Researchers at the National Eye Institute (NEI) have identified distinct differences between the cells that make up a tissue in the retina that is essential for human visual perception.
According to the NEI, the team discovered five subpopulations of retinal pigment epithelium (RPE) — a layer of tissue that nourishes and supports the retina’s light-sensitive photoreceptors. Using artificial intelligence, the researchers analyzed images of RPE at single-cell resolution to create a reference map that locates each subpopulation in the eye. A report on the research published in Proceedings of the National Academy of Sciences.1
“These results provide a unique framework for understanding different subpopulations of RPE cells and their vulnerability to retinal disorders, and for developing targeted therapies to treat them,” said Michael F. Chiang, MD, director of the NEI, part from the National Institutes of Health.
“The findings will help us develop more accurate cell and gene therapies for specific degenerative eye diseases,” said the study’s principal investigator, Kapil Bharti, PhD, who leads the division of NEI Ocular and Stem Cell Translational Research.
Vision begins when light hits the rod and cone photoreceptors that line the retina at the back of the eye. Once activated, photoreceptors send signals through a complex network of other retinal neurons that converge at the optic nerve before traveling to different centers in the brain. The RPE sits below the photoreceptors as a monolayer, one cell deep.
Age and disease can cause metabolic changes in RPE cells that can lead to photoreceptor degeneration. The visual impact of these RPE changes varies widely with severity and where the RPE cells are located in the retina. For example, late-onset retinal degeneration (L-ORD) primarily affects the peripheral retina and thus peripheral vision. Age-related macular degeneration (AMD), a leading cause of vision loss, primarily affects RPE cells in the macula, which is crucial for central vision.
According to the NEI, Bharti and colleagues sought to determine whether there are distinct RPE subpopulations that could explain the broad spectrum of retinal disease phenotypes.
The NEI team used artificial intelligence (AI) to analyze RPE cell morphometry, the external shape and dimensions of each cell. They trained a computer using fluorescently labeled images of RPE to analyze the entire human RPE monolayer from nine cadaver donors with no history of significant eye disease.
Morphometry characteristics were calculated for each RPE cell – on average about 2.8 million cells per donor; A total of 47.6 million cells were analyzed.
The algorithm assessed each cell’s area, aspect ratio (width to height), hexagonality, and number of neighbors. Previous studies had suggested that RPE function is related to the tightness of cellular connections; the busier, the better for indicating cellular health.
Based on morphometry, they identified five different subpopulations of RPE cells, designated P1-P5, organized in concentric circles around the fovea, the center of the macula, and the most light-sensitive area of the retina.
Compared to RPE in the periphery, foveal RPE tend to be perfectly hexagonal and more compact, with a greater number of adjacent cells.
Unexpectedly, they found that the peripheral retina contains a ring of RPE cells (P4) with a cell area very similar to RPE in and around the macula.
“The presence of the P4 subpopulation highlights diversity within the retinal periphery, suggesting that there may be functional differences between RPE that we are not currently aware of,” the study’s lead author, Davide Ortolan, PhD, a research associate in the NEI Ocular and Stem Cell Translational Research Section, said in the press release. “Future studies are needed to help us understand the role of this subpopulation.”
Next, the NEI team analyzed RPE from cadavers containing AMD. Foveal (P1) RPE was mostly absent due to disease damage and the differences between cells in the P2-P5 subpopulations were not statistically significant. In general, the AMD RPE subpopulations tended to be elongated relative to RPE cells unaffected by AMD.
To further test the hypothesis that different retinal degenerations affect specific RPE subpopulations, they analyzed ultrawide-field fundus autofluorescence images of patients with choroideremia, L-ORD or a retinal degeneration with no identified molecular cause. Although these studies were conducted at a single point in time, they still showed that different RPE subpopulations are vulnerable to different types of retinal degenerative diseases.
“Overall, the results suggest that AI can detect changes in RPE cell morphometry prior to the development of visibly visible degeneration,” Ortolan said in the release.
The NEI concluded in the press release that age-related morphometric changes may also occur in some RPE subpopulations before being detectable in others. This finding will aid future studies using non-invasive imaging technologies, such as adaptive optics, that resolve retinal cells in unprecedented detail and could potentially be used to predict changes in RPE health in living patients.
The study was funded by the NEI Intramural Research Program.
1. Ortolan D, Sharma R, Volkov A, Maminishkis A, Hotaling NA, Huryn LA, Cukras C, Di Marco S, Bisti S, Bharti K. “Single cell resolution map of human retinal pigment epithelium helps discover differential subpopulations Disease Susceptibility” Published in PNAS May 6, 2022. https://doi.org/10.1073/pnas.2117553119