Research Converging on How Young Blood Improves Old Brains
Give an old mouse the blood of a young one, and the aged creature will experience a youthful resurgence of sharp thinking alongside the growth of new brain cells. It has been more than 150 years since scientists started documenting the brain-restoring properties of this kind of blood transfer. Ever since, they have been working to uncover the details about what is in the young blood that can help the older brain.
Now, scientists have converged on a molecule that appears to play a major role. In three new studies published simultaneously in August 2023, overlapping teams of researchers independently found evidence along three different lines implicating a protein called platelet factor 4 (PF4) in brain rejuvenation. Released by platelets into the blood as a result of various forms of activation, PF4 appears to mediate inflammation and stimulate neurogenesis, leading mice to do better on measures of memory and other forms of cognitive performance.
With an aging population, the work offers hope for developing new kinds of interventions that might treat both aging-related and disease-related cognitive decline. “Who would have thought that platelet factors play a role in helping the brain during aging? I think that’s just incredible,” says Dena Dubal, a neurologist and neuroscientist at the University of California, San Francisco (UCSF), the senior author on one of the three studies and an investigator with the Simons Collaboration on Plasticity and the Aging Brain (SCPAB). “All three of our findings really open the field to now understanding the role of platelet factors in the brain.”
Stimulating Platelets
Platelets are a component of blood best known for their role in clotting, but research in recent years has illuminated other functions beyond the traditional part they play in healing wounds, says Tara Walker, a neuroscientist at the University of Queensland in Brisbane, Australia. Each platelet contains more than 1,000 bioactive molecules, known as platelet factors, that get released in different combinations when platelets are activated, these studies show. A variety of stimuli can activate platelets, including infection, exercise and injury. And the kinds of factors they release depend on the type of activation they undergo.
Several years ago, Walker’s team discovered some of the first hints that one of those hundreds of molecules, PF4, might be particularly important. The group was trying to explain why exercise increases neurogenesis in the hippocampus, a brain region important for learning and memory. Hypothesizing that something happens in the blood after exercise that affects the brain, Walker and colleagues first screened the blood of adult mice after they started running on running wheels placed in their cages.
They found about 80 factors whose abundance in the blood changed significantly after exercise, Walker says, and PF4 was among those that rose the most. Platelet activation with exercise led to neurogenesis, the team reported in 2019, and PF4 was a leading contender to explain why. When they injected PF4 into mice that didn’t exercise, they were able to essentially mimic the brain-enhancing effects of exercise.
To follow up, Walker and colleagues looked at how PF4 affects the brains of older mice. As part of a series of experiments, they put mice on a rotating table that required them to move around to avoid a shock zone. Young mice are good at learning patterns and other cues that help them avoid the shock, sort of like using landmarks to get around a city, Walker says. But they get predictably worse at tasks like these as they age.
When the researchers injected PF4 into the bloodstream of older mice through their tails on a schedule of one injection every third day for 24 days, as the group reported in August 2023 in Nature Communications, the old mice behaved much more like young mice in the avoidance task and other memory tests. The findings echoed the cognitive benefits Walker’s group has seen with exercise, which included increased neurogenesis in the brains of the older mice treated with PF4.
Although exercise has powerful cognitive benefits, physical activity may not be practical for many people with Alzheimer’s disease, brain injuries, strokes or other conditions, Walker says, emphasizing the potential of a therapeutic based on PF4. With just a single factor, it appears to be possible to mimic the beneficial effects of exercise on neurogenesis and cognition.
“When we delivered platelet factor 4 to these aged mice,” she says, “it significantly enhanced their cognitive ability, taking them almost — not quite, but almost — back to what a younger mouse would look like in terms of the learning and memory function.”
Inflammatory Link
While Walker’s group was conducting its research linking exercise to PF4 and PF4 to cognition, a team of researchers at UCSF, including neuroscientists Adam Schroer and SCPAB investigator Saul Villeda, were trying to identify individual components in young blood that might explain its rejuvenating effects. They began by spinning blood from young mice in a centrifuge to separate plasma from blood cells. Plasma, the clear component of blood that contains soluble factors and some platelets, is typically injected in experiments like these. To see if platelets that stay behind in the young plasma might explain the substance’s benefits on aging, they took the process a step further, centrifuging the plasma again to separate out the platelets.
Next, they treated aged mice in three groups. Every third day for 24 days, mice received either young plasma, isolated platelets suspended in saline or saline only. In mice that got the plasma (which included platelets) and mice that got platelets alone, the researchers reported in Nature in August, performance on learning and memory tests improved. Using RNA sequencing to investigate changes in gene expression as a result of each treatment, they found that both young plasma and platelets affected a variety of overlapping genes, Schroer says, and many of those genes influence inflammatory processes.
Inflammation is known to increase with age. In the brain, inflammation activates a type of immune cells called microglia, ultimately leading to neuronal dysfunction and neurodegeneration. Previous studies have shown that injecting old blood into young mice increases inflammation in the young hippocampus. The new study was the first to show that young blood attenuates neuroinflammation and, in turn, reduces the activity of microglia, Schroer says. The injection of platelets alone was enough to have these effects. And the most common protein in the platelet solution, their analysis showed, was PF4.
Although PF4 didn’t appear to cross the blood-brain barrier, Villeda says, his team’s data show that PF4 affects the peripheral immune system — reducing the number of pro-aging immune factors in circulation, decreasing neuroinflammation and enhancing cognitive function. That sequence of events, Schroer says, suggests that both circulating immune factors and the peripheral immune system could be targets of future therapeutics.
“Generally, what we’re seeing is that this platelet fraction of plasma is in some way able to restore the microglia to a more youthful state where they’re less activated,” Schroer says. “In simple terms, it seemed to make the microglia happier.”
Klotho
Meanwhile, in a separate lab on a different campus of UCSF, Dena Dubal was pursuing research on a protein called klotho, which was named after Clotho, the Greek fate and daughter of Zeus who determines when we are born and when we die. Klotho declines with age, but studies show that people with genetic variants that lead them to produce higher levels of it tend to live longer and be more resistant to the diseases of aging. Mice that are engineered to be deficient in klotho die prematurely.
A decade ago, Dubal’s team linked klotho, often called a longevity protein, with better cognitive function in both young and old mice. Injecting the hormone, they found a few years later, boosted cognitive performance even in mice with dementia-like brain damage — even though klotho didn’t cross the blood-brain barrier. “It became a burning question to understand,” Dubal says. “How is klotho delivering this signal for better cognition to the brain if it is actually not getting into the brain?”
To search for messengers that might be mediators between klotho and the brain, Dubal and colleagues started by injecting klotho into mice and then analyzing their blood to see what changed. When they measured an increase in platelet factors — especially PF4 — they followed up by adding klotho directly to platelets. Those lab experiments confirmed that klotho mildly activates the blood cells. “It kind of tickles the platelets to release their contents,” including PF4, she says.
Next came the big test: they injected PF4 into mice and assessed cognition through a battery of maze tasks. The results, published in August 2023 in Nature Aging, showed that PF4 recapitulated the effects of klotho, improving brain function in both young and old mice. PF4 had benefits even in the absence of klotho. Much like the work in Walker’s and Villeda’s lab, the results seemed to show that PF4 could boost cognition.
The studies did not converge on all points. Whereas Villeda’s study suggested indirect effects of PF4 on the brain via the immune system, Dubal’s data offered evidence that PF4 acts directly on synaptic plasticity. Both mechanisms, Villeda says, could be true. “I think our data,” Villeda says, “point toward additive effects.”
Convergence
Dubal was well into her research when she sat down with Villeda. The two researchers have a collegial friendship, and they regularly meet to chat about work. Over coffee, one of them asked the other what they were studying, and the conversation quickly turned to PF4. “It was literally like, ‘We’re really excited, we’re working on this factor.’ And ‘Oh my god, we’re working on the same factor,’ and then staring at each other with our mouths open for a moment,” Dubal says. “Then there was a lot of excitement because it’s not often that biology converges.”
It was an exciting moment for Villeda, too. “We just thought, ‘This is wild,’” he says. “If you look at our papers, they just they start so differently, and then there’s these points where we come together.”
Soon after that conversation, both researchers presented their initial PF4 results at an early workshop for members of SCPAB. The group discussed the value of open communication in illuminating the convergence of data that they might not otherwise have known about, Villeda says.
Open communication also facilitated the connection to Walker’s work. Villeda had met Walker after writing a commentary on her 2019 paper about PF4 and exercise, and they had kept in touch ever since. When his work started to hone in on platelets, they started meeting to discuss their findings. Soon, Villeda, Dubal and Walker were talking regularly over Zoom, sharing notes and helping each other out. Given that their studies were progressing on similar timelines, they agreed to submit their papers together when they were all ready to go. All three were published simultaneously in three of the Nature family of journals.
That kind of cooperative experience was both enjoyable to be a part of and good for science, the researchers say, helping to validate and amplify the take-home messages: that PF4 enhances cognition, even in the face of aging, and that it may eventually be possible to develop drugs based on PF4 or the pathways involved. “At the same moment in time, the three of us had independently converged on the same factor, and we had all shown that it improves cognition, improves learning and memory in an old brain,” Dubal says. “When there are multiple lines of evidence, it means reproducibility. It also means something very valid about the biology.”
PF4 is far from the only molecule that communicates with the brain about what’s happening in the body, says Lee Rubin, a neuroscientist at Harvard University and a SCPAB investigator collaborating with Dubal, Villeda and Tony Wyss-Coray, a neuroscientist at Stanford University, on a project to identify the role of bloodborne factors in cognitive aging. Other work has illuminated a plethora of different factors that also appear to have rejuvenating effects. But by highlighting PF4 and its mechanisms as one important player, the new research opens a framework of possibilities for developing interventions that could slow brain deterioration and stimulate recovery through processes such as brain inflammation and other pathways. “It draws a lot of attention not only to the importance of this particular factor but to this complex signaling system,” Rubin says.
By flagging PF4 as a molecule to watch, the new papers open up a new wave of questions for researchers to address and details to refine. Walker and Villeda are collaborating on a follow-up study on PF4 in a mouse model of Alzheimer’s. Dubal continues to study klotho, PF4 and other approaches. “We hope that many, many, many more people will study platelet factors and understand how they work in the brain and understand what their potential is for treatments,” she says. “There’s so much more to do.”
