STEM CELL THERAPY AND CELL TRANSPLANTATION

What have we achieved?

Transplantation of retinal precursor cells
We have recently examined the ability of transplanted cells to connect to the retina in an adult eye. Instead of using stem cells we performed these experiments with retinal progenitors, cells from the eye of a newly born mouse that are about to turn into photoreceptors. When these cells were injected into the eye of another young mouse, a small percentage would integrate properly into the retina. This was a promising result, but not very surprising as the retina of the recipient mouse was being formed at the time. Therefore it was receptive to connecting up new cells. The next step was to inject these same progenitor cells into an adult eye, which was already fully formed. Again the cells appeared to integrate and connect properly into the retina. As our immature photoreceptors could integrate into an adult retina, we finally attempted to show that transplanted cells could improve the function of the retina. The retinal progenitor cells were injected into the retinas of adult mice with retinitis pigmentosa. As is the case in the adult normal mice a fraction of the transplanted retinal cells integrated into the retina. Before treatment the retinas of these mice are not sensitive to light, but we could show that after integration of the healthy progenitor cells, a flash of light resulted in a measurable electric response in the nerve that runs from the eye to the brain. We could also show that the pupil reflex, the closing of the pupil after shining light into the eye, was restored after the transplantation. These experiments for the first time provided proof that transplanted cells had wired up correctly and were signalling when they were stimulated with light.
These results are very encouraging, as they predict that if the right cells can be found, it is probable that some vision can be restored in patients. We are continuing these experiments to find methods that improve the integration process, thereby allowing higher numbers of healthy cells to connect to the retina.

Isolation and culture of stem cells
There are a number of requirements for a perfect stem cell source. Ideally we would like to be able to grow up stem cells from the patients’ own tissue, as that would solve problems such as rejection of the transplant. Secondly, the cells need to grow well in the laboratory, so that from a small number of cells we can grow up sufficient numbers to treat a substantial proportion of the retina. Finally, the cells need to be able to turn into photoreceptor cells when they are injected, a process called differentiation.

We are currently designing the methods to culture stem cells from several sources, such as the iris and the edge of the retina. At the moment these experiments use mainly tissue from various animals, but when the procedure will be performed on patients, a small biopsy might be taken from the patient’s eye for use as a source of cells. To grow up the animal stem cells, the correct culture conditions must be found that allow the cells to divide without them turning into various retinal cell types. When a sufficient number of cells has been obtained, the conditions are changed such that the cells mature to form photoreceptor cells.
These exact conditions are difficult to define and finding the optimal growth medium is a time consuming process. Our experiments have progressed to the extent that we can grow up stem cells from various mouse and pig eye tissues efficiently. These cells remain immature and able to turn into a variety of cell types when the conditions are changed. The next stage will be designing the methods to make the immature stem cells differentiate specifically into photoreceptor cells.

Differentiation of stem cells
Normally, the differentiation of immature cells into photoreceptor cells occurs only when the eye is formed in the embryo. Over the years many groups have studied the circumstances under which this occurs and we have a fairly good understanding of the process.

In order to induce differentiation of stem cells to photoreceptor cells, we have two tools that we can use. Firstly, we can add certain chemicals, called growth factors, which are present during the development of the eye. Many different growth factors exist and the combination, timing and amount will affect the efficiency of photoreceptor formation. A combination of recreating the mix of growth factors in the eye and trial and error may lead us to the find the optimal conditions for differentiation.
Secondly, we can use genes to trigger the differentiation to photoreceptors. Genes are encoded messages that tell the cell to perform a specific task. A more detailed description of ‘genes’ is given here. During the development of the retina, specific genes become active to force the cells to become immature retinal cells; later other genes cause them to become specific retinal cell types, such as photoreceptors. From previous research, we have a good understanding of which genes are involved in these processes. If we can temporarily activate the correct gene or combination of genes, we should be able to induce the differentiation to photoreceptor cells.

In practice it is likely that a combination of growth factors and genes is required for the optimal efficiency of photoreceptor differentiation.

This page last modified 18 December, 2012 by xxx