In recent years, the use of stem cell therapy has attracted much interest from researchers and clinicians. Similarly, patients are much more aware of stem cell therapy as development of specific treatments could potentially cure conditions which currently have no approved forms of treatment. Some ocular examples include retinitis pigmentosa and atrophic age-related macular degeneration.
Previous stem cell studies have looked at accurately deriving reliable sources of stem cells and their suitability in transplantation. Additional studies have analysed various methods of transplantation, exploring their advantages and disadvantages. Current ocular stem cell therapy has now further developed with experimental data from animal models being used to initiate clinical trials in human patients. This review aims to determine how far ocular stem cell therapy has progressed, with particular emphasis placed on corneal and retinal disease.
Definition of a stem cell
The term ‘stem cell’ was coined in 1868 by biologist Ernst Haeckel.1 In his early literature regarding embryology, he claimed that a fertilised egg could be classified as a stem cell as it has the ability to differentiate into all cell types. Although correct, subsequent studies in the field of stem cell research have given a much larger definition of the term.2
A stem cell is a unique cell type which differs considerably from a normal cell. Some have the capability to renew continuously to produce many cell types within the living organism. These stem cells are classified as pluripotent and are present within a developing embryo and umbilical cord tissue.3
Alternatively, stem cells can be controlled to differentiate into sub-types of cells related to the function of a specific tissue. These types of cells are classified as multipotent and can be found in limbal stem cells of the cornea and human umbilical tissue.4
Varieties of stem cell therapy
One familiar form of stem cell therapy is the use of embryonic stem cells (ESC) .They hold great clinical significance as they can differentiate to many ocular cell types following disease or injury.5,6 However, accurately controlling the differentiation of the pluripotent stem cell is a difficult procedure as well as there being moral and ethical concerns regarding the use of embryonic stem cells in research.7
Another form of therapy makes use of the present stem cells which can be cultured and expanded in vitro. Alternatively, tissue can be transplanted from one person to multiple sites within the individual (autologous stem cell transplantation) or from one individual to another (allograft stem cell transplantation).8,9
More interestingly, a development in 2006 allowed fully differentiated cells to transform into pluripotent stem cells.10 These cells are termed induced pluripotent stem cells (iPSC) and similarly to ESC, iPSC could be used to treat numerous ocular diseases.
[CaptionComponent="1569"]The retina
Anatomical overview
The retina is a multicellular structure which covers the posterior surface of the globe. The outermost layer of the retina consists of photoreceptors which transduce light into an electric impulse. This signal is then passed along to bipolar cells in the outer plexiform layer. After further processing, the signal is transferred to retinal ganglion cells in the inner plexiform layer before eventually leaving the eye via the ganglion cell layer.11 Within the inner layers is an additional blood retinal barrier as well as several glial cells. These include astrocytes, oligodendrocytes and Müller cells which perform important functions such as storage of neurotransmitter and provide nutrients to retinal cells.11
Retinitis pigmentosa and other retinal dystrophies
One example of a macular disease common in children is Stargardt’s (STGD). This can be caused by a genetic malformation in the ABCA4 gene inherited in an autosomal recessive fashion. This causes loss of function in a transporter protein found in photoreceptors.12 The visual impact is similar to that caused by AMD in which the central field is most affected. Other forms of Stargardt’s include STGD3 and STGD4 which are also caused by genetic mutations.13,14
Retinitis pigmentosa (RP) can also be caused by a number of genetic malformations in the RPE; these include RPE65 and MERTK genes. Similarly, it is inherited but can be either X-linked, dominant or recessive.15,16
The most common form of RP affects the gene coding for rhodopsin. For this reason, the rod photoreceptors are most impacted which explains why patients report symptoms of nyctalopia.17 Although peripheral visual field loss is common, central field visual field can also be affected.
Most degenerative retinal diseases stem from damage to the RPE. Due to its critical functions explained in a previous section, loss of these implicates the photoreceptor layer such as in STGD and RP. It has been shown that ganglion cells function even after the death of photoreceptors.18 Therefore replacement of photoreceptors through stem cell therapy could restore some, level of visual function
Current advancements and Complications
Current clinical trials have been conducted using a variety of stem cells. These include human embryonic stem cells, human umbilical tissue derived stem cells (hUTSCs) and bone marrow derived human/mesenchymal stem cells (BMHSC/BMMSC).
In a previously discussed trial, RPE cells derived from hESC were injected into a patient with Stargardt’s disease.19 Vision improved from 0 letters to 5 on the ETDRS chart and similarly there were no signs of rejection or tumour formation over a four-month period. More interestingly, there were signs of pigmentation where the suspension of cells had been injected, indicating that RPE cells were functioning. However, a further planned clinical trial may confirm these findings.
One animal study has looked at administering BMMSC in RCS rats induced with RP to see the effect they had on retinal degeneration. They recorded that over a three-month period, rod and cone cells were preserved whereas the control group only had a single layer of functioning cells.20 These results are similar to those conducted in other studies in which deterioration in visual function decreased and improvement in RPE and photoreceptor morphology was recorded.21,22
Moreover, a similar study was conducted on NOD-SCID mice (Non-Obese Diabetic-Severely combined immunodeficient) in which they wanted to establish the long-term safety of BMHSC’s in transplantation.23 These mice lacked T and B lymphocytes and can be used to transplant a wide variety of tissues into them which makes them ideal for innovative research purposes.24 Post cell injection showed that ERG traces were comparable to those left completely untreated suggesting improved retinal function. Additionally, there were no signs of tumour formation suggesting no immune reaction was generated against the transplanted cells.
Following these results, this team is recruiting for a clinical trial in which BMHSCs will be used in patients with RP and diabetic retinopathy. Although this seems to be taking a step forward in implementing these stem cells in clinical trials, the same group previously conducted a clinical trial in three patients with RP.25 Although no abnormal side effects occurred over a 10-month follow-up period, negligible improvements in visual parameters were reported with no change to visual field size. This suggests that although experiments in animal models continue to have positive outcomes, translating this success within humans is proving to be a more difficult task.
[CaptionComponent="1570"]Future directions
Studies have shown that photoreceptors can be generated from hESC26 and iPSC27 so replacing them theoretically should lead to a better visual outcome. Realistically, any transplant of the photoreceptors will require a healthy RPE for them to function effectively for a long period of time. For this reason, replacement of both the RPE and photoreceptors in a single transplant is one method which could be tested. One study has shown that photoreceptor progenitors were active for longer when cultured together with RPE derived from hESC.28 Additionally, another recent study confirmed optic cup formation derived from hESC in which both RPE and retina were present therefore implanting a similar type of graft is something which could be planned for future use29. However, stem cell therapy for use in the retina still requires further investigation in animal models before human clinical trials can commence. Moreover, re-establishing the complex retinal synapses remain a significant challenge.
Acknowledgment
The author would like to thank his supervisor, Dr Alison Binns, for her help and support.
Luqmaan Hafiz is currently completing his pre-registration optometry period.
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