High prevalence of cataracts in birds with pheomelanin-based colouration

The crystalline lens of the eyes of vertebrates focuses light on the retina. Therefore, maintaining the lens clear is necessary for proper visual function. However, oxidative damage to proteins of the lens leads to opacification and lens dysfunction, termed cataract. Antioxidants thus have a role in avoiding the development of cataracts through their reduction of oxidative stress, and glutathione (GSH), a key intracellular antioxidant, belongs to the primary antioxidant defence mechanism of the lens. Other physiological mechanisms that require GSH may compete with the antioxidant mechanism of the eye. Pheomelanin is a main type of melanin, the most common pigment in vertebrates, and its synthesis consumes GSH. Here, we use data on 81 bird species to test the hypothesis that species producing large amounts of pheomelanin should have diminished capacity to use GSH to protect their eyes and, as a consequence, higher prevalence of cataracts. As predicted, the proportion of pheomelanic plumage was positively associated with the proportion of individuals with cataracts across species, suggesting that production of pheomelanin may have profound fitness consequences, as birds with cataracts have limited ability to perform vital activities. This constitutes the first comparative study of cataracts in wild animals.     

A juvenile-onset, progressive cataract locus on chromosome 3q21-q22 is associated with a missense mutation in the beaded filament structural protein-2

Juvenile-onset cataracts are distinguished from congenital cataracts by the initial clarity of the lens at birth and the gradual development of lens opacity in the second and third decades of life. Genomewide linkage analysis in a multigenerational pedigree, segregating for autosomal dominant juvenile-onset cataracts, identified a locus in chromosome region 3q21.2-q22.3. Because of the proximity of the gene coding for lens beaded filament structural protein-2 (BFSP2) to this locus, we screened for mutations in the coding sequence of BFSP2. We observed a unique C-->T transition, one that was not observed in 200 normal chromosomes. We predicted that this led to a nonconservative R287W substitution in exon 4 that cosegregated with cataracts. This mutation alters an evolutionarily conserved arginine residue in the central rod domain of the intermediate filament. On consideration of the proposed function of BFSP2 in the lens cytoskeleton, it is likely that this alteration is the cause of cataracts in the members of the family we studied. This is the first example of a mutation in a noncrystallin structural gene that leads to a juvenile-onset, progressive cataract.     

Congenital cataracts and their molecular genetics

Cataract can be defined as any opacity of the crystalline lens. Congenital cataract is particularly serious because it has the potential for inhibiting visual development, resulting in permanent blindness. Inherited cataracts represent a major contribution to congenital cataracts, especially in developed countries. While cataract represents a common end stage of mutations in a potentially large number of genes acting through varied mechanisms in practice most inherited cataracts have been associated with a subgroup of genes encoding proteins of particular importance for the maintenance of lens transparency and homeostasis. The increasing availability of more detailed information about these proteins and their functions and is making it possible to understand the pathophysiology of cataracts and the biology of the lens in general.     

A novel spontaneous mutation of BCAR3 results in extrusion cataracts in CF#1 mouse strain

A substrain of mice originating from the CF#1 strain (an outbred colony) reared at Osaka Prefecture University (CF#1/lr mice) develops cataracts beginning at 4 weeks of age. Affected mice were fully viable and fertile and developed cataracts by 14 weeks of age. Histologically, CF#1/lr mice showed vacuolation of the lens cortex, swollen lens fibers, lens rupture and nuclear extrusion. To elucidate the mode of inheritance, we analyzed heterozygous mutant hybrids generated from CF#1/lr mice and wild-type BALB/c mice. None of the heterozygous mutants were affected, and the ratio of affected to unaffected mice was 1:3 among the offspring of the heterozygous mutants. For the initial genome-wide screening and further mapping, we used affected progeny of CF#1/lr × (CF#1/lr × BALB/c) mice. We concluded that the cataracts in CF#1/lr mice are inherited through an autosomal recessive mutation and that the mutant gene is located on mouse chromosome 3 between D3Mit79 and D3Mit216. In this region, we identified 8 genes associated with ocular disease. All 8 genes were sequenced and a novel point mutation (1 bp insertion of cytosine) in exon 7 of the Bcar3 gene was identified. This mutation produced a premature stop codon and a truncated protein. In conclusion, we have identified the first spontaneous mutation in the Bcar3 gene associated with lens extrusion cataracts. This novel cataract model may provide further knowledge of the molecular biology of cataractogenesis and the function of the BCAR3 protein.     

Elevated Frequency of Cataracts in Birds from Chernobyl

Background

Radiation cataracts develop as a consequence of the effects of ionizing radiation on the development of the lens of the eye with an opaque lens reducing or eliminating the ability to see. Therefore, we would expect cataracts to be associated with reduced fitness in free-living animals.

Methodology/Principal Findings

We investigated the incidence of lens opacities typical of cataracts in more than 1100 free-living birds in the Chernobyl region in relation to background radiation. The incidence of cataracts increased with level of background radiation both in analyses based on a dichotomous score and in analyses of continuous scores of intensity of cataracts. The odds ratio per unit change in the regressor was 0.722 (95% CI 0.648, 0.804), which was less than odds ratios from investigations of radiation cataracts in humans. The relatively small odds ratio may be due to increased mortality in birds with cataracts. We found a stronger negative relationship between bird abundance and background radiation when the frequency of cataracts was higher, but also a direct effect of radiation on abundance, suggesting that radiation indirectly affects abundance negatively through an increase in the frequency of cataracts in bird populations, but also through direct effects of radiation on other diseases, food abundance and interactions with other species. There was no increase in incidence of cataracts with increasing age, suggesting that yearlings and older individuals were similarly affected as is typical of radiation cataract.

Conclusions/Significance

These findings suggest that cataracts are an under-estimated cause of morbidity in free-living birds and, by inference, other vertebrates in areas contaminated with radioactive materials.     

Moderate caloric restriction delays cataract formation in the Emory mouse

Eye lens senile cataract is a major cause of blindness, affecting the elderly in particular. The etiology of the disorder has been elusive, and attempts to delay the onset of senile cataracts have been unsuccessful. The need for more information is underscored by epidemiologists who estimate that the ability to delay cataract formation in humans by only 10 years would eliminate the need for 50% of the cataract extractions performed annually in the United States. The Emory mouse provides the best model for human senile cataracts. Feeding Emory mice a diet that was restricted in calories by approximately 21% delayed the onset of cataracts. This is the first study that demonstrates in vivo the delay of senile-type cataracts. In these animals, aging and cataracts are associated with diverse changes in the proportion of various proteins (particularly 21, 22, 31-34 kDa) and with transformation of proteins from a soluble to an insoluble state. In advanced cataracts, there is a loss of total protein. Within a cataract grade, there is no difference between restricted and nonrestricted animals in relative proportion of specific lens proteins or in amounts of total or soluble proteins. The transition from a clear to cataractous lens appears when the soluble-to-total protein ratio falls below about 0.58. The exclusive use of gamma-crystallin as an indicator of lens viability is questioned. To the extent that cataract formation is due to lens protein oxidation and/or an inability to proteolytically remove damaged protein, it would appear that caloric restriction results in enhanced protection against lens oxidative stress or in prolonged proteolytic function.     

Gap junctions or hemichannel-dependent and independent roles of connexins in cataractogenesis and lens development

In the last decade or so, increasing evidences suggest that the mutations of two connexin genes, GJA3 and GJA8, are directly linked to human congenital cataracts in North and Central America, Europe and Asia. GIA3 and GIA8 genes encode gap junction-forming proteins, connexin (Cx) 46 and Cx50, respectively. These two connexins are predominantly expressed in lens fiber cells. Majority of identified mutations are missense, and the mutated sites are scattered across various domains of connexin molecules. Genetic deletion of either of these two genes leads to the development of cataracts; however, the types of cataracts developed are distinctive. More interestingly, microphthalmia is only developed in Cx50, but not Cx46 deficient mice, suggesting the unique role of Cx50 in lens cell growth and development. Knockin studies with the replacement of Cx46 or Cx50 at their respective gene locus further demonstrate the unique properties of these two connexins. Furthermore, the function of Cx50 in epithelial-fiber differentiation appears to be independent of its conventional role in forming gap junction junction channels. Due to their specific functions in maintaining lens clarity and development, and their malfunctions resulting in lens cataractogenesis and developmental impairment, connexin molecules could be developed as potential drug targets for therapeutic intervention for treatment of cataracts and other eye disorders. Recent advances in basic research of lens connexins and the discoveries of clinical disorders as a result of lens connexin dysfunctions are summarized and discussed here.     

Increased O-GlcNAc causes disrupted lens fiber cell differentiation and cataracts

Diminished proteolytic functionality in the lens may cause cataracts. We have reported that O-GlcNAc is an endogenous inhibitor of the proteasome. We hypothesize that in the lens there is a cause-and-effect relationship between proteasome inhibition by O-GlcNAc, and cataract formation. To demonstrate this, we established novel transgenic mouse models to over-express a dominant-negative form of O-GlcNAcase, GK-NCOAT, in the lens. Expression of GK-NCOAT suppresses removal of O-GlcNAc from proteins, resulting in increased levels of O-GlcNAc in the lenses of our transgenic mice, along with decreased proteasome function. We observed that transgenic mice developed markedly larger cataracts than controls and lens fiber cell denucleation was inhibited. Our study suggests that increased O-GlcNAc in the lens could lead to cataract formation and attenuation of lens fiber cell denucleation by inhibition of proteasome function. These findings may explain why cataract formation is a common complication of diabetes since O-GlcNAc is derived from glucose.     

Focus on lens connexins

The lens is an avascular organ composed of an anterior epithelial cell layer and fiber cells that form the bulk of the organ. The lens expresses connexin43 (Cx43), connexin46 (Cx46) and connexin50 (Cx50). Epithelial Cx50 has critical roles in cell proliferation and differentiation, likely involving growth factor-dependent signaling pathways. Both Cx46 and Cx50 are crucial for lens transparency; mutations in their genes have been linked to congenital and age-related cataracts. Congenital cataract-associated connexin mutants can affect protein trafficking, stability and/or function, and the functional effects may differ between gap junction channels and hemichannels. Dominantly inherited cataracts may result from effects of the connexin mutant on its wild type isotype, the other co-expressed wild type connexin and/or its interaction with other cellular components.     

Cataract induction by products of lipid peroxidation

Liposome suspension prepared from the unsaturated phospholipids exposed to lipid peroxidation (LPO) induced posterior subcapsular cataracts after injection into the posterior vitreous of rabbit eyes. In the background of this model lies a type of lens opacity formed during retinal degeneration when toxic peroxide substances diffuse anteriorly through the vitreous body resulting in vitreous opacities and complicated cataracts. Saturated liposomes (prepared from beta-oleoyl-gamma-palmitoyl) L-alpha-lecithin) did not induce lens opacities, which is the evidence that a lipid peroxidation mechanism may be responsible for the posterior cataracts. Along with cataract formation accumulation of LPO fluorescent products in vitreous, aqueous humor and lens was observed. It was followed by a decreased level of reduced glutathione in the lens. The obtained results strongly support the hypothesis of LPO initial role in cataracts.     

Whales,lifespan, phospholipids,and cataracts

This study addresses the question: why do rats get cataracts at 2 years, dogs at 8 years, and whales do not develop cataracts for 200 years? Whale lens lipid phase transitions were compared with the phase transitions of other species that were recalculated. The major phospholipids of the whale lens were sphingolipids, mostly dihydrosphingomyelins with an average molar cholesterol/phospholipid ratio of 10. There was a linear correlation between the percentage of lens sphingolipid and lens lipid hydrocarbon chain order until about 60% sphingolipid. The percentage of lens sphingolipid correlated with the lens lipid phase transition temperature. The lifespan of the bowhead whale was the longest of the species measured and the percentage of whale lens sphingolipid fit well in the correlation between the percentage of lens sphingolipid and lifespan for many species. In conclusion, bowhead whale lens membranes have a high sphingolipid content that confers resistance to oxidation, allowing these lenses to stay clear relatively longer than many other species. The strong correlation between sphingolipid and lifespan may form a basis for future studies, which are needed because correlations do not infer cause. One could hope that if human lenses could be made to have a lipid composition similar to whales, like the bowhead, humans would not develop age-related cataracts for over 100 years.     

遗传性白内障动物模型的研究进展

白内障是严重影响公众健康的重大疾病。人群中, 大部分遗传性白内障是外显率较高的常染色体显性遗传, 但也有X连锁和常染色体隐性遗传存在。随着分子生物学技术的发展, 出现了许多白内障遗传动物模型, 这些模型有助于揭示白内障的发病机制, 为晶状体的发育和生理学研究提供新的见解, 还有助于进一步了解遗传、环境和营养等因素对晶状体的作用方式, 并为白内障遗传病的诊断和治疗提供依据。文章综述了遗传性白内障动物模型的相关基因、突变形式及其研究进展。     

AlphaA-crystallin expression prevents gamma-crystallin insolubility and cataract formation in the zebrafish cloche mutant lens

Cataracts, the loss of lens transparency, are the leading cause of human blindness. The zebrafish embryo, with its transparency and relatively large eyes, is an excellent model for studying ocular disease in vivo. We found that the zebrafish cloche mutant, both the cloche(m39) and cloche(S5) alleles, which have defects in hematopoiesis and blood vessel development, also have lens cataracts. Quantitative examination of the living zebrafish lens by confocal microscopy showed significant increases in lens reflectance. Histological analysis revealed retention of lens fiber cell nuclei owing to impeded terminal differentiation. Proteomics identified gamma-crystallin as a protein that was substantially diminished in cloche mutants. Crystallins are the major structural proteins in mouse, human and zebrafish lens. Defects in crystallins have previously been shown in mice and humans to contribute to cataracts. The loss of gamma-crystallin protein in cloche was not due to lowered mRNA levels but rather to gamma-crystallin protein insolubility. AlphaA-crystallin is a chaperone that protects proteins from misfolding and becoming insoluble. The cloche lens is deficient in both alphaA-crystallin mRNA and protein during development from 2-5 dpf. Overexpression of exogenous alphaA-crystallin rescued the cloche lens phenotype, including solubilization of gamma-crystallin, increased lens transparency and induction of lens fiber cell differentiation. Taken together, these results indicate that alphaA-crystallin expression is required for normal lens development and demonstrate that cataract formation can be prevented in vivo. In addition, these results show that proteomics is a valuable tool for detecting protein alterations in zebrafish.     

The molecular refractive function of lens γ-Crystallins

γ-Crystallins constitute the major protein component in the nucleus of the vertebrate eye lens. Present at very high concentrations, they exhibit extreme solubility and thermodynamic stability to prevent scattering of light and formation of cataracts. However, functions beyond this structural role have remained mostly unclear. Here, we calculate molecular refractive index increments of crystallins. We show that all lens γ-crystallins have evolved a significantly elevated molecular refractive index increment, which is far above those of most proteins, including nonlens members of the βγ-crystallin family from different species. The same trait has evolved in parallel in crystallins of different phyla, including S-crystallins of cephalopods. A high refractive index increment can lower the crystallin concentration required to achieve a suitable refractive power of the lens and thereby reduce their propensity to aggregate and form cataracts. To produce a significant increase in the refractive index increment, a substantial global shift in amino acid composition is required, which can naturally explain the highly unusual amino acid composition of γ-crystallins and their functional homologues. This function provides a new perspective for interpreting their molecular structure.     

Dominant cataracts result from incongruous mixing of wild-type lens connexins

Gap junctions are composed of proteins called connexins (Cx) and facilitate both ionic and biochemical modes of intercellular communication. In the lens, Cx46 and Cx50 provide the gap junctional coupling needed for homeostasis and growth. In mice, deletion of Cx46 produced severe cataracts, whereas knockout of Cx50 resulted in significantly reduced lens growth and milder cataracts. Genetic replacement of Cx50 with Cx46 by knockin rescued clarity but not growth. By mating knockin and knockout mice, we show that heterozygous replacement of Cx50 with Cx46 rescued growth but produced dominant cataracts that resulted from disruption of lens fiber morphology and crystallin precipitation. Impedance measurements revealed normal levels of ionic gap junctional coupling, whereas the passage of fluorescent dyes that mimic biochemical coupling was altered in heterozygous knockin lenses. In addition, double heterozygous knockout lenses retained normal growth and clarity, whereas knockover lenses, where native Cx46 was deleted and homozygously knocked into the Cx50 locus, displayed significantly deficient growth but maintained clarity. Together, these findings suggest that unique biochemical modes of gap junctional communication influence lens clarity and lens growth, and this biochemical coupling is modulated by the connexin composition of the gap junction channels.     

Crystallins and hereditary cataracts: molecular mechanisms and potential for therapy

Hereditary childhood cataracts can arise from single-point mutations in genes encoding crystallins, the major protein components of the lens. The cataracts are most commonly inherited by an autosomal dominant mechanism. The nature of the changes in the lens resulting from these point mutations in crystallin genes has not been fully characterised. While aggregation and light scattering associated with expression of the mutant crystallin protein may be an end point, it is also necessary to determine the progression of changes induced at the level of development and differentiation. A key finding in recent work is that cell death or cytotoxicity is associated with mutations in alpha A-crystallin. The variable morphology or localisation of the cataract in different pedigrees, even with the identical crystallin gene mutation, has led to the idea that other environmental or genetic factors interact to give the final lens phenotype. The study of mechanisms of formation of hereditary cataracts may lead to a greater understanding of the mechanisms that lead to age-related cataracts, a very common cause of blindness in the ageing population.     

Pediatric cataracts

Congenital and acquired pediatric cataracts are recognized by many as being among the visually devastating pathologies of children. They remain one of the leading causes of legal blindness for children in under developed countries world wide. Diagnosed in approximately 400–500 patients per year in the United States alone, their presence requires agressive management with concurrent amblyopia therapy. Although most pediatric opacities are idiopathic, systemic etiologies which include genetic inheritance (Fabry's disease, Lowe's syndrome, Conradi's syndrome) metabolic disturbances (Galactosemia, Hypocalcemia, hypoglycemia, retinitis pigmentosa), infectious diseases (maternal syphilis, varicella virus, herpes virus, cytomegalovirus), toxic substances and trauma must be investigated. We reviewed the literature for pertinent information on the differential diagnosis of congenital and acquired cataracts in the pediatric population. The modalities and techniques of the pediatric examination are presented along with a discussion of the maladies, most recent advances in surgical intervention, optical or contact lens correction and amblyopia therapy associated with pediatric cataracts.     

Effect of the isoflavone genistein against galactose-induced cataracts in rats

Worldwide, ocular cataracts are a major cause of human blindness. A key goal of cataract-related research is to identify simple, cost-efficient but effective ways to prevent cataract formation or progression. Genistein is a naturally occurring dietary isoflavone with well-documented estrogenic, antioxidant, and protein tyrosine kinase inhibitor activity, which in turn modulates the activity of several enzymes involved in cell signaling and proliferation. Furthermore, many isoflavones have been shown to be potent inhibitors of aldose reductase, which is an important rate-limiting enzyme in the process of cataract induction in the metabolic disease galactosemia. In order to assess the potential for genistein to mitigate cataract formation, we have studied its effects in the animal model of dietary galactose-induced cataracts in adult male rats. Our experimental hypothesis was that dietary genistein would prevent or delay the progression of cataracts induced by high dietary intake of galactose. Our results show that the isoflavone genistein was not able to completely prevent galactose-induced cataract formation, but genistein did delay the progression of cataracts induced by dietary galactose. In addition, we found that dietary galactose decreased concentrations of serum somatostatin, while adding genistein decreased the serum glucose level but increased the serum testosterone level. As an initial inquiry into the mechanisms by which the partial protective effect of genistein could be mediated, we found that genistein increased the expression of connexin (Cx) 43 in the lens but did not affect the expression of soluble guanylyl cyclase (sGC) subunits. This finding suggests that the partial protective effect of genistein on cataract induction appears to be unrelated to sGC but may be mediated by enhanced expression of Cx43 and changed metabolic state.     

Diverse roles of Eph/ephrin signaling in the mouse lens

Recent genetic studies show that the Eph/ephrin bidirectional signaling pathway is associated with both congenital and age-related cataracts in mice and humans. We have investigated the molecular mechanisms of cataractogenesis and the roles of ephrin-A5 and EphA2 in the lens. Ephrin-A5 knockout (-/-) mice often display anterior polar cataracts while EphA2(-/-) lenses show very mild cortical or nuclear cataracts at weaning age. The anterior polar cataract of ephrin-A5(-/-) lenses is correlated with multilayers of aberrant cells that express alpha smooth muscle actin, a marker for mesenchymal cells. Only select fiber cells are altered in ephrin-A5(-/-) lenses. Moreover, the disruption of membrane-associated β-catenin and E-cadherin junctions is observed in ephrin-A5(-/-) lens central epithelial cells. In contrast, EphA2(-/-) lenses display normal monolayer epithelium while disorganization is apparent in all lens fiber cells. Immunostaining of ephrin-A5 proteins, highly expressed in lens epithelial cells, were not colocalized with EphA2 proteins, mainly expressed in lens fiber cells. Besides the previously reported function of ephrin-A5 in lens fiber cells, this work suggests that ephrin-A5 regulates β-catenin signaling and E-cadherin to prevent lens anterior epithelial cells from undergoing the epithelial-to-mesenchymal transition while EphA2 is essential for controlling the organization of lens fiber cells through an unknown mechanism. Ephrin-A5 and EphA2 likely interacting with other members of Eph/ephrin family to play diverse functions in lens epithelial cells and/or fiber cells.