Fourier analysis of cytoplasmic texture in nuclear fiber cells from transparent and cataractous human and animal lenses

In Vivo Quasi-Elastic Light Scattering Eye Scanner Detects Molecular Aging in Humans

The absence of clinical tools to evaluate individual variation in the pace of aging represents a major impediment to understanding aging and maximizing health throughout life. The human lens is an ideal tissue for quantitative assessment of molecular aging in vivo. Long-lived proteins in lens fiber cells are expressed during fetal life, do not undergo turnover, accumulate molecular alterations throughout life, and are optically accessible in vivo. We used quasi-elastic light scattering (QLS) to measure age-dependent signals in lenses of healthy human subjects. Age-dependent QLS signal changes detected in vivo recapitulated time-dependent changes in hydrodynamic radius, protein polydispersity, and supramolecular order of human lens proteins during long-term incubation (~1 year) and in response to sustained oxidation (~2.5 months) in vitro. Our findings demonstrate that QLS analysis of human lens proteins provides a practical technique for noninvasive assessment of molecular aging in vivo.

Selenite and ebselen supplementation attenuates D-galactose-induced oxidative stress and increases expression of SELR and SEP15 in rat lens

Selenite and ebselen supplementation has been shown to possess anti-cataract potential in some experimental animal models of cataract, however, the underlying mechanisms remain unclear. The present study was designed to evaluate the anti-cataract effects and the underlying mechanisms of selenite and ebselen supplementation on galactose induced cataract in rats, a common animal model of sugar cataract. Transmission electron microscopy images of lens fiber cells (LFC) and lens epithelial cells (LEC) were observed in D-galactose-induced experimental cataractous rats treated with or without selenite and ebselen, also redox homeostasis and expression of proteins such as selenoprotein R (SELR), 15kD selenoprotein (SEP15), superoxide dismutase 1 (SOD1), catalase (CAT), β-crystallin protein, aldose reductase (AR) and glucose-regulated protein 78 (GRP78) were estimated in the lenses. The results showed that D-galactose injection injured rat lens and resulted in cataract formation; however, selenite and ebselen supplementation markedly alleviated ultrastructural injury of LFC and LEC. Moreover, selenite and ebselen supplementation could mitigate the oxidative damage in rat lens and increase the protein expressions of SELR, SEP15, SOD1, CAT and β-crystallin, as well as decrease the protein expressions of AR and GRP78. Taken together, these findings for the first time reveal the anti-cataract potential of selenite and ebselen in galactosemic cataract, and provide important new insights into the anti-cataract mechanisms of selenite and ebselen in sugar cataract.

Electron tomography of fiber cell cytoplasm and dense cores of multilamellar bodies from human age-related nuclear cataracts

Human nuclear cataract formation is a multi-factorial disease with contributions to light scattering from many cellular sources that change their scattering properties over decades. The aging process produces aggregation of cytoplasmic crystallin proteins, which alters the protein packing and texture of the cytoplasm. Previous studies of the cytoplasmic texture quantified increases in density fluctuations in protein packing and theoretically predicted the corresponding scattering. Multilamellar bodies (MLBs) are large particles with a core of crystallin cytoplasm that have been suggested to be major sources of scattering in human nuclei. The core has been shown to condense over time such that the refractive index increases compared to the adjacent aged and textured cytoplasm. Electron tomography is used here to visualize the 3D arrangement of protein aggregates in aged and cataractous lens nuclear cytoplasm compared to the dense protein packing in the cores of MLBs. Thin sections, 70 nm thick, were prepared from epoxy-embedded human transparent donor lenses and nuclear cataracts. Tilt series were collected on an FEI T20 transmission electron microscope (TEM) operated at 200 kV using 15 nm gold particles as fiducial markers. Images were aligned and corrected with FEI software and reconstructed with IMOD and other software packages to produce animated tilt series and stereo anaglyphs. The 3D views of protein density showed the relatively uniform packing of proteins in aged transparent lens nuclear cytoplasm and less dense packing of aged cataractous cytoplasm where many low-density regions can be appreciated in the absence of the TEM projection artifacts. In contrast the cores of the MLBs showed a dense packing of protein with minimal density fluctuations. These observations support the conclusion that, during the nuclear cataract formation, alterations in protein packing are extensive and can result in pronounced density fluctuations. Aging causes the MLB cores to become increasingly different in their protein packing from the adjacent cytoplasm. These results support the hypothesis that the MLBs increase their scattering with age and nuclear cataract formation.

Texture analysis of TEM micrographs of alginate gels for cell microencapsulation

In this work, the morphological characteristics of a calcium alginate gel and a binary (gel) mixture composed of (calcium) alginate and lactose-modified chitosan (chitlac) are evaluated and compared to quantify the differences between the two three-dimensional (3D) structures. A set of textural descriptors based on histogram analysis as well as on gray level co-occurrence matrix and on fractal dimension is extracted from transmission electron microscopy micrographs to describe the morphological differences that the images present. The obtained results reveal significant quantitative morphological differences between the calcium alginate gel and the binary gel mixture that were already inferred from rheological experiments, so as to provide a structural basis for developing new encapsulation systems based on such mixed polymer gels.

Multilamellar spherical particles as potential sources of excessive light scattering in human age-related nuclear cataracts

The goal of this project was to determine the relative refractive index (RI) of the interior of multilamellar bodies (MLBs) compared to the adjacent cytoplasm within human nuclear fiber cells. MLBs have been characterized previously as 1-4 μm diameter spherical particles covered by multiple lipid bilayers surrounding a cytoplasmic core of variable density. Age-related nuclear cataracts have more MLBs than transparent donor lenses and were predicted to have high forward scattering according to Mie scattering theory, assuming different RIs for the MLB and cytoplasm. In this study quantitative values of relative RI were determined from specific MLBs in electron micrographs of thin sections and used to calculate new Mie scattering plots. Fresh lenses were Vibratome sectioned, immersion fixed and en bloc stained with osmium tetroxide and uranyl acetate, or uranyl acetate alone, prior to dehydration and embedding in epoxy or acrylic resins. Thin sections 70 nm thick were cut on a diamond knife and imaged without grid stains at 60 kV using a CCD camera on a transmission electron microscope (TEM). Integrated intensities in digital electron micrographs were related directly to protein density, which is linearly related to RI for a given substance. The RI of the MLB interior was calculated assuming an RI value of 1.42 for the cytoplasm from the literature. Calculated RI values for MLBs ranged from 1.35 to 1.53. Thus, some MLBs appeared to have interior protein densities similar to or less than the adjacent cytoplasm whereas others had significantly higher densities. The higher density MLBs occurred preferentially in older and more advanced cataracts suggesting a maturation process. The bilayer coats were more often observed in MLBs from transparent donors and early stage cataracts indicating that bilayer loss was part of the MLB maturation, producing large low-density spaces around dense MLB cores. These spaces were frequently observed in advanced cataracts from India as large low-density crescents and annular rings. Predicted scattering from Mie plots using particles with dense cores and low-density rims was higher than reported previously, although the most important factor was the relative RI, not the MLB coat or lack thereof. In conclusion, the measurements confirm the high protein density and RI of some MLB interiors compared to adjacent cytoplasm. This high RI ratio used in the Mie calculations suggests that for 2000 MLBs/mm³, about half that reported for early stage nuclear cataracts from the US, the forward scattering could be more than 30% of the incident light. Therefore, the extent of forward scattering and its influence on macular visual acuity could be important components of ophthalmological evaluations of cataract patients.

Measurement of lens protein aggregation in vivo using dynamic light scattering in a guinea pig/UVA model for nuclear cataract

The role of UVA radiation in the formation of human nuclear cataract is not well understood. We have previously shown that exposing guinea pigs for 5 months to a chronic low level of UVA light produces increased lens nuclear light scattering and elevated levels of protein disulfide. Here we have used the technique of dynamic light scattering (DLS) to investigate lens protein aggregation in vivo in the guinea pig/UVA model. DLS size distribution analysis conducted at the same location in the lens nucleus of control and UVA-irradiated animals showed a 28% reduction in intensity of small diameter proteins in experimental lenses compared with controls (P < 0.05). In addition, large diameter proteins in UVA-exposed lens nuclei increased five-fold in intensity compared to controls (P < 0.05). The UVA-induced increase in apparent size of lens nuclear small diameter proteins was three-fold (P < 0.01), and the size of large diameter aggregates was more than four-fold in experimental lenses compared with controls. The diameter of crystallin aggregates in the UVA-irradiated lens nucleus was estimated to be 350 nm, a size able to scatter light. No significant changes in protein size were detected in the anterior cortex of UVA-irradiated lenses. It is presumed that the presence of a UVA chromophore in the guinea pig lens (NADPH bound to zeta crystallin), as well as traces of oxygen, contributed to UVA-induced crystallin aggregation. The results indicate a potentially harmful role for UVA light in the lens nucleus. A similar process of UVA-irradiated protein aggregation may take place in the older human lens nucleus, accelerating the formation of human nuclear cataract.

Ultrastructural analysis of damage to nuclear fiber cell membranes in advanced age-related cataracts from India

The primary goal was to characterize the structural alterations that occur at the fiber cell interfaces in nuclei of fully opaque cataracts removed by extracapsular cataract surgery in India. The dark yellow to brunescent nuclei, ages 38-78 years, were probably representative of advanced age-related nuclear cataracts. Thick tissue slices were fixed, en bloc stained and embedded for transmission electron microscopy. Stained thin sections contained well-preserved membranes and junctions, although the complex cellular topology often made it necessary to tilt the grid extensively to visualize the membranes. Damage to the fiber cell membranes was noted in all regions of the nucleus. The most important damage occurred within undulating membrane junctions where the loss of membrane segments was common. These membrane breaks were not sites of fusion as membrane edges were detected and cytoplasm appeared to be in contact with extracellular space, which was enlarged in many regions. Dense deposits of protein-like material were frequently observed within the extracellular space and appeared to be similar to protein in the adjacent cytoplasm. The deposits were often 20-50 nm thick, variable in length and located on specific sites on plasma membranes and between clusters of cells or cell processes. In addition, low density regions were seen within the extracellular space, especially within highly undulating membranes where spaces about 100 nm in diameter were observed. The membrane damage was more extensive and extracellular spaces were larger than in aged transparent donor lenses. Because high and low density regions contribute equally to the fluctuations in refractive index, the changes in density due to the observed damage near membranes are likely to produce significant light scattering based on theoretical analysis. The dimensions of the fluctuations in the range 20-100 nm imply that the scattering is probably similar to that of small particles that would increase high-angle scattering visible in the slit lamp. Such damage to membranes would be expected to contribute to the total opacification of the nucleus as the cataract matures. The main sources of the fluctuations appear to be the degradation of membranes and adjacent cytoplasmic proteins, as well as the redistribution of proteins and fragments.

Analysis of nuclear fiber cell cytoplasmic texture in advanced cataractous lenses from Indian subjects using Debye-Bueche theory

Alterations in ultrastructural features of the lens fiber cells lead to scattering and opacity typical of cataracts. The organelle-free cytoplasm of the lens nuclear fiber cell is one such component that contains vital information about the packing and organization of crystallins critical to lens transparency. The current work has extended analysis of the cytoplasmic texture to transparent and advanced cataractous lenses from India and related the extent of texturing to the nuclear scattering observed using the Debye-Bueche theory for inhomogeneous materials. Advanced age-related nuclear cataracts (age-range 38-75 years) and transparent lenses (age-range 48-78 years) were obtained following extracapsular cataract removal or from the eye bank, at the L.V. Prasad Eye Institute. Lens nuclei were Vibratome-sectioned, fixed and prepared for transmission electron microscopy using established techniques. Electron micrographs of the unstained thin sections of the cytoplasm were acquired at 6500x and percent scattering for wavelengths 400-700 nm was calculated using the Debye-Bueche theory. Electron micrographs from comparable areas in an oxidative-damage sensitive (OXYS) rat model and normal rat lenses preserved from an earlier study were used, as they have extremely textured and smooth cytoplasms, respectively. The Debye-Bueche theoretical approach produces plots that vary smoothly with wavelength and are sensitive to spatial fluctuations in density. The central lens fiber cells from advanced cataractous lenses from India and the OXYS rat, representing opaque lens nuclei, produced the greatest texture and scattering. The transparent human lenses from India had a smoother texture and less predicted scattering, similar to early cataracts from previous studies. The normal rat lens had a homogeneous cytoplasm and little scattering. The data indicate that this method allowed easy comparison of small variations in cytoplasmic texture and robustly detected differences between transparent and advanced cataractous human lenses. This may relate directly to the proportion of opacification contributed by the packing of crystallins. The percent scattering calculated using this method may thus be used to generate a range of curves with which to compare and quantify the relative contribution of the packing of crystallins to the loss of transparency and scattering observed.

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.

Light scattering in lens research: an essay on accomplishments and promises

This paper briefly reviews light scattering methodologies in lens research. In the phenomenological sense cataract formation (lens opacities or turbidities) in its early stages can be described by enhanced scattering of light. In the analytical sense information is obtained on the molecular entities involved in light scattering. In Section 2, different methodologies (mainly static and dynamic) of light scattering experiments are described, which had been used successfully in lens research. In Section 3 the problem of interpretation of light scattering data in condensed phase is considered. It is pointed out that due to interparticle interactions the concentration dependence of the data must be considered. If dilution or thin sectioning of samples is not an option, it is better to report molecular parameters in terms of diffusion coefficients or decay times. In Section 4, a case is made to encourage light scattering experiments in the polarized/depolarized modes both in static and dynamic light scatterings. Preliminary data obtained with polarized/depolarized dynamic light scattering measurements on alpha-crystallin and interpreted in view of model systems and literature data imply that the molecule is a compact sphere with somewhat restricted segment mobility. The preliminary nature of this information is due to the unavailability of high power lasers and efficient polarizers in my lab at the time of these experiments.

Cataract formation in a strain of rats selected for high oxidative stress

The primary purpose of this study was to define the clinical and morphological features of cataractogenesis in the OXYS strain of rats that generate excess reactive oxygen species. Rats were sequentially examined from birth to the development of mature cataracts with slit lamp biomicroscopy. Morphology of selected stages of cataract development was studied using light and transmission electron microscopy (TEM), immunohistochemical localization of the lipid peroxidation product 4-hydroxynonenal (HNE) and fluorescent antibody labeling for DNA oxidation products. Lenses from age-matched normal rats were used as controls. OXYS rats developed cataracts as young as two weeks of age with progression to maturity by 1 year. Clinically, cataracts appeared initially either as nuclear or sub-capsular cortical changes and progressed to pronounced nuclear cataracts within months. TEM confirmed the light microscopic impression of region-specific alterations in both fiber cell cytoplasmic protein matrix and membrane structure. The outer adult nuclear region showed extensive cellular damage similar to osmotic cataracts, which is consistent with the postulated high uptake of glucose in the OXYS strain. The adult and outer fetal nuclear cells displayed several types of focal damage. The inner fetal and embryonic nuclear cells demonstrated textured cytoplasm, suggesting protein degradation or redistribution. Staining for HNE was increased in epithelium, cortex and nucleus compared to control lenses. Fluorescent antibody probes demonstrated increased levels of DNA oxidation products in OXYS rat lenses compared to age-matched controls. Fourier analysis of nuclear cytoplasm revealed significant components with corresponding sizes greater than 100 nm and, using a new theoretical approach, the texturing of the cytoplasm was shown to be sufficient to cause opacification of the nucleus. The OXYS rat appears to be an ideal model for oxidative stress cataractogenesis. The potential oxidative damage observed is extensive and characteristic of the developmental region. The source of oxidative damage may in part be a response to elevated levels of glucose. Because oxidative stress is thought to be a major factor in cataract formation in both diabetic and non-diabetic aging humans, this animal model may be a useful tool in assessing efficacy of antioxidant treatments that may slow or prevent cataract formation.

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.

Ultrastructural characterization and Fourier analysis of fiber cell cytoplasm in the hyperbaric oxygen treated guinea pig lens opacification model

The structural characteristics of differentiated fiber cells in control and hyperbaric oxygen (HBO)-treated guinea pig lenses were examined by transmission electron microscopy (TEM). Emphasis was placed on cell damage, membrane integrity, and cytoplasmic texture. Given the faint gross opacities observed in HBO-treated lenses in previous studies, it was hypothesized that subtle but significant morphological differences due to oxidative damage exist between control and treated animals. Experimental animals received either 70 or 85 treatments with HBO (2.5 atm of 100% O(2) for 2.5 hr, 3 times per week for 5-7 months). All specimens were obtained within 24 hr of death. Freshly cut Vibratome lens sections were fixed and processed for low and high-magnification thin-section TEM analysis. Cytoplasmic texture was analyzed using Fourier and autocorrelation image processing techniques. Low-magnification analysis revealed relatively insignificant differences in general appearance between the fiber cells of the inner fetal and embryonic nuclei in control and HBO-treated guinea pigs. Both groups demonstrated cells of similar morphology with equivalent membrane complexity and homogeneous cytoplasmic texture. Evidence of any major cellular damage or extracellular space debris was not obvious. High-magnification analysis of the cytoplasm of the treated lenses exhibited a mild, yet detectable increase in texture compared with controls and was confirmed by Fourier analysis. Cytoplasmic texture increased in complexity with additional treatments. The absence of major cellular damage in the lenses of HBO-treated animals suggests a less conspicuous source of light scattering. The small changes in cytoplasmic organization observed between treated and control animals may entirely account for the increase in nuclear light scattering observed by slit lamp. The results obtained with this guinea pig/HBO model parallel many of the morphological data associated with human nuclear cataracts. The high-angle scattering observed in the lens of the HBO-treated guinea pig may represent the type of cytoplasmic reorganization that occurs with mild oxidation, effectively making it a valuable model for human lens aging.