Basal membrane complex architecture is disrupted during posterior subcapsular cataract formation in Royal College of Surgeons rats.

PURPOSE: Previous studies detailing the development of posterior subcapsular cataracts (PSC) in Royal College of Surgeons (RCS) rats have shown that aberrant fiber-end migration underlies the structural compromise. This investigation was conducted to examine the distribution of select basal membrane complex (BMC) components and to assess the intravitreal levels of specific cytokines during PSC formation. METHODS: Lenses from 52 RCS dystrophic rats (RCS/Lav) and 28 genetically matched control animals (RCS-rdy(+)/Lav) from 2 to 8 weeks old were used. After enucleation, vitreous was collected for eventual cytokine level analyses; lenses were then removed and processed for immunocytochemical localization of actin, cadherin, ß integrin, vinculin, and cell nuclei. RESULTS: At 2-3 weeks postnatal, dystrophic lenses showed normal BMC distribution of actin, cadherin, and vinculin; however ß integrin distribution was altered as compared to controls. By 4-6 weeks of age, F-actin was visible as bright foci arranged in a "rosette" pattern around fiber-end profiles. Concurrently, vinculin was rearranged into a diffuse pattern within the BMC. Cadherin delineated the fiber ends in dystrophic lenses until 5 weeks postnatal, after which it displayed diffuse cytoplasmic staining with more definitive labeling at the BMC periphery. ß integrin was initially distributed as punctuate spots at 2-3 weeks postnatal; however, by 4-6 weeks it was co-localized with F-actin around the periphery of fiber ends. The distribution of F-actin, cadherin, and ß integrin components did not undergo further changes after 6 weeks of age; however, vinculin was present predominantly at the periphery of the BMC in 7-8-week-old dystrophic lenses. Intravitreal cytokine levels were assessed for interleukin (IL)-1α, IL-4, IL-6, IL-8, tumor necrosis factor (TNF), and interferon (IFN)-γ. Levels of IL-1α, IL-4, TNF, and IFN-γ demonstrated a similar pattern, with concentrations increasing from 2 to 6 weeks postnatal and then decreasing slightly up to 8 weeks of age. IL-4 and TNF had the highest average concentrations, with peaks of 148.00 pg/ml and 34.20 pg/ml, respectively. CONCLUSIONS: The data indicate that defined rearrangements of normal BMC architecture precede and characterize the structural changes that culminate in the PSC. These are consistent with modifications of adhesion mechanics involving cell-cell attachment, cell-matrix adhesion, and timely fiber-end detachment. Further, the results suggest that pro-inflammatory cytokines are potential initiating factors in aberrant fiber-end migration and subsequent PSC formation.

Abnormal fiber end migration in Royal College of Surgeons rats during posterior subcapsular cataract formation.

PURPOSE: Prior structural studies of posterior subcapsular cataract (PSC) development in Royal College of Surgeons (RCS) rats suggest that migration of basal fiber ends was disrupted, ultimately resulting in a PSC. Therefore the goal of this study was to assess the overall migration patterns as well as changes to the structure and cytoskeleton of basal fiber ends during PSC development. METHODS: Lenses from 48 RCS dystrophic rats (RCS/Lav) and 24 genetically matched control animals (RCS-rdy(+)/Lav) from 2 to 8 weeks old were examined. Equatorial diameters were measured and suture patterns were photographed immediately following enucleation/dissection. Right eye lenses were fixed and processed to visualize the actin cytoskeleton via laser scanning confocal microscopy (LSCM), left eye lenses were decapsulated, fixed and processed for scanning electron microscopy (SEM). Scaled 3D-computer assisted drawings (CADs) and animations were constructed from the data to depict the changes in suture patterns and fiber end architecture. RESULTS: At 2 weeks, dystrophic lenses displayed an inverted Y suture on the posterior, and by 3 weeks most lenses had at least one sub-branch. Additional sub-branches were observed with time, opacities being visible as early as 4 weeks and progressing into PSC plaques by 6 weeks. Control lenses displayed inverted Y sutures at all ages and were transparent. SEM of dystrophic lenses revealed fiber ends with normal size, shape, arrangement, and filopodia at 2 weeks; scattered areas of dome-shaped fiber ends and small filopodia were present at 3 weeks. At 4 weeks the irregularly arranged domed fiber ends had extremely long filopodia with 'boutons' at their tips. By 6 weeks all fiber ends within plaques displayed rounded or domed basal membranes and lacked filopodial extensions. Control lenses at all time points had comparable ultrastructure to the 2 week old dystrophic lenses. F-actin arrangement within the basal membrane complex (BMC) of control lenses showed the expected peripheral pattern of labeling at all ages. Dystrophic RCS lenses at 2 weeks were comparable to controls, however by 3-4 weeks they displayed scattered foci of F-actin within the BMC. At all time points thereafter, F-actin was rearranged into a 'rosette' pattern of prominent foci at cell vertices. CONCLUSIONS: The data are consistent with the hypothesis that migration of basal fiber ends is altered in a two stage process wherein initially, migration patterns undergo a rapid shift resulting in abnormal suture sub-branch formation. Subsequent cytological alterations are consistent with an eventual cessation of migration, precluding proper targeting of basal ends to their sutural destinations and leading to cataract plaque formation.

A novel terminal web-like structure in cortical lens fibers: architecture and functional assessment.

This study describes a novel cytoskeletal array in fiber cells of the ocular lens of the rat and shows its relationship to the classical terminal web of other epithelial tissues. Naive adult Sprague-Dawley rats (n = 28) were utilized. F-actin, fodrin, myosin IIA, and CP49 distribution was assessed in anterior and posterior polar sections. For functional analysis, lenses were cultured with or without cytochalasin-D for 3 hr, then processed for confocal microscopy or assessed by laser scan analysis along sutures. Phalloidin labeling demonstrated a dense mesh of F-actin adjacent to posterior sutural domains to a subcapsular depth of 400 µm. Anterior polar sections revealed a comparable actin structure adjacent to anterior suture branches however, it was not developed in superficial fibers. Fodrin and myosin were localized within the web-like actin apparatus. The data was used to construct a model showing that the cytoskeletal array is located within the blunt, variable-width fiber ends that abut at sutures such that the "terminal web" flanks the suture on either side. Treatment with cytochalasin-D resulted in partial disassembly of the "terminal web" and perturbed cellular organization. Laser scan analysis revealed that cytochalasin-D treated lenses had significantly greater focal variability than control lenses (P = 0.020). We conclude that cortical fibers of rat lenses contain a bipolar structure that is structurally and compositionally analogous to classical terminal webs. The results indicate that the lens "terminal web" functions to stabilize lens fiber ends at sutures thus minimizing structural disorder, which in turn, promotes the establishment and maintenance of lens transparency.

Aberrant basal fiber end migration underlies structural malformations in a streptozotocin-induced diabetic rat model.

This study characterized early structural changes at posterior fiber ends in the crystalline lens after diabetic induction. Wistar rats (n = 49), randomized into one naïve control group and four experimental groups, were rendered diabetic via streptozotocin injection. Animals were euthanized at 1 week intervals, blood glucose levels recorded and lenses were evaluated grossly, by SEM and by confocal microscopy. Scoring Indices were developed to assess structural alterations and for statistical correlations between the scores and the duration of hyperglycemic exposure as well as blood glucose levels. Average blood glucose levels increased progressively from 98.5 mg/dL (controls) to 331.4 mg/dL (4 weeks). Diabetic lenses displayed abnormal suture sub-branches and opacity formation beginning late in the first week post-injection and rapidly progressing in severity through four weeks. SEM analyses showed gradual elongation of fiber ends and filopodia which comprised a disorganized configuration and a loss of recognizable migration patterns. Structural alterations culminated in foci of fiber degeneration by the third to fourth weeks. The F-actin distribution at basal fiber ends was significantly altered as compared to naïve controls. Cadherin distribution was altered as compared to controls, but largely at later time points. The grading system clearly shows increased structural compromise with elevated blood glucose levels in streptozotocin-induced diabetes. Further, although the initial rise in blood glucose levels was associated with pathological changes, their progression depended to a larger extent on continued hyperglycemic exposure. The data suggests that hyperglycemia initially disrupts fiber end migration, resulting in structural alterations and eventual fiber degeneration.

Distribution of basal membrane complex components in elongating lens fibers.

PURPOSE: To localize specific components of the Basal Membrane Complex (BMC) of elongating lens fibers at defined points in their migration to the posterior sutures. METHODS: Normal, juvenile (4-6 week old) Sprague-Dawley rat lenses (n=46) were utilized. Lenses were either decapsulated to obtain whole mounts of lens capsules or sectioned with a vibrating knife microtome. Sections (100 microm thick) were cut parallel to the equatorial plane, beginning at the posterior pole. On both sections and whole mounts, F-actin was localized using phalloidin-FITC while myosin, cadherins, and beta1 integrin were localized using immunofluorescent labeling. Specimens were visualized on a laser scanning confocal microscope. RESULTS: F-actin labeling in the equatorial and peri-sutural regions was predominately localized to the periphery of basal fiber ends (consistent with our prior results). At sutures, labeling for F-actin in the BMC was rearranged into numerous small profiles. Furthermore, labeling intensity for F-actin was increased at sutures. Myosin was present in the BMC in all locations examined as a diffuse plaque at fiber ends. Similarly, beta1 integrin was also distributed throughout the BMC within the actin-rich borders in all regions except adjacent to and at the suture branches. In that location immunofluorescence for beta1 integrin appeared to be reduced. In the equatorial, lateral-posterior, and peri-sutural regions, cadherin showed strong localization around the periphery of basal fiber ends. However, cadherin labeling was markedly reduced in the BMC as fibers detached from the capsule and abutted to form sutures (i.e. in the sutural region). Cadherin was concentrated along the short faces of elongating fiber mid-segments. CONCLUSIONS: It appears that F-actin, cadherin and beta1 integrin components of the BMC undergo controlled rearrangements in the final stages of migration and detachment from the capsule.

Age-related compaction of lens fibers affects the structure and optical properties of rabbit lenses.

BACKGROUND: The goal of this investigation was to correlate particular age-related structural changes (compaction) to the amount of scatter in rabbit lenses and to determine if significant fiber compaction occurred in the nuclear and inner cortical regions. METHODS: New Zealand White rabbits at 16-20 months old (adult; n = 10) and at 3.5-4 years old (aged; n = 10) were utilized for this study. Immediately after euthanising, scatter was assessed in fresh lenses by low power helium-neon laser scan analysis. Scatter data was analyzed both for whole lenses and regionally, to facilitate correlation with morphometric data. After functional analysis, lenses were fixed and processed for scanning electron microcopy (SEM; right eyes) and light microscopy (LM; left eyes). Morphometric analysis of SEM images was utilized to evaluate compaction of nuclear fibers. Similarly, measurements from LM images were used to assess compaction of inner cortical fibers. RESULTS: Scatter was significantly greater in aged lenses as compared to adult lenses in all regions analyzed, however the difference in the mean was slightly more pronounced in the inner cortical region. The anterior and posterior elliptical angles at 1 mm (inner fetal nucleus) were significantly decreased in aged vs. adult lenses (anterior, p = 0.040; posterior, p = 0.036). However, the average elliptical angles at 2.5 mm (outer fetal nucleus) were not significantly different in adult and aged lenses since all lenses examined had comparable angles to inner fetal fibers of aged lenses, i.e. they were all compacted. In cortical fibers, measures of average cross-sectional fiber area were significantly different at diameters of both 6 and 7 mm as a function of age (p = 0.011 and p = 0.005, respectively). Accordingly, the estimated fiber volume was significantly decreased in aged as compared to adult lenses at both 6 mm diameter (p = 0.016) and 7 mm diameter (p = 0.010). CONCLUSION: Morphometric data indicates that inner cortical fibers undergo a greater degree of age-related compaction than nuclear fibers. Increased scatter appears to be only tentatively correlated with regions of fiber compaction, suggesting that it is simply one of an array of factors that contribute to the overall decreased transparency in aged rabbit lenses.

Lens structure in MIP-deficient mice.

In this study we used correlative light, scanning, and transmission (freeze-etch) electron microscopy to characterize lens structure in normal mice and compare it with that in mice deficient in the major intrinsic protein (MIP) of fiber cells. Grossly, wild-type lenses were transparent and had typical Y sutures at all of the ages examined. These lenses had fibers of uniform shape (hexagonal in cross section) arranged in ordered concentric growth shells and radial cell columns. In addition, these fibers had normal opposite end curvature and lateral interdigitations regularly arrayed along their length. Ultrastructural evaluation of these fibers revealed anterior and posterior end segments characterized by square array membrane on low-amplitude wavy fiber membrane. Approximately 13% of the equatorial or mid segments of these same fibers were specialized as gap junctions (GJs). In contrast, heterozygote lenses, while initially transparent at birth, were translucent by 3 weeks of age, except for a peripheral transparent region that contained fibers in the early stages of elongation. This degradation in clarity was correlated with abnormal fiber structure. Specifically, although the mid segment of these fibers was essentially normal, their end segments lacked normal opposite end curvature, were larger than normal, and had a distinct non-hexagonal shape. As a result, these fibers failed to form typical Y sutures. Furthermore, the nuclear fibers of heterozygote lenses were even larger and lacked any semblance of an ordered packing arrangement. Grossly, homozygote lenses were opaque at all ages examined, except for a peripheral transparent region that contained fibers in the early stages of elongation. All fibers from homozygote lenses lacked opposite end curvature, and thus failed to form any sutures. Also, these fibers were essentially devoid of interlocking devices, and only 7% of their mid segment was specialized as GJs. The results of this study suggest that MIP has essential roles in the establishment and maintenance of uniform fiber structure, and the organization of fibers, and as such is essential for lens function.

Morphology and organization of posterior fiber ends during migration.

PURPOSE: To characterize structural parameters of the basal membrane complex (BMC) and to determine the arrangement and organization of posterior fiber ends during elongation/migration in lenses with branched sutures. METHODS: Lenses from normal, juvenile (4-6 week old) Sprague-Dawley rats (n=16) were utilized. Posterior fiber ends were assessed on both whole mounts of lens capsules and on decapsulated lenses. The size, shape and organization of migrating fiber ends was assessed by scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM) along the entire posterior surface. The area of the BMC was measured using image analysis software and subjected to statistical analysis. RESULTS: Posterior fiber ends had a characteristic regional arrangement during elongation and migration along the capsule. These regions were termed the equatorial, the lateral-posterior (posterior from the equator to within 150 microm of the sutures), the peri-sutural (150 microm surrounding the sutures), and the sutural. The area of fiber ends (seen by SEM) was compared to the area of fluorescent F-actin profiles (seen by LSCM). There was no significant difference (p=0.324) between the average basal end area (40.21 microm2) and the average area of F-actin profiles (40.65 microm2). The average fiber end area in the lateral-posterior, peri-sutural, and sutural regions was 63.19 microm2, 71.95 microm2, and 25.75 microm2, respectively. In the equatorial region, footprints were aligned in rows oriented toward the posterior pole, consistent with the arrangement of straight, meridional rows. Initially, fiber ends within the lateral-posterior region were arranged in short irregular rows having variable orientation with respect to the posterior pole. The remainder of these ends were randomly arranged. In the peri-sutural region, fiber ends approaching suture branches were aligned in short rows oriented at angles to the posterior pole. At the sutures, fiber ends appeared to become rounded, presumably during detachment from the capsule. CONCLUSIONS: The results confirm that F-actin profiles delineate the BMC of posterior fiber ends. Furthermore, the average area, shape and arrangement of fiber ends varies in a predictable pattern during migration. The data suggests that elongating fiber ends follow defined migration patterns along the posterior capsule to their sutural destinations. This controlled process is crucial to the formation of ordered suture patterns, thereby minimizing their adverse effects on lens optical quality.

Discordant expression of the sterol pathway in lens underlies simvastatin-induced cataracts in Chbb: Thom rats.

Simvastatin rapidly induced cataracts in young Chbb:Thom (CT) but not Sprague Dawley (SD) or Hilltop Wistar (HW) rats. Oral treatment for 14 but not 7 days committed CT rat lenses to cataract formation. The cholesterol to phospholipid molar ratio in lenses of treated CT rats was unchanged. Differences between strains in serum and ocular humor levels of simvastatin acid poorly correlated with susceptibility to cataracts. No significant differences were found between rat strains in the capacity of simvastatin acid to inhibit lens-basal sterol synthesis. Prolonged treatment with simvastatin comparably elevated HMG-CoA reductase protein and enzyme activity in lenses of both cataract resistant and sensitive strains. However, in contrast to SD and HW rats, where sterol synthesis was markedly increased, sterol synthesis in CT rat lenses remained at baseline. Discordant expression of sterol synthesis in CT rats may be due to inadequate upregulation of lens HMG-CoA synthase. HMG-CoA synthase protein levels, and to a much lesser extent mRNA levels, increased in lens cortex of SD but not CT rats. Because upregulation of the sterol pathway may result in increased formation of isoprene-derived anti-inflammatory substances, failure to upregulate the pathway in CT rat lenses may reflect an attenuated compensatory response to injury that resulted in cataracts.

Analysis of nuclear fiber cell compaction in transparent and cataractous diabetic human lenses by scanning electron microscopy.

BACKGROUND: Compaction of human ocular lens fiber cells as a function of both aging and cataractogenesis has been demonstrated previously using scanning electron microscopy. The purpose of this investigation is to quantify morphological differences in the inner nuclear regions of cataractous and non-cataractous human lenses from individuals with diabetes. The hypothesis is that, even in the presence of the osmotic stress caused by diabetes, compaction rather than swelling occurs in the nucleus of diabetic lenses. METHODS: Transparent and nuclear cataractous lenses from diabetic patients were examined by scanning electron microscopy (SEM). Measurements of the fetal nuclear (FN) elliptical angles (anterior and posterior), embryonic nuclear (EN) anterior-posterior (A-P) axial thickness, and the number of EN fiber cell membrane folds over 20 microns were compared. RESULTS: Diabetic lenses with nuclear cataract exhibited smaller FN elliptical angles, smaller EN axial thicknesses, and larger numbers of EN compaction folds than their non-cataractous diabetic counterparts. CONCLUSION: As in non-diabetic lenses, the inner nuclei of cataractous lenses from diabetics were significantly more compacted than those of non-cataractous diabetics. Little difference between diabetic and non-diabetic compaction levels was found, suggesting that diabetes does not affect the degree of compaction. However, consistent with previous proposals, diabetes does appear to accelerate the formation of cataracts that are similar to age-related nuclear cataracts in non-diabetics. We conclude that as scattering increases in the diabetic lens with cataract formation, fiber cell compaction is significant.