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From the Department of Anatomy and Histology and Institute for Biomedical Research (F13), The University of Sydney, Sydney, New South Wales, Australia 2006.
| Abstract |
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METHODS. TGFß was injected into the vitreous of the left eye of anesthetized adult male Wistar rats. The right eye received sterile vehicle as a control. Three to four months after injection, animals were killed, and lenses were enucleated and examined for cataractous changes.
RESULTS. All lenses from control eyes remained transparent and maintained normal cellular architecture throughout. In contrast, lenses from TGFß-injected eyes displayed cloudiness in the cortex. In some lenses, distinct opacities were also apparent at the equator and extending some distance toward the anterior and posterior poles. Histologically, the opacities corresponded to subcapsular plaques containing aberrant cells and accumulations of extracellular matrix. In addition, cortical fibers in the anterior and posterior of all lenses displayed variable degrees of swelling, and many retained their nuclei. In some regions, the fiber cells appeared to have degenerated to form large homogeneous areas. The cellular architecture of the equator of these lenses was also disrupted and, in the most severe case, no bow zone was apparent with nucleated cells extending to the posterior pole.
CONCLUSION. The introduction of active TGFß into the vitreous induced lenses to undergo cataractous changes. In addition to the TGFß-induced changes in the epithelium that were reported previously, cataractous changes observed in this study also involved the lens fiber cells and resembled changes observed in human posterior subcapsular and cortical cataracts.
| Introduction |
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-smooth muscle actin and cell
death.1
2
Furthermore, distinct anterior opacities develop
in whole lenses cultured with TGFß.3
4
Histologically,
the opacities correspond with subcapsular plaques, which contain
spindle-shaped cells and accumulations of extracellular matrix,
including type I collagen.3
Morphologic and molecular
features similar to those induced by TGFß in vitro have been
described in human anterior and posterior subcapsular cataract and in
aftercataract (posterior capsular opacification).5
6
7
8
9
10
11
12
13
These findings have led to the hypothesis that TGFß may be involved
in the etiology of these forms of cataract. Analysis of ocular media of patients who are to undergo cataract surgery suggests that TGFß levels are elevated compared with ocular media collected from eye bank eyes.14 15 However, it is not clear whether the elevated level of TGFß in these patients was the inducing factor for the cataract or a consequence of the cataract. The purpose of this study was to investigate whether elevating active TGFß levels in the ocular media in vivo induces cataractous changes in the lens similar to those previously reported in studies in vitro. A whole animal model was used, in which TGFß was injected into the vitreous of adult male rats. Lenses from these animals have been shown to be both highly responsive and sensitive to the cataractogenic influence of TGFß.4 Here, we describe the cellular and molecular changes that are induced in the lens by intravitreal injection of TGFß.
| Materials and Methods |
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Intraocular Injection Protocol
All experimental procedures used in this study conformed to the
ARVO Statement for the Use of Animals in Ophthalmic and Vision
Research. Thirteen adult Wistar rats (9-months old) were used in this
study. Because adult male rats had been shown to be highly susceptible
to the cataractogenic influence of TGFß, these animals were
exclusively used in this study.4
Each rat was anesthetized
using 5% halothane in 70% NO2-30%
O2 then maintained in 1.5% halothane in the same
gas mixture for the duration of all surgical procedures. The rat was
positioned on its right side on an operating table under a dissecting
microscope with its left eye uppermost. Using a fine needle (Insulin
Syringe, Becton Dickinson, Franklin Lakes, NJ; external diameter, 360
µm), a small puncture was made in the region of the limbus. A very
fine needle (Hamilton, Reno, NV; external diameter, 200 µm) attached
to a 10-µl syringe was immediately lowered through the puncture hole
into the vitreous using a micromanipulator, and 3 µl (approximately
60 ng) TGFß, was slowly injected into the vitreous. The needle was
left in position for 30 to 60 seconds and then slowly withdrawn to
minimize fluid loss from the eye. Injections were performed under a
dissecting microscope to ensure correct positioning of the needle and
to monitor loss of fluid from the eye. It should be noted that the
maximum effective dose of TGFß to which lens cells were exposed as a
result of this procedure is unknown. Injection would generate a
positive pressure, which would probably lead to displacement of some
TGFß out of the vitreous compartment. In addition, the outcome would
be influenced by inhibitors of TGFß known to be present in the
vitreous.16
After injection, each animal was placed in a warmed (26°C), humidified recovery box and allowed to regain consciousness. Each rat was then anesthetized for a second time, and the same protocol was used to inject the right eye with 3 µl control vehicle, either acetonitrile-trifluoroacetic acid or HCl-phosphate-buffered saline (PBS)-BSA (depending on the source of TGFß). Rats were monitored regularly (at least twice weekly) for infection associated with the injection site. Most rats were killed at 15 weeks, one was killed at 6 weeks, and another was killed at 12 weeks. No time-dependent differences were noted.
Collection of Lenses and Tissue Processing
After the appropriate period after injection, rats were euthanized
by CO2 asphyxiation, and eyes were removed and
placed in culture dishes containing Medium 199 (Trace Biosciences,
Sydney, Australia) with BSA and antibiotics.16
Both eyes
from four rats were fixed whole for routine histology to determine the
influence of TGFß on all ocular tissues. The lenses were removed from
remaining eyes, as described for whole lens cultures,3
examined for the presence of opacities, and photographed.
Representative lenses and whole eyes were fixed in Carnoys
(1:3 acetic acid:ethanol) and processed for routine histology and
immunolocalization.3
Briefly, entire lenses and whole eyes
were serially sectioned and collected onto subbed glass slides.
Every fifth slide was stained with hematoxylin-eosin, and sections were
examined for evidence of plaque formation and/or fiber deterioration.
Sections corresponding to those that displayed histologic changes were
used for immunolocalization of ß-crystallin and the extracellular
matrix proteins laminin and type I collagen. Nuclei were counterstained
(Hoechst 33258; BoehringerMannheim, La Jolla, CA).
| Results |
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Influence of TGFß on Lens Transparency
For every rat examined in this study, the lens from the right eye
(vehicle-treated control) remained transparent (Fig. 1A
). Of the nine lenses dissected from eyes that were injected with
TGFß, four showed distinct subcapsular opacities (for example, Fig. 1B ). In these lenses, the opacities tended to be localized in a region
around the lens equator but also extending some distance toward the
anterior (Fig. 1B)
and/or posterior (Fig. 1C) poles of the lens. A
generalized more diffuse clouding, which appeared to involve the
cortical fiber cells, was a feature of all lenses from eyes injected
with TGFß. This was most readily detected in the region surrounding
the reflection of the light source (for example, see Fig. 1B
).
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Another form of anterior opacification was also observed. A large vacuole was observed funneling from the lens capsule at the anterior pole deep into the cortical fibers (Fig. 4A) . The overlying capsule displayed evidence of remodeling; nucleated cells appeared to be embedded in the extracellular matrix of the capsule, which was irregular in its thickness, forming a small residual plaque (Fig. 4A) . The fibers surrounding this vacuolated region and associated plaque were very swollen and highly disrupted, with complete loss of regular alignment (Fig. 4B) . It is interesting to note that this is the only rat in which opacification of the lens in the TGFß-injected eye was apparent in vivo.
TGFß-Induced Changes in the Posterior Region
Nucleated cells were observed at the posterior pole of some
of the lenses from eyes injected with TGFß. Nuclei were present in
fiber cells located directly adjacent to the posterior capsule and
extending deeper into the cortex (Fig. 5B
). The nucleated fiber cells often appeared enlarged and swollen,
reminiscent of bladder cells.17
In some areas these cells
appeared to have degenerated forming homogeneous regions in the cortex
(Fig. 5D) . No nucleated, swollen, or degenerating cells were detected
in the posterior of any of the control lenses (Figs. 5A
5C)
.
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| Discussion |
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Intravitreal injection of TGFß induced disruption of the normally highly aligned arrangement of the fiber cells. Fibers appeared swollen and retained their nuclei, and in some regions there was evidence of degeneration of fibers into rounded fragments and large homogeneous areas. Swollen, degenerating fibers were apparent from the anterior through the equator and into the posterior of the lens. TGFß also induced severe disruption of the ordered arrangement of nuclei at the lens equator and in the bow region of these lenses. Furthermore, many fiber cells atypically retained their nuclei beyond the bow region. In all these respects, changes in lenses from eyes injected with TGFß resemble those associated with human cortical cataract.20 21
Lenses from eyes injected with TGFß also displayed changes characteristic of posterior subcapsular cataract. In lenses exposed to TGFß, nucleated cells were observed extending from the disrupted equator to the posterior pole of the lens as described in posterior subcapsular cataract.17 22 Furthermore, swelling and deterioration of the fiber cells extending from beneath the posterior capsule to deep within the cortex of the lens, another feature of posterior subcapsular cataract, was common.
Epidemiologic studies indicate that posterior subcapsular cataract and cortical cataract commonly occur together in mixed cataracts.23 24 25 This has resulted in some confusion in establishing exactly which morphologic features arise in each form of cataract. Subsequently, there appears to be considerable overlap in the features of cortical cataract and posterior subcapsular cataract. For example, both these forms of cataract contain swollen and degenerated fibers that have lost their regular alignment.20 22 17 Results presented in the present study suggest that both forms may share a common etiology in which TGFß is implicated.
The introduction of active TGFß into the vitreous compartment also resulted in the formation of anterior opacities, indicating that diffusion of TGFß from the vitreous to aqueous compartments had occurred. The opacities corresponded with subcapsular plaques and were surrounded by regions of severe fiber disruption. Accumulations of extracellular matrix were common within the plaques. These opacities are similar to the anterior subcapsular cataracts described previously in lenses cultured with TGFß.3 4 The present study, therefore, provides additional strong support for the role of TGFß in the etiology of anterior subcapsular cataract.
Exposure to TGFß in vivo appears to induce more pronounced cataractous changes in the posterior and cortical regions of the lens than those observed when lenses are cultured with TGFß in vitro. Generally, changes induced by TGFß in vitro are restricted to the epithelium. Lenses cultured with TGFß develop numerous distinct opacities across the anterior surface that correspond with subcapsular plaques.3 4 Some subtle changes in the posterior and cortical fibers have been observed previously in lenses cultured with TGFß but only in lenses from ovariectomized rats, which are particularly susceptible to the cataractogenic effects of TGFß.4
Several factors may contribute to the observed differences in the effects of TGFß on the lens in vivo and in vitro. In studies in vitro, the entire lens is immersed in medium containing TGFß. In contrast, in the present study in vivo, TGFß was introduced directly into the vitreous chamber; therefore, the initial and presumably greatest TGFß insult was inflicted on the posterior region of the lens. Another factor may be the duration of the experiment. In studies in vitro, the time that lenses may be cultured without deterioration is limited (up to 7 days for lenses from adult rats), whereas lenses were generally not examined until 15 weeks after exposure to TGFß in vivo. TGFß-induced changes, either direct or secondary, may take longer to manifest in cortical fibers than in epithelial cells. Cells would also have more time to migrate posteriorly along the capsule in the study in vivo.
Differentiating fiber cells in the cortical region have the potential to respond directly to TGFß. TGFß receptors have been detected by immunolocalization and in situ hybridization in these cells as well as in the equatorial region and the lens epithelium.26 However, the observed swelling of cortical fibers may be a secondary response, the result of osmotic imbalance due to TGFß-induced disruption of the epithelium. This phenomenon does not appear to be the result of nonspecific damage associated with the injection procedure, because no such changes were observed in control lenses. Further investigation is required to determine the precise mechanisms involved in the cortical fiber response.
TGFß is known to be present in the ocular environment. The ocular
media contain TGFß and both mRNA and protein have been detected
within the lens.16
18
Normally, TGFß bioavailability to
the lens appears to be tightly regulated. Mechanisms that contribute to
suppression of TGFß activity include its production in a latent form
and binding to regulatory molecules.27
28
16
Recent work
in this laboratory has indicated that the ocular media contain
inhibitors of TGFß. Vitreous in particular has been shown to strongly
inhibit TGFß activity, demonstrated by its effectiveness in blocking
the morphologic changes induced by TGFß in lens epithelial explants
in vitro.16
At least part of the inhibitory activity of
the ocular media may be attributable to the presence of
-2-macroglobulin.16
Although the ocular media contain TGFß inhibitory molecules, it is clear that their effects can be overcome. For example, in the present study rats that received a single intravitreal injection of TGFß subsequently developed cataracts. Similarly, in humans it has been reported that patients who received intraocular TGFß to promote healing of macular holes later exhibited an unusually high incidence of cataract.29 Furthermore, transgenic mice that overexpress TGFß specifically in the lens developed subcapsular cataracts.19 Elucidating the mechanisms and molecules that are involved in regulating TGFß activity in the ocular media, and in the eye in general, represents an important area for future study that may lead to new strategies for preventing or slowing TGFß-induced cataract.
The present study adds further support to the hypothesis that TGFß is involved in the etiology of subcapsular cataract, in both the anterior and posterior hemispheres of the lens. In addition, the results indicate that TGFß may be involved in the etiology of at least some cortical cataracts. Having now established this in vivo model for TGFß-induced cataract, it will be possible to use it in future studies to assess the effects of cataract prevention strategies in the whole animal.
| Acknowledgements |
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| Footnotes |
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Submitted for publication March 1, 1999; revised June 10, 1999; accepted July 6, 1999.
Commercial relationships policy: N.
Corresponding author: John W. McAvoy, Department of Anatomy and Histology (F13), University of Sydney, Sydney, NSW, Australia 2006. E-mail: johnmca{at}anatomy.usyd.edu.au
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-smooth muscle actin, a marker for subcapsular cataracts Curr Eye Res 13,885-890[Medline][Order article via Infotrieve]
-2-macroglobulin Invest Ophthalmol Vis Sci 37,1509-1519This article has been cited by other articles:
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