At roughly 70 years human age, the mice looked elderly and unremarkable. Yet hidden underneath was a youthful cellular clock, turned back in time based on a Nobel-Prize-winning strategy. It’s also the latest bet for finding the fountain of youth, backed by heavy-hitter anti-aging startups in Silicon Valley.
At the center is partial cellular reprogramming. The technique, a sort of gene therapy, forces cells to make four proteins, collectively dubbed the Yamanaka factors. Like erasers, the factors wipe a cell’s genetic history clean, reverting adult cells—for example, skin cells—to a stem cell-like identity, giving them back the superpower to turn into almost any type of cell.
The process isn’t all-or-nothing. In a twist, scientists recently found that they can use the factors to rewind a cell’s genetic history tape rather than destroying it altogether. And if they stop at the right point, the cell dramatically loses its age, becoming more youthful but retaining its identity. The results spurred a wave of interest in moving the therapy to humans, with Calico Life Sciences—a sister company to Google—and Altos Labs, backed by Jeff Bezos, in the race.
But Yamanaka factors have a dark side. Too much, and the body develops nightmarish tumors called teratomas, an agglomeration of tissues often including partially-developed teeth, bone, and muscle. How to induce partial reprogramming without pushing cells all the way back to stem cells also remains enigmatic.
A new study, led by Dr. Juan Carlos Izpisua at the Salk Institute and Altos Labs, is cracking the code. Testing three different therapy schedules in mice, beginning at either middle or late age, the team found that brief bursts of Yamanaka factors rejuvenated both skin and kidneys in mice receiving long-term treatments. Their gene expression profile resembled that of much younger mice, with signs of a youthful metabolism.
The biggest win was that the therapy left no hints of teratomas or other health problems. “What we really wanted to establish was that using this approach for a longer time span is safe,” said study author Dr. Pradeep Reddy.
Refreshing cells in aging humans will be far trickier, given the dangers of serious side effects. Scientists are working on alternatives to gene therapy for the Yamanaka factors. If successful, the pursuit could launch radically new treatments to slow or reverse diseases that pop up with age, like osteoporosis, diabetes, and dementia.
“Our end goal is to find new forms of helping everyone to slow or even reverse the processes that lead to disease,” said Izpisua to El País. “I am convinced that within two decades we will have tools that not only treat symptoms, but also can predict, prevent, and treat diseases and aging through cellular rejuvenation.”
Tick-Tock Goes the Epigenetic Clock
How do you tell a cell’s age?
One answer lies in the epigenetic clock. If our genes are sentences, epigenetics are chemical “markers” that, like editing notes, tell a gene when to turn on or off. It’s how our cells—say neurons and skin cells—have the same DNA but look and function entirely differently.
These notes aren’t random. As we age, certain DNA letters are more susceptible to edits. One particularly strong “pen” is methylation, which adds a chemical group onto select parts of the DNA and effectively shuts a gene off. These patterns strongly correlate with chronological age (the number of years you’ve lived), so much so that they’re widely used as a biomarker for aging. In a way, these chemical markers represent a cell’s life history.
Enter Yamanaka factors. The soup of proteins that regulate DNA expression—Oct4, Sox2, Klf4, and c-Myc—are named after Dr. Shinya Yamanaka. First described in 2006, the factors erase a cell’s epigenetic landscape—including methylation patterns—and transform grown cells back into an embryonic state. The Nobel Prize-winning study heralded the era of induced pluripotent stem cells (iPSCs), the ingredients for mini-brains, lab-made embryos, and bioprinted organs.
Longevity research has a long historical crossover with the stem cell field, and Yamanaka factors soon caught the scientists’ eyes. But they asked a separate question: what would happen if we gave aging tissues just a dash of the rejuvenating potion?
The answer: a dip into the fountain of youth. In 2016, Izpisua Belmonte’s team showed that brief bursts of the factors countered signs of aging and increased lifespan in a genetic mouse model for rapid aging. Tantalizingly, the treatment also regenerated muscles and metabolism in 12-month-old mice, which are equivalent to middle-aged humans. Subsequent work also found that the factors improved heart, optic nerve, and brain function, gaining widespread interest.
“We’re investing in this area [because] it is one of the few interventions we know of that can restore youthful function in a diverse set of cell types,” said Dr. Jacob Kimmel at Calico to Nature Biotechnology.
A Recipe for Youth
To build a partial reprogramming regime, the team asked a few questions. When should we start the treatment? How long should it go on for?
They worked with three different groups of mice. One study was short, beginning treatment at 25 months old—the equivalent of roughly 80 years in human age—for just a month. The other two took the longer road. One group started around middle age, and the last at roughly 35 in human years. Both received treatments until 22 months, or about 70 years old in humans. The mice were all genetically altered so that the Yamanaka factors could be turned on by spiking their drinking water with a chemical for two days a week.
The good news? None of the mice showed signs of teratomas. The mice were also normal in their blood profile and showed similar stress and anxiety behaviors as non-treated peers.
The bad news? Short-term treatment with the factors didn’t do much. Their epigenetic clocks remained stuck in “aging mode,” with no visible improvements in bodily functions. The reason for the failure was unclear. The short-term bursts might not be enough to rejuvenate cells, or the aged mice’s genomes could be locked in a “frozen” state during aging, making reprogramming ineffective.
The long-haul mice had better luck. Their epigenetic clocks were assessed for several organs: the liver, kidneys, skin, muscles, spleen, and lungs. The skin had the best response to the treatment, with epigenetic age reversed. In a wound-healing test, the treatment bolstered the mice’s ability to heal their skin without scarring, which normally becomes an issue in elderly age. Genetically profiling the tissues, the team found upregulated genes involved in battling oxidative stress—a cellular process that damages tissues and increases with age—and a further boost in genes to dampen inflammation and senescence.
Profiling the mice’s metabolism, the treatment prevented the senior rodents from dangerous blood fatty lipid levels—a common gauge of health during aging—and a better metabolic profile. Future work needs to figure out if these “reflect healthy metabolism,” wrote Arianna Markel and Dr. George Q. Daley at Boston Children’s Hospital and Harvard University, who weren’t involved in the study,. For example, the gene expression changes could be able to fight off a whirlwind of metabolic turmoil that normally occurs with age, and combat diabetes, high cholesterol, or other age-related metabolic diseases.
Where Does This Leave Us?
The study, for the first time, showed that it’s possible to rewind the epigenetic clock in normally-aging mice with pulses of Yamanaka factors without the threat of cancer. But it leaves plenty of questions.
At the top of the list is why not all tissues were rejuvenated. The liver, muscle, spleen, and lung tissue retained their aged epigenetic programming. While it’s possible that different tissues may need personalized treatment regimes to combat aging, it’s also possible that each may have a mysterious “point of no return,” after which a tissue no longer responds to cellular reprogramming.
To Markel and Daley, who co-wrote an opinion piece, the study also didn’t report the crème de la crème of aging research: did the mice live longer?
Another problem is long-term and highly complex gene therapy. If used in humans, it adds a layer of complexity given our far longer lifespans. Several labs, including Daley’s, are trying out single factors with restorative powers, nixing the need for a four-gene therapeutic soup. Others are deciphering the biological basis of Yamanaka factors with the aim of developing drugs that could mimic the process.
“At the end of the day, we want to bring resilience and function back to older cells so that they are more resistant to stress, injury, and disease,” said Reddy. “This study shows that, at least in mice, there’s a path forward to achieving that.”
Image Credit: Nick Fewings on Unsplash
