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Regenerating retinal cells in mice

Regenerating Muller glia (yellow) in the mouse retina.Tom Reh, Ph.D.

The retina is the light-sensitive layer of tissue that lines the inside of the eye and sends visual messages through the optic nerve to the brain. Eye injuries and certain eye diseases that damage retinal tissue can lead to blindness. In the mammalian eye, injured retinal cells can’t regenerate.

Retinal cells do naturally regenerate in zebrafish. Müller glia, sometimes referred to as the stem cells of the fish eye, are the cells from which all other types of retinal cells are regenerated in the fish. These cells support the health and functioning of neighboring neurons and have an innate regenerative ability.

A transcription factor called Ascl1 activates a suite of genes involved in regeneration and is present in high amounts after retinal damage in zebrafish. Previous findings have shown that Asc11 can direct Müller glia to become retinal neurons in newborn mice. However, by the time the mice reach adulthood, the regions of the genetic code that are targeted by Ascl1 are no longer accessible.

A team of scientists led by Dr. Tom Reh at UW Medicine in Seattle investigated ways to encourage neuron regeneration in the adult mouse eye. The study was funded in part by NIH’s National Eye Institute (NEI). Results were published in the July 26, 2017, issue of Nature.

The team screened a library of small molecules to find one that could reopen access to the genetic regions targeted by Ascl1 in the adult mouse. They found that an anti-cancer agent called trichostatin A (TSA) altered the DNA to make the regions accessible again. The researchers used adult mice that were genetically engineered to express Ascl1 in Müller glia in response to a drug. A green fluorescent protein gene was inserted next to Ascl1, so that all cells expressing Ascl1 were labeled fluorescent green. The drug turned on Ascl1, and GFP marked the cells where Ascl1 was expressed.

After receiving a 5-day treatment to express Ascl1, the genetically engineered mice were injected with a toxin that causes cell death in retinal neurons. They were then given TSA. The team observed the shape and behavior of the fluorescent green-labeled cells for evidence of regeneration over several weeks. Genetic evidence showed that the cells that were once Müller glia were genetically reprogrammed into showing characteristics of interneurons, the cells that transmit signals from photoreceptors to the brain.

The team also found that the cells had formed functioning synapses—connections between neurons—and responded to light in a way that’s typical of a type of interneuron. The cells had also integrated with retinal cells that convey signals to the brain.

“The findings are significant because they suggest the feasibility of a novel approach for encouraging regeneration in the mammalian retina, the light sensitive tissue at the back of the eye that dies in many blinding diseases,” says Dr. Tom Greenwell, program director at NEI. “Importantly, the investigation also demonstrates that newly generated cells in the mouse retina not only look and behave like neurons, they also wire correctly to the existing neural circuitry at the back of the eye.”