iPSC-Derived Retinal Endothelial Cells Offer Platform for Studying Diseases

Biomedical engineers at Duke University have for the first time used induced pluripotent stem cells (iPSCs) to grow specialized blood vessel cells critical to retinal health.

When injected into mouse models of retinal disease, these “retinal endothelial cells” (iRECs) integrated into the damaged tissue to regenerate blood vessels and restore retinal function. The team also demonstrated these cells’ ability to form functional retinal vascular tissue in a lab-grown environment, providing a pathway to model and research various eye diseases.

The results point toward the potential of using these retinal cells and models to develop new methods of impactful vision loss treatments and eye disorder research. “Retinal vascular diseases affect millions of people in the U.S., but our understanding remains limited, hindering our ability to discover and develop new therapeutics,” said Sharon Gerecht, PhD, the Paul M. Gross Distinguished professor and chair of Biomedical Engineering at Duke. “Using human stem cells, we generated the cells found in retinal blood vessels, paving the way for new therapeutic approaches.”

Gerecht is senior and corresponding author of the researchers’ published paper in Nature Biomedical Engineering, titled “Derivation of functional retinal endothelial cells from human pluripotent stem cells for therapeutics and modeling.” In their report the authors suggested that their iREC differentiation strategy will “… advance cell therapy and disease modeling, accelerating the discovery of treatments for retinal microvascular diseases.”

The old saying that the eyes are windows into the soul is more accurate than one might think. Neurons from the retina—the back part of the eye that detects light—extend directly to the brain, technically making the eyes part of the central nervous system.

Also like the brain, the retina has a blood barrier that strictly controls what gets in and out including oxygen, nutrients, water and pharmaceuticals. While this barrier keeps the retina healthy and relatively protected from disease-causing agents, it also makes treating the retina difficult. “Retinal tissue has the highest energy and oxygen usage in the body due to the retina’s intense and continuous neuronal activity,” the authors further explained. “This demand leads to a crucial reliance on the inner blood–retina barrier (iBRB) to maintain ocular homeostasis.”

The barrier is formed by blood vessel tissue comprising a tight network of retinal endothelial cells, which form the inner layer of blood vessels, in concert with other specialized cells called pericytes and astrocytes. “Retinal endothelial cells (RECs) in the iBRB are continuous endothelial cells (ECs) that form tight junctions to regulate the diffusion of small molecules, such as ions and water, across their cell–cell interface,” the investigators continued. The specificity of these cells and the fact that they do not form in other areas of the body make the complex tissue difficult to heal or to grow from scratch.

These retinal endothelial cells are currently collected and grown from real patients, making them relatively expensive with a limited supply. “A renewable source of human iBRB endothelium is thus vital for advancing eye research and treatment development,” the team noted in their paper.

To expand access, reduce cost and control variability, the Gerecht lab wanted to see if they could grow them from iPSCs. These are essentially mature adult cells reprogrammed to become primal versions of themselves that can then grow into a wide variety of other cell types.

To do this, Esswein and Ying-Yu Lin, PhD, a former PhD student in Gerecht’s lab, took commercial iPSCs and used a well-established procedure to get them to grow into common endothelial cells that form the inner layer of most of the body’s blood vessels. The researchers then used a specialized cocktail of growth factors to coax the cells into becoming the specific type of endothelial cells found in the retina. “… we differentiated human induced pluripotent stem cells into retinal endothelial cells (iRECs) via the Wnt–β-catenin pathway, namely Norrin–Frizzled4 signaling,” they explained.

Once successful, the researchers put their development to the test. In benchtop experiments, the team was able to get the iRECs to form the same networks and structures that they do within the body. The team then subjected these lab-grown tissues to low oxygen and high glucose levels, which are detrimental conditions often seen within real people. These conditions are fundamental causes of diabetic retinopathy (DR), the leading cause of vision loss in working-age people in the United States, and caused the tissue barrier to break down just like it does in patients. They wrote in summary, “Overall, we were able to robustly recapitulate the DR phenotype in 2D and 3D with our iRECs, exemplifying their ability to be utilized for in vitro disease modeling and to elucidate aberrant pathways and therapeutic targets.”

The researchers then tried their lab-grown cells as a therapy for mouse models with weak, unstructured retinal blood vessels. When injected into the mice before any actual vision loss occurred, these cells successfully integrated into the existing tissue and helped develop strong blood vessels with strong barriers. “When injected into oxygen-induced retinopathy mice, iRECs integrated into the host vascular network and revascularized the ischemic eye, rescuing the tissue,” they stated.

“The tests showed that these lab-grown cells have promise for preventative treatments, especially since they should be easier and cheaper to obtain using our technique,” Esswein said. “And while our benchtop experiments did not attempt to model a wide variety of specific eye diseases in these studies, we’re confident we can create excellent human tissue models in the lab to help better understand these diseases and uncover therapies.”

Moving forward, the researchers are planning to explore these potential uses for their retinal endothelial cells both in their laboratory and through emerging industry partnerships. The group also has a patent pending that covers both the stem cell-based therapeutics and in vitro modeling for drug discovery and testing. In their paper they concluded “Our study establishes functional human iRECs and microphysiological iBRB models that facilitate mechanistic studies aimed at identifying therapeutic targets and promoting the revascularization of injured retinas, thereby supporting treatment advancement.”

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