Incurable eye disorders like glaucoma and macular degeneration are the world’s leading causes of vision loss. In the United States alone, approximately 11 million people suffer from some form of macular degeneration and another 3 million are living with glaucoma. Vision loss costs national economies billions of dollars every year in direct health care expenses and lost productivity.
At the University of Washington, Tom Reh PhD, a Professor of Biological Structure and a faculty member at the Institute for Stem Cell and Regenerative Medicine (ISCRM), uses stem cell-derived retinal organoids to study how diseases of the retina form and how they can be treated. Organoids are an attractive alternative to fetal tissue, another common disease-modeling tool, because organoids closely approximate human tissue without many of the ethical questions and supply limitations that complicate the use of fetal tissue.
But exactly how well do retinal organoids stack up against fetal tissue? New research from the Reh Lab published in the journal Cell Reports sheds light on this question. “Retinal organoids look similar to fetal retina under the microscope, at the cellular level,” explains Reh. “But to better understand whether organoids resemble fetal retina at the molecular level we needed to use newly developed single cell molecular tools.”
To determine how closely stem cell-derived retinal organoids model development, the researchers used powerful RNA sequencing technology to track the development of individual cells by mapping which cells were expressing which genes as the retina developed. The Reh Lab partnered with Cole Trapnell, Assistant Professor of Genome Sciences, on the RNA sequencing effort. Additional support came from Ian Glass, Professor of Genetic Medicine, and Olivia Bermingham-McDonogh, Professor of Biological Structure, who helped to validate the data.
“With single cell sequencing, you can analyze thousands of genes in each cell and account for all the cell types present,” says Reh. “That’s much more comprehensive than traditional cell labeling techniques, which allow us to see only a few genes at a time. This kind of comparison had not been done in such an unbiased way before.” The result, according to Reh, is the first-ever single cell atlas for normal human retina development.
How Well do Organoids Match Fetal Tissue as Disease Models?
Akshaya Sridhar, a Research Scientist in the Reh Lab and the lead author of the study, breaks down what she and Reh hoped to see. “We wanted to know what cell types were being produced, whether they are being produced at the same time in organoids as in fetal retina, and in the same order, and whether the cells express the same maturity of genes over time.”
According to Sridhar, the results help affirm the viability of organoids as models for studying, and perhaps, treating retinal diseases. “Our results show that organoids faithfully mimic the order and cellular composition of the fetal retina. The use of single cell technology has helped us to recreate the cellular atlas of the fetal retina, and has led us to uncover transitional cell states in the fetal retina as cells go from being progenitors to more differentiated cells. It was exciting to find that these transition populations are retained in organoids. This indicates that differentiation in organoids also proceeds through this transition as it would in fetal tissue.”
For Sridhar, the findings detailed in the paper are years in the making, and were the result of early support from ISCRM. The preliminary development of the organoids used in the study was funded by a state-funded ISCRM Fellowship awarded to Sridhar in 2017.
Reh emphasizes the novel nature of the study. “There have been other comparisons of organoids and adult retina. This was the first comparison of organoids to cultured fetal tissue, termed retinospheres, that has allowed us to define the degree of maturation that cells in organoids can achieve. One outcome is that we’ve created new methods of culturing fetal retina, and a new standard by which our peers can measure their organoids, which brings more standardization to organoid field.”
Of particular interest to Reh and Sridhar were the ganglion cells, among the earliest cell types to form in the retina. Someday, they hope, it will be possible to grow these cells in organoids for transplantation to patients losing their vision to glaucoma and other degenerative eye disorders.
“For therapy it would be prohibitively difficult to ever obtain enough fetal tissue to derive ganglion cells for transplantation,” says Reh. “We need to know how good the ganglion cells are in organoids, and what we see is very promising. If we figure this out, we’ll reduce our dependency of fetal tissue. And, we’ll have a new benchmark for organoids.”
There was one caveat. The research team found that after approximately 100 days, the stem cell-derived organoids began to diverge from the fetal retina tissue, notably in the organization of the neurons in the inner retina, which failed to form the same quality of circuitry seen in fetal retinas.
Nonetheless, Reh is excited about the momentum. “We took a big step closer to modeling glaucoma, and the better we can model it, and the better we can treat it. Ultimately, that’s the real measure of success.”
Acknowledgements: This project was funded by an ISCRM training grant (to A.S.) and a distinguished investigator award (to T.A.R.) from the Paul G. Allen Family Foundation