The effects of hyperbaric oxygen on the crystallins of cultured rabbit lenses: a possible catalytic role for copper

Refolding Increases the Chaperone-like Activity of αH-Crystallin and Reduces Its Hydrodynamic Diameter to That of α-Crystallin

αH-Crystallin, a high molecular weight form of α-crystallin, is one of the major proteins in the lens nucleus. This high molecular weight aggregate (HMWA) plays an important role in the pathogenesis of cataracts. We have shown that the chaperone-like activity of HMWA is 40% of that of α-crystallin from the lens cortex. Refolding with urea significantly increased-up to 260%-the chaperone-like activity of α-crystallin and slightly reduced its hydrodynamic diameter (Dh). HMWA refolding resulted in an increase in chaperone-like activity up to 120% and a significant reduction of Dh of protein particles compared with that of α-crystallin. It was shown that the chaperone-like activity of HMWA, α-crystallin, and refolded α-crystallin but not refolded HMWA was strongly correlated with the denaturation enthalpy measured with differential scanning calorimetry (DSC). The DSC data demonstrated a significant increase in the native protein portion of refolded α-crystallin in comparison with authentic α-crystallin; however, the denaturation enthalpy of refolded HMWA was significantly decreased in comparison with authentic HMWA. The authors suggested that the increase in the chaperone-like activity of both α-crystallin and HMWA could be the result of the correction of misfolded proteins during renaturation and the rearrangement of protein supramolecular structures.

Proposed Role for Internal Lens Pressure as an Initiator of Age-Related Lens Protein Aggregation Diseases

The process that initiates lens stiffness evident in age-related lens protein aggregation diseases is thought to be mainly the result of oxidation. While oxidation is a major contributor, the exposure of lens proteins to physical stress over time increases susceptibility of lens proteins to oxidative damage, and this is believed to play a significant role in initiating these diseases. Accordingly, an overview of key physical stressors and molecular factors known to be implicated in the development of age-related lens protein aggregation diseases is presented, paying particular attention to the consequence of persistent increase in internal lens pressure.

Biochemistry of Eye Lens in the Norm and in Cataractogenesis

The absence of cellular organelles in fiber cells and very high cytoplasmic protein concentration (up to 900 mg/ml) minimize light scattering in the lens and ensure its transparency. Low oxygen concentration, powerful defense systems (antioxidants, antioxidant enzymes, chaperone-like protein alpha-crystallin, etc.) maintain lens transparency. On the other hand, the ability of crystallins to accumulate age-associated post-translational modifications, which reduce the resistance of lens proteins to oxidative stress, is an important factor contributing to the cataract formation. Here, we suggest a mechanism of cataractogenesis common for the action of different cataractogenic factors, such as age, radiation, ultraviolet light, diabetes, etc. Exposure to these factors leads to the damage and death of lens epithelium, which allows oxygen to penetrate into the lens through the gaps in the epithelial layer and cause oxidative damage to crystallins, resulting in protein denaturation, aggregation, and formation of multilamellar bodies (the main cause of lens opacification). The review discusses various approaches to the inhibition of lens opacification (cataract development), in particular, a combined use of antioxidants and compounds enhancing the chaperone-like properties of alpha-crystallin. We also discuss the paradox of high efficiency of anti-cataract drugs in laboratory settings with the lack of their clinical effect, which might be due to the late use of the drugs at the stage, when the opacification has already formed. A probable solution to this situation will be development of new diagnostic methods that will allow to predict the emergence of cataract long before the manifestation of its clinical signs and to start early preventive treatment.

Hyperbaric oxygen as a model of lens aging in the bovine lens: The effects on lens biochemistry, physiology and optics

Age related nuclear (ARN) cataracts in humans take years to form and so experimental models have been developed to mimic the process in animals as a means of better understanding the etiology of nuclear cataracts in humans. A major limitation with these animal models is that many of the biochemical and physiological changes are not typical of that seen in human ARN cataract. In this review, we highlight the work of Frank Giblin and colleagues who established an in vivo animal model that replicates many of the changes observed in human ARN cataract. This model involves exposing aged guinea pigs to hyperbaric oxygen (HBO), which by causing the depletion of the antioxidant glutathione (GSH) specifically in the lens nucleus, produces oxidative changes to nuclear proteins, nuclear light scattering and a myopic shift in lens power that mimics the change that often precedes cataract development in humans. However, this model involves multiple HBO treatments per week, with sometimes up to a total of 100 treatments, spanning up to eight months, which is both costly and time consuming. To address these issues, Giblin developed an in vitro model that used rabbit lenses exposed to HBO for several hours which was subsequently shown to replicate many of the changes observed in human ARN cataract. These experiments suggest that HBO treatment of in vitro animal lenses may serve as a more economical and efficient model to study the development of cataract. Inspired by these experiments, we investigated whether exposure of young bovine lenses to HBO for 15 h could also serve as a suitable acute model of ARN cataract. We found that while this model is able to exhibit some of the biochemical and physiological changes associated with ARN cataract, the decrease in lens power we observed was more characteristic of the hyperopic shift in refraction associated with ageing. Future work will investigate whether HBO treatment to age the bovine lens in combination with an oxidative stressor such as UV light will induce refractive changes more closely associated with human ARN cataract. This will be important as developing an animal model that replicates the changes to lens biochemistry, physiology and optics observed in human ARN cataracts is urgently required to facilitate the identification and testing of anti-cataract therapies that are effective in humans.

Inhibition of copper-induced aggregation of human γD-crystallin by rutin and studies on its role in molecular level for enhancing the chaperone activity of human αA-crystallin by using multi-spectroscopic techniques

Oxidative aggregation of γ-crystallins induced by copper in aged lens increases the lens opacity and causes cataract formation. Therefore, chelation of free Cu²⁺ by small molecules can inhibit metal-mediated aggregation of γ-crystallin. In this work, the inhibition potency of several naturally occurring flavonoid compounds was studied against aggregation of human γD-crystallin (HGD) mediated by copper ions. Among them, rutin demonstrated ~20% inhibition of HGD aggregation induced by Cu²⁺ through its metal chelation ability. Not only that, the chaperone activity of lens chaperone, human αA-crystallin (HAA) was found to be enhanced in the presence of rutin. Subsequently, the molecular interactions between HAA and rutin were investigated using fluorescence and CD spectroscopy to understand the molecular basis of the chaperone activity enhancement by rutin. Quenching of HAA fluorescence by rutin with a quenching constant in the order of ~10⁵ M^-1 depicts a complexation between them. Entropy driven process of complexation between HAA and rutin suggests significant involvement of hydrophobic interactions. Fluorescence resonance energy transfer between protein and ligand can occur at a distance of 2.73 nm. Synchronous fluorescence and circular dichroism spectroscopy revealed that protein-ligand interaction does not cause any notable conformational changes in HAA. Experimental observations have been well substantiated by docking.

Mild hyperbaric oxygen: mechanisms and effects

Adequate oxygen supply by exposure to mild hyperbaric oxygen at appropriately high atmospheric pressure (1266-1317 hPa) and increased oxygen concentration (35-40% oxygen) has a possibility of improving the oxidative metabolism in cells and tissues without barotrauma and excessive production of reactive oxygen species. Therefore, metabolic syndrome and lifestyle-related diseases, including type 2 diabetes and hypertension, in rats were inhibited and/or improved by exposure to mild hyperbaric oxygen. It accelerated the growth-induced increase in oxidative capacity of the skeletal muscle in rats and inhibited the age-related decrease in oxidative capacity of the skeletal muscle in mice. A decrease in dopaminergic neurons in the substantia nigra of mice with Parkinson's disease was inhibited by exposure to mild hyperbaric oxygen. This review describes the beneficial effects of exposure to mild hyperbaric oxygen on some metabolic diseases and their perspectives.

Unloading-induced atrophy and decreased oxidative capacity of the soleus muscle in rats are reversed by pre- and postconditioning with mild hyperbaric oxygen

Our aim was to determine the effects of pre- and/or postconditioning with mild hyperbaric oxygen (1.25 atmospheric pressure, 36% oxygen for 3 h/day) on the properties of the soleus muscle that was atrophied by hindlimb suspension-induced unloading. Twelve groups of 8-week-old rats were housed under normobaric conditions (1 atmospheric pressure, 20.9% oxygen) or exposed to mild hyperbaric oxygen for 2 weeks. Ten groups then were housed under normobaric conditions for 2 weeks with their hindlimbs either unloaded via suspension or not unloaded. Six groups subsequently were either housed under normobaric conditions or exposed to mild hyperbaric oxygen for 2 weeks: the suspended groups were allowed to recover under reloaded conditions (unrestricted normal cage activity). Muscle weights, cross-sectional areas of all fiber types, oxidative capacity (muscle succinate dehydrogenase activity and fiber succinate dehydrogenase staining intensity) decreased, and a shift of fibers from type I to type IIA and type IIC was observed after hindlimb unloading. In addition, mRNA levels of peroxisome proliferator-activated receptor γ coactivator-1α decreased, whereas those of forkhead box-containing protein O1 increased after hindlimb unloading. Muscle atrophy and decreased oxidative capacity were unaffected by either pre- or postconditioning with mild hyperbaric oxygen. In contrast, these changes were followed by a return to nearly normal levels after 2 weeks of reloading when pre- and postconditioning were combined. Therefore, a combination of pre- and postconditioning with mild hyperbaric oxygen can be effective against the atrophy and decreased oxidative capacity of skeletal muscles associated with hindlimb unloading.

Mild hyperbaric oxygen inhibits the decrease of dopaminergic neurons in the substantia nigra of mice with MPTP-induced Parkinson's disease

We examined whether exposure to mild hyperbaric oxygen inhibits the decrease of dopaminergic neurons in the substantia nigra of a neurotoxic animal model with Parkinson's disease. Mice injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride and probenecid twice a week were divided into two groups: mice with mild hyperbaric oxygen and those without. The mice with mild hyperbaric oxygen were exposed to 1317hPa with 45% oxygen for 3h, three times a week. The decrease in dopaminergic neurons of mice with Parkinson's disease was inhibited by 11 weeks of exposure to mild hyperbaric oxygen. We conclude that exposure to mild hyperbaric oxygen is effective in preventing the progression of Parkinson's disease.

Comparison of lens oxidative damage induced by vitrectomy and/or hyperoxia in rabbits

AIM

To compare of lens oxidative damage induced by vitrectomy and/or hyperoxia in rabbit.

METHODS

Sixteen New Zealand rabbits (2.4-2.5 kg) were randomly divided into two groups (Group A, n=12; Group B, n=4). In Group A, the right eyes were treated with vitrectomy and systemic hyperoxia (oxygen concentration: 80%-85%, 1 ATA, 4h/d) (Group A-right), and the left eyes were treated with hyperoxia without vitrectomy surgery (Group A-left). Four rabbits in group B (eight eyes) were untreated as the controls. Lens transparency was monitored with a slit lamp and recorded before and after vitrectomy. After hyperoxic treatment for 6mo, the eyeballs were removed and the lens cortices (containing the capsules) and nuclei were separated for further morphological and biochemical evaluation.

RESULTS

Six months after treatments, there were no significant morphological changes in the lenses in any experimental group when observed with a slit lamp. However, the levels of water-soluble proteins and ascorbate, and the activities of catalase and Na⁺-K⁺-ATPase were significantly reduced, whereas the levels of malondialdehyde and transforming growth factor β2 (TGF-β2) were significantly elevated, in both the cortices and nuclei of eyes treated with vitrectomy and hyperoxia. The increase in protein-glutathione mixed disulfides and the reduction in water-soluble proteins were more obvious in the lens nuclei. The levels of ascorbate in the vitreous fluid were also reduced after vitrectomy, whereas TGF-β2 increased after vitrectomy and hyperoxia. Systemic hyperoxia exposure increased these effects.

CONCLUSION

Removal of the intact vitreous gel with vitrectomy and exposing the lens to increased oxygen from the retina induce lens oxidation and aggregation. Thus, an intact vitreous gel structure may protect the lens from oxidative insult and maintain lens transparency.

Spatial distributions of glutathione and its endogenous conjugates in normal bovine lens and a model of lens aging

Glutathione (GSH) is the archetypal antioxidant, and plays a central role in the protection of the ocular lens from cataract formation. High levels of GSH are maintained in the transparent lens, but with advancing age, GSH levels fall in the lens nucleus relative to outer cortical cells, thereby exposing the nucleus of the lens to the damaging effects of oxygen radicals, which ultimately leads to age-related nuclear (ARN) cataract. Under normal conditions, GSH also forms endogenous conjugates to detoxify the lens of reactive cellular metabolites and to maintain cell homeostasis. Due to the intrinsic gradient of lens fibre cell age, the lens contains distinct regions with different metabolic requirements for GSH. To investigate the impact of fibre cell and lens aging on the varied roles that GSH plays in the lens, we have utilised high mass resolution MALDI mass spectrometry profiling and imaging analysis of lens tissue sections. High Dynamic Range (HDR)-MALDI FTICR mass spectrometry was used as an initial screening method to detect regional differences in lens metabolites from normal bovine lenses and in those subjected to hyperbaric oxygen as a model of lens aging. Subsequent MALDI imaging analysis was used to spatially map GSH and its endogenous conjugates throughout all lenses. Accurate mass measurement by MALDI FTICR analysis and LC-MS/MS mass spectrometry of lens region homogenates were subsequently used to identify endogenous GSH conjugates. While the distribution and relative abundance of GSH-related metabolic intermediates involved in detoxification pathways remained relatively unchanged upon HBO treatment, those involved in its antioxidant function were altered under conditions of oxidative stress. For example, reduced glutathione levels were decreased in the lens cortex while oxidised glutathione levels were elevated in the lens outer cortex upon HBO treatment. Interestingly, cysteineglutathione disulfide, was detected in the inner cortex of the normal lens, but was greatly decreased in the HBO-treated lenses. These results contribute to our understanding of the multiple roles that GSH plays in maintenance of lens transparency and in the age-related metabolic changes that lead to lens cataract formation.

Parkin elimination of mitochondria is important for maintenance of lens epithelial cell ROS levels and survival upon oxidative stress exposure

Age-related cataract is associated with oxidative stress and death of lens epithelial cells (LECs) whose survival is dependent on functional mitochondrial populations. Oxidative stress-induced depolarization/damage of LEC mitochondria results in increased reactive oxygen species (ROS) levels and cell death suggesting the need for a LEC mechanism to remove mitochondria depolarized/damaged upon oxidative stress exposure to prevent ROS release and LEC death. To date, a mechanism(s) for removal of depolarized/damaged LEC mitochondria has yet to be identified and the importance of eliminating oxidative stress-damaged mitochondria to prevent LEC ROS release and death has not been established. Here, we demonstrate that Parkin levels increase in LECs exposed to H₂O₂-oxidative stress. We establish that Parkin translocates to LEC mitochondria depolarized upon oxidative stress exposure and that Parkin recruits p62/SQSTM1 to depolarized LEC mitochondria. We demonstrate that translocation of Parkin results in the elimination of depolarized/damaged mitochondria and that Parkin clearance of LEC mitochondria is dependent on its ubiquitin ligase activity. Importantly, we demonstrate that Parkin elimination of damaged LEC mitochondria results in reduced ROS levels and increased survival upon oxidative stress exposure. These results establish that Parkin functions to eliminate LEC mitochondria depolarized/damaged upon oxidative stress exposure and that elimination of damaged mitochondria by Parkin is important for LEC homeostasis and survival. The data also suggest that mitochondrial quality control by Parkin could play a role in lens transparency.

Copper and Zinc Ions Specifically Promote Nonamyloid Aggregation of the Highly Stable Human γ-D Crystallin

Cataract is the leading cause of blindness in the world. It results from aggregation of eye lens proteins into high-molecular-weight complexes, causing light scattering and lens opacity. Copper and zinc concentrations in cataractous lens are increased significantly relative to a healthy lens, and a variety of experimental and epidemiological studies implicate metals as potential etiological agents for cataract. The natively monomeric, β-sheet rich human γD (HγD) crystallin is one of the more abundant proteins in the core of the lens. It is also one of the most thermodynamically stable proteins in the human body. Surprisingly, we found that both Cu(II) and Zn(II) ions induced rapid, nonamyloid aggregation of HγD, forming high-molecular-weight light-scattering aggregates. Unlike Zn(II), Cu(II) also substantially decreased the thermal stability of HγD and promoted the formation of disulfide-bridged dimers, suggesting distinct aggregation mechanisms. In both cases, however, metal-induced aggregation depended strongly on temperature and was suppressed by the human lens chaperone αB-crystallin (HαB), implicating partially folded intermediates in the aggregation process. Consistently, distinct site-specific interactions of Cu(II) and Zn(II) ions with the protein and conformational changes in specific hinge regions were identified by nuclear magnetic resonance. This study provides insights into the mechanisms of metal-induced aggregation of one of the more stable proteins in the human body, and it reveals a novel and unexplored bioinorganic facet of cataract disease.

The effects of age on lens transport

PURPOSE

Age-related nuclear cataracts involve denaturation and aggregation of intracellular proteins. We have documented age-dependent changes in membrane transport in the mouse lens to see what might initiate changes in the intracellular milieu.

METHODS

Microelectrode-based intracellular impedance studies of intact lenses were used to determine gap junction coupling conductance, fiber and surface cell membrane conductances, effective extracellular resistivity, and intracellular voltage. Fiber cell connexin expression was detected by Western blotting. Intracellular hydrostatic pressure was measured with a microelectrode/manometer system. Concentrations of intracellular sodium and calcium were measured by intracellular injection of sodium-binding benzofuran isophthalate and Fura2, respectively.

RESULTS

In adult lenses, as age increased: fiber cell gap junction coupling conductance declined significantly, correlating with decreases in Cx46 and Cx50 labeling in Western blots; fiber and surface cell membrane conductances did not change systematically; effective extracellular resistivity increased monotonically; center to surface gradients for intracellular pressure, sodium, calcium, and voltage all increased, but in an interdependent manner that moderated changes. In newborn pup lenses, there were changes that did not simply fit with the above paradigm.

CONCLUSIONS

In newborn pup lenses, the observed changes may relate to growth factors that are not related to age-dependent changes seen in adult lenses. The major change in adult lenses was an age-dependent decrease in gap junction coupling, probably due to oxidative damage leading to degradation of connexin proteins. These changes clearly lead to compromise of intracellular homeostasis and may be a causal factor in age-related nuclear cataracts.

Oxygen concentration-dependent oxidative stress levels in rats

Introduction. We determined derivatives of reactive oxygen metabolites (dROMs) as an index of oxidative stress level (oxidant capacity) and biochemical antioxidant potential (BAP) as an index of antioxidant capacity in rats exposed to different oxygen concentrations. Methods. Male Wistar rats were exposed to 14.4%, 20.9%, 35.5%, 39.8%, 62.5%, and 82.2% oxygen at 1 atmosphere absolute for 24 h. Serum levels of dROMs and BAP were examined by using a free radical and antioxidant potential determination device. The morphological characteristics of red blood cells were examined by phase contrast microscopy. Results. There were no differences in the levels of dROMs in rats exposed to 14.4%, 20.9%, and 35.5% oxygen. However, the levels of dROMs increased in the rats exposed to 39.8% and 62.5% oxygen. The levels of dROMs were the highest in the rats exposed to 82.2% oxygen. There were no differences in the levels of BAP with respect to the oxygen concentration. Morphological changes in the red blood cells induced by oxidative attack from reactive oxygen species were observed in the rats exposed to 39.8%, 62.5%, and 82.2% oxygen. Conclusion. Our results suggest that exposure to oxygen concentrations higher than 40% for 24 h induces excessive levels of oxidative stress in rats.

Effect of exposure to hyperbaric oxygen on diabetes-induced cataracts in mice

BACKGROUND

The growth-associated increase in the blood glucose level of animals with Type 2 diabetes is inhibited by moderate hyperbaric exposure at 1.25 atmospheres absolute (ata) with 36% oxygen, presumably due to an increase in oxidative metabolism. However, there are no data available regarding the effect of moderate hyperbaric oxygen (HBO) on diabetes-induced cataracts.

METHODS

Four-week-old mice with Type 2 diabetes and cataracts were exposed to 1.25 ata with 36% oxygen, 6 h daily, for 12 weeks, followed by normal conditions at 1 ata with 21% oxygen for 16 weeks (cataract + hyperbaric group). Levels of blood glucose and derivatives of reactive oxygen metabolites (dROMs), used as an index of oxidative stress, and the turbidities of the lenses from these mice at 4, 8, 12, 16, and 32 weeks of age were compared with those of control and diabetic (cataract group) mice not exposed to HBO.

RESULTS

Non-fasting and fasting blood glucose levels were lower in the cataract + hyperbaric group at 12, 16, and 32 weeks of age than in the age-matched cataract group. The levels of dROMs were lower in the cataract + hyperbaric group at 16 and 32 weeks of age than in the age-matched cataract group. The turbidities of the peripheral and central regions of the lenses were lower in the cataract + hyperbaric group at 12, 16, and 32 weeks of age than in the age-matched cataract group.

CONCLUSIONS

Hyperbaric exposure at 1.25 ata with 36% oxygen delays cataract development and progression in mice with Type 2 diabetes.

Mechanism of lysine oxidation in human lens crystallins during aging and in diabetes

Oxidative mechanisms during nuclear sclerosis of the lens are poorly understood, in particular metal-catalyzed oxidation. The lysyl oxidation product adipic semialdehyde (allysine, ALL) and its oxidized end-product 2-aminoadipic acid (2-AAA) were determined as a function of age and presence of diabetes. Surprisingly, whereas both ALL and 2-AAA increased with age and strongly correlated with cataract grade and protein absorbance at 350 nm, only ALL formation but not 2-AAA was increased by diabetes. To clarify the mechanism of oxidation, rabbit lenses were treated with hyperbaric oxygen (HBO) for 48 h, and proteins were analyzed by gas and liquid chromatography mass spectrometry for ALL, 2-AAA, and multiple glycation products. Upon exposure to HBO, rabbit lenses were swollen, and nuclei were yellow. Protein-bound ALL increased 8-fold in the nuclear protein fractions versus controls. A dramatic increase in methyl-glyoxal hydroimidazolone and carboxyethyl-lysine but no increase of 2-AAA occurred, suggesting more drastic conditions are needed to oxidize ALL into 2-AAA. Indeed the latter formed only upon depletion of glutathione and was catalyzed by H(2)O(2). Neither carboxymethyl-lysine nor glyoxal hydroimidazolone, two markers of glyco-/lipoxidation, nor markers of lenticular glycemia (fructose-lysine, glucospane) were elevated by HBO, excluding significant lipid peroxidation and glucose involvement. The findings strongly implicate dicarbonyl/metal catalyzed oxidation of lysine to allysine, whereby low GSH combined with ascorbate-derived H(2)O(2) likely contributes toward 2-AAA formation, since virtually no 2-AAA formed in the presence of methylglyoxal instead of ascorbate. An important translational conclusion is that chelating agents might help delay nuclear sclerosis.

Aggregation of lens crystallins in an in vivo hyperbaric oxygen guinea pig model of nuclear cataract: dynamic light-scattering and HPLC analysis

PURPOSE

The role of oxygen in the formation of lens high-molecular-weight (HMW) protein aggregates during the development of human nuclear cataract is not well understood. The purpose of this study was to investigate lens crystallin aggregate formation in hyperbaric oxygen (HBO)-treated guinea pigs by using in vivo and in vitro

METHODS

methods. Guinea pigs were treated three times weekly for 7 months with HBO, and lens crystallin aggregation was investigated in vivo with the use of dynamic light-scattering (DLS) and in vitro by HPLC analysis of water-insoluble (WI) proteins. DLS measurements were made every 0.1 mm across the 4.5- to 5.0-mm optical axis of the guinea pig lens.

RESULTS

The average apparent diameter of proteins in the nucleus (the central region) of lenses of HBO-treated animals was nearly twice that of the control animals (P < 0.001). Size distribution analysis conducted at one selected point in the nucleus and cortex (the outer periphery of the lens) after dividing the proteins into small-diameter and large-diameter groups, showed in the O2-treated nucleus a threefold increase in intensity (P < 0.001) and a doubling in apparent size (P = 0.03) of large-diameter aggregate proteins, compared with the same control group. No significant changes in apparent protein diameter were detected in the O2-treated cortex, compared with the control. The average diameter of protein aggregates at the single selected location in the O2-treated nucleus was estimated to be 150 nm, a size capable of scattering light and similar to the size of aggregates found in human nuclear cataracts. HPLC analysis indicated that one half of the experimental nuclear WI protein fraction (that had been dissolved in guanidine) consisted of disulfide cross-linked 150- to 1000-kDa aggregates, not present in the control. HPLC-isolated aggregates contained alphaA-, beta-, gamma-, and zeta-crystallins, but not alphaB-crystallin, which is devoid of -SH groups and thus does not participate in disulfide cross-linking. All zeta-crystallin present in the nuclear WI fraction appeared to be there as a result of disulfide cross-linking.

CONCLUSIONS

The results indicate that molecular oxygen in vivo can induce the cross-linking of guinea pig lens nuclear crystallins into large disulfide-bonded aggregates capable of scattering light. A similar process may be involved in the formation of human nuclear cataract.

Surfactant and UV-B-induced damage of the cultured bovine lens

PURPOSE

To evaluate in vitro methods for testing the toxicity of the surfactants, sodium dodecyl sulfate (SDS) and benzalkonium chloride (BAK), and Ultraviolet (UV)-B radiation to the bovine lens.

METHODS

Lenses were dissected from bovine eyes--obtained from a local abattoir--and incubated in M199 culture medium at 37 degrees C, with 4% CO(2) and 96% air atmosphere. For the SDS and BAK experiments, the lenses (n = 153) were exposed directly to 0.001%, 0.01%, 0.1%, and 1.0% solutions for 15 min. These lenses were then rinsed five times each with saline and medium. Another group of lenses (n = 36) was irradiated with broadband UV-B at energy levels of 1.0 and 2.0 J/cm(2) (0.445 and 0.89 J/cm(2) in the biologically effective energy levels). For all of the above experiments, lens optical quality and cellular viability of lens epithelial cells were evaluated.

RESULTS

The analysis of optical quality, using a scanning laser in vitro assay system, of exposed lenses treated with SDS and BAK at concentrations of 0.01%, 0.1%, and 1.0%, and with UV-B at energy levels of 0.445 and 0.89 J/cm(2) showed a dose- and time-dependent increase in back vertex distance (BVD) variability, indicating loss of sharp focus in comparison with control lenses. Both 0.001% SDS and 0.001% BAK-treated lenses did not show any optical damage until 8-days after exposure. Lenses treated with 0.01% SDS showed recovery from optical damage 6-days later after exposure. Optical damage was not shown immediately for UV-B-exposed lenses. The Alamar Blue assay data for SDS, BAK and UV-B-exposed lenses, except the 0.001% SDS treated lens group, showed also dose- and time-dependent decreases in cellular viability in comparison with the control lenses, and there was no cellular recovery during the entire culture period. Lenses treated with 0.001% SDS did not show biological damage until 8-days after exposure. It appears that cellular changes appeared earlier than optical changes.

CONCLUSIONS

The findings suggest that cultured bovine lenses can be evaluated by assays that probe optical properties and cellular function after exposure to surfactants and UV-B irradiation, and that the optical and biological assay methods are valuable for in vitro mild ocular toxicity research.

Oxygen tension in the rabbit lens and vitreous before and after vitrectomy

Oxygen is believed to be one of the potential causative agents for the development of nuclear cataract following vitrectomy. The aim of this study was to determine the partial pressure of oxygen (pO2) in different compartments of the rabbit eye, and to describe the changes following vitrectomy. Twenty-six rabbits (3.5-5.3 kg) were anesthetized and oxygen tension was probed using a fiber-optic oxygen sensor system (optode). A micromanipulator was employed to ascertain the exact position of the probe within the eye. Measurements were taken pre- and post-vitrectomy at several defined positions within the vitreous, the lens and the anterior chamber. Follow-up measurements were performed 2 and 8 weeks after vitrectomy. The contralateral eye served as a control. Measurements in the normal rabbit eye showed that oxygen tension in the globe is asymmetrical with the lowest pO2 in the nucleus of the lens (10.4 mmHg+/-3.0). The region of the lens near the posterior capsule has an oxygen tension close to the values of the vitreous directly behind the posterior capsule (12.4 mmHg+/-3.1). The highest pO2 within the posterior compartment of the eye was measured close to the retinal surface (40-l60 mmHg) depending on neighboring large vessels. The tension drops off rapidly to 20 mmHg some 0.5 mm from the retina. From that position to the posterior surface of the lens there is a shallow gradient of decreasing pO2. Immediately following vitrectomy the pO2 in the BSS replacement varied from ca. 90-140 mmHg, and decreased over approximately 30 min. to levels that were 2-3 times that of normal vitreous. Two weeks after vitrectomy the pO2 values in the lens were 2-3 times as high as in the control eye (p < 0.05). In addition there is no longer a gradient in the vitreous cavity, except close to the retina. Eight weeks after vitrectomy, pO2 levels in the lens were decreased but still remained higher than in the normal eye (13.83 mmHg+/-0.02). The pO2 gradient in the vitreous was not detectable anymore. Overall the results provide evidence that oxygen levels in the lens increase significantly after vitrectomy in rabbits. If this occurs in humans it may contribute to cataract formation following surgery.

Alpha-crystallin

Alpha A and alpha B-crystallins are a major protein component of the mammalian eye lens. Being a member of the small heat-shock protein family they possess chaperone-like function. The alpha-crystallins and especially alpha B is also found outside the lens having an extensive tissue distribution. Alpha B-crystallin is found to be over-expressed in many neurological diseases, and mutations in alpha A or B-crystallin can cause cataract and myopathy. This review deals with some of the unique properties of the alpha-crystallins emphasizing especially what we don't know about its function and structure.

Evaluation of a porcine lens and fluorescence assay approach for in vitro ocular toxicological investigations

Cell biology, as monitored with the fluorescent indicator dyes Alamar Blue and 5-carboxyfluorescein diacetate acetoxymethyl ester (CFDA-AM), and lens optical quality, as measured with an in vitro scanning laser system, have been used to evaluate in vitro the condition of porcine lenses after being placed in a culture medium. The measurements, beginning from week one of culture, were compared statistically. Optical quality and cellular viability, as measured with either dye, were unchanged in lenses that had been maintained for 6 weeks in modified M199 medium. Some lenses were treated with 0.152J/cm(2) UVB radiation, and a decline was observed after 48 hours in both optical and metabolic capabilities, as indicated by a decreased capacity of the lenses to reduce Alamar Blue. The measurements with CFDA-AM did not show complete concordance with the other indicators of lens health after UV treatment, making this dye less reliable as applied currently to lens cultures. Overall, the findings suggest that porcine lenses can be maintained for weeks in culture, and that their condition can be evaluated quantitatively by assays that probe cellular functions and optical properties. Such a system should prove valuable for in vitro ocular pharmacotoxicological research.