Photoreceptor transplantation: anatomic, electrophysiologic, and behavioral evidence for the functional reconstruction of retinas lacking photoreceptors

Addition of Chk1 inhibitor and BMP4 cooperatively promotes retinal tissue formation in self-organizing human pluripotent stem cell differentiation culture

Background

The BMP signaling pathway plays a key role in growth, differentiation and patterning during neural development. Recent work on the generation of a self-organization of three-dimensional retinal organoid (3D-retina) from human pluripotent stem cells (hPSCs) revealed that addition of recombinant human BMP4 (rhBMP4) promotes retinal differentiation in the early neural differentiation stage. For clinical application, efficient differentiation from hPSCs to retinal cells with minimal numbers of off-target non-retinal cells is desirable. We therefore aimed to further improve an efficient retinal differentiation method for future up-scaling of cell production.

Methods

hPSCs were differentiated into 3D-retina using a modified SFEBq method. The effect of rhBMP4 with or without Checkpoint kinase 1 (Chk1) inhibitor (PD407824), a modulator of BMP signaling pathway, at day 3 was compared by characterizing the differentiating 3D-retina by the use of the hPSCs and immunohistochemical analysis.

Results

The Chk1 inhibitor treatment promoted retinal differentiation from hPSCs, in combination with low-concentration rhBMP4. Addition of a Chk1 inhibitor generated a unique type of organoid with neural retina (NR) encapsulated in retinal pigment epithelium (RPE), possibly by promoting phosphorylation of SMAD1/5/9 in the cells inside the early aggregates. We confirmed that the Chk1-inhibitor-treated hPSC-3D-retina differentiated into rod and cone photoreceptor precursors and other types of retinal neurons, in long-term culture.

Conclusions

In this study, we found that combined use of rhBMP4 and a Chk1 inhibitor cooperatively promoted retinal differentiation from hPSCs. Our new retinal differentiation method is a promising option for the stable supply and up-scaling of production of 3D-retina for future cell therapy.

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Chk1 inhibitor cooperates with low-concentration rhBMP4 to promote hPSC-retinal differentiation.

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Combined rhBMP4 and Chk1 inhibitor treatment generated NR-RPE organoids with NR tissue encapsulated in RPE.

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In long-term culture, the Chk1 inhibitor-treated 3D-retina produces rod and cone photoreceptor precursors and other types of retinal neurons.

Cell therapy with hiPSC-derived RPE cells and RPCs prevents visual function loss in a rat model of retinal degeneration

Photoreceptor loss is the principal cause of blindness in retinal degenerative diseases (RDDs). Whereas some therapies exist for early stages of RDDs, no effective treatment is currently available for later stages, and once photoreceptors are lost, the only option to rescue vision is cell transplantation. With the use of the Royal College of Surgeons (RCS) rat model of retinal degeneration, we sought to determine whether combined transplantation of human-induced pluripotent stem cell (hiPSC)-derived retinal precursor cells (RPCs) and retinal pigment epithelial (RPE) cells was superior to RPE or RPC transplantation alone in preserving retinal from degeneration. hiPSC-derived RPCs and RPE cells expressing (GFP) were transplanted into the subretinal space of rats. In vivo monitoring showed that grafted cells survived 12 weeks in the subretinal space, and rats treated with RPE + RPC therapy exhibited better conservation of the outer nuclear layer (ONL) and visual response than RPE-treated or RPC-treated rats. Transplanted RPE cells integrated in the host RPE layer, whereas RPC mostly remained in the subretinal space, although a limited number of cells integrated in the ONL. In conclusion, the combined transplantation of hiPSC-derived RPE and RPCs is a potentially superior therapeutic approach to protect retina from degeneration in RDDs.

Transplantation of Mature Photoreceptors in Rodents With Retinal Degeneration

Purpose

To demonstrate survival and integration of mature photoreceptors transplanted with the retinal pigment epithelium (RPE).

Methods

Full-thickness retina with attached RPE was harvested from healthy adult rats. Grafts were implanted into two rat models of retinal degeneration, Royal College of Surgeons (RCS) and S334ter-3. Survival of the host and transplanted retina was monitored using optical coherence tomography (OCT) for up to 6 months. The retinal structure and synaptogenesis between the host and transplant was assessed by histology and immunohistochemistry.

Results

OCT and histology demonstrated a well-preserved photoreceptor layer with inner and outer segments, while the inner retinal layers of the transplant largely disappeared. Grafts, including RPE, survived better than without and the transplanted RPE appeared as a monolayer integrated with the native one. Synaptogenesis was observed through sprouting of new dendrites from the host bipolar cells and synaptic connections forming with cells of the transplant. However, in many samples, a glial fibrillary acidic protein–positive membrane separated the host retina and the graft.

Conclusions

Presence of RPE in the graft improved the survival of transplanted photoreceptors. Functional integration between the transplant and the host retina is likely to be further enhanced if formation of a glial seal could be prevented. Transplantation of the mature photoreceptors with RPE may be a practical approach to restoration of sight in retinal degeneration.

Translational Relevance

This approach to restoration of sight in patients with photoreceptor degeneration can be rapidly advanced to clinical testing. In patients with central scotoma, autologous transplantation of the peripheral retina can be an option.

Surgical Approaches for Cell Therapeutics Delivery to the Retinal Pigment Epithelium and Retina

Developing successful surgical strategies to deliver cell therapeutics to the back of the eye is an essential pillar to success for stem cell-based applications in blinding retinal diseases. Within this chapter, we have attempted to gather all key considerations during preclinical animal trials.Guidance is provided for choices on animal models, options for immunosuppression, as well as anesthesia. Subsequently we cover surgical strategies for RPE graft delivery, both as suspension as well as in monolayers in small rodents, rabbits, pigs, and nonhuman primate. A detailed account is given in particular on animal variations in vitrectomy and subretinal surgery, which requires a considerable learning curve, when transiting from human to animal. In turn, however, many essential subretinal implantation techniques in large-eyed animals are directly transferrable to human clinical trial protocols.A dedicated subchapter on photoreceptor replacement provides insights on preparation of suspension as well as sheet grafts, to subsequently outline the basics of subretinal delivery via both the transscleral and transvitreal route. In closing, a future outlook on vision restoration through retinal cell-based therapeutics is presented.

Sheets of human retinal progenitor transplants improve vision in rats with severe retinal degeneration

Loss of photoreceptors and other retinal cells is a common endpoint in retinal degenerate (RD) diseases that cause blindness. Retinal transplantation is a potential therapy to replace damaged retinal cells and improve vision. In this study, we examined the development of human fetal retinal sheets with or without their retinal pigment epithelium (RPE) transplanted to immunodeficient retinal degenerate rho S334ter-3 rats. Sheets were dissected from fetal human eyes (11-15.7 weeks gestation) and then transplanted to the subretinal space of 24-31 d old RD nude rats. Every month post surgery, eyes were imaged by high-resolution spectral-domain optical coherence tomography (SD-OCT). SD-OCT showed that transplants were placed into the subretinal space and developed laminated areas or rosettes, with clear development of plexiform layers first seen in OCT at 3 months post surgery. Several months later, as could be expected by the much slower development of human cells compared to rat cells, transplant photoreceptors developed inner and later outer segments. Retinal sections were analyzed by immunohistochemistry for human and retinal markers and confirmed the formation of several retinal subtypes within the retinal layers. Transplant cells extended processes and a lot of the cells could also be seen migrating into the host retina. At 5.8-8.6 months post surgery, selected rats were exposed to light flashes and recorded for visual responses in superior colliculus, (visual center in midbrain). Four of seven rats with transplants showed responses to flashes of light in a limited area of superior colliculus. No response with the same dim light intensity was found in age-matched RD controls (non-surgery or sham surgery). In summary, our data showed that human fetal retinal sheets transplanted to the severely disturbed subretinal space of RD nude rats develop mature photoreceptors and other retinal cells, integrate with the host and induce vision improvement.

Regenerative medicine in the retina: from stem cells to cell replacement therapy

Following the fast pace of the growing field of stem cell research, retinal cell replacement is finally emerging as a feasible mean to be explored for clinical application. Although neuroprotective treatments are able to slow the progression of retinal degeneration caused by diseases such as age-related macular degeneration and glaucoma, they are insufficient to fully halt disease progression and unable to recover previously lost vision. Comprehensive, technological and intellectual advances over the past years, including the in vitro differentiation of retinal cells at manufacturing scale from embryonic stem (ES) cell and induced pluripotent stem (iPS) cell cultures, progress within the area of retinal disease modeling, and the first clinical trials have started to shape the way towards addressing this treatment gap and translating retinal cell replacement to the clinic. Here, summarize the most recent advances within retinal cell replacement from both a scientific and clinical perspective, and discuss the remaining challenges towards the delivery of the first retinal cell products.

Assessment of Safety and Functional Efficacy of Stem Cell-Based Therapeutic Approaches Using Retinal Degenerative Animal Models

Dysfunction and death of retinal pigment epithelium (RPE) and or photoreceptors can lead to irreversible vision loss. The eye represents an ideal microenvironment for stem cell-based therapy. It is considered an “immune privileged” site, and the number of cells needed for therapy is relatively low for the area of focused vision (macula). Further, surgical placement of stem cell-derived grafts (RPE, retinal progenitors, and photoreceptor precursors) into the vitreous cavity or subretinal space has been well established. For preclinical tests, assessments of stem cell-derived graft survival and functionality are conducted in animal models by various noninvasive approaches and imaging modalities. In vivo experiments conducted in animal models based on replacing photoreceptors and/or RPE cells have shown survival and functionality of the transplanted cells, rescue of the host retina, and improvement of visual function. Based on the positive results obtained from these animal experiments, human clinical trials are being initiated. Despite such progress in stem cell research, ethical, regulatory, safety, and technical difficulties still remain a challenge for the transformation of this technique into a standard clinical approach. In this review, the current status of preclinical safety and efficacy studies for retinal cell replacement therapies conducted in animal models will be discussed.

Chondroitinase ABC Treatment Enhances Synaptogenesis between Transplant and Host Neurons in Model of Retinal Degeneration

Although recent studies revealed chondroitinase ABC (ChABC), an enzyme that degrades chondroitin sulfate proteoglycans, promotes CNS regeneration in vivo, the usefulness of its application for transplantation is not clear. We investigated if treatment with ChABC can promote synapse formation between graft and host neurons following retinal transplantation. Dissociated retinal cells were prepared from neonatal Nrl-GFP transgenic mice in which rod photoreceptors and their progenitor cells are labeled with GFP. Each cell suspension with or without ChABC (Nrl/ChABC group and Nrl group, respectively) was injected subretinally into the eyes of mice following chemically induced photoreceptor degeneration. The survival and functional integration of the transplanted photoreceptors were examined by histologically and electrophysio-logically. Up to 4 weeks after transplantation, almost all the grafted GFP+ photoreceptor cells were widely distributed at the outer margin of the host retina where the photoreceptor layer was located originally. In the Nrl/ChABC group, 33.6% of the GFP+ photoreceptors elaborated neurites horizontally or vertically, and 4.6% elaborated neurites toward the retina. These neurites extended over the glial seal at the graft–host interface, and established synaptic contacts with neurons in the host retina as determined by confocal microscopy and three-dimensional analysis. Although 30.7% cells (p = 0.68) elaborated neurites in the Nrl group, only 1.2% cells (p < 0.05) projected neurites towards the host tissue and synaptic contacts were rare. Our results illustrate the potential utility of ChABC for enhancing synaptogenesis between transplanted neurons and host retina.

Neuroprotective Strategy in Retinal Degeneration: Suppressing ER Stress-Induced Cell Death via Inhibition of the mTOR Signal

The retina is a specialized sensory organ, which is essential for light detection and visual formation in the human eye. Inherited retinal degenerations are a heterogeneous group of eye diseases that can eventually cause permanent vision loss. UPR (unfolded protein response) and ER (endoplasmic reticulum) stress plays an important role in the pathological mechanism of retinal degenerative diseases. mTOR (the mammalian target of rapamycin) kinase, as a signaling hub, controls many cellular processes, covering protein synthesis, RNA translation, ER stress, and apoptosis. Here, the hypothesis that inhibition of mTOR signaling suppresses ER stress-induced cell death in retinal degenerative disorders is discussed. This review surveys knowledge of the influence of mTOR signaling on ER stress arising from misfolded proteins and genetic mutations in retinal degenerative diseases and highlights potential neuroprotective strategies for treatment and therapeutic implications.

Neuronal Differentiation of Induced Pluripotent Stem Cells on Surfactant Templated Chitosan Hydrogels

The development of effective tissue engineering materials requires careful consideration of several properties beyond biocompatibility, including permeability and mechanical stiffness. While surfactant templating has been used for over a decade to control the physical properties of photopolymer materials, the potential benefit of this technique with regard to biomaterials has yet to be fully explored. Herein we demonstrate that surfactant templating can be used to tune the water uptake and compressive modulus of photo-cross-linked chitosan hydrogels. Interestingly, templating with quaternary ammonium surfactants also hedges against property fluctuations that occur with changing pH. Further, we demonstrate that, after adequate surfactant removal, these materials are nontoxic, support the attachment of induced pluripotent stem cells and facilitate stem cell differentiation to neuronal phenotypes. These results demonstrate the utility of surfactant templating for optimizing the properties of biomaterials intended for a variety of applications, including retinal regeneration.

Cellular strategies for retinal repair by photoreceptor replacement

Loss of photoreceptors due to retinal degeneration is a major cause of blindness in the developed world. While no effective treatment is currently available, cell replacement therapy, using pluripotent stem cell-derived photoreceptor precursor cells, may be a feasible future treatment. Recent reports have demonstrated rescue of visual function following the transplantation of immature photoreceptors and we have seen major advances in our ability to generate transplantation-competent donor cells from stem cell sources. Moreover, we are beginning to realise the possibilities of using endogenous populations of cells from within the retina itself to mediate retinal repair. Here, we present a review of our current understanding of endogenous repair mechanisms together with recent progress in the use of both ocular and pluripotent stem cells for the treatment of photoreceptor loss. We consider how our understanding of retinal development has underpinned many of the recent major advances in translation and moved us closer to the goal of restoring vision by cellular means.

Mesenchymal stem cells from trabecular meshwork become photoreceptor-like cells on amniotic membrane

Stem cell therapy is a promising approach for treatment of degenerative retinal disorders such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD). In this study, human mesenchymal stem cells (MSCs) were isolated from the trabecular meshwork (TM), the major functional tissue of the anterior chamber angle in the eye, were characterized and differentiated into photoreceptor cells on amniotic membrane (AM). After isolation of trabecular meshwork and culture of the stromal segment of this tissue, fibroblast-like cells (CD105(+), CD90(+), CD44(+), CD166(+) cells) capable of differentiation toward mesenchymal and photoreceptor lineages were obtained. The isolated cells were seeded on amniotic membrane and were treated with induction medium. Immunocytochemistry and quantitative real time RT-PCR (qPCR) were used to detect expression of photoreceptor genes such as rhodopsin, recoverin, CRX, and peripherin; and the bipolar cell marker protein kinase C alpha (PKC-alpha). As a result, immunocytochemistry revealed that the differentiated TMMSCs expressed rhodopsin, CRX and PKC proteins. qPCR showed the expression of rhodopsin (rod like photoreceptor-specific marker), and CRX genes were significantly higher in TMMSCs differentiated on AM than those differentiated on tissue culture polystyrene (TCPS). In conclusion, our findings suggested that a combination of TMMSCs (as a new source) and basement membrane support from AM might be a suitable source of cells for subretinal transplantation in regenerative therapy for retinal disorders such as AMD and RP.

Cell replacement and visual restoration by retinal sheet transplants

Retinal diseases such as age-related macular degeneration (ARMD) and retinitis pigmentosa (RP) affect millions of people. Replacing lost cells with new cells that connect with the still functional part of the host retina might repair a degenerating retina and restore eyesight to an unknown extent. A unique model, subretinal transplantation of freshly dissected sheets of fetal-derived retinal progenitor cells, combined with its retinal pigment epithelium (RPE), has demonstrated successful results in both animals and humans. Most other approaches are restricted to rescue endogenous retinal cells of the recipient in earlier disease stages by a 'nursing' role of the implanted cells and are not aimed at neural retinal cell replacement. Sheet transplants restore lost visual responses in several retinal degeneration models in the superior colliculus (SC) corresponding to the location of the transplant in the retina. They do not simply preserve visual performance - they increase visual responsiveness to light. Restoration of visual responses in the SC can be directly traced to neural cells in the transplant, demonstrating that synaptic connections between transplant and host contribute to the visual improvement. Transplant processes invade the inner plexiform layer of the host retina and form synapses with presumable host cells. In a Phase II trial of RP and ARMD patients, transplants of retina together with its RPE improved visual acuity. In summary, retinal progenitor sheet transplantation provides an excellent model to answer questions about how to repair and restore function of a degenerating retina. Supply of fetal donor tissue will always be limited but the model can set a standard and provide an informative base for optimal cell replacement therapies such as embryonic stem cell (ESC)-derived therapy.

Stem cells as a therapeutic tool for the blind: biology and future prospects

Retinal degeneration due to genetic, diabetic and age-related disease is the most common cause of blindness in the developed world. Blindness occurs through the loss of the light-sensing photoreceptors; to restore vision, it would be necessary to introduce alternative photosensitive components into the eye. The recent development of an electronic prosthesis placed beneath the severely diseased retina has shown that subretinal stimulation may restore some visual function in blind patients. This proves that residual retinal circuits can be reawakened after photoreceptor loss and defines a goal for stem-cell-based therapy to replace photoreceptors. Advances in reprogramming adult cells have shown how it may be possible to generate autologous stem cells for transplantation without the need for an embryo donor. The recent success in culturing a whole optic cup in vitro has shown how large numbers of photoreceptors might be generated from embryonic stem cells. Taken together, these threads of discovery provide the basis for optimism for the development of a stem-cell-based strategy for the treatment of retinal blindness.

A tissue-engineered approach towards retinal repair: scaffolds for cell transplantation to the subretinal space

BACKGROUND

Several mechanisms of retina degeneration result in the deterioration of the outer retina and can lead to blindness. Currently, with the exception of anti-angiogenic treatments for wet age-related macular degeneration, there are no treatments that can restore lost vision. There is evidence that photoreceptors and embryonic retinal tissue, transplanted to the subretinal space, can form new synapses with surviving host neurons. However, these transplants have yet to result in a clinical treatment for retinal degeneration.

METHODS

This article reviews the current literature on the transplantation of scaffolds with retinal and retinal pigmented epithelial (RPE) cells to the subretinal space. We discuss the types of cells and materials that have been investigated for transplantation to the subretinal space, summarize the current findings, and present opportunities for future research and the next generation of scaffolds for retinal repair.

RESULTS

Challenges to cell transplantation include limited survival upon implantation and the formation of abnormal cell architectures in vivo. Scaffolds have been shown to enhance cell survival and direct cell differentiation and organization in a number of models of retinal degeneration.

CONCLUSIONS

The transplantation of cells within a scaffold represents a possible treatment to repair retinal degeneration and restore vision in effected patients. Materials have been developed for the delivery of retinal and RPE cells separately however, the development of a combined tissue-engineered scaffold targeting both cell populations represents a promising direction for retinal repair.

Structure and Function of Embryonic Rat Retinal Sheet Transplants

PURPOSE

To evaluate retinal sheet transplants in S334ter-line-3 retinal degenerate rats by comparing visual responses recorded electrophysiologically with morphology based on light and electron microscopy.

METHODS

S334ter-line-3 retinal degenerate rats (n = 7) received retinal sheet transplants between postnatal days 28 and 31. The donor tissue was derived from transgenic embryonic day 19 (E19) rat retinae expressing human placental alkaline phosphatase (hPAP). Fresh retinal sheets were gently transplanted into the subretinal space of the left eye with the help of a custom-made implantation tool. Selected rats (n = 5) were subjected to electrophysiologic evaluation of visual responses from the superior colliculus about 84-121 days after surgery. Transplanted eyes were processed for light microscopy (LM) and electron microscopy (EM) evaluations.

RESULTS

All the transplanted rats that were evaluated for visual responses in the brain showed responses to very low light stimulation (-3.42 to -2.8 log cd/m(2)) of the eye in a small area of the superior colliculus corresponding with the placement of the transplant in the host retina. Histologic evaluation showed that most of the transplants contained well-laminated areas with correct polarity in the subretinal space. Inside the transplant areas, rosettes of photoreceptors with inner and outer segments were found. In the laminated areas, the outer segments of photoreceptors were facing the host retinal pigment epithelium (RPE). Immunohistochemical evaluation of hPAP donor cells revealed areas with specific staining of the transplants in the subretinal space. Electron microscopic evaluation showed a glial demarcation membrane between the host and the transplant, however, processes originating from the transplant were observed inside the host retina.

CONCLUSIONS

Sheets of E19 rat retina transplanted into the subretinal space of S334ter-line-3 rats survived without immune rejection and continued to show visual function when tested after 3 months. Well-developed photoreceptors and many synapse types were seen within the transplants. hPAP staining showed a certain degree of integration between the host retina and the transplant suggesting that transplanted photoreceptors contributed to the restored light sensitivity.

BDNF-treated retinal progenitor sheets transplanted to degenerate rats: improved restoration of visual function

The aim of this study was to evaluate the functional efficacy of retinal progenitor cell (RPC) containing sheets with BDNF microspheres following subretinal transplantation in a rat model of retinal degeneration. Sheets of E19 RPCs derived from human placental alkaline phosphatase (hPAP) expressing transgenic rats were coated with poly-lactide-co-glycolide (PLGA) microspheres containing brain-derived neurotrophic factor (BDNF) and transplanted into the subretinal space of S334ter line 3 rhodopsin retinal degenerate rats. Controls received transplants without BDNF or BDNF microspheres alone. Visual function was monitored using optokinetic head-tracking behavior. Visually evoked responses to varying light intensities were recorded from the superior colliculus (SC) by electrophysiology at 60days after surgery. Frozen sections were studied by immunohistochemistry for photoreceptor and synaptic markers. Visual head tracking was significantly improved in rats that received BDNF-coated RPC sheets. Relatively more BDNF-treated transplanted rats (80%) compared to non-BDNF transplants (57%) responded to a "low light" intensity of 1cd/m2 in a confined SC area. With bright light, the onset latency of SC responses was restored to a nearly normal level in BDNF-treated transplants. No significant improvement was observed in the BDNF-only and no surgery transgenic control rats. The bipolar synaptic markers mGluR6 and PSD-95 showed normal distribution in transplants and abnormal distribution of the host retina, both with or without BDNF treatment. Red-green cones were significantly reduced in the host retina overlying the transplant in the BDNF-treated group. In summary, BDNF coating improved the functional efficacy of RPC grafts. The mechanism of the BDNF effects--either promoting functional integration between the transplant and the host retina and/or synergistic action with other putative humoral factors released by the RPCs--still needs to be elucidated.

Delay of Photoreceptor Cell Degeneration in rd Mice by Systemically Administered Phenyl-N-tert-butylnitrone

PURPOSE

To study the effect of systemic administration of phenyl-N-tert-butylnitrone (PBN) on the degeneration of photoreceptor cells in rd mice.

METHODS

PBN was injected intraperitoneally into FVB/rd mice on postnatal days (P) 5 to 14 (group A), and P10 to 18 (group B). At days P14, 16, 18, 20 and 27, morphological changes and apoptosis were analyzed by staining with hematoxylin and eosin or DAPI. The effect of PBN on apoptosis was analyzed in retinal pigment epithelial (RPE) cells by the measurement of caspase-3 activity.

RESULTS

In control and group B mice, the outer nuclear layer (ONL) of the retina was composed of 8-10 rows at P12, and rapidly decreased to one row at P18. In group A mice, the ONL was preserved with 5-7 rows at P18, and decreased to one row at P22. PBN inhibited caspase-3 activity in cultured RPE cells.

CONCLUSIONS

PBN delayed, but did not block, the degeneration of photoreceptor cells in rd mice. PBN may exert its inhibitory effect during the early phase of photoreceptor cell degeneration.

Restoration of visual responses following transplantation of intact retinal sheets in rd mice

PURPOSE

To correlate the functional outcomes with histologic findings following transplantation of fetal retinal sheets in rd mice, and to investigate the mechanisms of visual function restoration.

METHODS

Twenty-one postnatal day 31-38 rd/rd (C3H/HeJ) mice were transplanted in one eye with retinal sheets (1.0 x 0.4 mm) obtained from embryonic day (E) 17 enhanced-green-fluorescent protein (eGFP) mice. Five mice underwent sham surgery without insertion of tissue. Four to five weeks after transplantation, visual responses to a light flash were recorded across the superior colliculus (SC) in seven eyes of seven transplanted mice that had clear corneas and lenses, and in all five sham surgery mice. Following the SC recording, the eyes were enucleated and processed for immunohistochemistry and examined using confocal microscopy.

RESULTS

In three out of the seven eyes (43%), positive responses were recorded in the SC in an area topographically corresponding to the placement of the transplant in the host retina. No responses were recorded in the untreated eyes of 5-week-old and 9-week-old rd/rd mice, and in the 9-week-old sham surgery mice. In contrast, visual responses were recorded over the entire SC in normal eyes. The response onset latencies of the 3 transplanted mice with responses were similar to those of normal control mice. The organization of the graft did not appear to correlate as expected with the electrophysiology results, as eyes with well-organized, laminated grafts showed no response whereas the three light-responsive eyes had rosetted or disorganized grafts. All three light-responsive eyes demonstrated much higher levels of recoverin immunoreactivity in the host retina overlying the graft compared with untreated age-matched rd/rd mice.

CONCLUSION

Restoration of the SC visual response does not appear to depend on a well-organized transplant in the rd mouse. Increased recoverin-staining in the host retina in light-responsive animals suggested that host cone rescue was the likely mechanism of vision restoration in this transplant model.

Photoreceptor transplantation in retinitis pigmentosa: short-term follow-up

PURPOSE

To explore the use of adult human photoreceptor transplantation as a treatment for advanced retinitis pigmentosa (RP).

DESIGN

Prospective noncomparative case series.

PARTICIPANTS

Eight patients with advanced RP.

INTERVENTION

Transplantation of adult human cadaver photoreceptor sheets harvested with the excimer laser. No immunosuppression was used postoperatively. Patients were followed for 12 months postoperatively.

MAIN OUTCOME MEASURE

Visual acuity and retinal function measured by psychophysical, electrophysiologic, and clinical testing.

RESULTS

Best-corrected visual acuity (Bailey-Lovie chart), median reading speed, contrast sensitivity, and visual fields for the operated eye were not statistically significantly improved postoperatively. The amplitude and latency of the maculoscope electroretinogram, as well as the log threshold for dark adaptation, did not change between the operated and control (unoperated) eye. There was no detectable homograft reaction on slit-lamp biomicroscopy or fluorescein angiography. The only adverse effect observed was one patient who complained of monocular diplopia after retinal transplantation and subsequent cataract surgery.

CONCLUSIONS

Allogeneic adult human photoreceptor transplantation is feasible in RP but was not associated with rescue of central vision or a delay in visual loss. However, any possible slowing in the rate of retinal degeneration will take many years to determine.

Transplantation of intact sheets of fetal neural retina with its retinal pigment epithelium in retinitis pigmentosa patients

PURPOSE

To show the safety of transplanting sheets of fetal neural retina together with its retinal pigment epithelium (RPE) to patients with retinitis pigmentosa.

DESIGN

Interventional case series.

METHODS

Sheets of fetal neural retina and RPE were transplanted together into the subretinal space near the fovea unilaterally in the eyes of five patients with retinitis pigmentosa who had only light perception in both eyes. The patients were followed for 6 months. The main outcome measures were tissue typing of both donors and recipients, fluorescein angiography, multifocal electroretinogram (mfERG) testing, and clinical examination. No immunosuppressive medications were given.

RESULTS

No evidence of rejection was observed. Up to 6 months there was no evidence of tissue disintegration, retinal edema, or scarring. There was no change in vision both by Snellen acuity and with mfERGs. Growth of the transplant was noted in two of five patients at 6 months vs. 2 weeks. All patients typed were HLA mismatched with donor tissue.

CONCLUSIONS

This study indicates that fetal retina can be transplanted together with its RPE and survive for at least 6 months without evidence of rejection. However, no improvements in vision were observed, possibly due to the severe retinal degeneration of the patients.

Retinal transplantation--advantages of intact fetal sheets

Retinal transplantation aims to prevent blindness and to restore eyesight, i.e., to rescue photoreceptors or to replace damaged photoreceptors with the hope of reestablishing neural circuitry. Retinal donor tissue has been transplanted as dissociated cells or intact sheets. A promising experimental paradigm is the subretinal transplantation of sheets of fetal retina with or without its attached retinal pigment epithelium (RPE) into recipient rats with retinal degeneration. As long as healthy RPE either from the host or from the graft is present, such transplants can develop lamination resembling a normal retina. Different methods have been used to demonstrate transplant/host connectivity. In two different rat retinal degeneration models, visually evoked responses can be demonstrated in an area of the superior colliculus corresponding to the placement of the transplant in the retina. In summary, sheets of fetal retina can morphologically repair an area of a degenerated retina, and there is evidence to suggest that transplants form synaptic connections with the host and restore visual responses in blind rats.

Cell transplantation as a treatment for retinal disease

It has been shown that photoreceptor degeneration can be limited in experimental animals by transplantation of fresh RPE to the subretinal space. There is also evidence that retinal cell transplants can be used to reconstruct retinal circuitry in dystrophic animals. Here we describe and review recent developments that highlight the necessary steps that should be taken prior to embarking on clinical trials in humans.

Neuronal differentiation and morphological integration of hippocampal progenitor cells transplanted to the retina of immature and mature dystrophic rats

Attempts to repopulate the retina with grafted neurons have been unsuccessful, in large part because donor cells prefer not to integrate with those of the host. Here we describe the first use of neural progenitor cells in the diseased adult retina. Adult rat hippocampal progenitor cells were injected into the eyes of rats with a genetic retinal degeneration. After survival times up to 16 weeks, the retinae of 1-, 4-, and 10-week-old recipients exhibited widespread incorporation of green fluorescent protein-expressing (GFP+) donor cells into the host retina. The 18-week-old recipients showed a similar pattern, but with fewer cells. Grafted cells expressed the mature neuronal markers NF-200, MAP-5, and calbindin. GFP+ cells extended numerous neurites into the host plexiform layers and these processes were intimately associated with synaptophysin+ profiles. GFP+ neurites also extended into the host optic nerve head. These results demonstrate the differentiation of substantial numbers of new neurons within the mature dystrophic retina.

Photoreceptor layer reconstruction in a rodent model of retinal degeneration

We have examined the potential of retinal cell transplantation to dystrophic retinal degeneration mice as a way of replacing photoreceptors lost because of an intrinsic genetic defect. Early postnatal retinae which had been gently dissociated survived for at least 6 weeks after transplantation to the subretinal space. Over a significant area of distribution, transplanted cells formed outer segments which lay in close apposition to the host retinal pigment epithelial cell layer. The grafts integrated with the remaining host retina, sufficient at least to mediate a simple light-dark preference. A new synaptic layer was seen at the graft-host interface, which contained substantial numbers of photoreceptor synapses. This and the fact that the behavior could be elicited at low luminance levels argue for functional circuit reconstruction between grafted cells and host retina.

Photoreceptor function of retinal transplants implicated by light-dark shift of S-antigen and rod transducin

The aim was to demonstrate functional properties of transplanted histologically normal photoreceptors. Subretinal intact-sheet transplants of fetal E17-E20 rat retinas to light-damaged albino rat eyes were fixed in light or dark, 2 to 42 weeks after transplantation, and stained immunohistochemically for certain phototransduction proteins. In light adapted transplants, transducin was predominantly found in inner segments of parallel-organized photoreceptors. Transducin shifted to the outer segments with dark-adaptation. S-antigen distribution was opposite to transducin. Rhodopsin distribution did not change. The shift of signal transduction proteins correlated to the light conditions indicates that normal phototransduction processes were established in photoreceptors of transplanted retinal sheets.

The transplantation of human fetal neuroretinal cells in advanced retinitis pigmentosa patients: results of a long-term safety study

The purpose of this study was to determine the long-term safety of transplanting human fetal neuroretinal cells (14 to 18 week gestational age) into a series of patients with advanced retinitis pigmentosa (RP). After obtaining informed consent, both hosts and mothers of donors were screened for transmissible diseases. Pre- and postoperative clinical exams, visual acuity, electroretinograms, and fluorescein angiograms were performed and visual field testing was attempted in each case. Surgically, an anterior approach through pars plana ciliaris was used. A retinotomy was performed in the paramacular area and a two-function cannula was introduced into the subretinal space to deliver a suspension of donor cells. The cell suspension carried approximately 4000 cells/microl; the volume injected did not exceed 150 microl. The patients were examined for periods ranging from 12 to 40 months posttransplantation. To date, no evidence of inflammation, infection, or overt rejection of the graft was noted in the host eye, neither was any change observed in the contralateral, unoperated eye. In conclusion, neuroretinal cells were injected into the subretinal space of 14 patients with advanced RP with no clinical appearance of detrimental effects at the time of surgery or up to 40 months postinjection except in 1 patient who developed retinal detachment. This sets the stage for a phase II clinical trial to determine the possible beneficial effects of this procedure in patients blinded by degenerative retinal disease.

Management of hereditary retinal degenerations: present status and future directions

Research on hereditary retinal degenerations has considerably improved our understanding of these disorders, although much remains to be learned about the exact mechanism involved in the pathogenesis. The advent of recombinant DNA technology will refine diagnostic capabilities, which have so far been based on the manifestations of the disease to localization of the molecular defects. The correlation of the molecular defects with the phenotype of the disease will result in better prognostic counseling for patients. In certain forms of retinitis pigmentosa, such as Refsum disease, gyrate atrophy of the choroid and retina, and abetalipoproteinemia, exact biochemical defects have been identified and specific treatments have been applied with some success. In other forms of retinitis pigmentosa, various investigations have suggested the possibilities of arresting the progress of degeneration by means such as the use of growth factors and controlling apoptosis. Efforts to alter the expression of the mutated gene or to introduce a normal gene into the genome are in their infancy, but results are encouraging. Vitamin A has been tried in patients with retinitis pigmentosa, and the results demonstrate statistically significant beneficial effects of this vitamin, suggesting that the course of the disease can be decelerated to some extent. Another interesting research area with potential for therapeutic application is the replacement of the retinal pigment epithelium or the degenerated neural retina by transplantation of the respective cell types. Clinical trials are being conducted both with retinal pigment epithelium and neuroretinal transplants.

Retinal degeneration and transplantation in the Royal College of Surgeons rat

The Royal College of Surgeons rat provides a valuable animal model for examining the ontogeny of inherited or acquired photoreceptor degeneration and for assessing various treatment paradigms. Here we describe a sequence of events in which photoreceptor loss induces secondary changes that ultimately result in a progressive loss of retinal ganglion cells. The functional consequences of photoreceptor loss are described and compared with those observed in dystrophic animals that received grafts of pigment epithelial cells at an early stage in the degenerative process. The results of this work suggest that transplantation might slow or halt the progress of photoreceptor loss in certain human retinal degenerative conditions, provided suitable safeguards have been put in place.

The retinal ganglion cells that drive the pupilloconstrictor response in rats

It is well established that the pupillary light reflex (PLR) in rats is mediated by a direct retinal projection to the olivary pretectal nucleus (OPN). Although several authors have commented on the specific subpopulation of retinal ganglion cells (RGC) that project to the rat pretectum, much of this evidence is circumstantial, and depends mostly upon electrophysiological data (e.g., conduction velocity). Here, we have used microinjections of Fluoro-Gold into the OPN (pretectum and superior colliculus as controls) to retrogradely label RGCs projecting to this region. The retinae were whole-mounted, viewed under fluorescence, and the regional distribution pattern, laterality of projection, and cell soma sizes determined. The results show OPN injections label a small subpopulation of RGCs. In the contralateral retinae, labeled RGCs were most numerous and widespread, with 97% projecting to the contralateral pretectum. The highest density of cells in the contralateral retinae was found in the inferior and nasal retinal quadrants. In the ipsilateral retinae, the small number of labeled cells were concentrated in the periphery of the inferior and nasal retinal quadrants. A striking feature of both ipsilateral and contralateral retinae was the paucity of labeled cells found in the dorsal hemiretina (lower visual field). Cell size measurements indicate 90-95% of labeled RGCs had diameters of 9-13 microm, while most of the remaining cells had diameters of 20-25 microm. This would suggest class III cells may be the predominant RGC type mediating pupilloconstriction, although a smaller population of larger cells (e.g., class I and/or II) may also contribute to this pathway. The recent reports utilizing the PLR as an assay for the efficacy of intraretinal grafts has highlighted the significance of the regional distribution results. The extremely low number of labeled cells in the dorsal hemiretina would argue for the placement of such grafts in the ventral hemiretina.

An in vivo model of human cytomegalovirus retinal infection

PURPOSE

To develop an animal model system in which human retina implanted in the anterior chamber of the eyes of rats would support human cytomegalovirus replication. Cytomegalovirus retinitis currently represents the most common cause of posterior uveitis in many urban areas in North America. Despite the tremendous interest in cytomegalovirus retinitis as a result of the acquired immunodeficiency syndrome (AIDS) epidemic, human cytomegalovirus infection has been difficult to model in vivo because of its extreme species-specificity.

METHODS

Human retina was introduced into the anterior chamber of athymic rats and allowed to attach to the rat iris. A human cytomegalovirus mutant carrying a beta-galactosidase indicator gene was then injected into the anterior chamber to infect the implanted tissue. After 4 weeks, the eyes were removed, sectioned, and developed with a chromogenic substrate to demonstrate the presence and location of beta-galactosidase expression.

RESULTS

Multiple spreading foci of beta-galactosidase expression were found in the retinal implants, indicating that human cytomegalovirus replication had occurred within the human tissue. There was no infection of rat tissue.

CONCLUSIONS

This model allows human cytomegalovirus infection of human retina to be established in vivo and sustained long enough to permit multiple cycles of viral replication to occur. The model thus has potential for evaluating antiviral therapies directed against human cytomegalovirus retinal disease.

The first decade of continuous progress in retinal transplantation

In recent months, neural fetal retina has been transplanted into blind human patients affected by Retinitis Pigmentosa. Initial success, as documented by improved visual activity, has been reported (del Cerro et al., Neuroscience Abstract, 1996). With the rapid progress in human patients, additional questions are arising concerning transplantation issues. Additional answers and further success in treating clinical disease will necessarily come from new laboratory research in animal models as well as in vitro systems. This increases the need for evaluation of the data already gathered over the first decade of retinal transplantation. The extensive experimental background work that preceded the current wave of human retinal transplants is reviewed in this paper, with particular emphasis given to the work dealing with the transplantation of neural retina.

Morphologic and neurochemical target selectivity of regenerating adult photoreceptors in vitro

Regenerating adult central nervous system (CNS) neurons must re-establish synaptic circuits in an environment very different from that present during development. However, the complexity of CNS circuitry has made it extremely difficult to assess the selectivity and mechanisms of synaptic regeneration at the cellular level in vivo. The synaptic preferences of adult photoreceptors were examined by using a defined cell culture system known to support regenerative process growth, presynaptic varicosity formation, and establishment of functional synapses. Immunolabeling for synaptic vesicle protein 2 and ultrastructural analysis demonstrated that cell-cell contacts made by photoreceptors were synaptic in nature. Target selectivity was determined by quantitative analysis of contacts onto normal and novel target cell types in cultures in which opportunities to contact all retinal cell types were present. Target cells were identified by morphology and immunolabeling for the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine. Regenerating photoreceptors showed a strong preference for novel multipolar cell targets (amacrine and ganglion cells) over normal photoreceptor, horizontal, and bipolar cell targets. Additionally, photoreceptors were selective for targets containing the transmitter GABA. These results indicate first, that the normal synaptic partners for photoreceptors are not intrinsically the optimal targets for regenerative synapse formation, and second, that GABA may modulate synaptic targeting by adult photoreceptors.

Injury-induced remodelling and regeneration of the ribbon presynaptic terminal in vitro

The neuronal response to axonal injury may relate to the type of insult incurred. Recently, neuritic and presynaptic varicosity regeneration by isolated adult salamander photoreceptors was demonstrated. We have used this system to compare the rod photoreceptor response to two types of injury: denervation/detargeting, the removal of pre- and postsynaptic partners from the axon terminal, and axotomy, the removal of the axon terminal itself. Cells were followed with time-lapse video microscopy for 24-48 h in culture and immunolabelled for SV2 or synaptophysin to identify synaptic vesicle-containing varicosities. Although all injured cells responded with regenerative growth, denervated/detargeted photoreceptors (i.e. neurons which retain their axon terminal) grew 80% more processes and fourfold more presynaptic varicosities than axotomized neurons. In cells which retained their original axon and terminal, varicosity formation generally began with axon retraction. Retraction was followed by elaboration of a lamellipodium and, by 48 h, development of varicosity-bearing neurites from the lamellipodium. Synaptic vesicle protein localization in denervated/detargeted cells paralleled axon terminal reorganization. Axotomized cells, in contrast, lacked synaptic vesicle protein immunoreactivity during this period. To detect synaptic protein synthesis, photoreceptors were examined for colocalization of synaptic vesicle protein with rab6, a Golgi marker, by confocal microscopy. As expected, synaptic vesicle protein staining was present in the Golgi complex during regeneration; however, in cells with an axon, new synaptic vesicle protein-labelled varicosities were found at early stages, prior to the appearance of immunolabel in the Golgi complex. The data demonstrate remarkable plasticity in the ribbon synapse, and suggest that in adult rod cells with an intact axon terminal, synaptic vesicle protein synthesis is not a prerequisite for the formation of new presynaptic-like terminals. We propose that preexisting axonal components are reutilized to expedite presynaptic renewal as an early response to denervation/detargeting.

Reversed ratio of color-specific cones in rabbit retinal cell transplants

Recently, we have reported on the emergence of various retinal cell types in embryonic rabbit retina transplanted to adult rabbits. When comparing the relative numbers of the spectrally different cone types in the transplants to those in the host or age-matched control retinas, a surprising shift was observed. While in the normal rabbit retina the middle-wavelength-sensitive (M) cones are considerably more abundant than the short-wave-sensitive (S) cones, the S/M cone ratio was found to be the opposite in the graft. The number of rosettes containing only S-cones in high density was found to be considerably higher than that of M-cone rich rosettes. The number of S-cones also exceeded that of the M-cones in each rosette that contained both cell types. Our results were obtained from the systematic immunocytochemical analysis of 15 different transplants derived from transplantations of embryonic rabbit retinas into adult hosts of the same species. The emergence and proportion of the two cone types were followed between 14 and 63 days after transplantation (between 29 and 78 postconceptional days of the donor tissue). Sections from various parts of the transplants were reacted with the monoclonal antibodies COS-1 and OS-2, specific for the middle- and short-wavelength-sensitive cones, respectively. The explanation for the reverse cone ratio in these transplants is not known yet, however, the observed phenomenon may indicate differences between the specification of the two basic cone types.

Neurophysiology of central retinal degeneration in cat

Receptive fields of ganglion cells have been studied in cats possessing a chronic, arrested lesion of central retinal degeneration. Lesions were characterized by an ophthalmoscopically sharp border separating apparently normal retina from the region of the lesion. Under direct ophthalmoscopic guidance, a succession of recordings was obtained from ganglion cells having cell bodies at various positions relative to the lesion. Cells located more than 1 deg outside the ophthalmoscopic border had normal visual sensitivity as assessed by area-threshold experiments. Inside the lesion cells within 1 deg of the border had reduced sensitivity which often precluded functional classification by the usual visual tests. Ganglion cells located more than 1 deg inside the border of large lesions were blind and some had abnormal patterns of maintained discharge of action potentials. Nevertheless, the antidromic latencies of these blind cells fell into the familiar conduction groups (T1/T2/T3). Receptive-field maps of cells near the border of the lesion often appeared truncated, with the missing portion of the field covered by the lesion. These observations were consistent with the abnormal form of area-threshold curves. Although the responsiveness of cells near the lesion was abnormally low for grating stimuli, cutoff spatial frequency and orientation bias of these cells were within normal limits.