December 21, 2021
Written by: Nitsan Goldstein
If you’ve been following the developments in COVID-19 treatments over the last year, you’ve probably heard of monoclonal antibodies being used to treat people with severe cases of COVID. Monoclonal antibodies are produced and circulated through the blood when your body fights off an infection. Scientists have figured out how to extract these antibodies from the blood of patients who have recovered from COVID and inject them into patients that are in earlier stages of the disease. While treatments like these are certainly amazing medical feats, it is not all that surprising that contents from the blood of a person who has successfully fought off an infection might help someone else fighting the same virus. It turns out, though, that antibodies are not the only proteins circulating in your blood that could improve someone’s health.
We’ve known for hundreds of years that certain qualities about a person make them more or less likely to have strong cognitive skills like memory. Being young, for example, means your memory is likely strong, while being older or suffering from diseases like Alzheimer’s means your memory is weaker. Another such factor is the degree to which you exercise. Exercise improves cognitive skills and can help improve memory in people suffering from dementia. For many years, scientists have asked how these factors actually improve brain function. If we can figure out how things like youth and exercise improve memory, perhaps we can use those same pathways to develop treatments for dementia and related disorders.
In 2014, a study was published showing that simply taking blood from young mice and infusing it into older mice could improve the older mice’s performance on memory tasks1. This result was exciting because it suggested that not only could memory impairments of older mice be reversed, but also that they could be reversed by some molecule that was circulating in the blood of young mice. This month, the same group published another study showing that much like the blood of young mice, the blood of mice that had been exercising regularly could also improve other mice’s ability to perform a memory task2. Recent technological advancements allowed them to identify one of the specific proteins in the blood that was mediating the effects in the brain.
The researchers started by infusing plasma, or the liquid part of blood, that was taken from mice that had access to a running wheel for 28 days into mice that did not have access to a wheel. They found that sedentary mice that received plasma infusions from exercised mice (1) performed better on memory tasks, (2) displayed increased neurogenesis, a process believed to be important for memory, and (3) showed evidence of decreased inflammation in the brain. This last finding was intriguing considering the negative impact neuroinflammation can have on learning and memory and the strong link between neurodegenerative diseases like Alzheimer’s and a heightened inflammatory state3,4.
To further probe the relationship between inflammation and memory, the group decided to inject an inflammatory agent and measure changes in the brain with or without plasma taken from exercised mice. They focused on the hippocampus, a region that is crucial for the formation of memories and is prone to degeneration in neurodegenerative diseases. The scientists examined the changes in gene expression, or the proteins that will be produced by cells in the hippocampus, after injecting mice with lipopolysaccharide (LPS), which causes an inflammatory response. LPS injection caused changes in gene expression in the hippocampus, but many of these changes were reversed after treatment with plasma from exercised mice. Next, the researches wanted to pinpoint the protein or proteins in the blood that are responsible for reversing these LPS-induced changes in gene expression. After identifying several candidate proteins, they repeated the experiment, only this time some mice got plasma where one of the candidate proteins was removed before the infusion. They found that one protein in particular, clusterin, was essential for the beneficial effects of exercised plasma on neuroinflammation. When clusterin was removed from exercised plasma, many of the effects on LPS-induced inflammation in the hippocampus were gone. Even more convincing, the researchers found that injecting clusterin alone was able reverse some neuroinflammation caused by LPS.
So what are we waiting for? How can we get our hands on clusterin so that we can reap the benefits of exercise from the comfort of our couches? Before you start looking for clusterin vendors on the internet, it’s important to keep a few things in mind. First, it’s important to remember that these studies were performed in mice. The authors of the study did, however, begin to look at some of these pathways in exercised humans. They exposed one group of veterans with mild cognitive impairment to an exercise regimen and found that some of the changes in protein levels that they observed in mice were also present in humans, including increased levels of clusterin. Much more work is needed to further characterize gene expression and protein changes in humans after exercise and to link these changes to improved cognition. Another important point is that the true biological basis of the cognitive benefits of age and exercise are almost guaranteed to be more complicated than a single or even a handful of proteins circulating in the blood. Moreover, altering gene expression or proteins involved in these very crucial pathways can carry risks independent of their effects on memory. Therefore, highly controlled clinical trials must first conclude that these treatments are safe before even considering their efficacy. There are, however, ongoing clinical trials using plasma from young donors to treat neurodegenerative diseases like Alzheimer’s and Parkinson’s Disease5, giving us hope that one day studies like these will lead to more informed and effective treatments for neurological diseases.
- Villeda SA, Plambeck KE, Middeldorp J, Castellano JM, Mosher KI, Luo J, et al. Young blood reverses age-related impairments in cognitive function and synaptic plasticity in mice. Nat Med. 20, 659-63 (2014).
- De Miguel Z, Khoury N, Betley MJ, Lehallier B, Willoughby D, Olsson N, Yang AC, et al. Exercise plasma boosts memory and dampens brain inflammation via clusterin. Nature. Epub ahead of print (2021).
- Monje ML, Toda H, Palmer TD. Inflammatory blockade restores adult hippocampal neurogenesis. Science 302, 1760-5 (2003).
- Glass CK, Saijo K, Winner B, Marchetto MC, Gage FH. Mechanisms underlying inflammation in neurodegeneration. Cell 140, 918-34 (2010).
- “Young Plasma.” Alzforum. https://www.alzforum.org/therapeutics/young-plasma
Cover image by roxanawilliams1920 from Pixabay https://pixabay.com/photos/running-woman-fitness-runner-6252827/