Subretinal transplantation of brain-derived precursor cells to young RCS rats promotes photoreceptor cell survival

Induced pluripotent stem cell-based therapy for age-related macular degeneration

INTRODUCTION

In age-related macular degeneration (AMD), stem cells could possibly replace or regenerate disrupted pathologic retinal pigment epithelium (RPE), and produce supportive growth factors and cytokines such as brain-derived neurotrophic factor.  Induced pluripotent stem cells (iPSCs)-derived RPE was first subretinally transplanted in a neovascular AMD patient in 2014. Areas covered: Induced PSCs are derived from the introduction of transcription factors to adult cells under specific cell culture conditions, followed by differentiation into RPE cells. Induced PSC-derived RPE cells exhibit ion transport, membrane potential, polarized VEGF secretion and gene expression that is similar to native RPE. Despite having similar in vitro function, morphology, immunostaining and microscopic analysis, it remains to be seen if iPSC-derived RPE can replicate the myriad of in vivo functions, including immunomodulatory effects, of native RPE cells.  Historically, adjuvant RPE transplantation during CNV resections were technically difficult and complicated by immune rejection. Autologous iPSCs are hypothesized to reduce the risk of immune rejection, but their production is time-consuming and expensive.  Alternatively, allogenic transplantation using human leukocyte antigen (HLA)-matched iPSCs, similar to HLA-matched organ transplantation, is currently being investigated. Expert opinion: Challenges to successful transplantation with iPSCs include surgical technique, a pathologic subretinal microenvironment, possible immune rejection, and complications of immunosuppression.

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.

Multipotent adult hippocampal progenitor cells maintained as neurospheres favor differentiation toward glial lineages

Adult hippocampal progenitor cells (AHPCs) are generally maintained as a dispersed monolayer population of multipotent neural progenitors. To better understand cell-cell interactions among neural progenitors and their influences on cellular characteristics, we generated free-floating cellular aggregates, or neurospheres, from the adherent monolayer population of AHPCs. Results from in vitro analyses demonstrated that both populations of AHPCs were highly proliferative under maintenance conditions, but AHPCs formed in neurospheres favored differentiation along a glial lineage and displayed greater migrational activity than the traditionally cultured AHPCs. To study the plasticity of AHPCs from both populations in vivo, we transplanted green fluorescent protein (GFP)-expressing AHPCs via intraocular injection into the developing rat eyes. Both AHPC populations were capable of surviving and integrating into developing host central nervous system, but considerably more GFP-positive cells were observed in the retinas transplanted with neurosphere AHPCs, compared to adherent AHPCs. These results suggest that the culture configuration during maintenance for neural progenitor cells (NPCs) influences cell fate and motility in vitro as well as in vivo. Our findings have implication for understanding different cellular characteristics of NPCs according to distinct intercellular architectures and for developing cell-based therapeutic strategies using lineage-committed NPCs.

The influence of immunosuppressive drugs on neural stem/progenitor cell fate in vitro

In allogenic and xenogenic transplantation, adequate immunosuppression plays a major role in graft survival, especially over the long term. The effect of immunosuppressive drugs on neural stem/progenitor cell fate has not been sufficiently explored. The focus of this study is to systematically investigate the effects of the following four different immunotherapeutic strategies on human neural progenitor cell survival/death, proliferation, metabolic activity, differentiation and migration in vitro: (1) cyclosporine A (CsA), a calcineurin inhibitor; (2) everolimus (RAD001), an mTOR-inhibitor; (3) mycophenolic acid (MPA, mycophenolate), an inhibitor of inosine monophosphate dehydrogenase and (4) prednisolone, a steroid. At the minimum effective concentration (MEC), we found a prominent decrease in hNPCs' proliferative capacity (BrdU incorporation), especially for CsA and MPA, and an alteration of the NAD(P)H-dependent metabolic activity. Cell death rate, neurogenesis, gliogenesis and cell migration remained mostly unaffected under these conditions for all four immunosuppressants, except for apoptotic cell death, which was significantly increased by MPA treatment.

Towards retinal ganglion cell regeneration

Traumatic optic nerve injury and glaucoma are among the leading causes of incurable vision loss across the world. What is worse, neither pharmacological nor surgical interventions are significantly effective in reversing or halting the progression of vision loss. Advances in cell biology offer some hope for the victims of optic nerve damage and subsequent partial or complete visual loss. Retinal ganglion cells (RGCs) travel through the optic nerve and carry all visual signals to the brain. After injury, RGC axons usually fail to regrow and die, leading to irreversible loss of vision. Various kinds of cells and factors possess the ability to support the process of axon regeneration for RGCs. This article summarizes the latest advances in RGC regeneration.

Localization and developmental expression patterns of CSPG-cs56 (aggrecan) in normal and dystrophic retinas in two rat strains

Proteoglycans have a number of important functions in the central nervous system. Aggrecan (hyaluronan-binding proteoglycan, CSPG-cs56) is found in the extracellular matrix of cartilage as well as in the developing brain. We compared the postnatal distribution of CSPG-cs56 in Long Evans (LE) and Royal College of Surgeons (RCS) rat retinas to determine if this proteoglycan played a role in the development of dystrophic retinas. CSPG-cs56 expression was examined in rat retinas aged between birth (postnatal day 0, P0) and P150 using immunofluorescence and Western-blots. Immunofluorescence was quantified using ImageJ. GFAP staining was used to compare Müller cell labeling and the distribution of CSPG-cs56. Both rat strains showed a significant rise in total retinal CSPG-cs56 between P0 and P21; values peaked on P21 in LE rats and P14 in RCS rats. CSPG-cs56 then significantly decreased to lower levels (P35) in both strains before reaching significantly higher levels by P90-P150. CSPG-cs56 positive staining was present in the ganglion cell layer at birth and clear layering of the inner plexiform layer was seen between P7 and P21 due to dendritic staining of retinal ganglion cells. Staining was less intense and diffuse within the outer plexiform over a similar time-course. Light CSPG-cs56 labeling in the region of the outer segments was present at (P14) and became more intense as the retina approached maturity. CSPG-cs56 in the outer segments was the main contributor to the higher expression in older animals. Substantial differences in CSPG-cs56 labeling were not seen between LE and RCS rats. There was no evidence to suggest that Müller cells were the source of CSPG-cs56 in either rat strain, although their staining distributions had a degree of overlap. The lack of significant differences between LE and RCS rats indicates that CSPG-cs56 may not be involved in the degenerative process or the reorganization of the RCS rat retina. We suggest that the main role of CPSG-cs56 is to maintain retinal ganglion cell dendritic structure in the inner plexiform layer and is closely related to providing adequate support and flexibility for the photoreceptor outer segments, which is necessary to maintain their function.

Immortalized human fetal retinal cells retain progenitor characteristics and represent a potential source for the treatment of retinal degenerative disease

Human fetal retinal cells have been widely advocated for the development of cellular replacement therapies in patients with retinal dystrophies and age-related macular degeneration. A major limitation, however, is the lack of an abundant and renewable source of cells to meet therapeutic demand, although theoretically this may be addressed through the use of immortalized retinal progenitor cell lines. Here, we have used the temperature-sensitive tsA58 simian virus SV40 T antigen to conditionally immortalize human retinal progenitor cells isolated from retinal tissue at 10-12 weeks of gestation. We show that immortalized human fetal retinal cells retain their progenitor cell properties over many passages, and are comparable with nonimmortalized human fetal retinal cultures from the same gestational period with regard to expression of certain retinal genes. To evaluate the capacity of these cells to integrate into the diseased retina and to screen for potential tumorigenicity, cells were grafted into neonatal hooded Lister rats and RCS dystrophic rats. Both cell lines exhibited scarce integration into the host retina and failed to express markers of mature differentiated retinal cells. Moreover, although immortalized cells showed a greater propensity to survive, the cell lines demonstrated poor long-term survival. All grafts were infiltrated with host macrophage/microglial cells throughout their duration of survival. This study demonstrates that immortalized human fetal retinal progenitor cells retain their progenitor characteristics and may therefore have therapeutic potential in strategies that demand a renewable and consistent supply of donor cells for the treatment of degenerative retinal diseases.

Current Prospects for Adult Stem Cell–Based Therapies in Ocular Repair and Regeneration

Recent advances in stem cell biology have led to the exploration of stem cell-based therapies to treat a wide range of human diseases. In the ophthalmic field, much hope has been placed on the potential use of these cells to restore sight, particularly in those conditions in which other established treatments have failed and in which visual function has been irreversibly damaged by disease or injury. At present, there are many limitations for the immediate use of embryonic stem cells to treat ocular disease, and as more evidence emerges that adult stem cells are present in the adult human eye, it is clear that these cells may have advantages to develop into feasible therapeutic treatments without the problems associated with embryonic research and immune rejection. Here we discuss the current prospects for the application of various adult ocular stem cells to human therapies for restoration of vision.

Intraretinal processing following photoreceptor rescue by non-retinal cells

Royal College of Surgeon (RCS) rats undergo retinal degeneration due to the inability of retinal pigment epithelial (RPE) cells to phagocytose shed outer segments. We explored the effect of introducing Schwann cells to the subretinal space of RCS rats (before the onset of retinal degeneration), by relying on electroretinogram (ERG) recordings and correlative retinal morphology. Scotopic ERGs recorded from cell-injected eyes showed preserved amplitudes of mixed a-wave b-wave, rod b-waves, and cone b-waves over controls (sham-injected eyes); photopic b-wave amplitudes and critical flicker fusion were also improved. Normal retinal morphology was found in areas of retinas that had received cell injections. Since Schwann cells have no phagocytic properties, their therapeutic effect is best explained through a paracrine mechanism (secretion of factors that ensure photoreceptor survival).

Molecular genetics of infantile-onset retinal dystrophies

Over the last decade there have been major advances in our understanding of the molecular pathology of inherited retinal dystrophies. This paper reviews recent advances in the identification of genetic mutations underlying infantile-onset inherited retinal disorders and considers how this knowledge may lead to novel therapeutic approaches.

A review of the potential to restore vision with stem cells

Vision research involving stem cells is a rapidly evolving field. Animal experiments have shown that in response to environmental cues, stem cells can repopulate damaged retinas, regrow neuronal axons, repair higher cortical pathways, and restore pupil reflexes, light responses and basic pattern recognition. Viable corneas have been grown from stem cells and transplanted into humans. Similarly, human trials to repair damaged retinas in retinitis pigmentosa and age-related macular degeneration patients have produced preliminary successes. This review attempts to place the collective contributions toward stem cell/vision research into a broader clinical model of how stem cells might ultimately be used to restore the entire visual pathway.

Chondroitin sulfate proteoglycans and microglia prevent migration and integration of grafted Müller stem cells into degenerating retina

At present, there are severe limitations to the successful migration and integration of stem cells transplanted into the degenerated retina to restore visual function. This study investigated the potential role of chondroitin sulfate proteoglycans (CSPGs) and microglia in the migration of human Müller glia with neural stem cell characteristics following subretinal injection into the Lister hooded (LH) and Royal College of Surgeons (RCS) rat retinae. Neonate LH rat retina showed minimal baseline microglial accumulation (CD68-positive cells) that increased significantly 2 weeks after transplantation (p < .001), particularly in the ganglion cell layer (GCL) and inner plexiform layer. In contrast, nontransplanted 5-week-old RCS rat retina showed considerable baseline microglial accumulation in the outer nuclear layer (ONL) and photoreceptor outer segment debris zone (DZ) that further increased (p < .05) throughout the retina 2 weeks after transplantation. Marked deposition of the N-terminal fragment of CSPGs, as well as neurocan and versican, was observed in the DZ of 5-week-old RCS rat retinae, which contrasted with the limited expression of these proteins in the GCL of the adult and neonate LH rat retinae. Staining for CSPGs and CD68 revealed colocalization of these two molecules in cells infiltrating the ONL and DZ of the degenerating RCS rat retina. Enhanced immune suppression with oral prednisolone and intraperitoneal injections of indomethacin caused a reduction in the number of microglia but did not facilitate Müller stem cell migration. However, injection of cells with chondroitinase ABC combined with enhanced immune suppression caused a dramatic increase in the migration of Müller stem cells into all the retinal cell layers. These observations suggest that both microglia and CSPGs constitute a barrier for stem cell migration following transplantation into experimental models of retinal degeneration and that control of matrix deposition and the innate microglial response to neural retina degeneration may need to be addressed when translating cell-based therapies to treat human retinal disease.

Transplantation of ex vivo cultured limbal epithelial stem cells: a review of techniques and clinical results

Ex vivo cultured limbal epithelial stem cells have been used successfully to treat corneal limbal stem cell deficiency. We identified 17 reports of the application of this novel cell-based therapy in humans. In addition we identified four reports of the use of culture oral mucosal epithelial cells to treat limbal stem cell deficiency. We examined these reports to discern the success rate, complication rate, visual outcome, whether there is an optimal technique and which patients are the most likely to benefit. We also discuss the different culture methods employed and the regulations governing cell banks that are providing this service. We found that the techniques used to cultivate and transplant cells varied, but that no individual method was clearly superior. The reported success rate is similar across all studies for both allografts and autografts. The clinical indications for this treatment are not clearly defined as indicated by the variety of disorders treated. Follow-up is limited and the long-term success rate is yet to be established. Nonetheless, we conclude that there is sufficient evidence to support the continued use and refinement of this procedure as a treatment for corneal stem cell deficiency.

Studies of host–graft interactionsin vitro

Progenitor and stem cell transplantation represent therapeutic strategies for retinal disorders that are accompanied by photoreceptor degeneration. The transplanted cells may either replace degenerating photoreceptors or secrete beneficial factors that halt the processes of photoreceptor degeneration. The present study analyzes whether rat retinal progenitor cells differentiated into photoreceptor phenotypic cells in neurospheres have a potential to interact with rat retinal explants. Immunocytochemistry for rhodopsin and synaptophysin indicated photoreceptor cell-like differentiation in neurospheres that were stimulated by basic fibroblast growth factor and epidermal growth factor. Differentiation into neural phenotypes including photoreceptor cells was effectively blocked by an addition of leukemia inhibitory factor. Grafting of neurospheres onto retinal explants demonstrated a consistent penetration of glial cell processes into the explanted tissue. On the other hand, the incorporation of donor cells into explants was very low. A general finding was that neurospheres grafting was associated with local decrease in Müller cell activation in the explants. Further characterization of these effect(s) could provide further insight into progenitor cell-based therapies of retinal degenerative disorders.

Subretinal transplantation of bone marrow mesenchymal stem cells delays retinal degeneration in the RCS rat model of retinal degeneration

Because there is no effective treatment for this retinal degeneration, potential application of cell-based therapy has attracted considerable attention. Several investigations support that bone marrow mesenchymal stem cells (MSCs) can be used for a broad spectrum of indications. Bone marrow MSCs exert their therapeutic effect in part by secreting trophic factors to promote cell survival. The current study investigates whether bone marrow MSCs secrete factor(s) to promote photoreceptor cell survival and whether subretinal transplantation of bone marrow MSCs promotes photoreceptor survival in a retinal degeneration model using Royal College of Surgeons (RCS) rats. In vitro, using mouse retinal cell culture, it was demonstrated that the conditioned medium of the MSCs delays photoreceptor cell apoptosis, suggesting that the secreted factor(s) from the MSCs promote photoreceptor cell survival. In vivo, the MSCs were injected into the subretinal space of the RCS rats and histological analysis, real-time RT-PCR and electrophysiological analysis demonstrated that the subretinal transplantation of MSCs delays retinal degeneration and preserves retinal function in the RCS rats. These results suggest that MSC is a useful cell source for cell-replacement therapy for some forms of retinal degeneration.

Morphological integration and functional assessment of transplanted neural progenitor cells in healthy and acute ischemic rat eyes

We have functionally and morphologically characterized the retina and optic nerve after neural progenitor cell transplants to healthy rat eyes and eyes damaged by acute elevation of intraocular pressure (IOP). Green fluorescent protein-expressing adult rat hippocampal progenitor cells (AHPCs) were transplanted by intravitreal injection into healthy eyes and eyes damaged with acute ocular hypertension. Pupil light reflexes (PLR) and electroretinograms (ERGs) were recorded preoperatively and postoperatively. Eyes were subsequently prepared for immunohistochemical analysis and confocal imaging. Transplanted AHPCs were found in 8 of 15 (53%) acute ischemic eyes 62 days after surgery and 5 of 10 (50%) healthy eyes 32 days after grafting. Analysis of PLR and ERG function in acute ischemic eyes revealed no statistically significant difference compared to controls after transplantation for all observed functional parameters. Transplant into healthy rat eyes revealed no PLR or ERG amplitude deficits between transplanted and non-transplanted (control) eyes. Morphological and immunohistochemical analysis revealed that transplanted AHPCs survived and differentiated in both normal and injured retinal environments. Morphological integration occurred primarily within the inner retinal layers of the acute ischemic eyes. AHPCs were found to express neuronal and glial markers following transplantation. Transplanted AHPCs have the ability to integrate and differentiate in ischemia damaged retinas. PLR and ERG analysis revealed no significant difference in functional outcome in transplant recipient eyes.

Intraocular injection of folate antagonist methotrexate induces neuronal differentiation of embryonic stem cells transplanted in the adult mouse retina

Transplanted embryonic stem (ES) cells can be integrated into the retinas of adult mice as well-differentiated neuronal cells. However, the integrated ES cells also have a tumorigenic effect just because they have the ability for multipotential differentiation to various types of tissues. Thus, control of neoplastic potentials of ES cells is very important for the treatment of degenerative or injured diseases. Mouse ES cells carrying the sequence for the green fluorescent protein (GFP) gene were transplanted into adult mouse retinas by intravitreal injections 20 h after intravitreal N-methyl-d-aspartate (NMDA) administration. One week after the ES cell injection, folate antagonist methotrexate (MTX) was injected intravitreally. Eyes were retrieved 4 weeks after ES cell transplantation for histologic analyses. Conventional histological analysis was performed by hematoxylin and eosin staining with the use of paraffin-embedded sections. Neuronal differentiation and teratogenic potential of ES cells were demonstrated by immunohistochemistry. The proliferative activity of transplanted cells was detected by mitotic index, proliferating cell nuclear antigen index and AgNOR count. The incorporation of transplanted ES cells in MTX-treated and non-treated retinas at 4 weeks after transplantation was observed in 8/16 eyes (50%) and 8/16 eyes (50%), respectively. Transplanted ES cells in MTX-treated retina showed increased neuronal differentiation and decreased expression of teratogenic markers, compared with ES cells in non-treated retina. The proliferative activity of transplanted ES cells in MTX-treated retina was lower than that in non-treated retina. These results suggest that intravitreal MTX treatment following transplantation can induce neuronal differentiation in the transplanted ES cells and decrease their proliferative activity.

Retinal neurospheres prepared as tissue for transplantation

The present work was conducted to study the cellular composition and developmental capacity of retinal neurospheres. Furthermore, the ability of grafted neurospheres to integrate into adult retinal tissue was studied in an in vitro model. Retinal progenitor cells isolated from rat embryos were expanded into neurospheres in vitro in the presence of basic fibroblast growth factor (bFGF), epidermal growth factor (EGF) and leukemia inhibitory factor (LIF). Neurospheres labeled with a lipophilic dye were placed onto explants, and tissue interactions were analyzed after 2-6 days of culture. Immunocytochemical analysis of neurospheres revealed the presence of neuronal and glial cells. Proliferating neuronal and glial cells were observed after 2 weeks, whereas the neuronal cell proliferation declined considerably after 4 weeks. Few apoptotic cells were observed in the neurospheres. Neurospheres cultured on explanted adult retina engrafted with the surrounding tissue, but progenitor cell migration into the explants was low. However, the grafted neurospheres appeared to limit the experimentally induced photoreceptor apoptosis in the surrounding explant tissue.

Immune problems in central nervous system cell therapy

Transplantation of cells and tissues to the mammalian brain and CNS has revived the interest in the immunological status of brain and its response to grafted tissue. The previously held view that the brain was an absolute "immunologically privileged site" allowing indefinite survival without rejection of grafts of cells has proven to be wrong. Thus, the brain should be regarded as a site where immune responses can occur, albeit in a modified form, and under certain circumstances these are as vigorous as those seen in other peripheral sites. Clinical cell transplant trials have now been performed in Parkinson's disease, Huntington's disease, demyelinating diseases, retinal disorders, stroke, epilepsy, and even deafness, and normally are designed as cell replacement strategies, although implantation of genetically modified cells for supplementation of growth factors has also been tried. In addition, some disorders of the CNS for which cell therapies are being considered have an immunological basis, such as multiple sclerosis, which further complicates the situation. Embryonic neural tissue allografted into the CNS of animals and patients with neurodegenerative conditions survives, makes and receives synapses, and ameliorates behavioral deficits. The use of aborted human tissue is logistically and ethically complicated, which has lead to the search for alternative sources of cells, including xenogeneic tissue, genetically modified cells, and stem cells, all of which can and will induce some level of immune reaction. We review some of the immunological factors involved in transplantation of cells to CNS.

Fabrication of degradable polymer scaffolds to direct the integration and differentiation of retinal progenitors

Retinal progenitor cells (RPCs) are self-renewing cells capable of differentiating into the different retinal cell types including photoreceptors, and they have shown promise as a source of replacement cells in experimental models of retinal degeneration. We hypothesized that a biodegradable polymer scaffold could deliver these cells to the subretinal space in a more organized manner than bolus injections, while also providing the graft with laminar organization and structural guidance channels. We fabricated highly porous scaffolds from blends of poly(L-lactic acid) and poly(lactic-co-glycolic acid) using a variety of techniques to produce pores oriented normal to the plane of the scaffold. RPCs were seeded on the polymer scaffolds and cultured for 14 days. Seeded scaffolds were then either fixed for characterization or used in an explant or in vivo rat model. The scaffolds were fully covered by RPCs in 3 days. Attachment of RPCs to the polymer scaffold was associated with down-regulation of immature markers and up-regulation of markers of differentiation. This suggests that the scaffold may promote differentiation of RPCs. The seeded cells elaborated cellular processes and aligned in the scaffold in conjunction with degenerating retinal explants. The cells also exhibited morphologies consistent with photoreceptors including a high degree of polarization of the cells. This data suggests that the scaffold may be a means to assist in the promotion of photoreceptor phenotypes. Implantation of the seeded scaffold into the rat eye is associated with increased RPC survival. Taken together, these data suggest that these polymer scaffolds provide a useful means for delivering RPCs to the subretinal space and may assist in the formation of retinal cell phenotypes, although it is clear that more cues are needed to direct the differentiation of RPCs into functional photoreceptors.

[Stem cell-based therapies for retinal disorders]

In the context of cell-based therapies for hereditary retinal dystrophies and other retinal disorders, interest has focussed on the therapeutic potential of embryonic and tissue-specific stem cells. Stem cells are characterised by their capacity for self-renewal and by their multipotentiality. Because of these properties, they can be expanded in vitro and eventually differentiated into "desired" specialized cell types. Stem cells are not only candidate cells for the development of cell replacement strategies, but are also interesting cells for the establishment of ex vivo gene therapies. Here, we discuss recent experimental work performed to evaluate the therapeutic potential of embryonic, mesenchymal, hematopoietic, neural and retinal stem cells for the treatment of inherited retinal dystrophies and other retinal diseases.

Stem cells and retinal repair

Retinal stem cells (RSCs) are multipotent central nervous system (CNS) precursors that give rise to the retina during the course of development. RSCs are present in the embryonic eyecup of all vertebrate species and remain active in lower vertebrates throughout life. Mammals, however, exhibit little RSC activity in adulthood and thus little capacity for retinal growth or regeneration. Because CNS precursors can now be isolated from immature and mature mammals and expanded ex vivo, it is possible to study these cells in culture as well as following transplantation to the diseased retina. Such experiments have revealed a wealth of unanticipated findings, both in terms of the instructive cues present in the mature mammalian retina as well as the ability of grafted CNS precursors to respond to them. This review examines current knowledge regarding RSCs, together with other CNS precursors, from the perspective of investigators who wish to isolate, propagate, genetically modify, and transplant these cells as a regenerative strategy with application to retinal disease.

Migratory capacity of the cell line RN33B and the host glial cell response after subretinal transplantation to normal adult rats

As previously reported, the brain-derived precursor cell line RN33B has a great capacity to migrate when transplanted to adult brain or retina. This cell line is immortalized with the SV40 large T-antigen and carries the reporter gene LacZ and the green fluorescent protein GFP. In the present study, the precursor cells were transplanted to the subretinal space of adult rats and investigated early after grafting. The purpose was to demonstrate the migration of the grafted cells from the subretinal space into the retina and the glial cell response of the host retina. Detachment caused by the transplantation method was persistent up to 4 days after transplantation, and then reattachment occurred. The grafted cells were shown to migrate in between the photoreceptor cells before entering into the plexiform layers. Molecules involved in migration of immature neuronal cells as the polysialylated neural cell adhesion molecule (PSA-NCAM) and the collapsing response-mediated protein 4 (TUC-4) was found in the plexiform layers of the host retina, but not in the grafted cells. The expression of the intermediate filaments GFAP, vimentin, and nestin was intensely upregulated immediately after transplantation. A less pronounced upregulation was observed on sham-operated animals. In summary, the RN33B cell line migrated promptly posttransplantation and settled preferably into the plexiform layers of the retina, the same layers where the migration cues PSA-NCAM and TUC-4 were established. In addition, both the transplantation method per se and the implanted cells caused an intense glial cell response by the host retina.

Survival and Long Distance Migration of Brain‐Derived Precursor Cells Transplanted to Adult Rat Retina

Neural precursor cells transplanted to adult retina can integrate into the host. This is especially true when the neural precursor rat cell line RN33B is used. This cell line carries the reporter genes LacZ and green fluorescent protein (GFP). In grafted rat eyes, RN33B cells are localized from one eccentricity to the other of the host retina. In the present study, whole-mounted retinas were analyzed to obtain a more appropriate evaluation of the amount of transgene-expressing cells and the migratory capacity of these cells 3 and 8 weeks post-transplantation. Quantification was made of the number of beta-galactosidase- and GFP-expressing cells with a semiautomatized stereological cell counting system. With the same system, delineation of the distribution area of the grafted cells was also performed. At 3 weeks, 68% of the grafted eyes contained marker-expressing cells, whereas at 8 weeks only 35% of the eyes contained such cells. Counting of marker-expressing cells demonstrated a lower number of transgene-expressing cells at 3 weeks compared with 8 weeks post-transplantation. The distribution pattern of marker gene-expressing cells revealed cells occupying up to 21% at 3 weeks and up to 68% at 8 weeks of the entire host retina post-grafting. The precursor cells survived well in the adult retina although the most striking feature of the RN33B cell line was its extraordinary migratory capacity. This capability could be useful if precursor cells are used to deliver necessary genes or gene products that need to be distributed over a large diseased area.

Differentiation and morphological integration of neural progenitor cells transplanted into the developing mammalian eye

Transplantation of neural stem/progenitor cells has been proposed as a novel approach for the replacement and repair of damaged CNS tissues. We have evaluated the influence of the host cellular microenvironment upon the survival, differentiation, and integration of neural progenitor cells transplanted into the CNS. Using this approach, we have investigated the fate of neural progenitor cells in vivo following transplantation into the developing mammalian eye. Murine brain progenitor cells (mBPCs) isolated from neonatal mice expressing the green fluorescent protein (GFP) transgene were transplanted into the eyes of Brazilian opossums (Monodelphis domestica). Monodelphis pups are born in an extremely immature, fetal-like state. The eyes of neonatal pups provide a fetal-like environment in which to study cellular interactions between host tissues and transplanted neural progenitor cells. mBPCs were transplanted by intraocular injection in hosts ranging in age from 5 days postnatal to adult. The transplanted cells were easily identified because of their GFP fluorescence. Extensive survival, differentiation, and morphological integration of mBPCs within the host tissue was observed. We found that the younger retinas provided a more supportive environment for the morphological integration of the transplanted mBPCs. Cells with morphologies characteristic of specific retinal cell types were observed. Moreover, some transplanted mBPCs were labeled with antibodies characteristic of specific neural/retinal phenotypes. These results suggest that the host environment strongly influences progenitor cell differentiation and that transplantation of neural progenitor cells may be a useful approach aimed at treating degeneration and pathology of the CNS.

Long-term survival and glial differentiation of the brain-derived precursor cell line RN33B after subretinal transplantation to adult normal rats

The potential use of in vitro-expanded precursor cells or cell lines in repair includes transplantation of such cells for cell replacement purposes and the activation of host cells to provide "self-repair." Recently, we have reported that cells from the brain-derived cell line RN33B (derived from the embryonic rat medullary raphe and immortalized through retroviral transduction of the temperature-sensitive mutant of the simian virus 40 ([SV40] large T-antigen) survive for at least 4 weeks, integrate, and differentiate after subretinal grafting to normal adult rats. Here, we demonstrate that grafts of these cells survive for at least 4 months after subretinal transplantation to adult, normal immunosuppressed rats. Implanted cells integrate into the retinal pigment epithelium and the inner retinal layers, and the anterior part of the optic nerve. In addition, the RN33B cells migrate within the retina, occupying the whole retina from one eccentricity to the other. A large fraction of the grafted cells differentiate into glial cells, as shown by double labeling of the reporter genes LacZ or green fluorescent protein, and several glial markers, including oligodendrocytes. However, the cells did not differentiate into retinal neurons, judging from their lack of expression of retinal neuronal phenotypic markers. A significant number of the implanted cells in the host retina were in a proliferative stage, judging from proliferative cell nuclear antigen and SV40 large T-antigen immunohistochemistry. To conclude, the cells survived, integrated, and migrated over long distances within the host. Therefore, our results may be advantageous for future design of therapeutic strategies, since such cells may have the potential of being a source of, for example, growth factor delivery in experimental models of retinal degeneration.