Loss of visual acuity is a common feature of aging, whether it’s from age-related conditions such as glaucoma, an impaired ability for cells to respond to damage, or degeneration. In an attempt to address these issues with a genetic intervention, scientists reprogrammed neurons in mouse eyes to regrow after inducing glaucoma or crushing the optic nerve, and restored visual acuity in healthy, middle-aged mice to that of younger mice by expressing a set of genes known to revert cells to a pluripotent state, according to a study published in Nature Wednesday (December 2). The before-and-after genome methylation profiles of treated neurons and the requirement of demethylation enzymes for the success of the treatment indicate that the epigenome may be critical to aging—and to efforts to reverse it.
“This study is very exciting because it’s really proposing an approach to rejuvenate neurons. . . and is going to make a difference in the field,” José-Alain Sahel, who chairs the University of Pittsburgh department of ophthalmology and was not involved with the study, tells The Scientist.
Harvard Medical School geneticist David Sinclair and his colleagues aimed to reset the biological clocks of a group of cells to see if they could rejuvenate injured and aging cells. They looked to genes that encode transcription factors that de-differentiate cells. These so-called Yamanaka factors, named after biologist Shinya Yamanaka, are widely used to generate induced pluripotent stem cells (iPSCs). In a 2016 Cellpaper, a different group of scientists had activated these transcription factors in mice with progeria, a condition that causes premature aging, and found that the treatment alleviated symptoms and extended their lifespan by a matter of weeks. These findings raised the possibility that Yamanaka factors could hold the key to counteract aging.
A risk of using Yamanaka factors in vivo is that dedifferentiation can cause cells to divide rapidly, so researchers in the Cell study only turned them on in short bursts. To avoid such out-of-control proliferation, which would risk causing cancerous growth, Sinclair’s team eliminated one of the Yamanaka factors from their study—MYC, which is a known oncogene—and fitted the remaining three into an adeno-associated virus and injected it into mice. The researchers found that the mice didn’t develop tumors, even after more than a year. The next challenge: to see if these transcription factors could successfully revert old and injured neurons back to a more youthful and healthy state. The eyes were a logical target because younger mice can regrow axons of ocular neurons, unlike older mice.
The team focused on retinal ganglion cells (RGCs), which are neurons whose axons make up the optic nerve and snake their way into the brain, transmitting information from light-sensitive photoreceptors. Although these cells can regenerate if injured during development, they typically lose their ability to heal within a few days of a mouse’s birth. To test whether adult mice can regain this ability, the researchers crushed the optic nerves of some mice and induced glaucoma in another set of mice by inserting tiny beads into their eyes, then injected the virus encoding the three transcription factors into all the mice’s eyes.
The treatment caused mice to grow new axons from RGCs that reached back into the brain and halted glaucoma progression. The rodents with glaucoma regained around half of their lost visual acuity, marking the first time that mice with a glaucoma-like condition have restored vision loss.
“Vision loss from glaucoma was not thought to be recoverable,” says Sinclair. “Bruce [Ksander, a collaborator] called me at ten o’clock at night and told me the news. It was difficult to believe.”
Many research groups are trying to regenerate new ganglion cells outside the eye to be transplanted back in, says Sahel, but “this is much better because the cells are already [in place]—they just need to regrow and reconnect.” He notes, however, that the researchers used an acute model of injury, and that it isn’t possible to regenerate ganglion cells that have already died. “You would like to see what happens in a more advanced situation that would mimic what we see in patients that are desperate for regaining vision,” he says.
In another set of experiments, the researchers injected the virus bearing the genes that encode the three transcription factors into the eyes of healthy, middle-aged (one-year-old) mice. Although they had scored worse on tests of visual acuity than did younger mice before treatment, a month afterward, the one-year-old mice had similar acuity scores to younger ones, yet the researchers did not find an increase in the number or density of retinal ganglion cells. Eighteen-month-old mice that received the injection did not show any differences in visual acuity compared with untreated rodents, a result the authors ascribe to an increase in corneal opacity as the mice aged.
University of Edinburgh geneticist Tamir Chandra, who also studies aging and was not involved in this study, says that while these results are interesting, it is unclear to what extent their results are indicative of rejuvenation rather than dedifferentiation. “That almost sounds like a semantic question, but it’s quite important because dedifferentiation always carries the risk of inducing cancer,” he says. “The fact that they didn’t induce tumors in the healthy mice is more conceivable in the absence of MYC, but that doesn’t mean that if you had a tumor-prone mouse, that you’d get the same result.”
Epigenetics at work
Scientists have long looked to the epigenome for clues into understanding disease susceptibility, behavior, and even mental health. Sinclair has been studying how aging affects the epigenome and found in a 2008 Cell study that a loss of epigenetic information over time contributed to aging.
“If epigenetic loss drives aging, can you recover it, thereby reversing the aging process, and if you do that, do you get the youthful function of a complex tissue back again?” he says.
When the researchers looked at DNA methylation patterns of the retinal ganglion cells, they saw that changes caused by injury resembled methylation patterns in the ganglion cells of older mice, and that treatment with the transcription factor–encoding virus reversed those changes. The intervention did not work in mice that did not have the enzymes necessary to remove methyl groups from DNA, suggesting that the demethylation process plays an important role in rejuvenating the neurons.
“It’s not just that gene A went off with aging and came back on with treatment,” says Sinclair. “There are hundreds and hundreds of genes that went down a certain level and then came back up a proportional level with the programming. So somehow the cell has recorded not just which genes should be altered, but the level [to which] they should be altered.” Still, Sinclair says, this study doesn’t show whether DNA methylation is directly responsible for rejuvenation or if it is more of a bystander.
Both Chandra and Sahel question the paper’s general relevance to aging, as the researchers used injury models rather than degeneration models, but they are encouraged by the potential new avenue of treatment.
“It’s already amazing that we are able to ask these questions based on this paper,” says Sahel.
