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 appears in the journal Nature.
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 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.
Researchers 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," author 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.
Next, teaming up with electrophysiologists looked at the cells' electrophysiological activity in the presence of light, taking advantage of the fact that Müller glia respond to light in a distinct, measurable pattern. About two weeks following injury, the cells responded to light as if they were interneurons.
"We're showing for the first time that Müller glia in the adult mouse can give rise to new neurons after injury, and these neurons have the gene expression pattern, the morphology, the electrophysiology, and the epigenetic program to look like interneurons instead of glia," author said.
The studied cells had formed functioning synapses - connections from one neuron to another - 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.
"These findings suggest that the regenerated cells were making synapses and integrating into both sides of the circuitry, presynaptically and postsynaptically," author said.