Transketolase gene expression in the cornea is influenced by environmental factors and developmentally controlled events

Antioxidant Defenses in the Human Eye: A Focus on Metallothioneins

The human eye, the highly specialized organ of vision, is greatly influenced by oxidants of endogenous and exogenous origin. Oxidative stress affects all structures of the human eye with special emphasis on the ocular surface, the lens, the retina and its retinal pigment epithelium, which are considered natural barriers of antioxidant protection, contributing to the onset and/or progression of eye diseases. These ocular structures contain a complex antioxidant defense system slightly different along the eye depending on cell tissue. In addition to widely studied enzymatic antioxidants, including superoxide dismutase, glutathione peroxidase, catalase, peroxiredoxins and selenoproteins, inter alia, metallothioneins (MTs) are considered antioxidant proteins of growing interest with further cell-mediated functions. This family of cysteine rich and low molecular mass proteins captures and neutralizes free radicals in a redox-dependent mechanism involving zinc binding and release. The state of the art of MTs, including the isoforms classification, the main functions described to date, the Zn-MT redox cycle as antioxidant defense system, and the antioxidant activity of Zn-MTs in the ocular surface, lens, retina and its retinal pigment epithelium, dependent on the number of occupied zinc-binding sites, will be comprehensively reviewed.

Genetic variants in TKT and DERA in the nicotinamide adenine dinucleotide phosphate pathway predict melanoma survival

BACKGROUND

Cutaneous melanoma (CM) is the most lethal type of skin cancers. Nicotinamide adenine dinucleotide phosphate (NADPH) plays an important role in anabolic reactions and tumorigenesis, but many genes are involved in the NADPH system.

METHODS

We used 10,912 single-nucleotide polymorphisms (SNPs) (2018 genotyped and 8894 imputed) in 134 NADPH-related genes from a genome-wide association study (GWAS) of 858 patients from The University of Texas MD Anderson Cancer Center (MDACC) in a single-locus analysis to predict CM survival. We then replicated the results in another GWAS data set of 409 patients from the Nurses' Health Study (NHS) and the Health Professionals Follow-up Study (HPFS).

RESULTS

There were 95 of 858 (11.1%) and 48 of 409 (11.7%) patients who died of CM, respectively. In multivariable Cox regression analyses, we identified two independent SNPs (TKT rs9864057 G > A and deoxyribose phosphate aldolase (DERA) rs12297652 A > G) to be significantly associated with CM-specific survival [hazards ratio (HR) of 1.52, 95% confidence interval (CI) = 1.18-1.96, P = 1.06 × 10^-3 and 1.51 (1.19-1.91, 5.89 × 10^-4)] in the meta-analysis, respectively. Furthermore, an increasing number of risk genotypes of these two SNPs was associated with a higher risk of death in the MDACC, the NHS/HPFS, and their combined data sets (P_trend<0.001, = 0.004 and <0.001, respectively). In the expression quantitative trait loci analysis, TKT rs9864057 G > A and DERA rs12297652 A > G were also significantly associated with higher mRNA expression levels in sun-exposed lower-leg skin (P = 0.043 and 0.006, respectively).

CONCLUSIONS

These results suggest that these two potentially functional SNPs may be valuable prognostic biomarkers for CM survival, but larger studies are needed to validate these findings.

Protein Analysis of the TGFBIR124H Mouse Model Gives Insight into Phenotype Development of Granular Corneal Dystrophy

PURPOSE

Mutations in the transforming growth factor β-induced protein (TGFBIp) are associated with TGFBI-linked corneal dystrophies, which manifests as protein deposits in the cornea. A total of 70 different disease-causing mutations have been reported so far including the common R124H substitution, which is associated with granular corneal dystrophy type 2 (GCD2). The disease mechanism of GCD2 is not known and the current treatments only offer temporary relief due to the reoccurrence of deposits.

EXPERIMENTAL DESIGN

The corneal protein profiles of the three genotypes (wild-type (WT), heterozygotes, and homozygotes) of a GCD2 mouse model are compared using label-free quantitative LC-MS/MS.

RESULTS

The mice do not display corneal protein deposits and the global protein expression between the three genotypes is highly similar. However, the expression of mutated TGFBIp is 41% of that of the WT protein.

CONCLUSIONS AND CLINICAL RELEVANCE

It is proposed that the lowered expression level of mutant TGFBIp protein relative to WT protein is the direct cause of the missing development of corneal deposits in the mouse. The overall protein profiles of the corneas are not impacted by the reduced amount of TGFBIp. Altogether, this supports a partial reduction in mutated TGFBIp as a potential treatment strategy for GCD2.

Histological Features of Postnatal Development of the Eye in White Rabbits

Rabbits are frequently used in studies assessing the toxicity of ophthalmic drugs; however, the postnatal histological changes that occur in the rabbit eye have not been fully described. To characterize postnatal ocular development in white rabbits, a histological investigation of the eyes and eyelids was sequentially performed between postnatal days (PNDs) 1 and 42. The eyes opened during PNDs10 to 12. Significant changes prior to eyelid opening included the proliferation of uveal and optic nerve cells, regression of the lenticular vasculature, and thinning of the retina with a decreasing number of retinal cells. After eyelid opening, several significant changes occurred in the anterior segment, including thickening of the cornea and the development of lacrimation-related tissues in the eyelid and conjunctiva. Additionally, the differentiation of retinal layer-derived cells and optic nerve thickening occurred. The lens size continued to increase throughout the postnatal period. The histological structure of the eyes and eyelids was nearly mature by PNDs28 to 42. This study characterizes the postnatal changes in the histological features of the eyes in juvenile white rabbits, providing fundamental knowledge on the appropriate design of histological studies of the eyes in juvenile rabbits, particularly ophthalmic drug evaluations.

Cyadox regulates the transcription of different genes by activation of the PI3K signaling pathway in porcine primary hepatocytes

Cyadox, a new derivative of quinoxalines, has been ascertained as an antibiotic with significant growth promoting, low poison, quick absorption, swift elimination, brief residual period, and noncumulative effect. Seven differential expressed genes, including Insulin-like Growth Factor-1 ( IGF-1), Epidermal Growth Factor ( EGF), Poly ADP-ribose polymerase ( PARP), the Defender Against Apoptotic Death 1 ( DAD1), Complement Component 3 ( C3), Transketolase ( TK) and a New gene, were induced by cyadox in swine liver tissues by messenger RNA differential display reverse transcription polymerase chain reaction (DDRT-PCR) in our laboratory. However, the signal mechanism that cyadox altered these genes expression is not completely elucidated. The signaling pathways involved in the expressions of seven genes induced by cyadox were determined in porcine primary hepatocytes by RT-qPCR and the application of various signal pathway inhibitors. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay revealed that cyadox could stimulate proliferation of porcine primary hepatocytes in a time-dependent manner. In porcine primary cultured hepatocytes, phosphoinositide 3-kinase (PI3K) and transforming growth factor-β (TGF-β) signal pathways were the main signal pathways involved in the expressions of seven genes induced by cyadox. Taken together, these results demonstrate for the first time that seven cyadox-related genes expressions in porcine primary hepatocytes treated with cyadox are mediated mainly through the PI3K signaling pathway, potentially leading to enhanced cell growth and cell immunity. EGF might be the early response gene of cyadox, and a primary regulator of the other gene expressions such as IGF-1 and DAD1, playing an important role in cell proliferation promoted by cyadox.

Modulation of human corneal stromal cell differentiation by hepatocyte growth factor and substratum compliance

Corneal wound healing is a complex process that consists of cellular integration of multiple soluble biochemical cues and cellular responses to biophysical attributes associated with the matrix of the wound space. Upon corneal stromal wounding, the transformation of corneal fibroblasts to myofibroblasts is promoted by transforming growth factor-β (TGFβ). This process is critical for wound healing; however, excessive persistence of myofibroblasts in the wound space has been associated with corneal fibrosis resulting in severe vision loss. The objective of this study was to determine the effect of hepatocyte growth factor (HGF), which can modulate TGFβ signaling, on corneal myofibroblast transformation by analyzing the expression of α-smooth muscle actin (αSMA) as a marker of myofibroblast phenotype particularly as it relates to biomechanical cues. Human corneal fibroblasts were cultured on tissue culture plastic (>1 GPa) or hydrogel substrates mimicking human normal or wounded corneal stiffness (25 and 75 kPa) in media containing TGFβ1 ± HGF. The expression of αSMA was analyzed by quantitative PCR, Western blot and immunocytochemistry. Cellular stiffness, which is correlated with cellular phenotype, was measured by atomic force microscopy (AFM). In primary human corneal fibroblasts, the mRNA expression of αSMA showed a clear dose response to TGFβ1. The expression was significantly suppressed when cells were incubated with 20 ng/ml HGF in the presence of 2 ng/ml of TGFβ1. The protein expression of αSMA induced by 5 ng/ml TGFβ1 was also decreased by 20 ng/ml of HGF. Cells cultured on hydrogels mimicking human normal (25 kPa) and fibrotic (75 kPa) cornea also showed an inhibitory effect of HGF on αSMA expression in the presence or absence of TGFβ1. Cellular stiffness was decreased by HGF in the presence of TGFβ1 as measured by AFM. In this study, we have demonstrated that HGF can suppress the myofibroblast phenotype promoted by TGFβ1 in human corneal stromal cells. These data suggest that HGF holds the potential as a therapeutic agent to improve wound healing outcomes by minimizing corneal fibrosis.

Corneal keratocyte transition to mesenchymal stem cell phenotype and reversal using serum-free medium supplemented with fibroblast growth factor-2, transforming growth factor-β3 and retinoic acid

Keratocytes of the corneal limbal stroma can derive populations of mesenchymal stem cells (MSC) when expanded in vitro. However, once a corneal MSC (cMSC) phenotype is achieved, regaining the keratocyte phenotype can be challenging, and there is no standardised differentiation medium. Here, we investigated the transition of keratocytes to cMSC and compared different supplements in their ability to return cMSC to a keratocyte phenotype. Immunofluorescence and quantitative reverse transcription polymerase chain reaction demonstrated in vivo keratocyte expression of aldehyde dehydrogenase 3A1, CD34 and keratocan, but not any of the typical MSC markers (CD73, CD90, CD105). As the keratocytes were expanded in vitro, the phenotypic profile reversed and the cells expressed MSC markers but not keratocyte markers. Differentiating the cMSC back to a keratocyte phenotype using nonsupplemented, serum-free medium restored keratocyte markers but did not maintain cell viability or support corneal extracellular matrix production. Supplementing the differentiation medium with combinations of fibroblast growth factor-2, transforming growth factor-β3 and retinoic acid maintained viability, restored expression of CD34, aldehyde dehydrogenase 3A1 and keratocan, and facilitated production of abundant extracellular matrix as shown by immunofluorescent staining for collagen-I and lumican, alongside quantitative assays for collagen and glycosaminoglycan production. However, no differentiation medium was able to downregulate the expression of MSC markers in the 21-day culture period. This study shows that the keratocyte to MSC transition can be partially reversed using serum-free media and supplementation with retinoic acid, fibroblast growth factor-2 and transforming growth factor-β3 and can enhance this effect. This is relevant for development of corneal regenerative strategies that require the production of a keratocyte phenotype. Copyright © 2016 John Wiley & Sons, Ltd.

Cellular and extracellular matrix modulation of corneal stromal opacity

Stromal transparency is a critical factor contributing to normal function of the visual system. Corneal injury, surgery, disease and infection elicit complex wound healing responses that serve to protect against insults and maintain the integrity of the cornea, and subsequently to restore corneal structure and transparency. However, in some cases these processes result in prolonged loss of corneal transparency and resulting diminished vision. Corneal opacity is mediated by the complex actions of many cytokines, growth factors, and chemokines produced by the epithelial cells, stromal cells, bone marrow-derived cells, lacrimal tissues, and nerves. Myofibroblasts, and the disorganized extracellular matrix produced by these cells, are critical determinants of the level and persistence of stromal opacity after corneal injury. Decreases in corneal crystallins in myofibroblasts and corneal fibroblasts contribute to cellular opacity in the stroma. Regeneration of a fully functional epithelial basement membrane (BM) appears to have a critical role in the maintenance of corneal stromal transparency after mild injuries and recovery of transparency when opacity is generated after severe injuries. The epithelial BM likely has a regulatory function whereby it modulates epithelium-derived growth factors such as transforming growth factor (TGF) β and platelet-derived growth factor (PDGF) that drive the development and persistence of myofibroblasts from precursor cells. The purpose of this article is to review the factors involved in the maintenance of corneal transparency and to highlight the mechanisms involved in the appearance, persistency and regression of corneal opacity after stromal injury.

Keeping an eye on decellularized corneas: a review of methods, characterization and applications

The worldwide limited availability of suitable corneal donor tissue has led to the development of alternatives, including keratoprostheses (Kpros) and tissue engineered (TE) constructs. Despite advances in bioscaffold design, there is yet to be a corneal equivalent that effectively mimics both the native tissue ultrastructure and biomechanical properties. Human decellularized corneas (DCs) could offer a safe, sustainable source of corneal tissue, increasing the donor pool and potentially reducing the risk of immune rejection after corneal graft surgery. Appropriate, human-specific, decellularization techniques and high-resolution, non-destructive analysis systems are required to ensure reproducible outputs can be achieved. If robust treatment and characterization processes can be developed, DCs could offer a supplement to the donor corneal pool, alongside superior cell culture systems for pharmacology, toxicology and drug discovery studies.

Characterization of corneal stromal stem cells with the potential for epithelial transdifferentiation

INTRODUCTION

The corneal stroma is being increasingly recognized as a repository for stem cells. Like the limbal and endothelial niches, stromal stem cells often reside in the peripheral cornea and limbus. These peripheral and limbal corneal stromal cells (PLCSCs) are known to produce mesenchymal stem cells in vitro. Recently, a common corneal stromal and epithelial progenitor was hinted at. This study aims to examine the stem cell potential of corneal stromal cells and to investigate their epithelial transdifferentiation ability.

METHODS

PLCSCs were grown in traditional Dulbecco modified Eagle medium (DMEM)-based keratocyte culture medium and an M199-based medium and analyzed for a profile of cell-surface markers by using flow cytometry and differentiated into mesenchymal phenotypes analyzed with quantitative polymerase chain reaction (qPCR) and histologic staining. PLCSCs in M199 were subsequently divided into subpopulations based on CD34 and CD105 expression by using fluorescence- activated cell sorting (FACS). Subpopulations were characterized by marker profile and mesenchymal differentiation ability. Both whole PLCSCs and subpopulations were also cultured for epithelial transdifferentiation.

RESULTS

Cells cultured in M199 demonstrated a more stem-like cell-surface marker profile, and the keratocyte marker CD34 was retained for several passages but absent in cells cultured in DMEM. Cells cultured in M199 also exhibited a greater mesenchymal differentiation potential, compared with DMEM. PLCSCs could be divided into CD34(+)CD105(+), CD34-CD105(+), and CD34-CD105- subpopulations, of which CD34(+)CD105(+) cells were the most stemlike with regard to marker expression and mesenchymal differentiation potential. Subpopulations of PLCSCs exhibited differing abilities to transdifferentiate into epithelial phenotypes. Cells that were initially CD34(+)CD105(+) showed the greatest differentiation potential, producing CK3(+) and CK19(+) cells, and expressed a range of both epithelial progenitor (HES1, FRZB1, DCT, SOD2, ABCG2, CDH1, KRT19) and terminally differentiated (DSG3, KRT3, KRT12, KRT24) genes.

CONCLUSIONS

Culture medium has a significant effect on the phenotype and differentiation capacity of PLCSCs. The stroma contains a heterogeneous cell population in which we have identified CD34(+) cells as a stem cell population with a capacity for mesenchymal and epithelial differentiation.

Ocular surface development and gene expression

The ocular surface-a continuous epithelial surface with regional specializations including the surface and glandular epithelia of the cornea, conjunctiva, and lacrimal and meibomian glands connected by the overlying tear film-plays a central role in vision. Molecular and cellular events involved in embryonic development, postnatal maturation, and maintenance of the ocular surface are precisely regulated at the level of gene expression by a well-coordinated network of transcription factors. A thorough appreciation of the biological characteristics of the ocular surface in terms of its gene expression profiles and their regulation provides us with a valuable insight into the pathophysiology of various blinding disorders that disrupt the normal development, maturation, and/or maintenance of the ocular surface. This paper summarizes the current status of our knowledge related to the ocular surface development and gene expression and the contribution of different transcription factors to this process.

Myofibroblast differentiation modulates keratocyte crystallin protein expression, concentration, and cellular light scattering

PURPOSE

The purpose of this study was to determine whether myofibroblast differentiation altered keratocyte crystallin protein concentration and increased cellular light scattering.

METHODS

Serum-free cultured rabbit corneal keratocytes and TGFβ (5 ng/mL) induced myofibroblasts were harvested and counted and protein/RNA extracted. Expression of myofibroblast and keratocyte markers was determined by real-time PCR and Western blot analysis. The cell volume of calcein AM-loaded keratocytes and myofibroblasts was determined by using nonlinear optical microscopy. Cellular light scattering of transformed myofibroblasts expressing human keratocyte crystallins was measured by reflectance confocal microscopy.

RESULTS

Differentiated myofibroblasts showed a significant decrease in RNA levels for the keratocyte markers ALDH1A1, lumican, and keratocan and a significant increase in the myofibroblast marker α-smooth muscle actin. Volumetric and protein measurements showed that myofibroblast differentiation significantly increased cytoplasmic volume (293%; P < 0.001) and water-soluble and -insoluble protein content per cell (respectively, 442% and 431%; P < 0.002) compared to keratocytes. Western blot analysis showed that the level of ALDH1A1 protein per cell was similar between myofibroblasts and keratocytes, but was substantially reduced as a percentage of total water-soluble protein. Light scattering measurements showed that induced expression of corneal crystallins significantly decreased light scattering.

CONCLUSIONS

These data suggest that myofibroblast differentiation leads to a marked increase in cell volume and dilution of corneal crystallins associated with an increase in cellular light scattering.

Lack of transketolase-like (TKTL) 1 aggravates murine experimental colitis

Transketolase-like (TKTL) 1 indirectly replenishes NADPH preventing damage induced by reactive oxygen species (ROS) formed upon intestinal inflammation. We investigated the function of TKTL1 during murine colitis and ROS detoxification for prevention of tissue damage. Mucosal damage in TKTL1(-/-) and wild-type (WT) mice was assessed by miniendoscopy and histology during dextran sodium sulfate (DSS) colitis. mRNA levels of interferon (IFN)-γ, inducible nitric oxide synthase (iNOS), interleukin (IL)-6, tumor necrosis factor (TNF), transketolase (TKT), and TKTL2 were determined by PCR and/or Western blotting. To assess oxidative and nitrosative stress nitrosylation, carbonylation and antioxidative enzymes catalase (Cat), superoxide dismutase 1 and 2, as well as glutathione (GSH) were determined. Myeloperoxidase (MPO) was determined for assessment of tissue neutrophils. TKTL1 knockout or DSS treatment did not influence TKT and TKTL2 mRNA or protein expression. Mucosal damage was significantly increased in TKTL1(-/-) mice indicated by miniendoscopy as well as a significantly shorter colon and more severe histological scores compared with WT mice during DSS colitis. This was associated with higher mRNA levels of IFN-γ, iNOS, IL-6, and TNF. In addition, iNOS protein expression was significantly enhanced in TKTL1(-/-) mice as well as MPO activity. Protein modification by nitric oxide (nitrotyrosine) was induced in TKTL1(-/-) mice. However, introduction of carbonyl groups by ROS was not induced in these mice. The expression of SOD1, SOD2, Cat, as well as GSH content was not significantly changed in TKTL1(-/-) mice. We conclude that induced colitis in TKTL1(-/-) mice was more severe compared with WT. This indicates a role of TKTL1 during mucosal repair and restoration.

Differential epithelial and stromal protein profiles in keratoconus and normal human corneas

The purpose of the study was to identify epithelial and stromal proteins that exhibit up- or down-regulation in keratoconus (KC) vs. normal human corneas. Because previous proteomic studies utilized whole human corneas or epithelium alone, thereby diluted the specificity of the proteome of each tissue, we selectively analyzed the epithelium and stromal proteins. Individual preparations of epithelial and stromal proteins from KC and age-matched normal corneas were analyzed by two independent methods, i.e., a shotgun proteomic using a Nano-Electrospray Ionization Liquid Chromatography Tandem Mass Spectrometry [Nano-ESI-LC-MS (MS)(2)] and two-dimensional-difference gel electrophoresis (2D-DIGE) coupled with mass spectrometric methods. The label-free Nano-ESI-LC-MS (MS)(2) method identified 104 epithelial and 44 stromal proteins from both normal and KC corneas, and also quantified relative changes in levels of selected proteins, in both the tissues using spectral counts in a proteomic dataset. Relative to normal corneal epithelial proteins, six KC epithelial proteins (lamin-A/C, keratin type I cytoskeletal 14, tubulin beta chain, heat shock cognate 71 kDa protein, keratin type I cytoskeletal 16 and protein S100-A4) exhibited up-regulation and five proteins (transketolase, pyruvate kinase, 14-3-3 sigma isoform, phosphoglycerate kinase 1, and NADPH dehydrogenase (quinone) 1) showed down-regulation. A similar relative analysis showed that three KC stromal proteins (decorin, vimentin and keratocan) were up-regulated and five stromal proteins (TGF-betaig h3 (Bigh3), serotransferrin, MAM domain-containing protein 2 and isoforms 2C2A of collagen alpha-2[VI] chain) were down-regulated. The 2D-DIGE-mass spectrometry followed by Decyder software analysis showed that relative to normal corneas, the KC corneal epithelium exhibited up-regulation of four proteins (serum albumin, keratin 5, L-lactate dehydrogenase and annexin A8) and down-regulation of four proteins (FTH1 [Ferritin heavy chain protein 1], calpain small subunit 1, heat shock protein beta 1 and annexin A2). A similar relative analysis of stroma by this method also showed up-regulation of aldehyde dehydrogenase 3A1 (ALDH3A1), keratin 12, apolipoprotein A-IV precursor, haptoglobin precursor, prolipoprotein and lipoprotein Gln in KC corneas. Together, the results suggested that the Nano-ESI-LC-MS(MS)(2) method was superior than the 2D-DIGE method as it identified a greater number of proteins with altered levels in KC corneas. Further, the epithelial and stromal structural proteins of KC corneas exhibited altered levels compared to normal corneas, suggesting that they are affected due to structural remodeling during KC development and progression. Additionally, because several epithelial and stromal enzymes exhibited up- or down-regulation in the KC corneas relative to normal corneas, the two layers of KC corneas were under metabolic stress to adjust their remodeling.

The transparent lens and cornea in the mouse and zebra fish eye

The lens and cornea combine to form a single optical element in which transparency and refraction are the fundamental biophysical characteristics required for a functional visual system. Although lens and cornea have different cellular and extracellular specializations that contribute to transparency and refraction, their development is closely related. In the embryonic mouse, the developing cornea and lens separate early. In contrast, zebra fish lens and cornea remain connected during early development and the optical properties of the cornea and lens observed by slit lamp and quasielastic laser light scattering spectroscopy (QLS) are more similar in the zebra fish eye than in the mouse eye. Optical similarities between cornea and lens of zebra fish may be the result of similarities in the cellular development of the cornea and lens.

The role of corneal crystallins in the cellular defense mechanisms against oxidative stress

The refracton hypothesis describes the lens and cornea together as a functional unit that provides the proper ocular transparent and refractive properties for the basis of normal vision. Similarities between the lens and corneal crystallins also suggest that both elements of the refracton may also contribute to the antioxidant defenses of the entire eye. The cornea is the primary physical barrier against environmental assault to the eye and functions as a dominant filter of UV radiation. It is routinely exposed to reactive oxygen species (ROS)-generating UV light and molecular O(2) making it a target vulnerable to UV-induced damage. The cornea is equipped with several defensive mechanisms to counteract the deleterious effects of UV-induced oxidative damage. These comprise both non-enzymatic elements that include proteins and low molecular weight compounds (ferritin, glutathione, NAD(P)H, ascorbate and alpha-tocopherol) as well as various enzymes (catalase, glucose-6-phosphate dehydrogenase, glutathione peroxidase, glutathione reductase, and superoxide dismutase). Several proteins accumulate in the cornea at unusually high concentrations and have been classified as corneal crystallins based on the analogy of these proteins with the abundant taxon-specific lens crystallins. In addition to performing a structural role related to ocular transparency, corneal crystallins may also contribute to the corneal antioxidant systems through a variety of mechanisms including the direct scavenging of free radicals, the production of NAD(P)H, the metabolism and/or detoxification of toxic compounds (i.e. reactive aldehydes), and the direct absorption of UV radiation. In this review, we extend the discussion of the antioxidant defenses of the cornea to include these highly expressed corneal crystallins and address their specific capacities to minimize oxidative damage.

Corneal crystallins and the development of cellular transparency

Past studies have established that the cornea like the lens abundantly expresses a few water-soluble enzyme/proteins in a taxon specific fashion. Based on these similarities it has been proposed that the lens and the cornea form a structural unit, the 'refracton', that has co-evolved through gene sharing to maximize light transmission and refraction to the retina. Thus far, the analogy between corneal crystallins and lens crystallins has been limited to similarities in the abundant expression, with few reports concerning their structural function. This review covers recent studies that establish a clear relationship between expression of corneal crystallins and light scattering from corneal stromal cells, i.e. keratocytes, that support a structural role for corneal crystallins in the development of transparency similar to that of lens crystallins that would be consistent with the 'refracton' hypothesis.

Aldehyde dehydrogenase (ALDH) 3A1 expression by the human keratocyte and its repair phenotypes

Transparency is essential for normal corneal function. Recent studies suggest that corneal cells express high levels of so-called corneal crystallins, such as aldehyde dehydrogenase (ALDH) and transketolase (TKT) that contribute to maintaining cellular transparency. Stromal injury leads to the appearance of repair phenotype keratocytes, the corneal fibroblast and myofibroblast. Previous studies on keratocytes from species such as bovine and rabbit indicate that the transformation from the normal to repair phenotype is accompanied by a loss of corneal crystallin expression, which may be associated with loss of cellular transparency. Here we investigated if a similar loss occurs with human keratocyte repair phenotypes. Human corneal epithelial cells were collected by scraping and keratocytes were isolated by collagenase digestion from cadaveric corneas. The cells were either processed immediately (freshly isolated keratocytes) or were cultured in the presence of 10% fetal bovine serum or transforming growth factor-beta to induce transformation to the corneal fibroblast and myofibroblast phenotypes, respectively. RT-PCR, western blotting and immunolabeling were used to detect mRNA and protein expression of ALDH isozymes and TKT. ALDH enzyme activity was also quantitated and immunolabeling was performed to determine the expression of ALDH3A1 in human corneal tissue sections from normal and diseased corneas. Human corneal keratocytes isolated from three donors expressed ALDH1A1 and ALDH3A1 mRNA, and one donor also expressed ALDH2 and TKT. Corneal epithelial cells expressed ALDH1A1, ALDH2, ALDH3A1 and TKT. Compared to normal keratocytes, corneal fibroblast expression of ALDH3A1 mRNA was reduced by 27% (n=5). ALDH3A1 protein expression as detected by western blotting was markedly reduced in passage zero fibroblasts and undetectable in higher passages (n=3). TKT protein expression was reduced in fibroblasts compared to keratocytes (n=2). ALDH3A1 enzyme activity was not detectable in corneal fibroblasts (n=6) but was readily detected in corneal epithelial cells (0.29+/-0.1U/mg protein, n=4) and keratocytes (0.05+/-0.009U/mg protein, n=7). ALDH3A1 expression was also reduced in corneal fibroblasts and myofibroblasts as determined by immunolabeling of the cells in culture (n=3) and in diseased corneal tissues in situ (n=2). We conclude that expression of the crystallin ALDH3A1 is decreased in repair phenotype human keratocytes, compared to normal human keratocytes. Extrapolating from studies of bovine and rabbit, the reduced expression of ALDH3A1 may contribute to the loss of corneal transparency experienced by human patients after injury and refractive surgeries.

Corneal keratocytes: phenotypic and species differences in abundant protein expression and in vitro light-scattering

PURPOSE

Previous studies suggest that corneal haze after injury involves changes in the light-scattering properties of keratocytes that are possibly linked to the abundant expression of water-soluble proteins. The purpose of this study was to determine the protein expression pattern of keratocytes from different species and different cultured rabbit keratocyte phenotypes and to assess differences in light-scattering in vitro.

METHODS

Water-soluble proteins were isolated from corneal epithelial cells and keratocytes of several species, including human (Hu), mouse (Mo), rabbit (Ra), chicken (Ch), and pig (P) and different cultured rabbit keratocyte phenotypes. Proteins were then characterized by SDS-PAGE, tryptic peptide sequence analysis, and Western blot analysis. Light-scattering and actin organization from cultured cells were determined with confocal reflectance and fluorescence microscopy, respectively.

RESULTS

Protein expression patterns varied substantially between species and cell types, with five new abundantly expressed proteins identified including, LDH (Ra, Ch), G3PDH (Hu, Ch), pyruvate kinase (Ch), Annexin II (Ch), and protein disulfide isomerase (Ch). Different rabbit keratocyte phenotypes also showed different levels of expression of ALDH1A1 and TKT, with myofibroblasts showing the greatest reduction. Myofibroblasts showed significantly greater (P < 0.05) light-scattering but also showed the greatest organization of actin filaments.

CONCLUSIONS

Abundant protein expression is a characteristic feature of corneal keratocytes that is lost when cells are phenotypically modulated in culture. Greater light-scattering by myofibroblasts also provides support for a link between cellular transparency and haze after injury that is possibly related to loss of protein expression or development of prominent actin filament bundles.

Abundant corneal gelsolin in Zebrafish and the 'four-eyed' fish, Anableps anableps: possible analogy with multifunctional lens crystallins

The cornea accumulates high proportions (can be up to 50%) of taxon-specific, water-soluble, cytoplasmic proteins (often enzymes) that have been considered analogous to the multifunctional lens crystallins. We have shown that gelsolin (an actin-severing protein) is the major water-soluble corneal protein of the zebrafish (Danio rerio) and the 'four-eyed' fish (Anableps anableps). Each Anableps eye contains one lens, an aquatic ventral cornea with an epithelium comprising 5-7 cell layers, and an air-exposed flatter dorsal cornea with an epithelium comprising >20 cell layers and appreciably enriched with glycogen. Gelsolin accounts for 38 and 21% of the dorsal and ventral cornea, respectively, suggesting that the abundance of gelsolin in the cornea is not incompatible with its function in air. The thicker, glycogen-enriched, air-exposed dorsal cornea may protect against UV irradiation and desiccation. Gelsolin comprises approximately 50% of the 5 cell-layer thick aquatic corneal epithelium of zebrafish. Reported zebrafish ESTs have indicated the presence of a second gelsolin gene in this species. We show by RT-PCR that the abundant corneal gelsolin (also expressed weakly in lens) (C/L-gelsolin) is also expressed in early development and differs from a ubiquitously expressed gelsolin (U-gelsolin) that is not specialized for cornea. Microinjection tests showed that overexpression of C/L-gelsolin dorsalizes the embryo and can lead to axis duplication, while interruption of C/L-gelsolin expression with a specific morpholino oligonucleotide ventralizes the embryo and interferes with brain and eye development. The evidence that C/L-gelsolin participates in the bone morphogenetic protein (BMP)/Smad dorsal-ventral signaling pathway is reviewed. Finally, we speculate that soluble C/L-gelsolin:actin complexes in the cornea may be analogous to soluble alphaA:alphaB-crystallin complexes in the lens. Together, our data are consistent with an analogy between the abundance of gelsolin in fish corneas and taxon-specific multifunctional crystallins in lenses.

Preferential transcription of rabbit Aldh1a1 in the cornea: implication of hypoxia-related pathways

Here we examine the molecular basis for the known preferential expression of rabbit aldehyde dehydrogenase class 1 (ALDH1A1) in the cornea. The rabbit Aldh1a1 promoter-firefly luciferase reporter transgene (-3519 to +43) was expressed preferentially in corneal cells in transfection tests and in transgenic mice, with an expression pattern resembling that of rabbit Aldh1a1. The 5' flanking region of the rabbit Aldh1a1 gene resembled that in the human gene (60.2%) more closely than that in the mouse (46%) or rat (51.5%) genes. We detected three xenobiotic response elements (XREs) and one E-box consensus sequence in the rabbit Aldh1a1 upstream region; these elements are prevalent in other highly expressed corneal genes and can mediate stimulation by dioxin and repression by CoCl(2), which simulates hypoxia. The rabbit Aldh1a1 promoter was stimulated fourfold by dioxin in human hepatoma cells and repressed threefold by CoCl(2) treatment in rabbit corneal stromal and epithelial cells. Cotransfection, mutagenesis, and gel retardation experiments implicated the hypoxia-inducible factor 3alpha/aryl hydrocarbon nuclear translocator heterodimer for Aldh1a1 promoter activation via the XREs and stimulated by retinoic acid protein 13 for promoter repression via the E-box. These experiments suggest that XREs, E-boxes, and PAS domain/basic helix-loop-helix transcription factors (bHLH-PAS) contribute to preferential rabbit Aldh1a1 promoter activity in the cornea, implicating hypoxia-related pathways.

Human aldehyde dehydrogenase 3A1 (ALDH3A1): biochemical characterization and immunohistochemical localization in the cornea

ALDH3A1 (aldehyde dehydrogenase 3A1) is expressed at high concentrations in the mammalian cornea and it is believed that it protects this vital tissue and the rest of the eye against UV-light-induced damage. The precise biological function(s) and cellular distribution of ALDH3A1 in the corneal tissue remain to be elucidated. Among the hypotheses proposed for ALDH3A1 function in cornea is detoxification of aldehydes formed during UV-induced lipid peroxidation. To investigate in detail the biochemical properties and distribution of this protein in the human cornea, we expressed human ALDH3A1 in Sf9 insect cells using a baculovirus vector and raised monoclonal antibodies against ALDH3A1. Recombinant ALDH3A1 protein was purified to homogeneity with a single-step affinity chromatography method using 5'-AMP-Sepharose 4B. Human ALDH3A1 demonstrated high substrate specificity for medium-chain (6 carbons and more) saturated and unsaturated aldehydes, including 4-hydroxy-2-nonenal, which are generated by the peroxidation of cellular lipids. Short-chain aliphatic aldehydes, such as acetaldehyde, propionaldehyde and malondialdehyde, were found to be very poor substrates for human ALDH3A1. In addition, ALDH3A1 metabolized glyceraldehyde poorly and did not metabolize glucose 6-phosphate, 6-phosphoglucono-delta-lactone and 6-phosphogluconate at all, suggesting that this enzyme is not involved in either glycolysis or the pentose phosphate pathway. Immunohistochemistry in human corneas, using the monoclonal antibodies described herein, revealed ALDH3A1 expression in epithelial cells and stromal keratocytes, but not in endothelial cells. Overall, these cumulative findings support the metabolic function of ALDH3A1 as a part of a corneal cellular defence mechanism against oxidative damage caused by aldehydic products of lipid peroxidation. Both recombinant human ALDH3A1 and the highly specific monoclonal antibodies described in the present paper may prove to be useful in probing biological functions of this protein in ocular tissue.

Neonatal corneal stromal development in the normal and lumican-deficient mouse

PURPOSE

The purpose of this study was to characterize temporally stromal growth and transparency in lumican-deficient and normal neonatal mice.

METHODS

Lumican-deficient mice and CD1 wild-type mice were evaluated by in vivo confocal microscopy through-focusing (CMTF) to quantify stromal and epithelial thickness and corneal light-scattering and by laser scanning CM to determine density of keratocytes from 1 day to 12 weeks after birth.

RESULTS

CD1 corneas showed a rapid loss of light-scattering, decreasing by 50% from day 1 to day 12, that paralleled a 60% decrease in density of keratocytes. By contrast, the stroma demonstrated a marked swelling from day 8 to day 12, followed by thinning at day 14. Compared to corneas from CD1 mice, lumican-deficient corneas showed significantly increased (P < 0.05) light-scattering beginning at week 3 that remained elevated above wild-type levels for the duration of the study. Stromal development was also markedly altered, with thinning detected at week 3, followed by no detectable stromal growth for the duration of the study. Density of keratocytes was significantly increased, but the total cell number was similar compared with that in the wild-type cornea, suggesting no effect on keratocyte differentiation.

CONCLUSIONS

Development of normal neonatal corneal transparency appears related to changes in density of keratocytes. The stroma, however, undergoes a marked swelling and thinning at the time of eyelid opening (days 8-14). In the lumican-deficient mouse, stromal swelling is abolished, indicating that this critical phase in stromal development is lumican dependent and essential for normal stromal growth and maintenance of stromal transparency.

Enigma of the abundant water-soluble cytoplasmic proteins of the cornea: the "refracton" hypothesis

It is accepted that the taxon-specific, multifunctional crystallins (small heat-shock proteins and enzymes) serve structural roles contributing to the transparent and refractive properties of the lens. The transparent cornea also accumulates unexpectedly high proportions of taxon-specific, multifunctional proteins particularly, but not only, in the epithelium. For example, aldehyde dehydrogenase 3 (ALDH3) is the main water-soluble protein in corneal epithelial cells of most mammals (but ALDH1 predominates in the rabbit), whereas gelsolin predominates in the zebrafish corneal epithelium. Moreover, some invertebrates (e.g., squid and scallop) accumulate proteins in their corneas that are similar to their lens crystallins. Pax-6, among other transcription factors, is implicated in development and tissue-specific gene expression of the lens and cornea. Environmental factors appear to influence gene expression in the cornea, but not the lens. Although no direct proof exists, the diverse, abundant corneal proteins may have evolved a crystallinlike role, in addition to their enzymatic or cytoskeletal functions, by a gene sharing mechanism similar to the lens crystallins. Consequently, it is proposed that the cornea and lens be considered as a single refractive unit, called here the "refracton," to emphasize their similarities and common function.