1 mutations are discussed. In addition, in vitro studies and animal models that clarify the pathophysiological mechanisms are reviewed.

New blood for hemoglobin in the lens: roles in stem cell differentiation and fibre cell organelle loss?

Abstract Purpose Evidence is emerging for haemoglobin (Hb) expression outside the vascular system. We previously demonstrated Hb expression in the mouse lens during post‐natal development and cataract progression. Here, we extended this work by carrying out a comprehensive spatio‐temporal analysis of Hb subunit expression during mouse lens development and maturation. Methods We used RT‐PCR, Western blotting and immunofluorescence to analyze Hb expression in mouse eyes (E16.5 to 9 wks). We also used a sensitive heme assay to test for the presence of heme in the lens by colourimetric assay and histological staining of paraffin‐embedded sections. Results Hb subunits were expressed in lens epithelial cells and cortical lens fibre cells. However, the heme assay revealed negligible levels of this prosthetic group in the lens. Hb immunofluorescence was also observed in other regions of the developing eye including the cornea, the retinal ganglion cell layer and the retinal pigment epithelium. Finally, we also observed Hb expression in early embryos by microarray and during differentiation of embryonic stem (ES) cells into embryoid bodies (EBs) in vitro. Conclusion These results suggest a paradigm shift: Hb subunits are expressed in the eye during development and in the adult and, therefore, may have novel roles in ocular development, physiology and pathophysiology. The absence of heme from the lens indicates that at least some of these functions may be independent of oxygen metabolism. The pattern of expression of Hb in lens epithelial cells and cortical lens fibre cells may indicate an involvement for Hb subunits in lens epithelial cell differentiation into lens fibre cells and/or lens fibre cell organelle loss.

New blood for hemoglobin in the lens: roles in stem cell differentiation and fibre cell organelle loss?

Purpose Evidence is emerging for haemoglobin (Hb) expression outside the vascular system. We previously demonstrated Hb expression in the mouse lens during post-natal development and cataract progression. Here, we extended this work by carrying out a comprehensive spatio-temporal analysis of Hb subunit expression during mouse lens development and maturation. Methods We used RT-PCR, Western blotting and immunofluorescence to analyze Hb expression in mouse eyes (E16.5 to 9 wks). We also used a sensitive heme assay to test for the presence of heme in the lens by colourimetric assay and histological staining of paraffin-embedded sections. Results Hb subunits were expressed in lens epithelial cells and cortical lens fibre cells. However, the heme assay revealed negligible levels of this prosthetic group in the lens. Hb immunofluorescence was also observed in other regions of the developing eye including the cornea, the retinal ganglion cell layer and the retinal pigment epithelium. Finally, we also observed Hb expression in early embryos by microarray and during differentiation of embryonic stem (ES) cells into embryoid bodies (EBs) in vitro. Conclusion These results suggest a paradigm shift: Hb subunits are expressed in the eye during development and in the adult and, therefore, may have novel roles in ocular development, physiology and pathophysiology. The absence of heme from the lens indicates that at least some of these functions may be independent of oxygen metabolism. The pattern of expression of Hb in lens epithelial cells and cortical lens fibre cells may indicate an involvement for Hb subunits in lens epithelial cell differentiation into lens fibre cells and/or lens fibre cell organelle loss.

Opticin is an anti‐angiogenic component of the vitreous humour of the eye

Angiogenesis plays a key role in ocular development and disease. Normally following regression of the hyaloid vascular system the vitreous humour is avascular and anti-angiogenic. However, in diseases such as proliferative diabetic retinopathy, new blood vessels grow from the retina into the vitreous humour potentially leading to blindness. Opticin is a glycoprotein that was first isolated from vitreous humour of the eye and is a member of the extracellular matrix small leucine-rich repeat proteoglycan/protein family. As it is highly expressed in the eye throughout life we hypothesised that it contributes towards the anti-angiogenic properties of the vitreous humour. Purified recombinant opticin was generated as previously described (Le Goff et al. JBC 2003, 278:45280). Using the recombinant opticin we show that opticin potently inhibits VEGF and FGF-2-induced human retinal endothelial cell and bovine aortic endothelial cell proliferation, migration, formation of tube-like structures in matrigel and collagen, and invasion. In vivo studies showed that opticin significantly decreased FGF-2 and VEGF induced angiogenesis in chick chorioallantoic membrane assays. We also demonstrate that opticin decreases signalling through ERK1/2. These observations suggest that opticin is a key regulator of angiogenesis in the eye and further studies will elucidate its mechanisms of action.

Proteases in eye development and disease

The eye is one of the classical systems in developmental biology. Furthermore, diseases of the eye, many of which have a developmental basis, have devastating effects that often result in blindness. Proteases have diverse roles in ocular physiology and pathophysiology. Here, a broad overview is provided of the recent literature pertaining to the involvement of proteases in various aspects of eye development and disease: lens development (focusing on apoptosis and lens fiber cell denucleation and organelle loss) and cataract progression, cornea development and disease, retina development and degeneration, sclera development and myopia, and the trabecular meshwork and glaucoma. Proteases discussed include caspases, calpains, matrix metalloproteases (MMPs), a disintegrin and metalloproteinases (ADAMs) and ADAM with thrombospondin motifs (ADAMTS), the ubiquitin-proteasome pathway (UPP), tissue plasminogen activator (tPA), and secretases. It is clear that proteases have diverse and important roles in ocular development and disease, and represent, in many cases, useful therapeutic targets for treating ocular conditions, which would otherwise lead to visual impairment.

Physiological role of collagen XVIII and endostatin

Collagen XVIII is a component of basement membranes (BMs) with the structural properties of both a collagen and a proteoglycan. Proteolytic cleavage within its C-terminal domain releases a fragment, endostatin, which has been reported to have anti-angiogenesis effects. Molecular studies demonstrated binding of the endostatin domain to heparan sulfate and to BM components like laminin and perlecan, but the functional role of these interactions in vivo remains unknown. Insights into the physiological function of collagen XVIII/endostatin have recently been obtained through the identification of inactivating mutations in the human collagen XVIII/endostatin gene (COL18A1) in patients with Knobloch syndrome, characterized by age-dependent vitreoretinal degeneration and occipital encephalocele. That collagen XVIII/endostatin has an essential role in ocular development and the maintenance of visual function is further demonstrated by the ocular abnormalities seen in mice lacking collagen XVIII/endostatin. Age-dependent loss of vision in these mutant mice is associated with pathological accumulation of deposits under the retinal pigment epithelium, as seen in early stages of age-related macular degeneration in humans. In addition, recent evidence suggests that lack of collagen XVIII/endostatin predisposes to hydrocephalus formation. These recent findings demonstrate an important role for collagen XVIII/endostatin in cell-matrix interactions in certain tissues that may be compensated for in other tissues expressing this collagen.