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

Long-Term Efficacy of GMP Grade Xeno-Free hESC-Derived RPE Cells Following Transplantation

Purpose

Retinal pigment epithelium (RPE) dysfunction underlies the retinal degenerative process in age-related macular degeneration (AMD), and thus RPE cell replacement provides an optimal treatment target. We characterized longitudinally the efficacy of RPE cells derived under xeno-free conditions from clinical and xeno-free grade human embryonic stem cells (OpRegen) following transplantation into the subretinal space of Royal College of Surgeons (RCS) rats.

Methods

Postnatal (P) day 20 to 25 RCS rats (n = 242) received a single subretinal injection of 25,000 (low)-, 100,000 (mid)-, or 200,000 (high)-dose xeno-free RPE cells. BSS+ (balanced salt solution) (vehicle) and unoperated eyes served as controls. Optomotor tracking (OKT) behavior was used to quantify functional efficacy. Histology and immunohistochemistry were used to evaluate photoreceptor rescue and transplanted cell survival at 60, 100, 150, and 200 days of age.

Results

OKT was rescued in a dose-dependent manner. Outer nuclear layer (ONL) was significantly thicker in cell-treated eyes than controls up to P150. Transplanted RPE cells were identified in both the subretinal space and integrated into the host RPE monolayer in animals of all age groups, and often contained internalized photoreceptor outer segments. No pathology was observed.

Conclusions

OpRegen RPE cells survived, rescued visual function, preserved rod and cone photoreceptors long-term in the RCS rat. Thus, these data support the use of OpRegen RPE cells for the treatment of human RPE cell disorders including AMD.

Translational Relevance

Our novel xeno-free RPE cells minimize concerns of animal derived contaminants while providing a promising prospective therapy to the diseased retina.

RPE transplantation and its role in retinal disease

Retinal pigment epithelial (RPE) transplantation aims to restore the subretinal anatomy and re-establish the critical interaction between the RPE and the photoreceptor, which is fundamental to sight. The field has developed over the past 20 years with advances coming from a large body of animal work and more recently a considerable number of human trials. Enormous progress has been made with the potential for at least partial restoration of visual function in both animal and human clinical work. Diseases that have been treated with RPE transplantation demonstrating partial reversal of vision loss include primary RPE dystrophies such as the merTK dystrophy in the Royal College of Surgeons (RCS) rat and in humans, photoreceptor dystrophies as well as complex retinal diseases such as atrophic and neovascular age-related macular degeneration (AMD). Unfortunately, in the human trials the visual recovery has been limited at best and full visual recovery has not been demonstrated. Autologous full-thickness transplants have been used most commonly and effectively in human disease but the search for a cell source to replace autologous RPE such as embryonic stem cells, marrow-derived stem cells, umbilical cord-derived cells as well as immortalised cell lines continues. The combination of cell transplantation with other modalities of treatment such as gene transfer remains an exciting future prospect. RPE transplantation has already been shown to be capable of restoring the subretinal anatomy and improving photoreceptor function in a variety of retinal diseases. In the near future, refinements of current techniques are likely to allow RPE transplantation to enter the mainstream of retinal therapy at a time when the treatment of previously blinding retinal diseases is finally becoming a reality.

Short-term study of retinal pigment epithelium sheet transplants onto Bruch's membrane

The purpose of this study is to investigate the survival and behaviour of retinal pigment epithelium sheets transplanted onto hydraulically debrided Bruch's membrane. Uncultured retinal pigment epithelium sheets obtained from male cats and sandwiched between two gelatin sheets were transplanted onto the tapetal area of female cats after native retinal pigment epithelium was debrided. For controls, the gelatin carrier was transplanted after debridement. Each transplant or control specimen was analyzed histologically and immunohistochemically. Transplanted male retinal pigment epithelial cells were identified by in situ labelling of the cat Y chromosome. Over half of the transplants appeared as retinal pigment epithelium multilayers in the subretinal space. Retinal pigment epithelium pigment dispersion into the subretinal space was seen in most of the transplants, and retinal pigment epithelium pigment infiltration into the neural retina was seen in all 7-day survival transplants. A few condensed darkly stained retinal pigment epithelium nuclei and Terminal Transferase dUTP Nick End Labelling-positive retinal pigment epithelium cells were observed in all transplants. Cellular retinaldehyde-binding protein was present up to day-7 in most transplanted RPE cells. In both transplant and control specimens, the antibody against the Ki-67 nuclear antigen labelled a few retinal pigment epithelium cells at day-3. Terminal Transferase dUTP Nick End Labelling-positive outer nuclear layer nuclei were most frequently observed at day-1 but were much less frequent at day-3 in both transplants and controls. The survival and effectiveness of retinal pigment epithelium sheet transplants appeared similar to the retinal pigment epithelium microaggregates transplants conducted previously in this model.

Differential expression of syntaxin-1 and synaptophysin in the developing and adult human retina

Synaptophysin and syntaxin-1 are membrane proteins that associate with synaptic vesicles and presynaptic active zones at nerve endings, respectively. The former is known to be a good marker of synaptogenesis; this aspect, however, is not clear with syntaxin-1. In this study, the expression of both proteins was examined in the developing human retina and compared with their distribution in postnatal to adult retinas, by immunohistochemistry. In the inner plexiform layer, both were expressed simultaneously at 11-12 weeks of gestation, when synaptogenesis reportedly begins in the central retina. In the outer plexiform layer, however, the immunoreactivities were prominent by 16 weeks of gestation. Their expression in both plexiform layers followed a centre-to-periphery gradient. The immunoreactivities for both proteins were found in the immature photoreceptor, amacrine and ganglion cells; however, synaptophysin was differentially localized in bipolar cells and their axons, and syntaxin was present in some horizontal cells. In postnatal-to-adult retinas, synaptophysin immunoreactivity was prominent in photo-receptor terminals lying in the outer plexiform layer; on the contrary, syntaxin-1 was present in a thin immunoreactive band in this layer. In the inner plexiform layer, however, both were homogeneously distributed. Our study suggests that (i) syntaxin-1 appears in parallel with synapse formation; (ii) synaptogenesis in the human retina might follow a centre-to-periphery gradient; (iii) syntaxin-1 is likely to be absent from ribbon synapses of the outer plexiform layer, but may occur at presynaptic terminals of photoreceptor and horizontal cells, as is apparent from its localization in these cells, which is hitherto unreported for any vertebrate retina.

The HNK-1 carbohydrate epitope in the eye: basic science and functional implications

The HNK-1 carbohydrate epitope is part of many cell membrane and extracellular matrix molecules. It has been implicated in cell to cell and cell to extracellular matrix adhesion, and antibodies to the HNK-1 epitope are emerging as a versatile tool in eye research. They have been used to identify a novel cell type in the human eye, the subepithelial matrix cells that reside in the inner connective tissue layer (ICTL) of the ciliary body. Although these cells resemble fibroblasts in ultrastructure, they form a distinct cell population that differs in its antigenic profile from fibroblasts of other tissues. These cells are associated with the elastic fiber system of the ICTL. Other structures in the human eye that harbor the HNK-1 epitope in a nonrandom pattern are the ciliary and iris epithelia, the zonular lamella, the lens capsule, the retina, glial cells of the optic and ciliary nerves, and scleral fibroblasts. The HNK-1 epitope in the eye appears early during embryonic development and is phylogenetically conserved, but many interspecies differences exist in its distribution. The role of the HNK-1 epitope may be to structurally stabilize the ciliary body and the retina, and to participate in zonular attachments. The HNK-1 epitope has been linked with many common eye diseases. The subepithelial matrix cells seem to be susceptible to undergo irreversible damage as a result of glaucoma, thermal injury, and tissue compression. This epitope has proved to be useful in identifying intraocular deposits of exfoliation syndrome. It can explain the adhesiveness of exfoliation material. Intraocular exfoliation material differs in HNK-1 immunoreactivity from the extraocular fibrillopathy of exfoliation syndrome and its presence in fellow eyes also argues against the concept of unilateral exfoliation syndrome. The HNK-1 epitope is found in the extracellular matrix of secondary cataract and anterior subcapsular cataract, and it may contribute to their pathogenesis. Finally, the HNK-1 epitope can be used to trace neuroepithelial derivatives of the optic vesicle in developmental anomalies and in tumors of the eye. Eventual identification of molecules that bear the HNK-1 epitope in the eye will likely shed light on many aspects of ocular physiology and pathobiology

Differentiation and transdifferentiation of the retinal pigment epithelium

The retinal pigment epithelium (RPE) lies between the retina and the choroid of the eye and plays a vital role in ocular metabolism. The RPE develops from the same sheet of neuroepithelium as the retina and the two derivatives become distinguished by different expression patterns of a number of transcription factors during embryonic development. As the RPE layer differentiates it expresses a set of unique molecules, many of which are restricted to certain regions of the cell. PRE cells undergo both a loss of polarity and a loss of expression of many of these cell type-specific molecules when placed in monolayer culture. The RPE of many species, including mammals, can be induced to transdifferentiate by growth factors such as basic fibroblast growth factor. Under the influence of such factors the RPE is triggered to alter expression of a wide array of molecules and to take on a retinal epithelium fate, from which differentiated retinal cell types including rod photoreceptors can be produced.

Photoreceptor repair in response to RPE transplants in RCS rats: outer segment regeneration

PURPOSE

We have previously shown that transplants of normal rat neonatal RPE cells rescued photoreceptor cells in retinas of Royal College of Surgeons (RCS) dystrophic rats for up to one year. In this study, we investigated the photoreceptor rescue effects in RCS rats within the first three weeks following transplantation in an attempt to determine if RPE transplants initiate repair mechanisms, specifically, outer segment (OS) regeneration.

METHODS

Freshly isolated RPE cells from neonatal pigmented Long Evans rats were transplanted into the subretinal space of 22-23 day-old RCS rats using a transscleral approach. For controls, vehicle was similarly injected.

RESULTS

When analyzed at 10 days post-transplantation, long inner segments were observed with short buds of outer segment growth in the area of the RPE-cell transplants. The outer segments were of insufficient length to be measured at 10 days, but by 14 and 21 days, OS were 2.02 +/- 0.32 microns and 18.80 +/- 2.78 microns, respectively. In vehicle-injected retinas from 10 to 21 days postsurgery, outer segments were not observed and the inner segments were three-fold shorter than in RPE-transplanted retinas. At 10 days post-transplantation, most RPE cells were seen in the subretinal space, but a few had attached to Bruch's membrane; however, by 21 days, many of the transplanted RPE cells had attached to Bruch's membrane, although a few were found free in the subretinal space.

CONCLUSIONS

This study has shown that transplants of normal rat neonatal RPE cells have the capacity to support not only photoreceptor cell survival but also initiate early repair mechanisms as exhibited by outer segment regeneration in RCS retinas. These results also conclusively show the important role that the RPE plays in outer segment growth and maturation.

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.