Retinal cells integrate into the outer nuclear layer and differentiate into mature photoreceptors after subretinal transplantation into adult mice

Adult donor rod photoreceptors integrate into the mature mouse retina

PURPOSE

Previous studies indicate that early postnatal mouse photoreceptors have the ability to integrate into the mature host retina after transplantation, while progenitors and fully differentiated photoreceptors do not. The authors sought to determine whether the decline in the ability of photoreceptors to integrate after transplantation with increasing age is related to a loss of migratory ability in the adult neurons or by a decrease in their survival.

METHODS

Dissociated retinal cells were transferred from green fluorescent protein-positive (GFP(+)) donor mice of ages ranging from embryonic day (E)12.5 to adults (>28 days postnatal [P]). Immunofluorescence was used to assess marker expression and the morphology of integrated cells. In vitro cultures of dissociated Nrl-GFP mice were used to assay survival.

RESULTS

It was confirmed in previous reports that neonatal rods integrate into adult hosts. However, in contrast to previous reports, the age of the donor cell was not as critical as previously reported, because it was found that donor cells older than P11 effectively integrated into adult host retina. Although fully adult photoreceptors (P28 and older) show a higher transplant failure rate than immature ones (P5), successful transplants had very similar numbers of integrated cells for both mature and immature donors. Integrated cells from all ages were indistinguishable from those of the host in morphology and marker expression.

CONCLUSIONS

Fully mature photoreceptors retain the ability to integrate into the mature retina. The authors propose that their potential for integration is limited primarily by their decreased survival after dissociation.

Distinct and conserved prominin-1/CD133-positive retinal cell populations identified across species

Besides being a marker of various somatic stem cells in mammals, prominin-1 (CD133) plays a role in maintaining the photoreceptor integrity since mutations in the PROM1 gene are linked with retinal degeneration. In spite of that, little information is available regarding its distribution in eyes of non-mammalian vertebrates endowed with high regenerative abilities. To address this subject, prominin-1 cognates were isolated from axolotl, zebrafish and chicken, and their retinal compartmentalization was investigated and compared to that of their mammalian orthologue. Interestingly, prominin-1 transcripts--except for the axolotl--were not strictly restricted to the outer nuclear layer (i.e., photoreceptor cells), but they also marked distinct subdivisions of the inner nuclear layer (INL). In zebrafish, where the prominin-1 gene is duplicated (i.e., prominin-1a and prominin-1b), a differential expression was noted for both paralogues within the INL being localized either to its vitreal or scleral subdivision, respectively. Interestingly, expression of prominin-1a within the former domain coincided with Pax-6-positive cells that are known to act as progenitors upon injury-induced retino-neurogenesis. A similar, but minute population of prominin-1-positive cells located at the vitreal side of the INL was also detected in developing and adult mice. In chicken, however, prominin-1-positive cells appeared to be aligned along the scleral side of the INL reminiscent of zebrafish prominin-1b. Taken together our data indicate that in addition to conserved expression of prominin-1 in photoreceptors, significant prominin-1-expressing non-photoreceptor retinal cell populations are present in the vertebrate eye that might represent potential sources of stem/progenitor cells for regenerative therapies.

Comparative analysis of the retinal potential of embryonic stem cells and amniotic fluid-derived stem cells

Photoreceptors have recently been generated from mouse and human embryonic stem cells (ESCs), although ethics concerns impede their utilization for cell replacement therapy for retinal disease. Extra-embryonic tissues have received attention as alternative therapeutic sources of stem cells. Human and mouse amniotic fluid-derived stem cells (AFCs) have been reported to be multipotent and express embryonic and adult stem cell markers. Here, in vitro conditions that generate retinal cells from ESCs were used to analyze and compare the retinal potential of murine AFCs and ESCs. We show that AFCs express pluripotency markers (Nanog, Sox2, and Oct3/4) as well as retinal transcription factor genes (Et, Lhx2, Tll1, Six6, Otx2, Pax6, and Fgf15). AFCs from amniotic fluid of Fgf15.gfp, Nrl.gfp, and Crx.gfp embryos cultured in retinal proliferation and differentiation conditions failed to switch on these retinal transgenes. AFCs cultured in retinal-promoting conditions, effective on ESCs, showed reduced expression of retinal markers. Retinal co-cultures activated retinal genes in ESCs but not in AFCs, and migration assays in retinal explants showed limited migration of AFCs compared with ESCs. Unlike ESCs, AFCs do not express the early embryonic ectodermal gene Utf1 and Western analysis of AFCs identified only the B isoform of Oct3/4, rather than the isoform A present in ESCs. We conclude that AFCs have restricted potential and differ considerably from ESCs and retinal progenitor cells. Reprogramming to induce pluripotency or new differentiation protocols will be required to confer retinal potential to AFCs as expression of a subset of pluripotency and retinal markers is not sufficient.

Stem cells therapy for retinal degeneration

Stem cell therapy is widely considered as a therapeutic approach for retinal degeneration. Retinal injury results in permanent visual disturbance or blindness. Repair of such damage by stem cells is one of the most feasible types of central nervous system repair. In this review, we consider how stem cells might be optimized for use as donor cells. We discuss the benefits of stem cells for transplantation in retinal degenerative disease. A wide range of stem cells from different sources is being investigated for the treatment of retinal degeneration. This study reviews the recent and old achievements about stem cells for retinal repair.

Long-term survival of photoreceptors transplanted into the adult murine neural retina requires immune modulation

Stem cell therapy presents an opportunity to replace photoreceptors that are lost as a result of inherited and age-related degenerative disease. We have previously shown that murine postmitotic rod photoreceptor precursor cells, identified by expression of the rod-specific transcription factor Nrl, are able to migrate into and integrate within the adult murine neural retina. However, their long-term survival has yet to be determined. Here, we found that integrated Nrl.gfp(+ve) photoreceptors were present up to 12 months post-transplantation, albeit in significantly reduced numbers. Surviving cells had rod-like morphology, including inner/outer segments and spherule synapses. In a minority of eyes, we observed an early, marked reduction in integrated photoreceptors within 1 month post-transplantation, which correlated with increased numbers of amoeboid macrophages, indicating acute loss of transplanted cells due to an inflammatory response. In the majority of transplants, similar numbers of integrated cells were observed between 1 and 2 months post-transplantation. By 4 months, however, we observed a significant decrease in integrated cell survival. Macrophages and T cells were present around the transplantation site, indicating a chronic immune response. Immune suppression of recipients significantly increased transplanted photoreceptor survival, indicating that the loss observed in unsuppressed recipients resulted from T cell-mediated host immune responses. Thus, if immune responses are modulated, correctly integrated transplanted photoreceptors can survive for extended periods of time in hosts with partially mismatched H-2 haplotypes. These findings suggest that autologous donor cells are optimal for therapeutic approaches to repair the neural retina, though with immune suppression nonautologous donors may be effective.

E13.5 retinal progenitors induce mouse bone marrow mesenchymal stromal cells to differentiate into retinal progenitor-like cells

BACKGROUND AIMS

Retinal progenitor cells (RPC) are an excellent resource for retinal replacement therapy but usually unavailable. We attempted to induce bone marrow mesenchymal stromal cells (BMSC) into RPC.

METHODS

BMSC and embryonic day 13.5 (E13.5) RPC derived from wild-type or enhanced green fluorescence protein (EGFP) transgenic (Egfp(+/+)) mice were co-cultured in a transwell or re-aggregation system. Gene and protein expressions were investigated by reverse transcription-polymerase chain reaction (PCR) and immunofluorescence, respectively. Spontaneous cell fusion was evaluated by Chloromethylbenzamido derivative of 1,1'- dioctadecyl-3,3,3',3' - tetramethylindocarbocyanine perchlorate (CM-DiI) labeling together with EGFP tracing.

RESULTS

BMSC from both wild-type and Egfp(+/+) mice displayed similar spindle shapes. The undifferentiated BMSC already expressed immature neural markers but did not express Nfl, Gfap or the retina-related genes Pax6, Math5 and Brn3b. When co-cultured with E13.5 RPC in the transwell system, BMSC displayed transient expression of early retinal development genes, including Pax6, Math5 and Brn3b at 3 days, as well as long-term expression of Nfl (up to 21 days). No expression of the late photoreceptor gene rhodopsin could be detected at any time. In re-aggregation co-culture, E13.5 RPC induced EGFP-positive BMSC to express not only the early retinal development genes but also the late gene rhodopsin. Furthermore, a small fraction of BMSC could be induced to express the synaptophysin protein. Re-aggregation co-culture of CM-DiI-labeled BMSC and EGFP-positive E13.5 RPC displayed minimal co-localization of the two fluorescence signals.

CONCLUSIONS

E13.5 RPC are capable of inducing BMSC towards an RPC fate. The differentiation is independent of cell fusion. Cytokines and cell-cell interactions exert this induction effect, but they have different functions.

Cone and rod photoreceptor transplantation in models of the childhood retinopathy Leber congenital amaurosis using flow-sorted Crx-positive donor cells

Retinal degenerative disease causing loss of photoreceptor cells is the leading cause of untreatable blindness in the developed world, with inherited degeneration affecting 1 in 3000 people. Visual acuity deteriorates rapidly once the cone photoreceptors die, as these cells provide daylight and colour vision. Here, in proof-of-principle experiments, we demonstrate the feasibility of cone photoreceptor transplantation into the wild-type and degenerating retina of two genetic models of Leber congenital amaurosis, the Crb1^rd8/rd8 and Gucy2e^−/− mouse. Crx-expressing cells were flow-sorted from the developing retina of CrxGFP transgenic mice and transplanted into adult recipient retinae; CrxGFP is a marker of cone and rod photoreceptor commitment. Only the embryonic-stage Crx-positive donor cells integrated within the outer nuclear layer of the recipient and differentiated into new cones, whereas postnatal cells generated a 10-fold higher number of rods compared with embryonic-stage donors. New cone photoreceptors displayed unambiguous morphological cone features and expressed mature cone markers. Importantly, we found that the adult environment influences the number of integrating cones and favours rod integration. New cones and rods were observed in ratios similar to that of the host retina (1:35) even when the transplanted population consisted primarily of cone precursors. Cone integration efficiency was highest in the cone-deficient Gucy2e^−/− retina suggesting that cone depletion creates a more optimal environment for cone transplantation. This is the first comprehensive study demonstrating the feasibility of cone transplantation into the adult retina. We conclude that flow-sorted embryonic-stage Crx-positive donor cells have the potential to replace lost cones, as well as rods, an important requirement for retinal disease therapy.

Adult ciliary epithelial cells, previously identified as retinal stem cells with potential for retinal repair, fail to differentiate into new rod photoreceptors

The ciliary margin in lower vertebrates is a site of continual retinal neurogenesis and a stem cell niche. By contrast, the human eye ceases retinal neuron production before birth and loss of photoreceptors during life is permanent and a major cause of blindness. The discovery of a proliferative cell population in the ciliary epithelium (CE) of the adult mammalian eye, designated retinal stem cells, raised the possibility that these cells could help to restore sight by replacing lost photoreceptors. We previously demonstrated the feasibility of photoreceptor transplantation using cells from the developing retina. CE cells could provide a renewable source of photoreceptors for transplantation. Several laboratories reported that these cells generate new photoreceptors, whereas a recent report questioned the existence of retinal stem cells. We used Nrl.gfp transgenic mice that express green fluorescent protein in rod photoreceptors to assess definitively the ability of CE cells to generate new photoreceptors. We report that CE cells expanded in monolayer cultures, lose pigmentation, and express a subset of eye field and retinal progenitor cell markers. Simultaneously, they continue to express some markers characteristic of differentiated CE and typically lack a neuronal morphology. Previously reported photoreceptor differentiation conditions used for CE cells, as well as conditions used to differentiate embryonic retinal progenitor cells (RPCs) and embryonic stem cell-derived RPCs, do not effectively activate the Nrl-regulated photoreceptor differentiation program. Therefore, we conclude that CE cells lack potential for photoreceptor differentiation and would require reprogramming to be useful as a source of new photoreceptors.

Using stem cells to mend the retina in ocular disease

Retinal degenerative diseases are the leading cause of incurable blindness worldwide. Furthermore, existing pharmacological and surgical interventions are only partially effective in halting disease progression, thus adjunctive neuroprotective strategies are desperately needed to preserve vision. Stem cells appear to possess inherent neuroprotective abilities, at least in part by providing neurotrophic support to injured neurons. Advances in stem cell biology offer the hope of new therapies for a broad range of neurodegenerative conditions, including those of the retina. Experimental cell-mediated therapies also hint at the tantalizing possibility of achieving retinal neuronal replacement and regeneration, once cells are lost to the disease process. This article summarizes the latest advances in cell therapies for neuroprotection and regeneration in neurodegenerative pathologies of both the inner and outer retina.

Induced pluripotent stem cell therapies for retinal disease

PURPOSE OF REVIEW

This review will discuss how recent advances with induced pluripotent stem (iPS) cells have brought the science of stem cell biology much closer to clinical application for patients with retinal degeneration.

RECENT FINDINGS

The ability to generate embryonic stem cells by reprogramming DNA taken from adult cells was demonstrated by the cloning of Dolly, the sheep, by somatic cell nuclear transfer, over 10 years ago. Recently, it has been shown that adult cells can be reprogrammed directly, without the need for a surrogate oocyte, through the generation of iPS cells. The method of reprogramming has since been optimized to avoid the use of retroviruses, making the process considerably safer. Last year, human iPS cells were isolated from an 80-year-old patient with neurodegenerative disease and differentiated into neurons in vitro.

SUMMARY

For stem cell therapies, the retina has the optimal combination of ease of surgical access, combined with an ability to observe transplanted cells directly through the clear ocular media. The question now is which retinal diseases are most appropriate targets for clinical trials using iPS cell approaches.

Targeted disruption of outer limiting membrane junctional proteins (Crb1 and ZO-1) increases integration of transplanted photoreceptor precursors into the adult wild-type and degenerating retina

Diseases culminating in photoreceptor loss are a major cause of untreatable blindness. Transplantation of rod photoreceptors is feasible, provided donor cells are at an appropriate stage of development when transplanted. Nevertheless, the proportion of cells that integrate into the recipient outer nuclear layer (ONL) is low. The outer limiting membrane (OLM), formed by adherens junctions between Müller glia and photoreceptors, may impede transplanted cells from migrating into the recipient ONL. Adaptor proteins such as Crumbs homologue 1 (Crb1) and zona occludins (ZO-1) are essential for localization of the OLM adherens junctions. We investigated whether targeted disruption of these proteins enhances donor cell integration. Transplantation of rod precursors in wild-type mice achieved 949 +/- 141 integrated cells. By contrast, integration is significantly higher when rod precursors are transplanted into Crb1(rd8/rd8) mice, a model of retinitis pigmentosa and Lebers congenital amaurosis that lacks functional CRB1 protein and displays disruption of the OLM (7,819 +/- 1,297; maximum 15,721 cells). We next used small interfering (si)RNA to transiently reduce the expression of ZO-1 and generate a reversible disruption of the OLM. ZO-1 knockdown resulted in similar, significantly improved, integration of transplanted cells in wild-type mice (7,037 +/- 1,293; maximum 11,965 cells). Finally, as the OLM remains largely intact in many retinal disorders, we tested whether transient ZO-1 knockdown increased integration in a model of retinitis pigmentosa, the rho(-/-) mouse; donor cell integration was significantly increased from 313 +/- 58 cells without treatment to 919 +/- 198 cells after ZO-1 knockdown. This study shows that targeted disruption of OLM junctional proteins enhances integration in the wild-type and degenerating retina and may be a useful approach for developing photoreceptor transplantation strategies.

Use of Lectins to Enrich Mouse ES-Derived Retinal Progenitor Cells for the Purpose of Transplantation Therapy

Using the mouse ES cell line with green fluorescent protein knocked-in at the Rx locus (Rx-KI ES cell), we previously showed that photoreceptors can be efficiently obtained in defined culture conditions by enriching Rx-positive retinal progenitor cells. We aimed to explore a protocol applicable for non-Rx-labeled stem cell lines for subsequent enrichment of retinal photoreceptor precursors for transplantation. The Rx-KI ES cell line was differentiated according to the serum-free suspension conditions with serum-free suspension/Dkk1/LeftyA/serum/activin method (SFEB/DLFA) described previously. Enrichment efficacy by negative selection was compared among 20 different lectins and the lectin combination that effectively enriched the Rx-positive cells by selecting the lectin low-binding population was determined. Subsequent differentiation efficiency to photoreceptor precursors and the contamination of Nanog or Oct3/4+ cells in the culture were evaluated between the cell cultures using negative selection with lectins and Rx positive selection. The effect of cytarabine (Ara-C) for minimizing the contamination of undifferentiated cells after the selection was also studied. The combination of the lectins, wheat germ agglutinin (WGA), and Erythrina crista-galli agglutinin (ECA) enabled us to enrich the Rx-positive population by approximately twice the original Rx percentage. The selection also minimized the percentage of Oct3/4+ cells. The lectin-selected cells produced a comparable percentage of Crx/rhodopsin-positive colonies with Rx-positive selection and were differentiated into photoreceptors. The Ara-C treatment on differentiating days 24–26 decreased Nanog and Oct3/4 expression in subsequent cultures. Enrichment of Rx-positive cells using WGA and ECA was comparable to Rx-positive selection, and the method could be applied to achieve efficient photoreceptor differentiation from other ES or iPS cell lines in which the Rx gene is not marked.

Stem-cell therapy in retinal disease

PURPOSE OF REVIEW

Stem-cell research is being investigated for the treatment of retina diseases. Cell replacement strategies have the potential to improve vision in patients who were previously considered to be untreatable. This review summarizes progress within the field and obstacles which must be overcome to make stem-cell therapy a viable treatment for select retinal disease.

RECENT FINDINGS

Researchers have demonstrated that stem-cell transplants can survive, migrate, differentiate, and integrate within the retina. Stem cells from various developmental stages have been used in these experiments, including embryonic stem cells, neural stem cells, mesenchymal stem cells, retinal stem cells, and adult stem cells from the ciliary margin. Not only can these transplants adopt retina-like morphologies and phenotypes, but they have also shown evidence of synaptic reconnection and visual recovery in both animal and human studies. Still, work must be done to achieve higher yields of functioning retinal neurons and to promote better integration within the host retina.

SUMMARY

Although many obstacles remain, stem-cell-based therapy is a promising treatment to restore vision in patients with retina disease.

Cell transplantation strategies for retinal repair

Cell transplantation is a novel therapeutic strategy to restore visual responses to the degenerate adult neural retina and represents an exciting area of regenerative neurotherapy. So far, it has been shown that transplanted postmitotic photoreceptor precursors are able to functionally integrate into the adult mouse neural retina. In this review, we discuss the differentiation of photoreceptor cells from both adult and embryonic-derived stem cells and their potential for retinal cell transplantation. We also discuss the strategies used to overcome barriers present in the degenerate neural retina and improve retinal cell integration. Finally, we consider the future translation of retinal cell therapy as a therapeutic strategy to treat retinal degeneration.

Transplantation prospects for the inner retina

Transplantation of stem or progenitor cells is an attractive new approach for treating neurodegenerative conditions of the central nervous system, which aims to protect or replace neurons and improve function. Proof of principle has already been shown in the retina that photoreceptors may be replaced by transplantation of neural progenitor cells. However, the task of retinal ganglion cell replacement is much more complex, as new cells will need to establish complex connections within the retina and also extend axons to precise targets in the brain. Although progress has been made in this field, it is likely that neuroprotective clinical applications will be established more quickly. Our laboratory has focused on the intraocular transplantation of cells to treat inner retinal disease, either by neuronal replacement or neuroprotection of existing cells. We have investigated the efficacy and effects of transplanting a variety of cell types, including human Müller stem cells (MIO-M1), oligodendrocyte precursor cells (OPCs), and bone marrow-derived mesenchymal stromal cells (MSCs) in a rat model of experimentally induced glaucoma. We also have developed and characterized a novel in vitro organotypic retinal explant culture system for exploring the methods of enhancing the efficacy of cell transplantation for the inner retina. In this review, we discuss the potentially beneficial effects of intraocular cell injections, identify current shortcomings of retinal stem cell therapy, and suggest directions for future research.

The association between multifocal electroretinograms and OCT retinal thickness in retinitis pigmentosa patients with good visual acuity

AIMS

To investigate relationships between retinal morphology and retinal function in patients with retinitis pigmentosa (RP) using optical coherence tomography (OCT) and multifocal electroretinography (mfERG).

METHODS

In all, 14 patients with RP who had visual acuities of 0.2 logMAR or better and Humphrey central fields of 10 degrees or larger participated in the study along with 16 normal control subjects. The amplitudes and timings of the mfERG responses were compared with spatially corresponding measures of retinal layer thickness from OCT within the macula region (central 12 degrees ).

RESULTS

Eyes with RP showed thinning of the photoreceptor retinal (PR) layer and thickening of mid-inner retinal (MIR) layers beyond the fovea. mfERG amplitude was reduced in all regions, whereas mfERG timing was only significantly delayed at a retinal eccentricity of 6-12 degrees and was otherwise preserved within the foveal and parafoveal retina (0-6 degrees). PR layer thickness was correlated with mfERG amplitude across the macula region. mfERG timing was correlated with the total change in retinal thickness (combined PR thinning and MIR thickening) at an eccentricity of 6-12 degrees.

CONCLUSIONS

The relationship between mfERG timing and retinal thickness in RP is dependent on the retinal eccentricity. Preserved timing in the central retina (0-6 degrees ), despite significant disruption to retinal laminar structure, could be suggestive of inner retinal remodelling or functional redundancy. Cone system activity derived from mfERG amplitude appears to be related to the thickness of the photoreceptor layer in the macula region.