I-122 Zdrav Vestn Supl | junij 2012 | Letnik 81 PregLednI čLanek/reVIew Cataractogenesis kataraktogeneza Sofija andjelić, Marko Hawlina Izvleček Kataraktogeneza je proces nastanka sive mrene. Katarakta, ki jo je mogoče opredeliti kot katero koli motnost leče, je prevladujoči vzrok odpra- vljive slepote po vsem svetu. Mehanizmi, ki so- delujejo v kataraktogenezi, še niso popolnoma znani. Namen tega članka je podati pregled so- dobnih raziskav o kataraktogenezi. Več dejavni- kov tveganja za nastanek katarakte je že oprede- ljenih, vključno s starostjo, gensko nagnjenostjo, oksidativnim stresom in izpostavljenostjo UV- -svetlobi. Katarakte so lahko prirojene, povezane s starostjo ali sekundarne. Sekundarna katarakta je lahko povezano z očesnimi stanji, kot so pi- gmentna retinopatija ali uveitis, ali sistemskimi stanji, kot je v primeru sladkorne bolezni ali ho- mocistinurije, ali jo pa lahko povzročajo zdravi- la, zlasti kortikosteroidi. Introduction Cataract is a significant problem throu- ghout the world and is responsible for the majority of visual impairments in adult hu- mans. As classified by predominant types, cataract can be congenital, age-related or secondary. Congenital or juvenile cataract may have serious visual consequences on visual maturation and development of am- blyopia, and account for approximately 30 % of blindness in infants. Age-related cataract is responsible for nearly half of all blindness worldwide.1 Cataract types 1. Congenital cataract Congenital cataract is rare and occurs in developed countries with a frequency of 30 per 100,000 births with a further 10 cases being diagnosed during childhood, main- ly as dominant form.2 Rates are likely to be higher in developing countries because of rubella infections and consanguinity, for the recessive forms.3 The knowledge about the mechanisms of cataractogenesis has been derived mostly from the genetic analysis of affected families and from the spontaneous or induced mutations in the mouse.4 Affected family analysis showed that the- re are contributions from genes coding for Abstract Cataractogenesis is the process of cataract forma- tion. Cataract, which can be defined as any opac- ity of the crystalline lens, is the leading cause of avoidable blindness worldwide. The mechanisms involved in cataractogenesis are not yet under- stood. The purpose of this review is to give an overview of contemporary research in cataracto- genesis. Several risk factors for cataract forma- tion have been identified, including increasing age, genetic predisposition, oxidative stress and exposure to UV light. Cataract can be congenital, age-related or secondary. Secondary cataract can be associated with ocular conditions such as ret- initis pigmentosa or uveitis, or systemic condi- tions as in the case of diabetes or homocistinuria, or can be also drug-induced, mainly by steroids. eye Hospital, University Medical Centre Ljubljana, grablovičeva 46, 1525 Ljubljana, Slovenia Korespondenca/ Correspondence: dr. sc. Sofija andjelić, eye Hospital, University Medical Centre Ljubljana, grablovičeva 46, 1525 Ljubljana, Slovenia Ključne besede: katarakta, kataraktogeneza Key words: cataract, cataractogenesis Citirajte kot/Cite as: Zdrav Vestn 2012; 81: I-122–32 Prispelo: 14. mar. 2012, Sprejeto: 23. apr. 2012 Zdrav Vestn Supl | Cataractogenesis I-123 PregLednI čLanek/reVIew least common. Whether these differences are related to differences in genetics, envi- ronment, diet or other factors is difficult to discern. Even age-related cataract, generally tho- ught to be due to multiple insults accumu- lated over many years, may have a genetic component, making certain individuals more vulnerable to the environmental in- sults.8 b. Clinical types i. Cortical cataract The first human mutations associated with age-related cortical cataract were re- cently identified in EPH2A, a gene encoding a transmembrane tyrosine kinase.10,11 Epithelial changes are associated with the cortical cataract. Epidemiological in- formation supports an association between exposure to UV radiation from sunlight and the development of cortical cataract in hu- mans.12 The opaque shades of cortical ca- taract represent cohorts of locally affected fibres segregated from unaffected neighbo- uring fibres.13-15 The lens epithelium is especially vulne- rable to oxidative stress. Being the anteri- ormost portion of the lens, it is the first site for the interaction with the UV radiation. In vivo studies of the UV radiation effects showed that decreased cell density of lens epithelial cells was observed in all three epi- thelial zones along with a decrease in the le- vels of soluble sulfhydryls (S-SH), glutathio- ne reductase, superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT).16 After UV radiation exposure, p53 and caspase-3 expression increased in lens epithelial cells. Apoptosis induced by UV radiation may be associated with increased p53 expression.17 Oxidative stress or UV ra- diation have also been reported to induce an aberrant in situ transglutaminase (TG) acti- vation in human lens epithelial cells.18 Tran- sglutaminase 2 (TG2) is a multifunctional calcium-dependent enzyme that catalyzes the post-translational protein crosslinking. The up-regulation and activation of TG2 have been reported in cataractogenesis.19 Another mechanism of premature lens senescence phenotype that triggers human transcription factors and structural proteins such as crystallins or connexins. Cataract may be inherited either as an isolated ocu- lar abnormality or as part of a syndrome,5 with the nonsyndromic cataract represen- ting a significant proportion of cases where many causative genetic mutations have been identified.6 Congenital cataract (in industri- alized countries) reflects mainly genetically caused developmental alterations in the lens. There is a broad genetic heterogeneity in that which clinicians usually simply refer to as a »cataract«. 2 The most frequent mutations in congenital cataract affect genes coding for γ-crystallins (gene symbol: Cryg). Some po- stnatal, progressive cataracts have been cha- racterized by mutations in the β-crystallin encoding genes (Cryb). Mutations in genes coding for membrane proteins like major intrinsic protein of lens fiber (MIP) or con- nexins and for transcription factors such as FoxE3, Maf, Sox1 and Six5 may also cause cataracts.7 There are also contributions from enzymes affecting sugar pathways, particu- larly the galactose pathway, and from axon guidance molecules such as ephrins and their receptors.4 The mouse cataract models contributed to the understanding of lens development rather than to the ageing process affecting the lens. Early events are influenced by genes coding for transcription factors. In maturing lens, mutations affecting the lens membra- nes (aquaporins/Mip, Lim-2 or connexins) or the structural proteins of the cytosol of the lens fiber cells (the crystallins) become more important.2 2. Age-related cataract a. Epidemiology Age-related cataract is the most frequent type of cataract. The lens is clear during in- fancy and remains clear until sometime after 45 years of age when progressive opacities begin to form8 starting the process of cata- ractogenesis. The prevalence of the 3 main subtypes of age-related cataracts, nuclear, cortical or posterior subcapsular catarac- ts (PSCs), differs in different regions of the world and in different racial groups.9 Howe- ver, in all areas and populations, PSCs are I-124 Zdrav Vestn Supl | junij 2012 | Letnik 81 PregLednI čLanek/reVIew Loss of protein sulfhydryl groups and the oxidation of methionine residues are pro- gressive and increase as the cataract worsens until > 90 % of cysteine and half the methi- onine residues are oxidised in the most ad- vanced form.22 Studies of the morphological structure and biophysical changes of the lens in human senile cataract have demonstrated the disappearance of normal fiber structure in the opaque region of the lens and the di- sintegration of the lens fiber plasma mem- brane in the lens tissue.13 Modified with oxygen, phospholipid molecules, accumula- ting in the lipid bilayer, change its geome- try and impair lipid-lipid and protein-lipid interactions in lenticular fiber membranes. Human lenses at various stages of age-rela- ted cataract studied by electron microscopy show that these disruptions are globules, va- cuoles, multilamellar membranes, and clu- sters of highly undulating membranes. In the mature cataract nucleus, other potential scattering centers include variations in sta- ining density between adjacent cells, enlar- ged extracellular spaces between undulating membrane pairs, and protein-like deposits in the extracellular space.13 c. Mechanisms of cataractogenesis of age-related cataracts i. Oxidative stress A common environmental factor in most age-related cataracts is believed to be oxida- tive stress.22,25 Cataract formation occurs when the rate of reactive oxygen species (ROS) production exceeds the rate of their removal.26 During environmental stress (e.g. UV or heat exposure), ROS levels can inc- rease dramatically. In vitro and in vivo stu- dies suggest that light penetration into the eye is a significant contributory factor in the cataractogenesis, with the major effect being through photochemical generation of ROS and consequent oxidative stress to the tis- sue.27 This may result in significant damage to cell structures such as damage of DNA, RNA, and proteins. Low levels of antioxi- dants, such as glutathione and vitamin C or inhibition of the antioxidant enzymes, such as catalase (CAT), superoxide dismutase (SOD) and various peroxidases, cause oxi- dative stress and may damage or kill cells. 28 cataractogenesis in human lens epithelial cells is believed to be erosion and shorte- ning of telomeres and the lack of telomerase activity. The rate of telomere shortening is modulated by oxidative stress and by chan- ged antioxidative defense capacity.20 Lipid peroxidation (LPO) may also be associated with cataractogenesis, initiated by enhanced promotion of oxygen free radicals in the eye fluids and tissues and impaired enzymatic and non-enzymatic antioxidant defenses of the crystalline lens. The increased concen- trations of primary molecular LPO produc- ts were detected in the lipid moieties of the aqueous humor samples obtained from pati- ents with senile and complicated cataracts.20 All these mechanisms may possible cause cortical cataract. ii. Nuclear cataract In nuclear cataract the nucleus is sub- stantially harder, more opaque and usually more brown than in persons of comparable age who did not develop cataract. Clinicians describe it as »nuclear sclerotic cataract« as an indication of the increased hardness of the cataractous nucleus. With increasing age, many types of modifications occur in the abundant crystallin proteins of the lens nucleus. When an internal barrier to the movement of small molecules, such as an- tioxidants, develops in the normal lens at middle age, the long-lived proteins in the lens centre become susceptible both to co- valent attachment of reactive molecules and to oxidation. These processes of prote- in modification may, over time, lead to lens opacification and cataract21 where effects of UV radiation and oxidative stress also play a role. These include protein truncati- on, cross-linking, denaturation, amino acid racemization, deamidation, glycation and oxidation. The exposure to increased levels of molecular oxygen and UV exposure acce- lerates the opacification of the lens nucleus, leading to nuclear cataract.1,22-24 Factors in the eye that maintain low oxygen partial pressure around the lens are, therefore, important in protecting the lens.1 The key factor in preventing oxidation se- ems to be the concentration of nuclear glu- tathione.22 Zdrav Vestn Supl | Cataractogenesis I-125 PregLednI čLanek/reVIew cs, we used Ca2+ indicator dyes that allow the tracking of changes in [Ca2+]i in real time. The lens epithelial cells respond to the bath application of acetylcholine (ACh) with a rise in intracellular calcium concentration, [Ca2+]i.41-43 In human anterior lens epitheli- al cell ACh binds to M1 muscarinic receptors and induces a rise in [Ca2+]i.29,43,44 We have recently described fast contractions of lens epithelial cells45 (Fig.1.) and found that these occur as a nonspecific response to ACh, wa- ter jet or mechanical contact. We have also found that [Ca2+]i may not be directly invol- ved in the changes of the shape of epithelial cells described, and therefore, some other mechanisms, such as activation of transi- ent receptor potential channels (TRP chan- nels),46-49 may be involved in this process. During contraction, the cells stay conected to each other at several points, presumably representing regions containing desmoso- mes and/ or gap junctions. The gaps forming between the epithelial cells would very likely cause influx of water and seriously impair the normal function of the lens epithelium in situ. Water influx through these gaps may be linked to cataractogenesis, for example in injury or phakic intraocular lenses that are in contact with the cristalline lens. As con- tractions may be a new mechanism associa- ted with cataractogenesis, there is a need for their further studies to assess the underlying physiological mechanisms and eventual the- rapeutic or prophylactic possibilities. 3. Secondary cataracts a. Secondary cataracts in systemic disorders i. Diabetes Diabetes is potential cause of cataract development in affected individuals,50,51 with cataract in diabetic patients being a major cause of blindness in developed and developing countries.52 Aldose reductase is involved in secondary diabetic complica- tions including cataractogenesis. It is a key enzyme of polyol pathway that catalyzes coensyme NADPH-dependent reduction of glucose to sorbitol (sorbitol pathway) and li- pid aldehydes to lipid alcohols. An excessive accumulation of intracellular sorbitol found in various tissues of diabetic animals and in ii. Calcium Calcium, Ca2+, has an important role in the development of human age-related cata- ract. The Ca2+ level controls homeostasis of entire lens. Alteration in Ca2+ homeostasis is associated with various types of human and experimental cataracts. The raised le- vels of Ca2+ in human lenses with cortical cataract play a major role in the opacifica- tion process.29-31 The very large increases in Ca2+ recorded in cortical cataract indicate that intracellular Ca2+ homeostasis breaks down and influx exceeds the ability of lens cells to remove Ca2+ from the cytosol.29 The epithelial cell breakdown in cortical cataract causes dysfunction of active transport of electrolytes, causing passive inward move- ment of water.32 Intracellular overload with Ca2+ in the epithelial cells triggers activati- on of Ca2+ dependent enzymes, irreversible breakdown of important structural proteins and cell death.33 Nuclear cataract does not involve major Ca2+ alteration in the lens.34,35 The role of calcium in cataractogenesis was reviewed previously.36 The lens epitheli- um is a subject of numerous studies because of its importance for functioning of the lens and its accessibility. A significant part of epi- thelial cell research is oriented towards the role of the altered Ca2+ signalling in lens epi- thelial cells and the effects this may have in cataract formation.37-39 The role of lens epi- thelial cells in controlling the lenticular Ca2+ is interesting since other components of lens do not possess intracellular Ca2+-store such as endoplasmic reticulum and mitochon- dria. In order for Ca2+ to act as a signalling molecule and to prevent the toxic effects of Ca2+ overload, intracellular Ca2+ is tightly re- gulated and the concentration of Ca2+ in the cytoplasm is kept low. In our previous studies, we have used the human anterior lens capsule preparation consisting of the monolayer of anterior lens epithelial cells lying on the basal lamina, ob- tained during routine cataract surgeries. We have shown that the human anterior lens capsule preparation is an adequate source for investigating cellular Ca2+ dynamics of lens epithelial cells in different forms of ca- taract.40 To investigate cellular Ca2+ dynami- I-126 Zdrav Vestn Supl | junij 2012 | Letnik 81 PregLednI čLanek/reVIew Figure 1: The contraction of the lens epithelial cells. a. Lens epithelial cells in non-contracted state, before stimulation. B. enlarged image of the selected region of interest (yellow, a) showing the contraction of the lens epithelial cells upon aCh stimulation. C. The time courses of the coefficient of variation, indicating the changes in morphology, are shown in red and the time courses of the 360 ⁄ 380 ratio, proportional to [Ca2+]i, are shown in blue. In this case, the change in morphology starts synchronously with the Ca-response. hyperglycemic cataract and the use of aldo- se reductase inhibitors in the prevention of cataractogenesis.61 ii. Homocystinuria Cataract is frequently associated with homocystinuria. Homocystinuria is an in- herited disorder in which the body is una- ble to process certain amino acids properly. There are multiple forms of homocystinuria, which are distinguished by their signs and symptoms and genetic cause. Homocystinu- ria is inherited in families as an autosomal recessive trait. Mutations in the genes for cystathionine beta synthase (CBS), methyle- netetrahydrofolate reductase (MTHFR), 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), 5-methyltetrahy- drofolate-homocysteine methyltransfera- se reductase (MTRR), and methylmalonic aciduria (cobalamin deficiency) cblD type, with homocystinuria (MMADHC), cause homocystinuria. Mutations in the CBS gene cause the most common form of homocy- stinuria.62,63 The CBS gene provides instruc- tions for producing enzyme cystathionine cells cultured under high glucose conditions has been proposed to be an important factor for the pathogenesis of diabetic complicati- ons.53 The intracellular accumulation of sor- bitol results in hyperosmotic effects that can induce cellular swelling that initiates a cascade of biochemical steps that result in lens opacification. Among these steps is the osmotic induction of endoplasmic reticular stress 54,55 which then can initiate an unfol- ded protein response that generates ROS and apoptotic signaling. As sorbitol accumulates primarily in the epithelium and superficial lens fibers of the diabetic lens, free radical production is increased and natural antioxi- dant defenses are compromised. This results in increased oxidative stress 56-59 and apop- totic signaling.60 This generation of ROS has been reported by many investigators.56,57 The rat lenses obtained from Aldose re- ductase knockdown rats were resistant to high glucose-induced lens opacification as compared to wild-type rat lenses. These experiments indicate a physiological role of aldose reductase in the pathophysiology of Zdrav Vestn Supl | Cataractogenesis I-127 PregLednI čLanek/reVIew autosomal recessive (ar) and all of these can develop cataract.65 Patients with RP typically develop a combined posterior cortical and posterior subcapsular cataract which, when combined with a restricted central visual field, may ca- use significant additional visual disability even when the opacity appears to be relati- vely minor.66 Cataract associated with reti- nitis pigmentosa may not occur until second or third decade.8,66-69 Based on a study of the Royal College of Surgeons (RCS) on rat model of retinal degeneration, posterior subcapsular cataract likely results from abnormal posterior lens fiber growth and suture formation deve- lopment. Posterior subcapsular cataract de- velops presumably in response to toxic lipid peroxides formed by degenerating rod outer segments.70 ii. Uveitis Cataract formation is one of the major complications of different forms of uveitis. Cataract development in uveitis results from chronic inflammation and as a consequence of long-term corticosteroid treatment.71 The etiologic cause and type of uveitis can influence not only disease course, but also the treatment response and rate of asso- ciated complications, such as cataract.72 For example, a diagnosis of juvenile idiopathic arthritis (JIA)-associated uveitis is conec- ted with an increased rate of cataract deve- lopment and often more complicated post- -extraction course in comparison to patients with other idiopathic diseases.73,74 Risk fac- tors for cataract include posterior synechiae and longstanding ocular inflammation.75 Cataract extraction with intraocular lens implantation with the control of inflamma- tion can optimize visual outcome in adults and children with uveitis.71 However, cata- ract surgery in uveitic patients is challen- ging. The unpredictable behavior of some uve- itic cataract capsules during capsulorhexis may occur due to changes in the capsule’s ultrastructure. In white uveitic cataracts, extensive epithelial and capsular-epithelial border changes and epithelial-mesenchymal transition in some fibrotic capsules were fo- beta-synthase that is responsible for con- verting the amino acid homocysteine to a molecule called cystathionine. As a result of this pathway, other amino acids, including methionine, are produced. Mutations in the CBS gene disrupt the function of cystathi- onine beta-synthase, preventing homocy- steine from being used properly. Rarely, ho- mocystinuria can be caused by mutations in several other genes. The enzymes made by the MTHFR, MTR, MTRR, and MMADHC genes play roles in converting homocysteine to methionine. Mutations in any of these ge- nes prevent the enzymes from functioning properly, which leads to a buildup of homo- cysteine in the body. The screening of patients for homocysti- nuria with and without cataract was done and analysed for homocystine and methio- nine.64 Out of 29 homocystinuric patients, 24 had cataract. Only one had appreciable amounts of methionine in his serum. He also had mental retardation associated with Type I. The other types (II, III or IV) did not have methionine but had only homocystine. There was no mental retardation or ectopia lentis. As there is excess methionine in Type I, with low cysteine, cataract may be due to deficiency of cysteine and reduced glutathi- one and might be averted by suitable thera- py, i.e., high–cysteine–low-methionine diet with B6. In other types with low methionine, cataract may be due to decreased availability of amino acids for the synthesis of lens pro- teins; the treatment of choice should be B12, and folate with methionine.64 b. Secondary cataracts in other eye diseases and conditions i. Retinitis pigmentosa Cataract is often also associated with the inherited retinal degenerations. Retini- tis pigmentosa (RP) is a type of progressive retinal dystrophy, clinically and genetically heterogeneous group of inherited retinal di- sorders in which abnormalities of the pho- toreceptors (rods and cones) or the retinal pigment epithelium of the retina lead to pro- gressive visual loss and eventually incurable blindness. The mode of inheritance can be X-linked (xl), autosomal dominant (ad), or I-128 Zdrav Vestn Supl | junij 2012 | Letnik 81 PregLednI čLanek/reVIew lens epithelial cell signaling pathways, mito- genactivated protein kinases (MAPKs) and phosphotidyl inositol-3-kinase/AKT (PI3K/ AKT) regulators, suggest that glucocorti- coids modulate several cellular functions.83 The chronic glucocorticoids treatment pos- sibly leads to prolonged modulation of these pathways and steroid-induced cataract.84 Glucocorticoid receptors (GR) have been shown to take part in the apoptotic process of human lens epithelial cells, but the GR antagonist RU486 does not rescue the cells fully.85 Dexamethasone, a steroid which has been used as a medical agent for a long time, increased expression of glucocorticoid re- ceptors, GRE-luciferase, the GR-α gene and GR-protein and, in contrast, decreased the viability of human lens epithelial cells. The nuclear morphology of human lens epithe- lial cells showed an obvious apoptotic phe- nomenon under greater concentrations of Dexamethasone.85,86 ii. Sex hormones The female sex hormone estrogen may modify rates of cataractogenesis. Ovarian hormones enhance radiation-induced cata- ract formation. However, estrogen both enhances and at- tenuates the rate of cataractogenesis, depen- ding on the time of hormone administration relative to the cataractogenic insult, as found with the use of a rat model of radiation-in- duced cataract.87,88 The major endogenous estrogen, 17β-estradiol, has a mitogenic and anti-oxidative effects at physiologic concen- trations, whereas pharmacological levels in- duce oxidative stress and act pro-apoptotic in cultured lens cells.89 Hormone supple- mentation experiments indicate that estro- gen is responsible for cataract formation.90 iii. Other drugs and cataract Cataractous changes can result from antipsychotics. Systemic administration of antipsychotics phenothiazines or the anti- -cancer drug busulfan induce cataractous changes in the anterior or posterior cortex, respectively.78 Antipsychotics chlorproma- zine or thioridazine, when used at high do- und. Uveitic capsules showed more extensi- ve and different ultrastructural changes that probably occurred because of inflammation in the eye than capsules of nuclear non- -uveitic cataracts.76 Intraocular inflamma- tion causes high levels of pro-inflammatory cytokines such as TGF-b, within the aqueous humor and vitreous. These can pass across the lens capsule and induce mesenchymal transition of the anterior lens epithelium.77 c. Drug toxicity and cataract Drugs can produce adverse reactions with the toxic effects of systemic and topi- cally applied drugs manifested as cloudiness of the lens.78 i. Corticosteroids Corticosteroids have long been associa- ted with cataract, regardless of how they are administered, but the direct injection into the eye of triamcinolone, often to treat ma- cular edema, frequently leads to a cataract.79 Long-term use of glucocorticoids produces a characteristic posterior subcapsular cata- ract (PSC) and, although the opacities may remain stationary or progress, they rarely regress upon drug withdrawal.78 Steroid- -induced posterior subcapsular cataract exhibits three main distinctive characteri- stics: association only with steroids posses- sing glucocorticoid activity, involvement of aberrant migrating lens epithelial cells, and a central posterior location.80 The evaluation of posterior capsule opacification (PCO) de- velopment after cataract surgery in eyes with or without a history of steroid use showed that steroid-induced posterior subcapsular cataract was associated with a higher risk for PCO after cataract surgery at the 1-year fol- low-up. 81 The mechanisms responsible for the opa- cification are unknown. One proposed me- chanism is that steroids do not directly act on the lens but rather affect the balance of ocular cytokines and growth factors.82 On the other hand, the finding of a clas- sical, specific, functional lens glucocorticoid receptor suggests that glucocorticoids tar- get lens epithelial cells directly, specifical- ly, and similar to what has been observed in other cell types. The distinct changes in Zdrav Vestn Supl | Cataractogenesis I-129 PregLednI čLanek/reVIew sages and for prolonged periods, frequently cause lenticular opacifications.91 Conclusions Cataract is responsible for the majority of visual impairments in adult humans. No clinically used non-surgical intervention exist that can prevent or treat age-related ca- taract. Several major risk factors for cataract formation have been identified, including increasing age, genetic predisposition, oxi- dative stress, exposure to UV light and other toxic agents, such as corticosteroids, and di- sease conditions. Understanding the process of cataract formation is of great importance. The mechanisms involved in cataractogene- sis are heterogenous and not yet understood. However, there is a better understanding of the complexity of this multifactorial conditi- on. There is a significant research work done and in progress by different groups in order to understand the cataractogenesis. Our work is also oriented in that direction. As we can conclude from this review, contem- porary research is giving a significant new insights into the mechanisms of cataracto- genesis, but the mechanisms do not seem to have a common denominator that would envisage possibilities of pharmacological treatment soon. Acknowledgements The study was supported by The Sloveni- an Research Agency (program P3–0333 and postdoctoral grant Z3–9689). I-130 Zdrav Vestn Supl | junij 2012 | Letnik 81 PregLednI čLanek/reVIew taminase 2 in age-related cataractogenesis. J Biol Chem 2004; 279: 15032–9. 19. Caccamo D, Curro M, Ferlazzo N, Condello S, Ientile R. Monitoring of transglutaminase2 under different oxidative stress conditions. Amino Acids Epub 2011 July 39. 20. Babizhayev MA, Vishnyakova KS, Yegorov YE. Telomere-dependent senescent phenotype of lens epithelial cells as a biological marker of aging and cataractogenesis: the role of oxidative stress inten- sity and specific mechanism of phospholipid hy- droperoxide toxicity in lens and aqueous. Fundam Clin Pharmacol 2011; 25: 139–62. 21. Truscott RJ. Human cataract: the mechanisms re- sponsible; light and butterfly eyes. Int J Biochem Cell Biol 2003; 35: 1500–4. 22. Truscott RJ. Age-related nuclear cataract-oxidati- on is the key. Exp Eye Res 2005; 80: 709–25. 23. Dong X, Ayala M, Lo¨ fgren S, So¨ derberg PG. Ultraviolet radiation-induced cataract: age and maximum acceptable dose. Invest Ophthalmol Vis Sci 2003; 44: 1150–4. 24. Lofgren S, Michael R, Soderberg PG. Impact of age and sex in ultraviolet radiation cataract in the rat. Invest Ophthalmol Vis Sci 2003; 44: 1629–33. 25. Chiu CJ, Taylor A. Nutritional antioxidants and age-related cataract and maculopathy. Exp Eye Res 2007; 84: 229–45. 26. Babizhayev MA, Yegorov YE. Reactive oxygen species and the aging eye: Specific role of meta- bolically active mitochondria in maintaining lens function and in the initiation of the oxidation-in- duced maturity onset cataract – A novel platform of mitochondria-targeted antioxidants with broad therapeutic potential for redox regulation and de- toxification of oxidants in eye diseases. Am J Ther 2010. 27. Varma SD, Kovtun S, Hegde KR. Role of ultravi- olet irradiation and oxidative stress in cataract formation-medical prevention by nutritional anti- oxidants and metabolic agonists. Eye Contact Lens 2011; 37: 233–45. 28. Aher VD, Wahi A, Pawdey AM, Sonawane A. Anti- oxidants as immunomodulator: an expanding re- search avenue. Int J Curr Pharm Res 2011; 3: 8–10. 29. Rhodes JD, Sanderson J. The mechanisms of calci- um homeostasis and signalling in the lens: a revi- ew. Exp Eye Res 2009; 88: 226–34. 30. Sanderson J, Marcantonio JM, Duncan G. A hu- man lens model of cortical cataract: Ca2+ induced protein loss, vimentin cleavage and opacification. Invest Ophthalmol Vis Sci 2000; 41: 2255–61. 31. Tang D, Borchman D, Yappert MC, Vrensen GF, Rasi V. Influence of age, diabetes, and cataract on calcium, lipid-calcium, and protein-calcium rela- tionships in human lenses. Invest Ophthalmol Vis Sci 2003; 44: 2059–66. 32. Delamere NA, Tamiya S. Expression, regulation and function of Na, KATP- ase in the lens. Prog Retin Eye Res 2004; 23: 593–615. 33. Gupta PD, Johar K, Vasavada A. Causative and preventive action of calcium in cataracto-genesis. Acta Pharmacol Sin 2004; 25: 1250–6. 34. Duncan G, Bushell AR. Ion analyses of human ca- taractous lenses. Exp Eye Res 1975; 20: 223–30. 35. Duncan G, Jacob TJ. Calcium and the physiology of cataract. Ciba Found Symp 1984; 106: 132–52. References 1. Beebe DC, Holekamp NM, Shui Y. Oxidative da- mage and the prevention of age-related cataracts. Ophthalmic Res 2010; 44: 155–65. 2. Graw J. Congenital hereditary cataracts. Int J Dev Biol 2004; 48: 1031–44. 3. Foster A, Gilbert C, Rahi J. Epidemiology of cata- ract in childhood: a global perspective. J Cataract Refract Surg 1997; 23 (Suppl. 1): 601–4. 4. Churchill A, Graw J. Clinical and experimental ad- vances in congenital and paediatric cataracts. Phi- los Trans R Soc Lond B Biol Sci 2011; 366: 1234–49. 5. Francis PJ, Berry V, Bhattacharya SS, Moore AT. The genetics of childhood cataract. J Med Genet 2000; 37: 481–8. 6. Reddy MA, Francis PJ, Berry V, Bhattacharya SS, Moore AT. Molecular genetic basis of inherited ca- taract and associated phenotypes. Surv Ophthal- mol. 2004; 49: 300–15. 7. Graw J. Mouse models of cataract. J Genet 2009; 88: 469–86. 8. Shiels A, Hejtmancik JF. Genetic origins of cata- ract. Arch Ophthalmol 2007; 125: 165–73. 9. Sasaki K, Sasaki H, Jonasson F, Kojima M, Cheng HM. Racial differences of lens transparency pro- perties with aging and prevalence of age-related cataract applying a WHO classification system. Ophthalmic Res 2004; 36: 332–40. 10. Jun G, Guo H, Klein BE, Klein R, Wang JJ, Mit- chell P, et al: EPHA2 is associated with agerelated cortical cataract in mice and humans. PLoS Genet 2009; 5:e1000584. 11. Shiels A, Bennett TM, Knopf HLS, Maraini G, Li A, Jiao X, Hejtmancik JF. The EPHA2 gene is associ- ated with cataracts linked to chromosome 1p. Mol Vis 2008; 14: 2042–55. 12. McCarty CA, Taylor HR. A review of the epidemi- ologic evidence linking ultraviolet radiation and cataracts. Dev Ophthalmol 2002; 35: 21–31. 13. Babizhayev MA. Potentiation of intraocular ab- sorption and drug metabolism of N-acetylcar- nosine lubricant eye drops: drug interaction with sight threatening lipid peroxides in the treatment for age-related eye diseases. Drug Metabol Drug Interact 2009; 24: 275–323. 14. Vrensen GF. Early cortical lens opacities: a short overview. Acta Ophthalmol 2009; 87: 602–10. 15. Vrensen G, Willekens D. Biomicroscopy and scan- ning electron microscopy of early opacities in the aging human lens. Invest Ophthalmol Vis Sci 1990; 31: 1582–91. 16. Johar SR, Rawal UM, Jain NK, Vasavada AR. Sequential effects of ultraviolet radiation on the histomorphology, cell density and antioxidative status of the lens epithelium – An In Vivo stu- dy. Photochemistry and Photobiology 2003; 78: 306–11. 17. Ayala M, Strid H, Jacobsson U, Söderberg PG. p53 Expression and apoptosis in the lens after ultravi- olet radiation exposure. Invest Ophthalmol Vis Sci 2007; 48: 4187–91. 18. Shin DM, Jeon JH, Kim CW, Cho SY, Kwon JC, Lee HJ, Choi KH, et al. Cell type-specific activation of intracellular transglutaminase 2 by oxidative stress or ultraviolet irradiation: implications of transglu- Zdrav Vestn Supl | Cataractogenesis I-131 PregLednI čLanek/reVIew sponse in lens epithelial cells from galactosemic rat lenses. Invest Ophthalmol Vis Sci 2006; 47: 3951–9. 56. Hegde KR, Varma SD. Combination of glycemic and oxidative stress in lens: implications in au- gmentation of cataract formation in diabetes. Free Radic Res 2005; 39: 513–17. 57. Chan AW, Ho YS, Chung SK, Chung SS. Synergi- stic effect of osmotic and oxidative stress in slow- -developing cataract formation. Exp Eye Res 2008; 87: 454–61. 58. Obara Y. The oxidative stress in the cataract for- mation. Nippon Ganka Gakkai Zasshi 1995; 99: 1303–41. 59. Kubo E, Miyoshi N, Fukuda M, Akagi Y. Cataract formation through the polyol pathway is associa- ted with free radical production. Exp Eye Res 1999; 68: 457–64. 60. Randazzo J, Zhang P, Makita J, Blessing K, Kador PF. Orally active multi-functional antioxidants delay cataract formation in streptozotocin (type 1) diabetic and gamma-irradiated rats. PLoS One 2011; 6: e18980. 61. Reddy AB, Tammali R, Mishra R, Srivastava S, Sri- vastava SK, Ramana KV. Aldose reductase defici- ency protects sugar-induced lens opacification in rats. Chem Biol Interact 2011; 191: 346–50. 62. Yap S. Classical homocystinuria: vascular risk and its prevention. J Inherit Metab Dis 2003; 26: 259–65. 63. Skovby F, Gaustadnes M, Mudd SH. A revisit to the natural history of homocystinuria due to cystathi- onine beta-synthase deficiency. Mol Genet Metab 2010; 99: 1–3. 64. Sulochana KN, Amirthalakshmi S, Vasanthi SB, Tamilselvi R, Ramakrishnan S. Homocystinuria with congenital/developmental cataract. Indian J Pediatr 2000; 67: 725–8. 65. Hamel C. Retinitis pigmentosa. Orphanet J Rare Dis 2006, 1: 40. 66. Jackson H, Garway-Heath D, Rosen P, Bird A C, Tuft S J. Outcome of cataract surgery in patients with retinitis pigmentosa. Br J Ophthalmol 2001; 85: 936–8. 67. Pruett RC. Retinitis pigmentosa: clinical observa- tions and correlations. Trans Am Acad Ophthal- mol Soc 1983; 81: 693–735. 68. Fishman GA, Anderson RJ, Loureco P. Prevalence of posterior subcapsular lens opacities in patients with retinitis pigmentosa. Br J Ophthalmol 1985; 69: 263–6. 69. Heckenlively J. The frequency of posterior subcap- sular cataract in the hereditary retinal degenerati- ons. Am J Ophthalmol 1982; 93: 733–8. 70. Al-ghoul KJ, Novak LA, Kuszak JR. The structu- re of posterior subcapsular cataracts in the Royal College of Surgeons (RCS) rats. Exp Eye Res 1998; 67: 163–77. 71. Jancevski M, Foster CS. Cataracts and uveitis. Curr Opin Ophthalmol. 2010; 21: 10–4. 72. Foster CS, Rashid S. Management of coincident cataract and uveitis. Curr Opin Ophthalmol 2003; 14: 1–6. 73. BenEzra D, Cohen E. Cataract surgery in children with uveitis. Ophthalmology 2000; 107: 1255–60. 74. Skarin A, Elborgh R, Edlund E, Bengtsson-Stigmar E. Long-term follow-up of patients with uveitis as- 36. Andjelić S, Zupančič G, Hawlina M. The prepara- tions used to study calcium in lens epithelial cells and its role in cataract formation. J Clinic Experi- ment Ophthalmol. S1: 002, 2011. 37. Duncan G, Wormstone IM. Calcium cell signal- ling and cataract: role of the endoplasmic reticu- lum. Eye 1999; 13: 480–3. 38. Churchill GC, Lurtz MM, Louis CF. Ca(2+) regu- lation of gap junctional coupling in lens epithelial cells. Am J Physiol Cell Phusiol 2001; 281: 972–81. 39. Yawata K, Nagata M, Narita A, Kawai Y Effects of longterm acidification of extracellular pH on ATP-induced calcium mobilization in rabbit lens epithelial cells. Jpn J Physiol 2001; 51: 81–7. 40. Andjelic S, Zupančič G, Perovšek D, Robič T, Ha- wlina M. Anterior lens capsule as a tool to stu- dy the physiology of human lens epithelial cells. Zdrav Vestn 2010; 79: I-123–130. 41. Williams MR, Duncan G, Riach RA, Webb SF. Ace- tylc holine-receptors are coupled to mobilization of intracellular calcium in cultured human lens cells. Exp Eye Res 1993; 57: 381–4. 42. Rafferty NS, Rafferty KA, Ito E. Agonist-induced rise in intracellular calcium of lens epithelial cells: effects on the actin cytoskeleton. Exp Eye Res 1994; 59: 191–201. 43. Collison DJ, Duncan G. Regional differences in functional receptor distribution and calcium mobilization in the intact human lens. Invest Ophthalmol Vis Sci 2001; 42: 2355–63. 44. Collison DJ, Coleman RA, James RS, Carey J, Dun- can G. Characterization of muscarinic receptors in human lens cells by pharmacological and molecu- lar techniques. Invest Ophthalmol Vis Sci 2000; 41: 2633–41. 45. Andjelic S, Zupančič G, Perovšek D, Hawlina M. Human anterior lens capsule epithelial cells con- traction. Acta Ophthalmol 2011; 89: e645-e653. 46. Liedtke W, Kim C. Functionality of the TRPV su- bfamily of TRP ion channels: add mechano-TRP and osmo-TRP to the lexicon! Cell Mol Life Sci 2005; 62: 2985–3001. 47. O Neil RG, Heller S. The mechanosensitive na- ture of TRPV channels. Pflügers Arch 2005; 451: 193–203. 48. Venkatachalam K, Montell C. TRP channels. Annu Rev Biochem 2007; 76: 387–417. 49. Hoffmann EK, Lambert IH, Pedersen SF. Physiolo- gy of cell volume regulation in vertebrates. Physiol Rev 2009; 89: 193–277. 50. Asbell PA, Dualan I, Mindel J, Brocks D, Ahmad M, Epstein S: Age-related cataract. Lancet 2005; 365: 599–609. 51. Obrosova IG, Chung SS, Kador PF: Diabetic ca- taracts: mechanisms and management. Diabetes Metab Res Rev 2010; 26: 172–80. 52. Pollreisz A, Schmidt-Erfurth U. Diabetic cataract- -pathogenesis, epidemiology and treatment. 2010 doi: 10.1155/2010/608751. 53. Suzen S, Buyukbingol E. Recent studies of aldose reductase enzyme inhibition for diabetic compli- cations. Curr Med Chem 2003; 10: 1329–52. 54. Kakehi T, Yabe-Nishimura C. NOX enzymes and diabetic complications. Semin Immunopathol 2008; 30: 301–14. 55. Mulhern ML, Madson CJ, Danford A, Ikesugi K, Kador PF, Shinohara T. The unfolded protein re- I-132 Zdrav Vestn Supl | junij 2012 | Letnik 81 PregLednI čLanek/reVIew signal transduction pathways in human lens epi- thelial cells. Invest Ophthalmol Vis Sci 2007; 48: 1724–34. 85. Wang L, Zhao W, Leng F, Ge J, Bu Z, Zhang Y, Liu P. Glucocorticoid receptors take part in the apop- totic process of human lens epithelial cells, but the glucocorticoid receptor antagonist RU486 does not rescue the cells fully. Mol BioSyst 2011; 7: 1926–37. 86. Petersen A, Carlsson T, Karlsson JO, Jonhede S, Zetterberg M. Effects of dexamethasone on hu- man lens epithelial cells in culture. Mol Vis 2008; 14: 1344–52. 87. Dynlacht JR, Tyree C, Valluri S, DesRosiers C, Caperell-Grant A, Mendonca MS, et al. Effect of estrogen on radiation-induced cataractogenesis. Radiat Res 2006; 165: 9–15. 88. Dynlacht JR, Valluri S, Lopez J, Greer F, Desrosiers C, Caperell-Grant A, et al. Estrogen protects again- st radiation-induced cataractogenesis. Radiat Res 2008; 170: 758–64. 89. Celojevic D, Petersen A, Karlsson JO, Behndig A, Zetterberg M. Effects of 17β-estradiol on prolife- ration, cell viability and intracellular redox status in native human lens epithelial cells. Mol Vis 2011; 17: 1987–96. 90. Bigsby RM, Valluri S, Lopez J, Mendonca MS, Ca- perell-Grant A, DesRosiers C, Dynlacht JR. Ova- rian hormone modulation of radiation-induced cataractogenesis: dose-response studies. Invest Ophthalmol Vis Sci 2009; 50: 3304–10. 91. Richa S, Yazbek JC. Ocular adverse effects of com- mon psychotropic agents: a review. CNS Drugs 2010; 24: 501–26. sociated with juvenile idiopathic arthritis: a cohort study. Ocul Immunol Inflamm 2009; 17: 104–8. 75. Angeles-Han S, Yeh S. Prevention and manage- ment of cataracts in children with juvenile idio- pathic arthritis-associated uveitis. Curr Rheuma- tol Rep 2012; 14: 142–9. 76. Stunf S, Hvala A, Vidovič Valentinčič N, Kraut A, Hawlina M. Ultrastructure of the anterior lens capsule and epithelium in cataracts associated with uveitis. Ophthalmic Res 2012; 48: 12–21. 77. De Iongh RU, Wederell E, Lovicu FJ, McAvoy JW: Transforming growth factor- b -induced epitheli- al-mesenchymal transition in the lens: a model for cataract formation. Cells Tissues Organs 2005; 179: 43–55. 78. Li J, Tripathi RC, Tripathi BJ. Drug-induced ocular disorders. Drug Saf 2008; 31: 127–41. 79. Jonas JB, Degenring R, Vossmerbauemer U, Kam- ppeter B. Frequency of cataract surgery after in- travitreal injection of high-dosage triamcinolone acetonide. Eur J Ophthalmol 2005; 15: 462–4. 80. James ER. The etiology of steroid cataract. J Ocul Pharmacol Ther. 2007; 23: 403–20. 81. Praveen MR, Shah GD, Vasavada AR, Shah AR, Jo- har K, Gami Y, et al. Posterior capsule opacificati- on in eyes with steroid-induced cataracts: Compa- rison of early results. J Cataract Refract Surg 2011; 37: 88–96. 82. Jobling AI, Augusteyn RC. What causes steroid cataracts? A review of steroid-induced posterior subcapsular cataracts. Clin Exp Optom 2002; 85: 61–75. 83. Gupta V, Wagner BJ. Search for a functional gluco- corticoid receptor in the mammalian lens. Exp Eye Res 2009; 88: 248–56. 84. Gupta V, Awasthi N, Wagner BJ. Specific activation of the glucocorticoid receptor and modulation of