(Na+ + K+)-ATPase and opsin in retinas of RCS dystrophic rats: time course study

Preservation of visual responsiveness in the superior colliculus of RCS rats after retinal pigment epithelium cell transplantation

The dystrophic RCS rat undergoes progressive photoreceptor degeneration due to a primary defect in retinal pigment epithelial (RPE) cells. This has a major impact on central visual responsiveness. Here we have examined how functional deterioration is contained by subretinal transplantation of immortalized human RPE cells. Transplantation was done at three to four weeks of age prior to significant photoreceptor loss and recipients were kept on cyclosporin. At six months of age, sensitivity maps and multi-unit response properties were obtained across the visual field by recording at 76 equidistant sites encompassing the whole superior colliculus.A significant degree of functional protection, both in terms of area of responsive retina and response characteristics was observed following RPE transplantation. At best, the sensitivity, latency of onset, and response rise time were all maintained within normal ranges and this was achieved with no more than half of the normal complement of photoreceptors. Although partial, the degree of anatomical preservation (both in terms of outer nuclear layer thickness and area of rescue) correlated well with the level of preserved visual sensitivities. Sham injections also resulted in rescue, though the area of preservation was strictly confined to the needle injury site and the response properties were significantly worse than with RPE injections. This study shows that central physiological responsiveness and correlated retinal morphology can be preserved in an animal model of retinal disease by implantation of an immortalized cell line. The use of retinal sensitivity measurements provides a background for assessing higher visual functions in these animals and a direct comparison for human perimetry measures.

Expression of p75(NTR) in photoreceptor cells of dystrophic rat retinas

Although a gene mutation in the Royal College of Surgeons (RCS) dystrophic rat results in defective phagocytosis and in accumulation of debris in the subretinal space, the molecular mechanisms leading to photoreceptor cell death remain unclear. In this study, the expression of p75(NTR), the low-affinity neurotrophin receptor incriminated in the apoptosis of developing neurons, was investigated at various stages of retinal degeneration in dystrophic rats using immunohistochemistry, in situ reverse transcription polymerase chain reaction (RT-PCR), Western blot, and relative RT-PCR. In normal adult retinas, p75(NTR) immunolabeling was observed mainly in the outer limiting membrane, with punctate labeling in the inner nuclear and ganglion cell layers. In 18- to 30-day-old dystrophic retinas, the immunostaining appeared to increase especially in the photoreceptor outer and inner segments. Dense staining was also observed in the retinal pigment epithelium (RPE) and choroid. In 60-day-old dystrophic rat retinas, the density of immunolabeling for p75(NTR) increased dramatically in the remaining inner retina, especially in the inner nuclear, inner plexiform, and ganglion cell layers. Post-embedding immunogold labeling of normal retinas verified the distribution of p75(NTR) in photoreceptor cells within the inner segments, cell bodies, and outer segments. The apparent increased intensity in p75(NTR) immunostaining in dystrophic retinas was verified by Western blots and densitometric analyses. In situ RT-PCR and relative RT-PCR further established increased synthesis of p75(NTR) in dystrophic retinas. The increased levels of p75(NTR) in the RPE and photoreceptor cells, the initial sites of injury, during retinal degeneration in dystrophic rats strongly suggest that altered expression of p75(NTR) may be directly involved in photoreceptor death.

Immunocytochemical characterisation of proteins secreted by retinal pigment epithelium in retinas of normal and Royal College of Surgeons dystrophic rats

In a previous study, an antigen consisting of proteins secreted by retinal pigment epithelial (RPE) cells was injected into a sheep and the specificity of the resulting antiserum was shown by Western blotting and its effects on retinal development were determined in vitro and in vivo. In the present study, the distribution of these secreted proteins was determined by light microscopy immunocytochemistry in cultured neonatal rat RPE cells and retinas of normal and Royal College of Surgeons (RCS) dystrophic rats and cerebrum of normal adult rats. Immunolabelling for these RPE-secreted proteins was detected in cytoplasmic vesicles surrounding nuclei and within processes of cultured normal and transformed rat RPE. In retinas of late postnatal and adult rats, dense immunostaining was found in the cytoplasm of RPE cells and ganglion cell bodies. In addition to RPE and ganglion cells, scattered photoreceptors within the thin outer nuclear layer and small structures within the debris zone were also densely immunoreactive in retinas of 2-mo-old RCS dystrophic rats. The numbers of immunostained ganglion cells appeared to decrease in retinas of older RCS rats, although the immunoreactivity within the RPE appeared to increase in density. No other neuron within the retina, i.e. bipolar, amacrine or horizontal, was immunoreactive for RPE-secreted proteins. In the cerebral cortex of adult rats, immunoreactivity for RPE-secreted proteins was primarily detected within large perikarya of pyramidal neurons and smaller granule neurons. In conclusion, we report an immunocytochemical analysis of an antiserum raised against secreted proteins of rat RPE. This antiserum recognised proteins within secretory-like vesicles of cultured neonatal normal and transformed rat RPE and showed a specificity for RPE and ganglion cells in normal rat retinas, that appeared to be developmentally regulated, and neuron perikarya in adult rat cerebrum.

Calpain activity in the retinas of normal and RCS rats

Calpains are calcium-activated proteinases which have been implicated in tissue differentiation and degeneration. The aims of the present study were: (1) to determine the relationship between postnatal age and calpain activity in the rat retina; (2) to test if calpain activity was aberrant in the RCS retina at different postnatal ages. Calpain activity was measured by a standard in vitro assay in fractions of retinas of rats, ranging in postnatal age of 2 to 42 days. Most retinal calpain activity was in the cytosolic fraction. Specific calpain activity declined with age. In the Long Evans rat, it was 8-fold higher on postnatal day 2 than on postnatal day 42. Comparison between RCS rats and their congenic controls showed that calpain activity was lower in the retinas of neonatal RCS rats. Specific calpain activity in RCS rat retinas was 46% lower on postnatal day 2 and 22% lower on postnatal day 3. It is concluded that during postnatal development of the retina, marked changes occurred in calpain activity. In addition, calpain activity is abnormal in the retina of the neonatal RCS rat--well before the onset of any morphological deterioration and preceding any other previously detected abnormality. Aberrant calpain activity appears to be a manifestation of very early events in processes that lead to retinal degeneration in the RCS rat.

Mullerian glia in dystrophic rodent retinas: an immunocytochemical analysis

Mullerian glia in retinas of Royal College of Surgeons (RCS) dystrophic rats and retinal degeneration (rd/rd) mice undergo biochemical and morphological alterations concomitant with photoreceptor loss. To follow the fate of Mullerian glia in these degenerating retinas, two Muller cell-specific markers, carbonic anhydrase-C (CAC) and cellular retinaldehyde-binding protein (CRALBP), were examined by light microscopic immunocytochemistry. In retinas of 1- to 12-month-old RCS dystrophic rats, cell bodies in the inner nuclear layer and radial processes were immunostained for CAC, but appeared to diminish with age. In addition, a material in the region of the retinal pigment epithelium (RPE), representing expansion of Muller cell processes into the subretinal space, was immunolabelled for CAC in retinas of 2-month-old and older RCS rats. The CAC-immunoreactive Muller cells seen in retinas of 12-month-old RCS rats were disorganized, as significant photoreceptor degeneration had occurred by this time. In retinas of 6-week-old RCS rats, Muller cells and their processes were immunolabelled for CRALBP, which spanned from the nerve fiber layer (NFL) through the outer nuclear layer. The density of this immunostaining increased, especially in the subretinal space, with advancing age in RCS rats, seen most prominently in retinas of 9-month-old RCS rats and decreased by 12 months. In retinas of rd/rd mice beginning by day 14, minimal CAC- and CRALBP-immunoreactive material was observed in the subretinal space. By 6 weeks, when a majority of the photoreceptors had degenerated, the CAC-staining pattern appeared significantly reduced and patchy. This study showed that Muller cells in degenerating retinas of RCS rats and rd/rd mice ultimately exhibited decreased immunolabelling for CAC and CRALBP at the more advanced stages of retinopathy, which coincided with the loss of photoreceptors. This is in contrast to the progressive increase in glial fibrillary acid protein (GFAP), an intermediate filament protein, throughout the retinal dystrophy in both animal models.

Synaptic and photoreceptor components in retinal pigment epithelial cell transplanted retinas of Royal College of Surgeons dystrophic rats

Plexiform layer synaptic and photoreceptor cell components were investigated in retinas of Royal College of Surgeons (RCS) dystrophic rats transplanted with normal retinal pigment epithelial (RPE) cells by immunocytochemistry using previously characterized monoclonal antibodies. In retinas of normal adult rats and RPE-cell transplanted retinas of 4 month-old RCS rats, HNK-1, a marker for a carbohydrate of the neural cell adhesion molecule (N-CAM), was detected immunocytochemically in the inner and outer plexiform layers and ganglion cell bodies and their axons. HNK-1 was also detected in the inner plexiform layer of nontreated retinas of 4 month-old RCS rats, but was reduced to scattered patches in the outer plexiform layer. In addition, immunoreactivity for the SVP-38 antibody recognizing synaptophysin was found in both plexiform layers of normal adult rat retinas and RPE-transplanted retinas of 4 month-old RCS rats. Furthermore, photoreceptor cell bodies and their inner and outer segments were immunostained for the opsin monoclonal antibody RET-P1 in retinas of normal adult rats and RPE-cell transplanted retinas of 4 month-old RCS rats. However, in nontreated retinas of 4-month-old RCS rats, only immunostained debris material was detected. These results strongly suggest that normal RPE transplants not only rescue photoreceptor cells in RCS rats, but also maintain an essential functional capacity, in this case, synaptic components in the plexiform layers.

Effects of RPE-cell factors secreted from permselective fibers on retinal cells in vitro

This study was undertaken to determine if retinal pigment epithelial (RPE) cells encased in permselective hollow fibers survive in a tissue culture environment and secrete a diffusible trophic factor(s) that may affect retinal cell survival in vitro. In this study, RPE cells were isolated from 6- to 8-day-old Long-Evans rats, then loaded into hollow fibers. The RPE-cell fibers were then cultured for at least one week in serum-containing medium. These RPE-cell fibers were subsequently co-cultured with cells isolated from retinas of day 2 Long-Evans rats in a defined medium. For at least 6 days in culture, opsin-positive cells were observed on the surface of larger flat cells. Over 80% of the small, round cells immunostained for opsin. However, opsin-immunostained cells were seldom seen in cultures with control fibers, that lacked RPE cells. In addition, conditioned medium collected from either the RPE-cell fibers or cultured RPE cells affected survival of opsin-positive retinal cells in culture in a manner similar to that of the RPE-cell fibers. Furthermore, selected growth factors such as epidermal, nerve and fibroblast growth factors, were unable to sustain retinal cell survival and affect morphological development as seen in RPE-CM supplemented cultures. In vivo companion developmental studies demonstrated that few opsin-positive cell bodies were observed in retinas of day 2 Long-Evans rats, the age corresponding to the stage of retinal cell isolation. In retinas of day 5 Long-Evans rats, the age corresponding to the end point of the in vitro assay, a dramatic increase in the number of opsin-immunostained cell bodies was noted, which corresponds to the developmental sequence also seen in culture. Light and electron microscopic examination revealed that the RPE cells cultured in the hollow tubes maintained an RPE-like structure for several months, in that these cells contained melanosomes and extended microvilli from their apical border and formed junctional complexes with adjacent cells. Results of this study confirm our earlier findings that RPE cells secrete an apparent novel factor(s) that affects retinal cell survival in vitro and, most significantly, the described encapsulation/secretion mechanism may provide a convenient method to deliver such factors for further in vivo testing of this phenomenon.

Transplantation to the diseased and damaged retina

Retinas of Royal College of Surgeons (RCS) dystrophic rats undergo a dramatic loss of photoreceptor cells as a result of defective retinal pigment epithelial (RPE) cells. These retinas are therefore a valuable model in the investigation of the role of the RPE on photoreceptor-cell survival and development. Also, rat retinas damaged by excessive light serve as a suitable environment to study survival of transplanted photoreceptor cells. Even though photoreceptor cells are lost in these retinas, a normal inner retinal structure is retained. Both models have recently been used in successful RPE-cell and/or photoreceptor-cell transplantation studies designed to replace defective or lost cells due to retinal disease or damage. These new approaches in the field of retinal transplantation offer unique and novel opportunities for the development of possible therapeutic strategies in human eye disease, and for improving our understanding of the normal relationships between retinal cells.

Ciliary body degeneration in the Royal College of Surgeons dystrophic rat

We have studied the pathological changes of the ciliary body in Royal College of Surgeons (RCS) rats with an inherited retinal degeneration. Morphometric analyses were performed on sectioned ciliary bodies by a computerized morphometry system. Age-matched non-pigmented Sprague-Dawley (SD) rats were used as the control animals. The ciliary body of 26-day-old RCS dystrophic rats showed normal structure. However, the length and height of the pars plicata of the ciliary body became shorter and the area became smaller with increased age. Significant decreases in the values of these three parameters were observed between 26-day-old and 3-month-old RCS dystrophic rats. These parameters also showed significant differences when values of 3-month-old RCS dystrophic rats were compared to those of 3-month-old control SD rats. The same trends were observed in the ciliary body measurements in RCS dystrophic rats up to 1 year of age. Scanning electron microscopic examination demonstrated the progressive thinning of the pars plicata of the ciliary body with age in the RCS dystrophic rats. The total volume of the ciliary process of 6-month-old RCS dystrophic rats appeared to be one-half that of 26-day-old RCS dystrophic rats. Transmission electron microscopy revealed progressive cellular degenerative changes in the non-pigmented and pigmented ciliary epithelium of the RCS dystrophic rats. It was apparent that the pigmented ciliary epithelium had more severe degenerative changes than the non-pigmented ciliary epithelium. Immunostaining for Na+ + K+ ATPase of the ciliary epithelium was found to be less in the RCS dystrophic rats than in age-matched controls. This result suggests a possible dysfunction of ion transport in the ciliary body of the RCS dystrophic rats, which may account for their increased incidence of cataract formation. Although the mechanisms for the ciliary body degeneration in RCS dystrophic rats remain speculative, these findings add a new area of interest in this model of inherited retinal dystrophy.

Development and distribution of opsin-like immunoreactivity in the dystrophic retinas of rdle mutant mice

The opsin-like immunoreactivity in the retinas of C57BL/6J rdle mutant mice has been studied by light-microscopic immunocytochemistry. Positively labeled cells were found in the normal heterozygous and homozygous mutant mouse outer nuclear layer (ONL) as early as postnatal day 3 (PN3). Beginning at PN10, in the retinas of the homozygous mutant mice, labeled photoreceptor cells rapidly decreased in number and disappeared after PN42. In the decreased ONL, remaining opsin-positive cells were labeled at higher density than those of controls. The retinal pigment epithelium was also moderately labeled during the loss of opsin-positive photoreceptor cells. In addition, sparse opsin-immunoreactive cells were demonstrated in the inner nuclear layer (INL) in the retinas of both the mutant and non-dystrophic mice as early as PN10 and are presumed to be ectopic photoreceptor cells. However, these displaced photoreceptor cells disappeared by PN28 in mutants along the same time course as those in the ONL but were still present in the PN28 retina of controls and seemed to be more abundant at later adult ages. There was no difference in the developmental regulation of opsin in the heterozygous and normal controls.