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

Photoreceptor replacement therapy: Challenges presented by the diseased recipient retinal environment

Vision loss caused by the death of photoreceptors is the leading cause of irreversible blindness in the developed world. Rapid advances in stem cell biology and techniques in cell transplantation have made photoreceptor replacement by transplantation a very plausible therapeutic strategy. These advances include the demonstration of restoration of vision following photoreceptor transplantation and the generation of transplantable populations of donor cells from stem cells. In this review, we present a brief overview of the recent progress in photoreceptor transplantation. We then consider in more detail some of the challenges presented by the degenerating retinal environment that must play host to these transplanted cells, how these may influence transplanted photoreceptor cell integration and survival, and some of the progress in developing strategies to circumnavigate these issues.

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.

Photoreceptor Differentiation following Transplantation of Allogeneic Retinal Progenitor Cells to the Dystrophic Rhodopsin Pro347Leu Transgenic Pig

Purpose. Transplantation of stem, progenitor, or precursor cells has resulted in photoreceptor replacement and evidence of functional efficacy in rodent models of retinal degeneration. Ongoing work has been directed toward the replication of these results in a large animal model, namely, the pig. Methods. Retinal progenitor cells were derived from the neural retina of GFP-transgenic pigs and transplanted to the subretinal space of rhodopsin Pro347Leu-transgenic allorecipients, in the early stage of the degeneration and the absence of immune suppression. Results. Results confirm the survival of allogeneic porcine RPCs without immune suppression in the setting of photoreceptor dystrophy. The expression of multiple photoreceptor markers by grafted cells included the rod outer segment-specific marker ROM-1. Further evidence of photoreceptor differentiation included the presence of numerous photoreceptor rosettes within GFP-positive grafts, indicative of the development of cellular polarity and self-assembly into rudiments of outer retinal tissue. Conclusion. Together, these data support the tolerance of RPCs as allografts and demonstrate the high level of rod photoreceptor development that can be obtained from cultured RPCs following transplantation. Strategies for further progress in this area, together with possible functional implications, are discussed.

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.

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.

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.

Retinal progenitor cell xenografts to the pig retina: immunological reactions

We evaluated the host response to murine retinal progenitor cells (RPCs) following transplantation to the subretinal space (SRS) of the pig. RPCs from GFP mice were transplanted subretinally in 18 nonimmunosuppressed normal or laser-treated pigs. Evaluation of the SRS was performed on hematoxylin-eosin (H&E)-stained sections. Serum samples were taken from naive and RPC-grafted pigs and mouse-reactive antibody responses were assessed. At 1 week, histology showed a few perivascular lymphocytes consistent with a mild retinal vasculitis, and depigmentation of the RPE with large numbers of mononuclear inflammatory cells in the choroid near the transplantation site. Large choroidal infiltrates were evident at 2-5 weeks. Serum from naive and RPC-xenografted pigs contained significant levels of preformed IgG and IgM antibodies against murine antigens. Xenogeneic RPCs transplanted to the porcine SRS induced mononuclear infiltration in the choroid with graft rejection occurring over 2-5 weeks. Serum analysis confirmed that mice and pigs are discordant species; however, a cell-mediated acute mechanism appears to be responsible, rather than an antibody-mediated rejection.

Gene therapy and transplantation in CNS repair: The visual system

Normal visual function in humans is compromised by a range of inherited and acquired degenerative conditions, many of which affect photoreceptors and/or retinal pigment epithelium. As a consequence the majority of experimental gene- and cell-based therapies are aimed at rescuing or replacing these cells. We provide a brief overview of these studies, but the major focus of this review is on the inner retina, in particular how gene therapy and transplantation can improve the viability and regenerative capacity of retinal ganglion cells (RGCs). Such studies are relevant to the development of new treatments for ocular conditions that cause RGC loss or dysfunction, for example glaucoma, diabetes, ischaemia, and various inflammatory and neurodegenerative diseases. However, RGCs and associated central visual pathways also serve as an excellent experimental model of the adult central nervous system (CNS) in which it is possible to study the molecular and cellular mechanisms associated with neuroprotection and axonal regeneration after neurotrauma. In this review we present the current state of knowledge pertaining to RGC responses to injury, neurotrophic and gene therapy strategies aimed at promoting RGC survival, and how best to promote the regeneration of RGC axons after optic nerve or optic tract injury. We also describe transplantation methods being used in attempts to replace lost RGCs or encourage the regrowth of RGC axons back into visual centres in the brain via peripheral nerve bridges. Cooperative approaches including novel combinations of transplantation, gene therapy and pharmacotherapy are discussed. Finally, we consider a number of caveats and future directions, such as problems associated with compensatory sprouting and the reformation of visuotopic maps, the need to develop efficient, regulatable viral vectors, and the need to develop different but sequential strategies that target the cell body and/or the growth cone at appropriate times during the repair process.

Neural Progenitor Cell Transplants into the Developing and Mature Central Nervous System

When developing cell transplant strategies to repair the diseased or injured central nervous system (CNS), it is essential to consider host-graft interactions and how they may influence the outcome of the transplants. Recent studies have demonstrated that transplanted neural progenitor cells (NPCs) can differentiate and integrate morphologically into developing mammalian retinas. Is the ability to differentiate and to undergo structural integration into the CNS unique to specific progenitor cells, or is this plasticity a function of host environment, or both? To address these issues we have used the developing retina of the Brazilian opossum and have compared the structural integration of brain and retinal progenitor cells transplanted into the eyes at different developmental stages. The Brazilian opossum, Monodelphis domestica, is a small pouchless marsupial native to South America. This animal's lack of a pouch and fetal-like nature at birth circumvents the need for in utero surgical procedures, and thus provides an ideal environment in which to study the interactions between developing host tissues and transplanted NPCs. To test whether NPCs affect visual function we transplanted adult hippocampal progenitor cells (AHPCs) into normal, healthy adult rat eyes and performed noninvasive functional recordings. Monitoring of the retina and optic nerve over time by electroretinography and pupillometry revealed no severe perturbation in visual function in the transplant recipient eyes. Taken together, our findings suggest that the age of the host environment can strongly influence NPC differentiation and that transplantation of neural progenitor cells may be a useful strategy aimed at treating neurodegeneration and pathology of the CNS.

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.

Preferential differentiation of neural progenitor cells into the glial lineage through gp130 signaling in N-methyl-d-aspartate-treated retinas

The purpose of this study was to investigate the differentiation of neural progenitor cells (NPCs) following retinal transplantation in N-methyl-D-aspartate (NMDA)-treated eyes. NMDA was injected into the vitreous cavity of adult rat eyes. NPCs were prepared from telencephalic neuroepithelium of enhanced green fluorescence protein (EGFP) transgenic mice on embryonic day 14.5. A cell suspension was injected into the vitreous cavity in experimental eyes. Immunohistochemistry was conducted at 1, 2 or 4 weeks after transplantation of NPCs in an effort to determine the survival and differentiation of transplanted NPCs. Similar experiments were conducted using glycoprotein (gp)130-null (-/-) mice. Examination of retinal sections revealed that transplanted NPCs could survive for at least 4 weeks in NMDA-treated retinas. Immunohistochemical studies for specific cell-type markers revealed that, among the transplanted NPCs at 2 weeks after transplantation, the mean percentage (+/-standard deviation) of glial fibrillary acidic protein (GFAP)-positive (glial) cells was 63.5 +/- 7.4%, demonstrating the differentiation of transplanted NPCs with a preference for the glial lineage. Furthermore, the mean percentage of betaIII-tubulin-positive (mature neuronal) cells was 18.8 +/- 4.5%. Following transplantation of NPCs isolated from gp130-/- mice into NMDA-treated retinas, the mean percentage of GFAP-positive cells (17.6 +/- 7.0%), was significantly lower than that in NPCs isolated from wild-type mice (59.1 +/- 6.0%, P = 0.04, Mann-Whitney U test). Preferential differentiation of NPCs into the glial lineage is induced through gp130 signaling in NMDA-treated eyes.

[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.

Transplantation of Neural Progenitor Cells into the Developing Retina of the Brazilian Opossum: An in vivo System for Studying Stem/Progenitor Cell Plasticity

In developing cell transplant strategies to repair the diseased or injured retina is essential to consider host-graft interactions and how they may influence the outcome of the transplants. In the present study we evaluated the influence of the host microenvironment upon neural progenitor cells (NPCs) transplanted into the developing and mature retina of the Brazilian opossum, Monodelphis domestica. Monodelphis pups are born in an extremely immature state and the neonatal pups provide a fetal-like environment in which to study the interactions between host tissues and transplanted NPCs. Three different populations of GFP-expressing NPCs were transplanted by intraocular injection in hosts ranging in age from 5 days postnatal to adult. Extensive survival, differentiation and morphological integration of NPCs were observed within the developing retina. These results suggest that the age of the host environment can strongly influence NPC differentiation and integration.

Fate of multipotent neural precursor cells transplanted into mouse retina selectively depleted of retinal ganglion cells

In some parts of the CNS, depletion of a particular class of neuron might induce changes in the microenvironment that influence the differentiation of newly grafted neural precursor cells. This hypothesis was tested in the retina by inducing apoptotic retinal ganglion cell (RGC) death in neonatal and adult female mice and examining whether intravitreally grafted male neural precursor cells (C17.2), a neural stem cell (NSC)-like clonal line, become incorporated into these selectively depleted retinae. In neonates, rapid RGC death was induced by removal of the contralateral superior colliculus (SC), in adults, delayed RGC death was induced by unilateral optic nerve (ON) transection. Cells were injected intravitreally 6-48 h after SC ablation (neonates) or 0-7 days after ON injury (adults). Cells were also injected into non-RGC depleted neonatal and adult retinae. At 4 or 8 weeks, transplanted cells were identified using a Y-chromosome marker and in situ hybridisation or by their expression of the lacZ reporter gene product Escherichia coli beta-galactosidase (beta-gal). No C17.2 cells were identified in axotomised adult-injected eyes undergoing delayed RGC apoptosis (n = 16). Donor cells were however stably integrated within the retina in 29% (15/55) of mice that received C17.2 cell injections 24 h after neonatal SC ablation; 6-31% of surviving cells were found in the RGC layer (GCL). These NSC-like cells were also present in intact retinae, but on average, there were fewer cells in GCL. In SC-ablated mice, most grafted cells did not express retinal-specific markers, although occasional donor cells in the GCL were immunopositive for beta-III tubulin, a protein highly expressed by, but not specific to, developing RGCs. Targeted rapid RGC depletion thus increased cell incorporation into the GCL, but grafted C17.2 cells did not appear to differentiate into an RGC phenotype.

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.

Retinal transplantation of neural progenitor cells derived from the brain of GFP transgenic mice

Neural progenitor cells isolated from the brains of neonatal GFP transgenic mice were grafted to the retina of RCS rats and rds and B6 mice. Expression of GFP and differentiation markers was evaluated at 1-4 weeks post-transplantation. Grafted cells maintained transgene expression throughout the 4-week period. At 1 week there was widespread migration of GFP+cells within the host retina and at 2 weeks evidence of neuronal differentiation (as shown by both marker expression and cell morphology), although integration at 4 weeks was limited to syngeneic recipients. Because brain-derived neural progenitor cells exhibit both neuronal and astrocytic differentiation in diseased and normal host retina, these cells provide a useful tool for studies of retinal regeneration.