Light scattering of human lens vesicles in vitro

Interaction of βL- and γ-Crystallin with Phospholipid Membrane Using Atomic Force Microscopy

Highly concentrated lens proteins, mostly β- and γ-crystallin, are responsible for maintaining the structure and refractivity of the eye lens. However, with aging and cataract formation, β- and γ-crystallin are associated with the lens membrane or other lens proteins forming high-molecular-weight proteins, which further associate with the lens membrane, leading to light scattering and cataract development. The mechanism by which β- and γ-crystallin are associated with the lens membrane is unknown. This work aims to study the interaction of β- and γ-crystallin with the phospholipid membrane with and without cholesterol (Chol) with the overall goal of understanding the role of phospholipid and Chol in β- and γ-crystallin association with the membrane. Small unilamellar vesicles made of Chol/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (Chol/POPC) membranes with varying Chol content were prepared using the rapid solvent exchange method followed by probe tip sonication and then dispensed on freshly cleaved mica disk to prepare a supported lipid membrane. The βL- and γ-crystallin from the cortex of the bovine lens was used to investigate the time-dependent association of βL- and γ-crystallin with the membrane by obtaining the topographical images using atomic force microscopy. Our study showed that βL-crystallin formed semi-transmembrane defects, whereas γ-crystallin formed transmembrane defects on the phospholipid membrane. The size of semi-transmembrane defects increases significantly with incubation time when βL-crystallin interacts with the membrane. In contrast, no significant increase in transmembrane defect size was observed in the case of γ-crystallin. Our result shows that Chol inhibits the formation of membrane defects when βL- and γ-crystallin interact with the Chol/POPC membrane, where the degree of inhibition depends upon the amount of Chol content in the membrane. At a Chol/POPC mixing ratio of 0.3, membrane defects were observed when both βL- and γ-crystallin interacted with the membrane. However, at a Chol/POPC mixing ratio of 1, no association of γ-crystallin with the membrane was observed, which resulted in a defect-free membrane, and the severity of the membrane defect was decreased when βL-crystallin interacted with the membrane. The semi-transmembrane or transmembrane defects formed by the interaction of βL- and γ-crystallin on phospholipid membrane might be responsible for light scattering and cataract formation. However, Chol suppressed the formation of such defects in the membrane, likely maintaining lens membrane homeostasis and protecting against cataract formation.

Lipid conformational order and the etiology of cataract and dry eye

Lens and tear film lipids are as unique as the systems they reside in. The major lipid of the human lens is dihydrosphingomylein, found in quantity only in the lens. The lens contains a cholesterol to phospholipid molar ratio as high as 10:1, more than anywhere else in the body. Lens lipids contribute to maintaining lens clarity, and alterations in lens lipid composition due to age are likely to contribute to cataract. Lens lipid composition reflects adaptations to the unique characteristics of the lens: no turnover of lens lipids or proteins; the lowest amount of oxygen of any tissue; and contains almost no intracellular organelles. The tear film lipid layer (TFLL) is also unique. The TFLL is a thin (100 nm) layer of lipid on the surface of tears covering the cornea that contributes to tear film stability. The major lipids of the TFLL are wax esters and cholesterol esters that are not found in the lens. The hydrocarbon chains associated with the esters are longer than those found anywhere else in the body (as long as 32 carbons), and many are branched. Changes in the composition and structure of the 30,000 different moieties of TFLL contribute to the instability of tears. The focus of the current review is how spectroscopy has been used to elucidate the relationships between lipid composition, conformational order and function, and the etiology of cataract and dry eye.

Changes in Intraocular Straylight and Visual Acuity with Age in Cataracts of Different Morphologies

Purpose

To investigate the significance of difference in straylight of cataract eyes with different morphologies, as a function of age and visual acuity.

Methods

A literature review to collect relevant papers on straylight, age, and visual acuity of three common cataract morphologies leads to including five eligible papers for the analysis. The effect of morphology was incorporated to categorize straylight dependency on the two variables. We also determined the amount of progression in a cataract group using a control group.

Results

The mean straylight was 1.22 log units ± 0.20 (SD) in nuclear (592 eyes), 1.26 log units ± 0.23 in cortical (776 eyes), and 1.48 log units ± 0.34 in posterior subcapsular (75 eyes) groups. The slope of straylight-age relationship was 0.009 (R² = 0.20) in nuclear, 0.012 (R² = 0.22) in cortical, and 0.014 (R² = 0.11) in posterior subcapsular groups. The slope of straylight-visual acuity relationship was 0.62 (R² = 0.25) in nuclear, 0.33 (R² = 0.13) in cortical, and 1.03 (R² = 0.34) in posterior subcapsular groups.

Conclusion

Considering morphology of cataract provides a better insight in assessing visual functions of cataract eyes, in posterior subcapsular cataract, particularly, in spite of notable elevated straylight, visual acuity might not manifest severe loss.

Refractive index degeneration in older lenses: A potential functional correlate to structural changes that underlie cataract formation

A major structure/function relationship in the eye lens is that between the constituent proteins, the crystallins and the optical property of refractive index. Structural breakdown that leads to cataract has been investigated in a number of studies; the concomitant changes in the optics, namely increases in light attenuation have also been well documented. Specific changes in the refractive index gradient that cause such attenuation, however, are not well studied because previous methods of measuring refractive index require transparent samples. The X-ray Talbot interferometric method using synchrotron radiation allows for measurement of fine changes in refractive index through lenses with opacities. The findings of this study on older human lenses show disruptions to the refractive index gradient and in the refractive index contours. These disruptions are linked to location in the lens and occur in polar regions, along or close to the equatorial plane or in lamellar-like formations. The disruptions that are seen in the polar regions manifest branching formations that alter with progression through the lens with some similarity to lens sutures. This study shows how the refractive index gradient, which is needed to maintain image quality of the eye, may be disturbed and that this can occur in a number of distinct ways. These findings offer insight into functional changes to a major optical parameter in older lenses. Further studies are needed to elicit how these may be related to structural degenerations reported in the literature.

Saturation with cholesterol increases vertical order and smoothes the surface of the phosphatidylcholine bilayer: a molecular simulation study

Molecular dynamics (MD) simulations of a mono-cis-unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer and a POPC bilayer containing 50mol% cholesterol (POPC-Chol50) were carried out for 200ns to compare the spatial organizations of the pure POPC bilayer and the POPC bilayer saturated with Chol. The results presented here indicate that saturation with Chol significantly narrows the distribution of vertical positions of the center-of-mass of POPC molecules and POPC atoms in the bilayer. In the POPC-Chol50 bilayer, the same moieties of the lipid molecules are better aligned at a given bilayer depth, forming the following clearly separated membrane regions: the polar headgroup, the rigid core consisting of steroid rings and upper fragments of the acyl chains, and the fluid hydrocarbon core consisting of Chol chains and the lower fragments of POPC chains. The membrane surface of the POPC-Chol50 bilayer is smooth. The results have biological significance because the POPC-Chol50 bilayer models the bulk phospholipid portion of the fiber-cell membrane in the eye lens. It is hypothesized that in the eye lens cholesterol-induced smoothing of the membrane surface decreases light-scattering and helps to maintain lens transparency.

Lipids and the ocular lens

The unusually high levels of saturation and thus order contribute to the uniqueness of human lens membranes. In addition, and unlike in most biomembranes, most of the lens lipids are associated with proteins, thus reducing their mobility. The major phospholipid of the human lens is dihydrosphingomyelin. Found in significant quantities only in primate lenses, particularly human ones, this lipid is so extremely stable that it was reported to be the only lipid remaining in a frozen mammoth 40,000 years after its death. Unusually high levels of cholesterol add peculiarity to the composition of lens membranes. Beyond the lateral segregation of lipids into dynamic domains known as rafts, the high abundance of cholesterol in the human lens leads to the formation of patches of pure cholesterol. Changes in human lens lipid composition with age and disease as well as differences among species are greater than those observed for any other biomembrane. The relationships among lens membrane composition, structure, and lipid conformation reviewed in this article are unique to the mammalian lens and offer exciting insights into lens membrane function. This review focuses on findings reported over the last two decades that demonstrate the uniqueness of mammalian lens membranes regarding their morphology and composition. Because the membranes of human lenses do undergo the most dramatic changes with age and cataractogenesis, the final sections of this review address our current knowledge of the unusual composition and organization of adult human lens membranes with and without opacification. Finally, the questions that still remain to be answered are presented.

X-ray imaging in advanced studies of ophthalmic diseases

Microscopic characterization of pathological tissues has one major intrinsic limitation, the small sampling areas with respect to the extension of the tissues. Mapping possible changes on vast tissues and correlating them with large ensembles of clinical cases is not a feasible procedure for studying most diseases, as for instance vision loss related diseases and, in particular, the cataract. Although intraocular lens implants are successful treatments, cataract still is a leading public-health issue that grows in importance as the population increases and life expectancy is extended worldwide. In this work we have exploited the radiation-tissue interaction properties of hard x-rays--very low absorption and scattering--to map distinct lesions on entire eye lenses. At the used synchrotron x-ray photon energy of 20 keV (wavelength lambda=0.062 nm), scattering and refraction are angular resolved effects. It allows the employed x-ray image technique to efficiently characterize two types of lesions in eye lenses under cataractogenesis: distributions of tiny scattering centers and extended areas of fiber cell compaction. The data collection procedure is relatively fast; allowing dozens of samples to be totally imaged (scattering, refraction, and mass absorption images) in a single day of synchrotron beam time. More than 60 cases of canine cataract, not correlated to specific causes, were investigated in this first application of x-rays to image entire lenses. Cortical opacity cases, or partial opacity, could be related to the presence of calcificated tissues at the cortical areas, clearly visible in the images, whose elemental contents were verified by micro x-ray fluorescence as very rich in calcium. Calcificated tissues were also observed at nuclear areas in some cases of hypermature cataract. Total opacity cases without distinguishable amount of scattering centers consist in 70% of the analyzed cases, where remarkable fissure marks owing to extended areas of fiber cell compaction are diagnosed.

alpha-Crystallin binding in vitro to lipids from clear human lenses

The association of alpha-crystallin to lens membranes increases with age and cataract. Lipid compositional changes also occur with age, cataract, and diabetes. In this study we determined the influence of lipid compositional differences on the binding capacity of alpha-crystallin to lipid vesicles in vitro. Lipids were extracted from pools of human lenses from younger (22+/-4 y, n=30) and older (69+/-3 y, n=26) nondiabetic donors as well as from diabetics taking insulin (60+/-9 y, n=26) and diabetics not taking insulin (58+/-9 y, n=20). Diabetics were insulin dependent for an average of 6 years. Extracted lipids were extruded into large unilamellar vesicles. alpha-Crystallin was mixed with the lipid at 36 degrees C, allowed to bind for about 12 h, and centrifuged at 14,000 g. This centrifugal force was low enough to not pellet free alpha-crystallin but high enough to pellet the lipid and bound alpha-crystallin. alpha-Crystallin-lipid binding was characterized by comparing the amount alpha-crystallin in the pellets of samples with and without lipid. Protein was measured using an assay that minimized interference from lipids. Lipid composition was determined by 31P-NMR spectroscopy. The binding capacity of alpha-crystallin to lipids was 12, 19, 8.9, 17 microg bound/mg lipid for lens lipids extracted from younger, older, insulin-treated and nontreated diabetic donors, respectively. The amount of alpha-crystallin in the pellet (bound alpha-crystallin) was significantly lower for the lipids from the younger group of lenses, p=0.033 and insulin-treated group, p=0.006, compared with the older group of lenses. Higher binding capacity was associated with a higher relative amount of sphingolipid and lower relative amounts of phosphatidylethanolamine-related lipid and phosphatidylcholine. The binding capacity of alpha-crystallin to lens lipids, measured in vitro, increases with age and decreases in diabetic donors that were treated with insulin. Our data support the idea that with age and perhaps certain types of diabetes, more alpha-crystallin is bound to the membrane and serves as a condensation point to which other crystallins bind and then become oxidized.

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.

Lens lipids and maximum lifespan

Unlike in most organs, the lipid composition of lenses varies dramatically among species and with age. The focus of this study is to assess how these changes relate to lifespan. Studies on cataract suggest that the lens may serve as a window into the processes leading to accelerated mortality. As a first step toward elucidating cellular processes in the lens that may serve as markers for accelerated mortality, we examined the correlation between species-dependent and age-related lens lipid compositional differences and maximum life span. We included data from camels, which, even in old age, rarely develop cataracts although they live under adverse conditions. Camel lens lipids were mainly composed of sphingolipids (77%) and phosphatidylcholines (23%). Bovine lens lipid composition was comparable to a previous study, and both bovine lens sphingolipids, phosphatidylcholines and camel lens phosphatidylcholines content fit well (within the 95% confidence limits) in the curve obtained by plotting maximum life spans of other species with sphingolipids and phosphatidylcholines. Lifespan was directly related to lens sphingolipid content and indirectly related to lens phosphatidylcholine content. The camel lens sphingolipid value was significantly above the curve for other species. Except for the camel lens nucleus, lipid order and sphingolipid content were linearly related, p < 0.005 with a slope of 0.85+/-0.07, and intercept of 6.9+/-3.8. Lipid phase transition temperature and sphingolipid content were also linearly related, p = 0.01 with a slope of 0.20+/-0.07, and intercept of 21.7+/-5.3. Our data support the hypothesis that humans have adapted so that their lens membranes have a high sphingolipid content that confers resistance to oxidation, allowing these membranes to stay clear for a relatively longer time than is the case in many other species. Age-related changes in human lens lipid composition may serve as a marker for oxidative stress and may reflect systemic oxidative insult, providing a window into the health of an individual.

Sphingolipids in human lens membranes: an update on their composition and possible biological implications

The unique nature of the most abundant phospholipids in human lens membranes remained overlooked until the 1990s when it was possible to discern dihydrosphingomyelins (DHSMs) from the more common sphingomyelins (SMs). Unlike in other mammalian membranes, DHSMs comprise nearly half of the phospholipids in adult human lenses. Compared to SMs with a trans double bond between carbons 4 and 5 of the sphingoid backbone, the absence of this unsaturation site in DHSMs allows the participation of the OH group on C3 in intermolecular H-bonds and leads to stronger interlipid interactions with both neighboring DHSMs and cholesterol. Phospholipid compositional changes with age and lens region observed in mammals with various life spans and lens growth rates, suggest that the highest levels of DHSMs along with the lowest amounts of phosphatidylcholines and SMs are found in lenses with the lowest growth rate, namely human lenses. The participation of phospholipid metabolites in the control of mitosis and elongation of lens cells is plausible and deserves investigation.

Distribution, spherical structure and predicted Mie scattering of multilamellar bodies in human age-related nuclear cataracts

PURPOSE

To characterize multilamellar bodies (MLBs), determine their distribution along the optic axis and predict their potential Mie scattering within human age-related nuclear cataracts. Previous studies restricted to the equatorial plane have shown that MLBs are rare spherical objects that are 1-4 microm in diameter and covered by multiple layers of thin lipid-rich membranes.

METHODS

Eight human aged transparent lenses were obtained from eye bank donors and eight human age-related nuclear cataracts were obtained immediately after extracapsular extraction. Each sample was Vibratome sectioned fresh into 200 microm thick sections that were fixed and embedded for light or electron microscopy. Light micrograph montages of the optic axis containing the juvenile, fetal and embryonic nuclei were examined. Mie scattering for random coated spherical particles was calculated based on assumed and measured particle parameters.

RESULTS

Cells along the optic axis of the cataract contained approximately 7.5 times more MLBs as similar regions of the aged transparent lens, although these MLBs occurred with extremely low frequency. Cells of the aged transparent lens contained 1.3 MLBs mm(-2), while those of the cataract contained 9.6 MLBs mm(-2), which are equivalent to calculated densities of 5.6 x 10(2) and 4.1 x 10(3)mm(-3), respectively. While some MLBs were located within the cytoplasm near cell membranes, others were found away from membranes. The MLBs are distinct from circular profiles resulting from finger-like projections between adjacent cells. MLBs displayed varying geometries and cytoplasmic textures, although predominately spherical with interiors similar to adjacent fiber cell cytoplasm. These results are in agreement with previous theoretical analysis of light scattering from human lenses and with previous morphological studies examining the equatorial plane of the lens. Potential Mie scattering of spherical particles with the average properties of the observed MLBs and assumed refractive index properties was calculated to be forward scattering of as much as 20% of the incident light.

CONCLUSIONS

The observed low frequency and absence of clustering of MLBs in the equatorial plane and along the optic axis suggests that MLBs are most likely uniformly distributed throughout the embryonic, fetal and juvenile nuclei of age-related cataracts. Because of their size, distribution, textured cytoplasm and calculated Mie scattering, MLBs probably cause local fluctuations in refractive index in human lens nuclei and, therefore, are potential sources of low-angle, forward light scattering that could impair image formation.