At a Glance

• Central vision loss caused by the degeneration and death of cone photoreceptors,  the cells in the retina of the eye responsible for sharp, detailed, and color vision, is currently irreversible and incurable.
• The Gamm lab is investigating ways to restore some or all vision by transplanting healthy photoreceptors derived from stem cells into the eye.
• They discovered a new protein marker on the surface of cone photoreceptors that allows for accurate and effective identification and purification to create a population of transplant-appropriate cells.
• The marker also identifies cells that have better nerve extension qualities that make them more likely to succeed once transplanted.
• The Gamm lab has started transplantation studies in animal models using the isolation and purification method that the new marker allows for. 
• The hope is that these cells will be able to integrate into the retina, form connections, and provide some measure of vision restoration, which would ultimately benefit in developing cell-therapy based treatment approaches for the treatment of blinding conditions such as cone-rod dystrophy and macular degeneration where central vision is essentially lost.

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A newly discovered protein marker on the surface of a retinal cell could help pave the way for cell therapies that restore vision in people with currently incurable forms of blindness.

Research in the lab of Waisman investigator David Gamm, MD, PhD, professor of ophthalmology and visual sciences at the University of Wisconsin-Madison, identified a protein called CD166/ALCAM that allows scientists to identify and purify a critical type of retinal photoreceptor for the first time. The discovery, published in Stem Cell Research and Therapy, overcomes a major obstacle in the development of stem cell-based therapies designed to replace cells lost in retinal disease.

Why the discovery matters

Cell therapies, medical interventions where healthy cells are put into a person’s body to treat or cure a condition, rely on having large numbers of healthy purified cells that are ready for transplantation. For years, the Gamm lab has worked to generate retinal cells from stem cells that could someday be transplanted into patients to restore lost vision. Until now, however, they lacked a reliable way to isolate and purify one particularly important cell type, L/M cone photoreceptors.

Photoreceptors are the cells in the retina of the eye that convert light signals to nerve signals that are then sent to the brain for processing. L/M cones are two specific types of photoreceptors that handle long and medium wavelengths of light respectively. Central vision loss caused by the degeneration and death of L/M cones is currently irreversible and incurable. The thought is that if by using cell therapy, new L/M cones could be transplanted into the eye to replace lost cells, vision could be partially or completely restored.

Finding the right cells

The Gamm lab has been able to grow L/M cones as a part of their retinal organoid stem cell models for some time, but has been unable to effectively identify and purify the cells to create a transplant ready population. “Retinal organoids are miniature versions of retinas grown in  culture flasks and these organoids have many other retinal cell types in them including the cone photoreceptors ,” says Praveen Susaimanickam, PhD, first author on the new paper and a scientist in Gamm’s lab. “When you want to do cell therapy, you don’t want to put any of the other cells that are not required into a patient. And before there was no way to purify these specific L/M cones from the organoids.”

Susaimanickam began to hunt for a marker that could be used to specifically identify L/M cones in retinal organoids. “We found a surface protein called CD166/ALCAM that is uniquely expressed on these cone photoreceptors and it’s expressed at an early developmental stage as well.” Using CD166/ALCAM as an anchoring point, the lab team can then attach microscopic magnetic beads and use a column of magnets to separate the L/M cones from the rest of the cells of the organoid.

The discovery of CD166/ALCAM allows for an identification and purification method that is simple, effective, and efficient. This lets researchers create a population of L/M cone cells that are appropriate for continued study into their viability for transplantation which is a crucial step toward developing an effective cell therapy.

Gamm, Susaimanickam, and the lab team then discovered that CD166/ALCAM has two other important benefits. It identifies L/M cones at an advantageous developmental stage and its uniqueness means that researchers no longer need to mark the cells with fluorescence. “CD166/ALCAM not only marks L/M cone precursors, [which are immature L/M cones], it marks a particular migratory subset of them which is very cool because you want this property in cells being used for transplantation.” Susaimanickam says. “The more movement and dynamicity the nerve extensions of these cells have the more potential they have to be able to form functional synaptic connections.”

Researchers, including the Gamm lab, often use fluorescence to identify particular kinds of cells and processes. One method involves editing the genome to insert a gene for fluorescence that will activate under certain desired conditions. This can be used to identify cone cells versus other types of photoreceptors. But once a cell has been modified with fluorescence, it cannot be used for transplantation in humans. “But now with this marker, you don’t need fluorescence or genetic modification anymore. You can use any cell line to purify L/M cones because they all express CD166/ALCAM on the surface,” Susaimanickam says.

Looking ahead

CD166/ALCAM is a foundational step toward vision-restoring cell therapies. With the ability to accurately identify and purify the correct cells with the preferred qualities, the Gamm lab team has started transplantation studies in animal models of central vision loss. The hope is that these cells will be able to integrate into the retina, form connections, and provide some measure of vision restoration, which would ultimately benefit in developing cell-therapy based treatment approaches for the treatment of blinding conditions such as cone-rod dystrophy and macular degeneration where central vision is essentially lost.