NEI–funded researchers use a clue from zebrafish to discover the cues that reprogram Müller glia into retinal neurons.
Cells within an injured mouse eye can be coaxed into regenerating neurons and those new neurons appear to integrate themselves into the eye’s circuitry, new research shows. The findings potentially open the door to new treatments for eye trauma and retinal disease.

The study appeared in the July 26 issue of the journal Nature.

The study’s lead investigator, Tom Reh, PhD, and his team at UW Medicine in Seattle, looked to the zebrafish for clues about how to encourage regeneration in the mouse eye. When a zebrafish injures its eye, cells within the eye naturally regenerate, allowing the fish to maintain vision. Mammals lack this regenerative ability.

In studying zebrafish the research team homed in on Müller glia, a type of retinal cell that supports the health and functioning of neighboring neurons, and that also exhibits an innate regenerative ability. Sometimes referred to as the stem cells of the zebrafish eye, Müller glia are the cells from which all other types of retinal cells are regenerated in the fish.

Earlier research from Reh’s lab showed that in newborn mice, Müller glia can be directed to become retinal neurons by activating a transcription factor called Ascl1, which in turn activates a suite of genes involved in regeneration. By the time the mice reach adulthood, however, regions of the genetic code that are targeted by Ascl1 and that are required for regeneration become inaccessible. In other words, in adult mice, regions of the genetic code that are critical for regeneration are closed for business.

Nikolas Jorstad and Matt Wilken, graduate students in Reh’s lab, screened a library of small molecules to find one that could reopen access to the genetic code in the adult mouse.

“We found that the commonly used anti–cancer agent trichostatin A (TSA) made critical regions of DNA accessible again. Ascl1 could then bind to those regions, which stimulated the regeneration of neurons in the adult mice,” Jorstad said.

The researchers used an adult mouse model genetically engineered to express Ascl1 in Müller glia in response to tamoxifen, a commonly used breast cancer drug. In this engineered mouse, the green fluorescent protein (GFP) gene is inserted next to Ascl1, so that all cells expressing Ascl1 are labeled fluorescent green. Tamoxifen turns on Ascl1, and GFP tracks the cells where Ascl1 is expressed.

The researchers injured the mice retinas with a toxin that causes cell death in retinal ganglion cells and interneurons, another type of retinal cell whose job it is to transmit signals from photoreceptors to the brain. They then injected the mice with TSA and tamoxifen. Over the next several weeks, the shape and behavior of the fluorescent green–labeled cells were observed to see if there was evidence of regeneration.

Proteins expressed by the observed cells were similar to those of interneurons. Analyses of genome structure further shored up evidence that the cells that were once Müller glia had been genetically reprogrammed and were now showing characteristics of interneurons.