A PUDGY BLACK MOUSE snuffles around a tiny tower of Legos, turns away, then comes back to snuffle again. He’s 18 months old—a senior citizen, in rodent terms. And it’s getting tough to keep it all straight. Do these blocks seem familiar to him? Has he seen this thing before?
He’s a bit muddled, but that’s not his fault. Few new neurons are being born inside his itty-bitty brain. The cells that once exuberantly branched, sending lush offshoots to interweave and connect with others, are now sparse and barren.
This Lego test indirectly measures those physical changes by monitoring his behavior. When mice of a certain age become forgetful, they spend more time checking out little trinkets they’ve seen before—objects that should warrant only a quick “Oh yeah, that thing again” glance. Cameras and laser-based detectors mounted on the ceiling capture and quantify those pauses and vacillations.
Alana Horowitz, the University of California, San Francisco graduate student conducting this FaceTime lab tour, puts her phone camera right up to the mouse’s muzzle. His eyes are bleary, like an old barfly’s. He probably hasn’t groomed himself recently, she says. His coat looks shabby and worn. You’ve likely never looked an elderly mouse in the face, but if you did, all of this—the thinning fur, the dim eyes, the hesitation—would be depressingly familiar. He inspires pity. Like sands through the hourglass, little fella.
But in this lab, headed up by neurobiologist Saul Villeda, nobody is sighing and moping over graybeard mice. Here, aging is not a sad fate to bemoan; it’s a problem to be solved. And for mice, at least, this team has already figured out how to reverse the damage time brings.
The secret is somewhere within those tiny veins. In a series of studies over the last 15 years [this article was written in 2021], Villeda and others in a few like-minded labs at places like Stanford and Harvard have shown that, when infused with blood from young mice, old ones heal faster, move quicker, think better, remember more. The experiments reverse almost every indicator of aging the teams have probed so far: It fixes signs of heart failure, improves bone healing, regrows pancreatic cells, and speeds spinal cord repair. “It sounds sensational, almost like pseudoscience,” says Villeda. It’s some of the most provocative aging research in decades.
These studies, which use a peculiar surgical method called parabiosis that turns mice into literal blood brothers, show that aging is not inevitable. It is not time’s arrow. It’s biology, and therefore something we could theoretically change. The attempt to turn back the clock in living bodies “is probably the most revolutionary experiment that biologists have done,” says Stanford professor of neurology Tony Wyss-Coray, who was Villeda’s graduate supervisor and still leads blood-based research on Alzheimer’s disease and cognitive decline. “It supports this notion that it is possible to reassemble and fix things that we thought are doomed to die.”
We know blood does indeed perform some kind of alchemy that can restore and remodel the flesh or hasten its decay.
Benjamin Button-ing, of course, isn’t natural. But Villeda counters that getting old isn’t either: “It is the most artificial construct.” Previously, only a very few rare individuals reached 90 or 100. Now, in wealthy nations, it’s becoming downright common. With antibiotics, vaccines, public health measures, and a steady food supply, the industrialized world made the long, slow goodbye of aging commonplace—and, along with it, the consequences, such as brittle bones, Alzheimer’s disease, diabetes, and heart failure. Young-blood research, like some gory fairy tale, whispers to us that there could one day be a magic pill that can fix it all. The plot twist: That bloody fountain of youth was inside our bodies all along.
Biologists like Villeda just haven’t yet figured out why all this trading works.
Blood itself will not become a treatment for old age. It’s too messy, too complicated, too dangerous. But because of these labs’ findings, we know that somewhere swirling around in young veins are signals that awaken the natural mechanisms to repair and restore the body. These mystery factors, once researchers can identify and fine-tune them, could become precious medicine.
Villeda’s group in particular is applying parabiosis to address the toughest task of all: fixing an elderly brain. His team is also testing whether other physiological benefits, like those brought on by fasting or exercise, could be spotted in blood and distilled into a remedy for the aged. “We know there’s a needle in this haystack,” he says. “We just have to figure it out.”
THE IDEA THAT BLOOD can impart vigor and vitality has a long and stomach-turning history. Pliny the Elder, writing in first-century Rome, describes people with epilepsy guzzling the gore of wounded gladiators. Similar motifs reappear frequently in European lore: The sickly 15th-century pope Innocent VIII allegedly traded blood with three shepherd boys; all four died shortly thereafter.
Once British physician William Harvey mapped the circulatory system in 1628, swapping fluids became a fad. Across France and England, enterprising proto-scientists linked animals to animals and animals to people, and on and bloody on. Their hypothesis was that blood could remodel the flesh. In 1666, for instance, the legendary natural philosopher Robert Boyle proposed that introducing blood from a cowardly dog into a fierce one might temper the savage beast’s nature.
In 1667, London’s Royal Society hosted a public experiment in which a surgeon paid a man suffering from mental illness to be linked to a living sheep for a few moments via feather quills and silver pipes. Perhaps the gentle lamb’s essence might ease his agitation, was the thinking. Afterward the fellow indeed “found himself very well,” at least according to the surgeon, and he allegedly went on to spend his fee in the tavern. (The sheep’s feelings were not recorded.)
Months later, a Frenchman died following a transfusion, taking the wind out of these blood-spattered sails. The pope himself (Innocent XI, this time) put an end to the practice in 1679.
A new round of transfusion science emerged in the early 19th century, this one with much more scientific rigor. These experiments helped establish the first real knowledge about how to keep injured soldiers from bleeding out or mothers from dying in labor. But it wasn’t until 1864 that a Parisian physician working on skin grafts developed true parabiosis: a sustained commingling of the blood supplies of two living creatures.
Knowing that the red stuff flows through every organ and tissue, scientists have used the technique ever since to study bodywide states like obesity and systemic diseases like radiation sickness. If you divert blood from a sickly animal into a healthy one, and that one also becomes ill, it suggests some soluble factor in the blood plays a role. That knowledge, in turn, helps you narrow down what causes the illness or condition. For example, in 1958, scientists linked up rats from a strain prone to tooth decay to rodents from another strain that’s naturally resistant to cavities, to test whether something in the blood might account for the differences. In this case, at least, blood swapping made no difference.
Heterochronic parabiosis, in which researchers pair two animals at different points in the lifespan, was first used to study aging in the 1950s. But by the 1990s, it was largely forgotten—until Stanford put it back on the map.
AGING AFFECTS EVERYTHING EVERYWHERE, all at once. The hair grows gray, the bones weaken, the heart falters. Inside cells, DNA replication glitches and stutters, and proteins clump up into sticky globs. Meanwhile, natural repair mechanisms like adult stem cells no longer scurry to replace dead or injured tissues. All this happens more or less in sync, as if some systemwide signal has told the whole body to go down the tubes.
This organized process of decrepitude was still largely an enigma in 1993, when biologist Cynthia Kenyon, then at UCSF, discovered that mutating just one gene in a roundworm doubled its lifespan. Her finding helped launch the modern study of aging, but it soon became clear that a one-gene or one-protein approach wasn’t going to work, at least not for mammals. “We started to realize that the human body is not a simple assembly of individual molecules, but an incredibly complex physiological machine,” says Stanford’s Wyss-Coray.
But what is it that coordinates this systemic ruin? Fellow Stanford neurologist Thomas Rando reasoned that it made sense to look in the blood, that witch’s brew of biochemical whatnot that bathes the body, pinkie toe to pointer finger. Mostly water, nutrients, and red blood cells, what runs through our veins also transports a huge variety of signaling molecules that coordinate metabolism, immune responses, fight-or-flight reactions, and myriad other activities.
On the theory that blood-borne factors might orchestrate the transitions of aging, Rando and two postdocs in his lab, the husband-and-wife team of Michael and Irina Conboy, turned to heterochronic parabiosis. In the creepy but simple procedure, the surgeon slits two anesthetized mice down their flanks, then sutures and staples them together, side by side. Because these lab animals are so inbred, their immune systems don’t attack one another. As the incisions heal, their blood vessels connect, and the two share a supply.
Conjoined, the Frankenmice learn to eat together, make their little nests together, and ramble around as if they’re in a three-legged race. Their bodies begin to change. The old mouse’s fur gets thicker and silkier. It scrapes together its bedding more quickly. The junior partner loses speed, becomes tentative.
The team’s 2005 findings, published in Nature, caused a stir. It’s like this: If an older mouse’s leg gets frozen with a piece of dry ice, the cells in charge of muscle repair don’t respond much; the number of active cells increases by just 10 percent or so. But after heterochronic parabiosis, twice as many cells activate in response to injury—a reaction like that of a young animal. Older mouse livers demonstrate a similarly sprightly cellular turnover.
The authors had brain data too, but it was too preliminary to be included in the paper. By 2005, the long-held dogma that adult brains cannot make new cells had softened: Research had shown that certain regions, including the hippocampus, could generate new neurons, but claims of actually restoring function still raised most eyebrows sky-high.
Soon after the Rando paper’s publication, Villeda, then just 25, was returning to his graduate studies in Wyss-Coray’s lab, one floor away in the same building at Stanford. The son of Guatemalan immigrants, Villeda had been educated in public schools in Los Angeles with little exposure to science until college, when he walked into a lab and saw a mouse embryo growing in a dish. It blew his mind. He loved science, the challenge, the craziness of it, the fun of it. He was curious and intellectually fearless. That is to say, exactly the type of person to grasp this particular third rail.
“It was very high risk,” says Wyss-Coray, who frequently collaborates with Rando. “Most people would say, ‘What does blood have to do with the brain? This will absolutely not work.’”
For three years, Villeda did the tiny surgeries and collected evidence. Soon, he could see that new brain cells were in fact surging in old mice. And they looked great.
“When a neuron is born in an old brain, it’s [usually] scrunched up,” he says, balling up his fist. “In these old brains they looked just like the young ones, beautiful,” he continues, stretching out his fingers. Those cells eagerly extended their long tendrils to make connections—the synapses that enable learning, memory, thinking, and everything else an elderly mind often struggles with.
In 2011, Villeda published a paper, also in Nature, showing that mature mice in parabiotic pairings sprout two to three times as many new neurons as usual. But the bigger splash came in Nature Medicine in 2014, where he demonstrated that the access to young blood not only remodeled old nerve cells so that they looked and responded like younger neurons but also improved aged mouse learning and memory. A group led by Harvard’s Amy Wagers published similar results in Science at the same time, bolstering both claims.
Wagers and others at places like Columbia Medical Center soon showed that parabiosis could improve the function of heart, bone, and other tissues. These teams worked together to establish a working definition of what really qualifies as rejuvenation, including changes in DNA modification, gene activation, or protein levels characteristic of younger bodies.
As Villeda drew blood, he also collected plasma—blood with the cells removed—from young mice, drop by teeny-tiny drop, and transfused it into older ones. The effect was the same, strongly suggesting that whatever the magic was, it was something dissolved in the fluid itself, some code or key that signaled a fresh start.
“The way I think about it is that there’s a lot of information in the blood,” he says. Now, at last, they could work on cracking that code—and hopefully doing for humans what they’d already done for mice.
JUST TO GET THIS OUT OF THE WAY: Nobody’s sewing humans together. Our immune systems would wallop one another, with potentially deadly consequences (the lovely technical term is parabiotic disharmony). Transfusing seniors with young blood isn’t practical either; people would probably need repeat treatments, with each bringing a risk of infection, allergic reaction, and even injury to the lungs (transfusions sometimes cause a poorly understood immune reaction that ravages their lining). Because the dosing would restart cell division, it might also spark cancerous growths. And we don’t even know whether it would produce the desired results in a human being—or what mechanism would be behind the transformation.
Nonetheless, those two 2014 papers inspired a lot of wild ambitions. Rando got calls from cosmetics companies developing elixirs for youthful skin and from men’s magazines seeking secrets for reinvigorated muscles. A billionaire invited Wyss-Coray to an Oscar party. (He didn’t go.) “You get offers of a lot of money and no oversight,” says Villeda; people who owned property in nations with lax regulatory supervision on human research made what he refers to as “indecent proposals.”
Longevity enthusiasts eagerly discussed the findings, even though there is little evidence that heterochronic parabiosis extends life; even in rodents, all we know for sure is that it undoes some late-in-life decay. Captains of the tech world also took note. Reputed interest from billionaire founder Peter Thiel inspired a spot-on subplot in the HBO comedy series Silicon Valley in which an aging mogul takes a meeting while getting pumped full of blood from a fresh-faced athlete.
Meanwhile, a cottage industry began selling young plasma. Around 2016, Ambrosia, a California company, offered to infuse customers as part of a clinical trial that charged participants $8,000 to join. (So far, the team has not published any findings in the scientific literature.) Other entities and individuals launched similar efforts, such as a proposed study that would charge large sums to frail elderly people for doses of young plasma.
This “therapeutic plasma exchange” is a legitimate treatment for certain rare autoimmune diseases and problems with coagulation, so these providers are not necessarily required to obtain explicit approval from the Food and Drug Administration so long as they make no unsubstantiated health claims about their regimen. But, of course, they did: Companies marketed benefits for people with memory loss, heart disease, and even Parkinson’s. The FDA, now stepping into the regulatory role of the 17th-century pope, released a stern memo in 2019 that curbed the trend.
Ultimately, these projects made no progress toward the real prize, which is to convert the knowledge gained into a convenient, powerful, and predictable form, such as a pill. “Everyone recognizes this is an incredibly important experiment,” says Eric Verdin, CEO of the Buck Institute for Research on Aging, who closely follows parabiosis. “What has been lagging is: How do you translate these discoveries?”
The most straightforward path would be to pinpoint a pro-aging factor in old blood, mouse and human, that a drug could block. Many groups have identified such elements. Villeda and his collaborators, for instance, found that a protein called CCL11 increases in aged humans and mice and is correlated with reduced brain cell birth.
The other obvious tactic is to identify youthful plasma’s secret formula and optimize it. The Conboys’ research suggests the hormone oxytocin might be a candidate; Wagers has identified the protein GDF11. Combination therapies are also under consideration; the biotech company Wyss-Coray founded is exploring mixtures of hundreds of blood-borne proteins as therapies for a variety of age-related diseases. Villeda is on its board.
It’s also possible that the rejuvenating effects seen in experiments don’t arise from one magic ingredient, or even from some combination of a dozen or a hundred compounds, but happen simply because the procedure dilutes some unknown harmful substances that accumulate in old blood. From this perspective, there’s no particular need for young stuff: Any form of plasma replacement will do. It’s sort of like changing the oil in your car.
The Conboys, now both at the University of California, Berkeley, suspect this is the case and are moving forward with tests of the idea. Their recent experiments, published in the journal Aging, replaced half the blood of some old mice with a mix of salt water and purified albumin (the main protein in plasma), which successfully rejuvenated the rodents’ hearts, livers, and brains. They too are starting a company and are aiming for human clinical trials to determine if simply flushing out the bloodstream can help with problems like frailty and declining cognition.
At this point, the quest for a treatment has no satisfying ending. We know blood does indeed perform some kind of alchemy that can restore and remodel the flesh or hasten its decay. But even as that core mystery lingers, Villeda and others are rushing forward with a bigger project: cracking all the other codes that might be written in blood.
SAUL VILLEDA IS NOW 40. His thick black hair still has no tinge of gray. He speaks quickly and laughs often and generally hums with energy. He still seems young, but he is no longer a newbie. At UCSF, he now oversees a group advancing a new era in rejuvenation research. It’s looking at other systemic bodily shifts, such as those caused by exercise or diet, to find the mechanisms that can turn back the clock—demonstrating that youthfulness alone is not the only fountain of youth
Soon after Villeda started his lab in 2013, his postdoc Shelly Fan was eager to begin a risky project. It’s well known that exercise can reduce some of the effects of aging on the brain, increasing blood flow to the organ and boosting cell birth in one of the few regions that produce new neurons. The junior researcher wanted to see whether plasma from an active animal could transmit those benefits to a sedentary one—but it would take many years of work to find out.
Villeda was now the senior scientist, fretting that his fearless young-blood collaborator was taking too big a risk. But he gave her the go-ahead. Shortly after the project got underway, Horowitz took over working with those Lego-snuffing critters. She’s spent three years watching mice age, watching them run, watching them remember and forget. “It’s long and grueling and tiring,” she says.
Mature mice were allowed to sprint as much as they wanted on little exercise wheels for six weeks (these critters typically like a nice, brisk jog). She then collected their plasma and delivered it to aged couch-potato equivalents. These older animals’ brains produced extra new neurons, and they aced memory tests. The paper was published in Science in summer 2020.
The surprise was that the effects seemed to flow through the liver, which ramped up several factors including an enzyme called GPLD1 that is also plentiful in active elderly humans. Rando and Wyss-Coray, with others, published similar results in Nature Metabolism; they found that serum (plasma with clotting factors and platelets removed) taken from exercising older mice restarted the systems responsible for muscle repair.
In addition to exercise, Villeda has played with a regimen known as caloric restriction that cuts food intake by 20 to 30 percent. Historically, the practice has improved age-related declines in brain, metabolism, and cardiac function in lab animals. At Stanford, Rando’s group is testing a high-fat, low-carb ketogenic diet. Others are interested in the effects of short-term physical stress (such as from bursts of intense exercise, or maybe even a small dose of radiation).
This story originally ran in the Fall 2021 Youth issue of PopSci.
Kat McGowan is a journalist and editor in California focused on healthcare, medicine, and biological research. She sees science reporting as a way to explore the zone where new ideas come into contact with politics, money, and power.