During this process the cells not only shed any memories of their previous identities, but they revert to a younger state. They accomplish this transformation by wiping their DNA clean of the molecular tags that not only differentiate, say, a skin cell from a heart muscle cell, but of other tags that accumulate as a cell ages.
Recently researchers have begun to wonder whether exposing the adult cells to Yamanaka proteins for days rather than weeks could trigger this youthful reversion without inducing full-on pluripotency. In fact, researchers at the Salk Institute for Biological Studies found in 2016 that briefly expressing the four Yamanaka factors in mice with a form of premature aging extended the animals’ life span by about 20%. But it wasn’t clear whether this approach would work in humans.
Sarkar and Sebastiano wondered whether old human cells would respond in a similar fashion, and whether the response would be limited to just a few cell types or generalizable for many tissues. They devised a way to use genetic material called messenger RNA to temporarily express six reprogramming factors — the four Yamanaka factors plus two additional proteins — in human skin and blood vessel cells. Messenger RNA rapidly degrades in cells, allowing the researchers to tightly control the duration of the signal.
The researchers then compared the gene-expression patterns of treated cells and control cells, both obtained from elderly adults, with those of untreated cells from younger people. They found that cells from elderly people exhibited signs of aging reversal after just four days of exposure to the reprogramming factors. Whereas untreated elderly cells expressed higher levels of genes associated with known aging pathways, treated elderly cells more closely resembled younger cells in their patterns of gene expression.
When the researchers studied the patterns of aging-associated chemical tags called methyl groups, which serve as an indicator of a cell’s chronological age, they found that the treated cells appeared to be about 1½ to 3½ years younger on average than untreated cells from elderly people, with peaks of 3½ years (in skin cells) and 7½ years (in cells that line blood vessels).
Comparing Hallmarks of Aging
Next they compared several hallmarks of aging — including how cells sense nutrients, metabolize compounds to create energy and dispose of cellular trash — among cells from young people, treated cells from old people and untreated cells from old people.
“We saw a dramatic rejuvenation across all hallmarks but one in all the cell types tested,” Sebastiano said. “But our last and most important experiment was done on muscle stem cells. Although they are naturally endowed with the ability to self-renew, this capacity wanes with age. We wondered, Can we also rejuvenate stem cells and have a long-term effect?”
When the researchers transplanted old mouse muscle stem cells that had been treated back into elderly mice, the animals regained the muscle strength of younger mice, they found.
Finally, the researchers isolated cells from the cartilage of people with and without osteoarthritis. They found that the temporary exposure of the osteoarthritic cells to the reprogramming factors reduced the secretion of inflammatory molecules and improved the cells’ ability to divide and function.
The researchers are now optimizing the panel of reprogramming proteins needed to rejuvenate human cells and are exploring the possibility of treating cells or tissues without removing them from the body.
“Although much more work needs to be done, we are hopeful that we may one day have the opportunity to reboot entire tissues,” Sebastiano said. “But first we want to make sure that this is rigorously tested in the lab and found to be safe.”
Other Stanford co-authors are former postdoctoral scholar Marco Quarta, PhD; postdoctoral scholar Shravani Mukherjee, PhD; graduate student Alex Colville; research assistants Patrick Paine, Linda Doan and Christopher Tran; Constance Chu, MD, professor of orthopaedic surgery; Stanley Qi, PhD, assistant professor of bioengineering and of chemical and systems biology; and Nidhi Bhutani, PhD, associate professor of orthopaedic surgery.
Researchers from the Veterans Affairs Palo Alto Health Care System, the University of California-Los Angeles and the Molecular Medicine Research Institute in Sunnyvale, California, also contributed to the study.
The research was supported by the National Institutes of Health (grants R01 AR070865, R01 AR070864, P01 AG036695, R01 AG23806, R01 AG057433 and R01 AG047820), the Glenn Foundation for Medical Research, the American Federation for Aging Research and the Department of Veterans Affairs.
Sarkar, Quarta and Sebastiano are co-founders of the startup Turn Biotechnologies, a company that is applying the technology described in the paper to treat aging-associated conditions. Rando is a member of the scientific advisory board.