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 Table of Contents  
REVIEW ARTICLE
Year : 2012  |  Volume : 2  |  Issue : 2  |  Page : 41-44

Retinal regeneration and stem cell therapy in retinitis pigmentosa


Department of Ophthalmology and Visual Sciences, University of Louisville, Louisville, KY, USA

Date of Web Publication19-May-2012

Correspondence Address:
Henry J Kaplan
301 East Muhammad Ali Boulevard, Louisville, KY 40202
USA
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Source of Support: None, Conflict of Interest: None


DOI: 10.1016/j.tjo.2012.03.003

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  Abstract 

Retinitis pigmentosa (RP) is the leading cause of hereditary blindness, and there is currently no available treatment that can either significantly slow the progression of this disease or restore lost vision. Amphibians and birds exhibit different strategies for retinal regeneration, including the proliferation of cells in the ciliary margin and transdifferentiation of the retinal pigment epithelium (RPE) and Muller glia. The mammalian retina does not have the innate ability to regenerate damaged retina, but research is actively exploring pathways that promote endogenous regeneration. Because the inner retinal architecture is largely preserved, even in advanced cases of RP, an alternative is to replace the degenerated photoreceptor cells, thereby replenishing the photoreceptor population. The transplantation of embryonic stem cells, induced pluripotent stem cells, embryonic retinal progenitors (postmitotic photoreceptor precursors), and hippocampal neuronal progenitors have been investigated for this purpose. The encouraging results demonstrate the integration and possible functional connection between the transplanted cells and the inner retinal circuitry of the host. In this review, we summarize recent advancements in this field and their potential for the treatment of RP and other retinal degenerations.

Keywords: retinal regeneration, retinal transplantation, retinitis pigmentosa, stem cells


How to cite this article:
Kaplan HJ, de Castro JP. Retinal regeneration and stem cell therapy in retinitis pigmentosa. Taiwan J Ophthalmol 2012;2:41-4

How to cite this URL:
Kaplan HJ, de Castro JP. Retinal regeneration and stem cell therapy in retinitis pigmentosa. Taiwan J Ophthalmol [serial online] 2012 [cited 2020 Jul 2];2:41-4. Available from: http://www.e-tjo.org/text.asp?2012/2/2/41/203720




  1. Introduction Top


Retinitis pigmentosa (RP) refers to a group of inherited retinal degenerations that result in severe visual impairment. Currently, 1.5 million people around the world are affected by RP, making it the leading cause of hereditary blindness.[1],[2] Abnormalities have been found in at least 179 genes (https://sph.uth.tmc.edu/retnet/sum-dis. htm), and autosomal dominant, autosomal recessive, and X-linked inheritance patterns have been described.[3],[4],[5],[6],[7] RP is also associated with several genetic syndromes.[8] Despite the genetic heterogeneity and wide variability in age of onset and severity, these syndromes all share a similar phenotypic expression that is characterized by the initial degeneration of rod followed by degeneration of the cone and photoreceptors and the subsequent involvement of the inner retina that leads to the loss of lamination, retinal vascular leakage, invasion of retinal pigment epithelium (RPE) cells into the neural retina, and, finally, the loss of ganglion cells.[9],[10]

Currently there is no effective therapy that stops the progression of RP or restores lost vision. Current medical management techniques try to slow down the progression of retinal degeneration, treat the various ocular complications that present such as cystoid macular edema and posterior subcapsular cataract, and provide psychological support and genetic counseling to patients.[11] Strategies that slow the progression of these complications include protection from light (because some retinopathies are light-dependent)[12] and vitamin A and E supplementation,[13] however these techniques remain controversial.

Exciting research is now being performed and new therapeutic approaches are being explored. These approaches can be divided into several broad groups according to the stage of the targeted disease.[14] The first approach targets the retina when it still has potentially functional photoreceptors and aims to correct the underlying biochemical abnormalities through genetic therapy. This approach can be further subdivided into gene augmentation therapies, where a normal gene is inserted to replace a diseased gene, and gene silencing therapies, where a mutated gene is inhibited using ribozyme or RNA interference. An example of a gene augmentation therapy is the correction of the retinoid cycle defect of Leber’s congenital amaurosis, a severe form of RP that is caused by a defect in the gene RPE65. Subretinal delivery of the normal RPE65 gene has been proven to produce sustained enhancement of the functions of both the rods and cones in dogs and humans.[15],[16],[17] The drawback of genetic therapy is that it is mutation-specific, so its success relies on knowing the precise genetic mutation and being able to develop individualized therapy. Additionally, most genetic defects are not null mutations, so the sole replacement of the missing enzyme or protein may not be sufficient to halt the progression of retinal degeneration. A second approach is drug therapy, where the mechanism of the disease is understood and there is the potential to replenish the deficit while the retina still has functional potential.[18],[19] Recent studies suggest that chromophore supplementation may achieve this result while avoiding drug toxicity.[20] The third approach is neuroprotection, which involves slowing down the degeneration of the photoreceptors using neurotrophic growth factors,[21] thereby inhibiting pro-apoptotic path-ways[22] and providing viability factors.[23] A fourth approach is electric stimulation of the visual pathways, which is used when there are few or no functional photoreceptors. The array of implants currently being developed include cortical visual prostheses[24] as well as suprachoroidal,[25] epiretinal,[26],[27] and subretinal implants that stimulate the degenerated neural retina.[28] The final approach, regenerative medicine, has recently reported important advances that may allow new therapies that can treat this family of diseases.


  2. Retinal cell regeneration Top


Amphibians have the ability to regenerate destroyed tissue, unlike mammals; for example, newts can regenerate a new retina from RPE. This process involves the dedifferentiation of the RPE and the subsequent proliferation and generation of two layers—a pigmented and a nonpigmented layer—that results in a functional retina with normal architecture.[29] This process is referred to as transdifferentiation.[30] Embryonic chick eyes are able to undergo a similar transdifferentiation process, even though the retinal progenitors maintain their polarity, thereby resulting in an inverted retina that has a photoreceptor layer along the inner surface of the retina near the vitreous and a ganglion cell layer that faces outward. Transdifferentiation in chicks can also arise from Muller glia, but only selected populations of retinal neurons are derived from these cells.[31] Fish have a similar ability to undergo transdifferentiation, regenerating a retina from Muller glia that can proliferate to form a blastema-like population next to the site of injury.[32]

Amphibians and fish also have a growth zone at the junction of the ciliary epithelium and the retina that is referred to as the ciliary marginal zone (CMZ). These animals are able to maintain their peripheral retina throughout life by adding concentric rings of neurons that originate from the stem cells at the CMZ.[32] Similarly, the embryonic chick retina can also undergo regeneration through cells within the CMZ. In contrast, the vast majority of the retinal cells in birds are produced during the first 10 days of embryonic development, but the CMZ is not spontaneously active during the posthatching phase.[31]

As already mentioned, the mammalian retina has only a limited capacity for neuronal regeneration. The Muller glia in the mature retina becomes reactive and hypertrophic following injury,[33] and only a few of these cells are able to reenter the mitotic cycle.[34] Although some progenitor genes can be activated after injury and mitotic provocation, many genes are not reexpressed, which explains at least in part the limited regeneration potential of the mammalian retina.[35] The age of the animal seems to determine the proliferative ability of the Muller glia, which declines shortly after birth.[36] However, a portion of the Muller glia may exhibit stem cell properties and become spontaneously immortalized under specific conditions in vitro.[37] While these findings are encouraging, the functional integration of these cells remains uncertain. Human ciliary epithelial cells are able to express retinal progenitor genes, but these cells do not differentiate into neurons in vitro.[38]


  3. Cell replacement strategies Top


Because endogenous retinal regeneration is limited in humans, the idea of replacing dysfunctional or dead photoreceptors with stem cells is currently being explored [Figure 1]. The mature central nervous system can effectively synapse with retinal grafts. When rat eyes are transplanted into young rat brains, they are able to connect to the visual pathway and respond to light.[39] Because the eye is an immune-privileged organ where neural grafts can survive long-term,[40] several laboratories are exploring the transplantation of stem cells into replace damaged photoreceptors after intravitreal or subretinal placement. Stem cell migration and integration into the outer retina (i.e., the outer nuclear layer [ONL]) after intravitreal transplantation has been observed in the developing rodent retina but not in healthy adults.[41] Although intravitreally injected stem cells appear to have some ability to migrate to the ONL, a subretinal transplant would present significant advantages for integration.
Figure 1: Schematic diagram of the origins of various stem cells. Adult somatic cells can be reprogrammed into iPSC, which differentiate into photoreceptors. ESC isolated from a blastocyst can also differentiate into photoreceptors. Embryonic retinal cells, as well as ciliary epithelial cells, have also been used to replenish photoreceptor populations. These cells can be transplanted into either the vitreous cavity or subretinal space. iPSC: induced pluripotent stem cells; ESC: embryonic stem cells.

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The rationale for stem cell transplantation after the onset of retinal degeneration is based on the existence of a functional inner retinal network, even when advanced disease is present. In a study on postmortem eyes from patients with advanced RP, only 4% of the photoreceptors remained (in comparison with normal retinas), but 66% of the bipolar cells and 28% of the ganglion cells were still present.[42] The observation that the inner neural retinal network remains intact in cases of advanced disease, and is presumably functional during the early stages of hereditary retinal degeneration, has given impetus to studies on photoreceptor replacement. Intact photoreceptor sheets have been transplanted into the subretinal space and have been proven as safe for use in humans.[43] These sheets have the theoretical advantage of a pre-established mono-layer morphology, but they develop rosettes after transplantation that restrict the integration of these cells.[44] Retinal spheres have also been transplanted into the subretinal space, but they tend to clump, which also inhibits integration into the host retina.[45]

Enzymatically dissociated cell suspensions have been injected into the subretinal space in order to avoid these problems. Murine retinal embryonic tissue has been dissociated and demonstrated both integration into the ONL and differentiation into photoreceptor cells. The generation of inner and outer segments in these cells, with synapses that connect to the host’s bipolar cells, is associated with the restoration of light sensitivity as measured by pupillometry and extracellular field potential recordings of the ganglion cell layer.[46] The age of the donor appears to be a critical factor in these experiments because neither progenitor cells nor adult photoreceptors have shown a similar ability to restore function. Only newly postmitotic photoreceptors have been shown to result in photoreceptor restoration.[47]

Human embryonic stem cells (hESC) are pluripotent–i.e., they have the ability to differentiate into any cell type in the body and are a promising source for use in retinal transplantation. Cells derived from hESC can be differentiated into either cone or rod photoreceptors. hESC have been transplanted into Crx−/− mice–a model of Leber’s congenital amaurosis–and have demonstrated functional integration and photoreceptor differentiation with recovery of electroretinogram (ERG) to light stimuli.[48] RPE derived from hESC was recently transplanted into patients with age-related macular degeneration. On a very short follow-up, no signs of tumorigenicity, ectopic tissue formation, or rejection were noted.[49] An alternative approach used hippocampal progenitor cells as a potential source of retinal neurons. The hippocampus is a site of active neurogenesis in adult mice; however, expanded hippocampal progenitor cells fail to show opsin expression after transplantation.[50]

Because of the limited access to human embryonic tissue, retinal progenitors have been expanded in vitro and differentiated into photoreceptors.[51] These cells can integrate into degenerated rodent retina and develop photoreceptor-like morphology capable of expressing recoverin, rhodopsin, and cone opsin.[52] Unfortunately, very few of these cells actually integrate into the rodent retina or develop both an inner and outer segment.

An alternative approach, which circumvents several of the political and ethical issues that arise from the use of human embryonic tissue, involves the use of induced pluripotent stem cells (iPSC). These are somatic cells that can be forced to overexpress a group of pluripotency genes, resulting in stem cell-like characteristics in these cells. iPSC have been differentiated in vitro into rods and transplanted into a swine model of retinal degeneration. They were able to integrate into the damaged retina, express rhodopsin, and exhibit projections that resemble the outer segments.[53] iPSC-derived photoreceptor precursors have also been transplanted into Rho-/- adult mice and been shown to integrate into the outer nuclear layer and increase the scotopic ERG response.[54] The ability to differentiate iPSC into rods offers the possibility of using these cells to recreate genetically diseased rod photoreceptors from human patients so that the underlying mechanisms of the disease, as well as its potential for reversal, can be studied.

Despite the significant advancements that have been made in the field of photoreceptor regeneration in the past few years, many challenges still remain. Large animal models of RP are an invaluable tool for assessing stem cell therapies. Although the rodent eye is very valuable for providing insight into the underlying mechanisms of retinal degeneration, as well as possible therapeutic alternatives, the significant differences between rodent and human eyes inhibit the translation of these observations into clinical therapies. New animal models of RP in swine[55] and rabbit[56] provide several advantages over the rodent: Namely, the larger size of the eyes in these species allows the development of surgical techniques that are applicable to humans, more complete electrophysiological testing is possible through the use of both full-field and multifocal electroretinography, and the existence of cone-rich visual streaks allows the study of the sequential effects of the disease on rod and cone photoreceptors.


  4. Conclusions Top


Recent advances in stem cell therapy for the treatment of RP and other hereditary retinal degenerations present new and exciting therapeutic pathways; however, which of these approaches will prove most useful is still unknown. It remains plausible that photoreceptor replacement through the use of stem cells will have a future role in the treatment of these diseases.



 
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