Researchers at the RIKEN Center for Biosystems Dynamics Research (BDR) in Japan, lead by Michiko Mandai, have improved human-derived retina transplants generated in the lab using genetic alteration.
Timed removal of particular cells from the grafts after transplant into damaged mouse retinas allowed for better connections to host retinas, resulting in improved light response in the damaged eyes.
Because the retinal sheets were created using human stem cells, this marks the final steps before the method may be tried in human clinical trials for the treatment of retinal degeneration.
The study was published in the scientific journal iScience.
One promising treatment is to replace a part of the retina at the back of the eye with a new retinal sheet made of stem cells that includes photoreceptors.
For this regenerative cell therapy to work, the new light receptors in the graft must connect to neurons in the host retina, allowing light from the outside world to be relayed to the brain, which is how we see.
Based on their previous studies, researchers at RIKEN BDR recognized that linking the transplanted sheet to bipolar cells in the host retina is crucial. But the retinal sheets naturally contain their own bipolar cells.
"Bipolar cells are inevitably born when the retinal sheet develops properly and photoreceptors mature," says Mandai. "But it's their very connection to the bipolar cells in the retinal sheet that prevents the photoreceptors from connecting to the bipolar cells in the host."
The solution was to engineer retinal sheets that would lose their bipolar cells during the final stages of photoreceptor maturation.
ISLET1, a gene required for the maturation of bipolar cells that connect to photoreceptors, was the focus of the study. They started with a batch of human stem cells and created clones that lacked ISLET1 genes.
The clones were then used to grow retinal organoid sheets. These retinal sheets started out looking exactly like those made from regular stem cells. All of the major retinal cell types, especially photoreceptors, were present and organized in the correct way.
As expected, the targeted bipolar cells eventually died off, which is what happens when they are not allowed to mature.
Following this success, the researchers put their theory to the test by transplanting the new type of retinal sheet into photoreceptor-depleted rat retinas.
Several experiments revealed that the photoreceptors in the retinal sheet matured properly after transplantation and made better contact with the host eye than conventional retinal sheets.
To test whether this actually led to better responses to light, the team recorded from retinal ganglion cells, which form the optic nerve and relay visual information from bipolar cells to the brain.
As a result, positive responses indicate that more photoreceptors in the graft sheet are linked. As expected, the response to light was better in these ganglion cells than in those that received a normal retinal sheet transplant.
"The genetic modification in human stem-cell derived retinas showed a substantial functional improvement compared with the wild-type graft retinas," says Mandai. "Additionally, we were able to make detailed observations of host-graft synapse formation in the absence of graft bipolar cells, which was difficult to do before."
Despite the fact that the team has previously published similar results using mouse stem cells, Mandai believes that extending the approach to human cells is a huge step forward.
"We can now move forward to applying this strategy in clinical studies," she says. "We expect it will improve clinical outcomes and be useful in general for stem-cell based therapies targeting retinal degeneration."