A Japanese research team has reported a major breakthrough in the development of stem cell therapies for retinal degeneration.
The report represents a world-first in deriving rod and cone photoreceptors from mouse, monkey, and human embryonic stem cells in a laboratory dish.
The findings, which appeared in the February issue of the journal Nature Biotechnology (2008;26:215-
224), create considerable hope for using stem cell therapy to treat such retinal degenerative conditions as retintis pigmentosa, age-related macular degereration, cone-rod dystrophies.
For those patients in whom the numbers of rod and cone photoreceptor cells have fallen below a critical level, one of the few potential therapeutic options may be a retinal tissue transplant from a healthy donor.
The principle of retinal tissue transplantation, while straightforward, has been dogged by challenging technical difficulties, including the maintaining of sufficient control over the recipient and donor environments and the securing of a sufficient pool of donor material.
The findings of the Japanese researchers may help overcome the second challenge and provide a sufficient pool of donor tissue.
Stem cells set themselves apart from all other cells in the body in two basic ways. First, they possess the unlimited ability for self-renewal; second, they maintain the capacity to develop into any one of the highly specialised cells of the body, such as lens epithelial cells or photoreceptor cells.This
developmental potential may hold wide-ranging therapeutic applications for a broad array of retinal degenerative conditions.
Embryonic stem cells are of particular value as they may allow the production of an infinite number of cells available for use in both research studies and therapeutic treatment. Up until Dr Takahashi’s breakthrough, the generation of photoreceptors from precursors and embryonic stem cells was relatively inefficient.
Clinical use of such photoreceptor material was not ideal, so many research teams have focused on developing the right conditions to generate the necessary specialised cells from an embryonic stem origin.
Research groups worldwide have achieved considerable successes in progressing the fields of embryonic stem cell biology and, specifically, ocular cell transplantation. A major goal of these groups has been to generate mature rod and cone photoreceptor cells from embryonic stem cells in defined culture conditions; however, the task has proved to be stubbornly elusive. Given that the retina exhibits many similarities to brain tissue, many researchers traditionally believed that retinal tissue, too, had limited ability to repair itself.
However, in recent years, particularly in animal models of retinal degeneration, a number of technical advances have provided renewed optimism that ophthalmic surgeons could stabilise or even improve retinal function by transplanting retinal progenitor or precursor cells to the retina.
UK research published in 2007 and reported in last July’s edition of EuroTimes, demonstrated the expression of photoreceptor cell markers following transplantation of neural retina. Despite that success, generating such photoreceptors under artificial lab conditions remained a highly challenging exercise. As in practically all fields of organ transplant medicine, the availability of donor tissue is a major limiting factor. Consequently, any technology that can provide a continuous supply of rod and cone photorceptor cells would create important advances for patient treatment.
One of the most difficult issues in the use of embryonic stem cell technologies is to control and direct the stem cells into the specific type of cell required for therapeutic intervention. Previously, many groups had reported the generation of neural and retinal pigment epithelium (RPE) cells from stem cell populations; however, generating fully differentiated mature rod or cone photoreceptors turned out to be a highly complex endeavour.
Against such a background, researchers led by Dr
Masayo Takahashi and based at the Laboratory for
Retinal Regeneration, in Kobe, Japan, have now grown putative photoreceptor cells in a laboratory dish from embryonic stem cells of mouse, monkey and human origin using defined culture conditions.
To generate such cells, the Japanese research team built on more than 10 years of experience in cell transplantation and stem cell work at their labs.
From previous research, the group had found that it could differentiate embryonic stem mouse cells to retinal progenitor cells by exposing the embryonic stem cells to a variety of cell signalling molecules that included “Wnt” and “Nodal” antagonists. Such antagonists are well characterised cell signalling proteins involved in cell differentiation.
From such a cell population, the team was able to isolate a subset of the retinal progenitor cells expressing a key transcriptional factor for specification of the retina known as “retinal homeobox” or “Rx”. Once an Rx enriched population of retinal progenitors was generated, the next step was to direct such cells into further differentiation along a photoreceptor cell lineage rather than progress the cells along a self-renewal pathway.
The researchers found that inhibition of “Notch,” a key cell signalling protein, promoted the differentiation of the Rx retinal progenitor cells into photoreceptor cell precursors.These photoreceptor precursors would then spontaneously differentiate into opsin expressing cone cells for colour vision or, upon further manipulation with growth factors, taurine, and retinoic acid, would differentiate into rhodopsin expressing rod cells for night vison.
This achievement of generating one of the most complex mature neural cells in the body through a defined protocol – particularly without co-culturing them with retinal tissues – represents a major advance and may eventually lead to new applications to treat patients diagnosed with a variety of retinal degenerations.
Not content with demonstrating this key achievement with mouse embryonic stem cells, the research group applied their techniques to both monkey and human embryonic stem cells to obtain similar results; however, the generation of rod and cone cells from human embryonic stem cells took between 200 and 300 days.
The next steps for this research will involve a thorough analysis of the embryonic stem approaches in the appropriate animal models as researchers ask themselves, “Can such tissue cultured derived photoreceptor cells orientate and function correctly in the physiological environment of a degenerating retina?”
The anwer could provide a milestone on the road to applying such exciting technology to the clinical setting.