In the search for treatments for degenerative eye disorders, many stem cell therapies have been tried for a few years. However, these tests are preceded by rigorous validation processes, at the risk of representing a danger for patients. In this sense, American researchers have spent nearly 10 years designing, from reprogrammed skin stem cells, organoids that can fully reproduce the functioning of retinal cells. The last step before the clinical trial was here to see if the cells of these organoids could correctly establish synaptic connections with those still (intact) from sick patients. The goal is that eventually, the clusters of organoids can replace damaged retinal cells in people suffering from ocular degeneration.
In primates, the cells that interact with light and those responsible for “high definition” vision are those in the shape of a cone, located in the fovea. In people with degenerative eye disorders, these cells die, fail to renew themselves, and cause partial or even total blindness. In previous studyresearchers from the University of Wisconsin-Madison (USA) succeeded in genetically reprogramming skin cells to act as stem cells and differentiate into retinal cells functionally similar to the cones of the fovea.
However, the greatest difficulty with stem cell therapies is not being able to ensure that once differentiated in vitro, they behave correctly after implantation in a patient. In 2017, patients were indeed victims of poorly controlled clinical trials, where fat stem cells were directly injected into their eyes the same day after the samples were taken. The results were disastrousbecause once injected, the stem cells had differentiated into myofibroblasts, causing retinal detachment and severe intraocular hemorrhage in three patients.
In the new study from the University of Wisconsin-Madison, more than ten years were spent establishing the right protocol in the laboratory, to ensure that stem cells properly differentiate into retinal cell organoids. ” We wanted to use cells from these organoids as spares for the same types of cells that have been lost in retinal diseases. “, explains in a communiqué David Gamm, professor of ophthalmology at the University of Wisconsin-Madison, director of the McPherson Eye Research Institute (the lab that developed the organoids) and co-lead author of the new study.
Once this step was carried out, it was necessary to determine if, once separated from the compact organoid, the cells would behave correctly and establish connections between them, in order to be able to transmit the light signals to the brain. Retinal cells communicate through synaptic connections, a common point with neurons. These are tiny spaces between each axon that carry sensory information between cells.
The cells from the organoids developed by the researchers therefore had to acquire the ability to correctly develop the equivalents of these axons and these synapses, in the form of small functional cords. ” The final piece of the puzzle was to see if the cords had the ability to plug into or bind to other types of retinal cells in order to communicate. “says Gamm.
Synaptic connections established
According to the results reported in the review PNAS, the researchers used genetically modified (and rendered harmless) rabies viruses to identify pairs of cells capable of forming synaptic connections between them. They previously isolated individual cells from the compact organoids, and gave them a week to develop axons.
Thanks to the fluorescent labeling of the viruses, the researchers were able to see that connections were established between the cells, because they could have been infected by the virus (passing through the synapses). Additionally, the researchers were also able to confirm that the cells that formed synapses and connections were photoreceptors, similar to the cone-rod cells of the retina.
The next step will now be to see how these cells behave once implanted in patients. ” All of this ultimately leads to human clinical trials, which are clearly the next step. “says Gramm. It should be remembered that this step remains delicate, insofar as it is possible that the cells behave differently once implanted, and that they do not perform the expected functions or even cause serious side effects.