The benefits of exercise on human and animal brains is well-established, and multiple studies have identified signals and changes within the body that might underlie these effects. A study published yesterday (December 8) in Nature identifies another, showing that the brain benefits of exercise can be transferred from active and sedentary mice via a plasma protein called clusterin.
“Previous research from my lab and others showed plasma factors that circulate in blood can affect the brain. So the question was; Are plasma factors induced by exercise affecting the brain during exercise, and in which way?” study coauthor and neuropsychologist Zurine De Miguel tells The Scientist.
To answer this question, De Miguel, then at Stanford University, and her colleagues kept mice in their habitats for 28 days with either a functional or locked wheel. Plasma was extracted from the blood of both groups, and an experimental group of sedentary mice was injected with “runner plasma” from active mice, while a control group of sedentary mice was injected with plasma from inactive mice every three days for the 28 days. A day after the final plasma doses were administered, the sedentary mice were given cognitive tests, and samples of hippocampal brain tissue from these mice were then removed, sectioned, and stained for imaging and RNA sequencing.
“We were looking for endpoints in the brain that have been traditionally known to be altered by exercise,” namely neuroplasticity and performance on cognitive tests, explains De Miguel, who is now at now at California State University, Monterey Bay. Imaging of the brain tissue extracted from sedentary mice revealed that “runner cell plasma increases cell proliferation in the hippocampus of the brain, which is an important area of the brain for learning and memory.”
Accordingly, in the tests, sedentary mice injected with runner plasma were more apt to freeze than were control mice injected with sedentary plasma in response to a cue they’d been trained to associate with an electric shock. Additionally, the runner plasma–injected mice performed better in a task that required learning the location of an underwater platform.
The scientists also found that mice infused with runner plasma had modified expression of genes associated with neuroinflammation. When the group induced inflammation in the brains of mice using bacterial lipopolysaccharides (LPS) and then injected runner plasma into those mice’s brains, they again saw reduced neuroinflammation and changed expression of associated genes compared to control mice with induced neuroinflammation which did not receive runner plasma.
The study authors suggest the findings could have implications for treating Alzheimer’s disease and other neurodegenerative conditions. Loughborough University neuropsychologist and epidemiologist Eef Hogervorst, who studies exercise and dementia but was not involved in the study, explains, “Alzheimer’s disease starts deep inside the brain, around the hippocampus, and then spreads out. And there is a good possibility that inflammatory factors play a large role in this in the pathology and outcome.”
A brain-boosting protein
To find out what component or components of plasma from active mice might be responsible for its neurological effects, the researchers used mass spectrometry to analyze the proteins in both sedentary and runner plasma. More than 200 proteins were found in different quantities between the plasma types, with about a quarter of these associated with the either the complement pathway, which is responsible for immune functions such as inflammation, or the coagulation pathway, which induces blood clotting. These biological cascades are thought to interact to mediate inflammatory responses, and the authors of the paper suggest these pathways are inhibited in runners compared with nonrunners. In particular, levels of clusterin—an inhibitor of the complement pathway—were significantly higher in runner plasma.
Looking for the mechanism by which runner plasma reduced inflammation in the brain, the researchers induced inflammation via LPS in sedentary mice and injected them with either normal runner plasma or runner plasma depleted of clusterin or one of three other differentially expressed proteins associated with the complement and coagulation pathways. Removing clusterin from runner plasma dampened its anti-inflammatory effects, while depleting the other proteins had no significant effect. Furthermore, when the researchers injected clusterin intravenously into mouse models of advanced Alzheimer’s, abnormal expression of genes linked to the Alzheimer’s pathology was reversed.
“Clusterin has also been linked with Alzheimer’s disease, and exercise has been linked to the complement-coagulation pathway, but the effects of exercise on the brain have not been linked to clusterin before,” De Miguel says.
Clusterin has been linked to many diseases, says Evangeline Foster, a University College London postdoctoral researcher who has studied clusterin proteins and Alzheimer’s disease but was not involved in this study. Clusterin is widely expressed, Foster says, and “has a lot of different functions. It’s involved in the transport of fats and lipids, helping clear the blood and the plasma of different fatty deposits. It’s also binding to proteins like [amyloid-beta], which is implicated in Alzheimer’s,” and could be regulating the clearance or buildup of amyloid-beta.
Cells in the liver are major producers of clusterin, the authors note in their paper, and this possible connection between the brain and liver via clusterin excites Saul Villeda, a University of California, San Francisco, molecular neuroscientist who studies the use of factors in blood to treat age-related brain impairments like Alzheimer’s but was not involved in the study. His own lab’s work has linked other factors from the liver to “rejuvenation in cognition and regeneration,” he says. “It’s sort of showing conserved mechanisms of biology.”
Clusterin in humans
To find out whether their findings are likely to apply to humans as well as mice, the researchers recruited 20 military veterans with mild cognitive impairment to participate in a six-month exercise regimen that included aerobic and resistance activities. Comparing plasma from the subjects at the end of the six months to samples collected before the regimen began, the researchers observed an increase in clusterin proportional to what they’d seen in the mouse studies.
Hogervorst maintains, however, that a lot more questions need to be answered in order to truly assess the transferability of the mouse clusterin findings to humans. She notes that some studies suggest clusterin is actually increased in people with Alzheimer’s disease, so “you can’t just go injecting [clusterin] in people because it’s already elevated, and it won’t work once you have Alzheimer’s.” Rather, “you need to do prevention as much as possible,” including exercising. “You need to be in there before extensive damage [is done]. . . . All these pathologies start very early. They start decades before we actually see cognitive impairment.”
As for the prospect of simulating the brain benefits of exercise by way of a drug, “I’d say we need good clinical trials to see what [clusterin] does in humans. Is it safe? What is the therapeutic dose? How do you administer it? Could you get a tablet? Could it be injected? When do you need to do this in humans?” Hogervorst says.
Foster calls the study “a great starting point” and says she’d like to see further investigation into the mechanisms of clusterin’s action. She adds, “it would be nice to look at proteins known to have similar functions to clusterin or that work with clusterin, and build up a bigger picture.”
De Miguel is currently studying the cognitive effects of exercise on the brains of children and says she hopes to couple that with further investigation into the mechanism behind clusterin’s influence on the brain. “I definitely think exercise is a wonderful tool we have to understand how the brain responds to signals from the periphery,” she says.