A cure for blindness is one step closer to reality, say researchers
A cure for blindness is one step closer to reality after researchers successfully transplanted light-sensitive photoreceptor cells from a synthetic retina that was grown from embryonic stem cells. So says a new study published July 21 in the journal Nature Biotechnology.
The photoreceptor cells were transplanted into the eyes of night-blind mice by researchers from University College London (UCL) and Moorfields Eye Hospital in the UK.
The cells, which developed normally, integrated into the existing retina in the mice and formed the required nerve connections that transmit visual information to the brain.
According to the study, embryonic stem cells could potentially be used to provide an "unlimited supply of healthy photoreceptors for retinal cell transplantations to treat blindness in humans."
What are photoreceptors and why is there a need for photoreceptor transplantations?
There are two types of photoreceptors, rods and cones, which are light-sensitive nerve cells found in the retina of the eye.
The cones are responsible for the eye's color sensitivity, and while the rods are not color sensitive, they are more sensitive to light and are especially crucial for allowing the eyes to see in the dark.
A leading cause of blindness in degenerative eye conditions like age-related macular degeneration, retinitis pigmentosa and diabetes-related sight loss, is the loss of photoreceptors in the eye.
In a study conducted last year, the researchers transplanted photoreceptors from healthy mice with normal sight into mice with sight loss due to retinal degeneration. At the time, the research team said that the method of transplantation they used back then would "not be practical for the thousands of patients in need of treatment."
However, they also said that they “are hopeful that we will soon be able to replicate this success with photoreceptors derived from embryonic stem cells and eventually to develop human trials."
They key, according to Professor Robin Ali of the Institute of Ophthalmology at UCL and Moorfields Eye Hospital, is to find a cell source.
"Much of this work has been done in mice in the past. Photoreceptor precursor cells taken from the developing mouse retina and pumped into adult mice shows that this can be effective in restoring vision for the mice that lack vision. This really gave the framework for our translation program,” said Ali.
“To make it practical, we needed to find a cell source from which we can get these photoreceptor precursors," Ali added.
"We have been working on trying to find ways of repairing the retina by transplanting photoreceptor cells, and we have demonstrated proof of concept of that development. They are not stem cells, they are not fully mature photoreceptor cells, but they are immature photoreceptor cells," the professor explained.
So how was the synthetic retina grown?
To grow the synthetic retina, the researchers used a new method recently developed in Japan, using 3D culture and variations of mouse embryonic stem cells.
This 3D culture method allowed the researchers to closely monitor the retinal precursor cells as they were growing while closely comparing them to normally developed cells at different stages of development.
Additionally, the researchers performed tests to make certain that the genes expressed by the two types of cells were "biologically equivalent" to each other; thus, enabling them to grow the synthetic retinas "in a dish" that contained all the nerve cells necessary for providing sight.
"What we have been able to do is build on work of a Japanese group from a study a couple of years ago, in order to make a synthetic retina from embryonic stem cells,” Ali explained. “We have adapted that, and we have shown for the first time that we can use embryonic stem cells to make a retina in a dish."
For this new study, the researchers injected around tens of thousands of artificially grown cells into the retinas of the night-blind mice. Three weeks after transplantation, the cells from the synthetic retina had "moved and integrated" within the mice retina and started to look like "normal mature rod cells" that continued to be present six weeks later.
The researchers also said that nerve connections developed. In other words, the transplanted cells were able to connect with the existing synapses within the retina, which gave them the cell source they needed.
"This now means we have a cell source. This has all been done with mouse embryonic stem cells, but if we do it with human embryonic stem cells then we can do this for the first time using an embryonic stem cell source," said Ali.
"That means we have got room to think about a human trial and repeat all this using human embryonic stem cells, and investigate whether we can repair the retina in conditions in which blindness is caused by loss of photoreceptor cells," he explained.
Although it will be a few years before this latest research will be used in a human trial, Ali says the research team has already started working with human embryonic stem cells.
"There are a number of ways that we can use this research to develop ways of treating blindness through gene therapy and artificial retinas. This is a very exciting approach because it has the ability to restore vision in patients who have very little vision, and the main cause of this in the developing world is loss of photoreceptors. Currently there is no treatment for that," he said.
SOURCE: Nature Biotechnology, “Photoreceptor precursors derived from three-dimensional embryonic stem cell cultures integrate and mature within adult degenerate retina”, published online July 21, 2013 (doi:10.1038/nbt.2643).