Research in animal models shows physical exercise can induce changes in the brain. In humans, studies also revealed changes in brain physiology and function resulting from physical exercise, including increased hippocampal and cognitive performance (1). Several studies in mice and rats also demonstrated that exercise can improve learning and memory and decrease neuroinflammation in models of Alzheimer’s disease and other neurodegenerative pathologies (2); these benefits are tied to increased plasticity and decreased inflammation in the hippocampus in mice (2). If regular time pounding the pavement does improve brain function, what is the underlying molecular biology of exercise-induced neuroprotection? Can we identify the cellular pathways and components involved? Can we detect important components in blood plasma? And, is the benefit of these components transferrable between organisms? De Miguel and colleagues set out to answer these questions and describe their results in a recent study published in Nature.
Does “Runner Plasma” improve cognition?
De Miguel et al. began by asking what happens when “runner plasma” (RP) from exercising male mice (male mice that had a running wheel in their cage) was transferred into young non-exercising litter mates (no access to a running wheel). Runner plasma from mice (28 days of running) increased cell survival among neurons, neural stem and progenitor cells, and astrocytes. The effect was most pronounced in 3-month-old mice. Injection of RP also increased learning and memory in mice as indicated by time freezing in a fear-conditioning test and improved performance in the Morris water maze. So yes, plasma from exercising mice, when injected into non-exercising mice did increase neural cell survival and improve cognitive performance.
Can scientists identify some of the components responsible for the changes in cognition and cell survival?
To begin to identify molecular components involved in the improved performance, De Miguel et al. asked if they could see differences in the genes transcribed in hippocampus of mice treated with RP compared to mice treated only control plasma (CP). Using RNA sequencing, they observed 1,952 genes that had differing transcription levels (differentially expressed genes); 61% of these were downregulated and 39% were upregulated after RP treatment. Some of the down-regulated genes included those involved in cell migration, adhesion, epithelial cell differentiation and proliferation, while some of the up-regulated genes were implicated in learning and memory, immune response, and neuroplasticity.
Does Runner Plasma mimic exercise-induced neuroprotection?
Lipopolysaccharide (LPS) is known to induce inflammation that is similar to the neuroinflammation associated with neurodegenerative diseases. When mice were treated with LPS, the researchers observed 4,936 genes that were differentially expressed compared to control mice. Giving LPS-treated mice Runner Plasma reversed the expression of many of the LPS differentially expressed genes, and the hippocampal gene expression profile of the mice treated with LPS and receiving Runner Plasma infusion was very close to that of the control mice that did not receive any LPS.
Can we identify key proteins affected by the running protocol in plasma?
The group turned their attention from gene expression and differential RNA analysis to differential protein analysis of plasma. They performed mass spec analyses on RP versus CP and found 35 unique proteins, 23 of which were down-regulated and 25 that were up-regulated in the RP. Among these proteins, 26% were associated with the complement and coagulation pathways. The complement pathway is a component of innate immunity that includes proteins that circulate in an inactive state, but can be activated to recruit phagocytes to the site of an injury or antigen, cause inflammation to further enhance immune response, and activate the membrane attack complex.
Next, the group took a closer look at four of the proteins that were differentially expressed in RP.
- Clusterin (CLU) also known as APOJ in humans. This protein is an extracellular chaperone and may also have functions in lipid transport and modulating the immune response. It has also been implicated in Alzheimer’s disease (3).
- Complement Factor H (FH): This is a soluble protein that is essential for regulating the alternative complement pathway (4).
- Glycoprotein pigment epithelium-derived factor (PEDF): This secreted protein has roles in neuroprotection, angiogenesis, and inflammation (5).
- Leukemia Inhibitory Factor Receptor (LIFR) This cell-surface receptor relays signals leading to cellular survival in neural cells and reducing inflammation in the immune response (6).
The group treated mice with LPS to induce inflammation as before and then treated with RP in which one of the four above proteins had been immunologically depleted or with a control, mock-depleted RP. Gene expression in the hippocampus was analyzed. In mice treated with LPS and RP depleted for CLU, expression studies of immune and inflammatory genes indicated that depleting CLU largely reversed the protective effect of RP.
Is CLU expression responsible for the exercise-induced neuroprotection of Runner Plasma in mice?
The authors moved to a mouse model for Alzheimer’s disease (APP mice) to study the effects of CLU. Seventeen-month-old APP mice were treated with recombinant CLU (rCLU) or saline control. APP mice exhibit advanced pathology at 17 months including amyloid deposition in the brain, neuronal and synaptic degradation, infiltration of microglia cells and learning and memory impairment. After treatment, the gene expression pattern of hippocampal brain endothelial cells was analyzed in rCLU-treated APP mice and mock-treated APP controls. The authors found that rCLU reversed the gene expression pattern of many genes that are abnormally expressed in the APP mouse model. Twenty transcripts that were commonly increased in after treatment with LPS or in the APP model were downregulated after treatment with rCLU. Some of these are genes important for responses to interferon gamma signaling and virus and epithelial cell proliferation, indicating that rCLU did result in downregulating pathways such as interferon and cytokine signaling that are linked to neuroinflammation.
How do these results compare to human biology?
This is a mouse study. The researchers asked if these results from mice translate to human biology. In a small study of 20 veterans who suffer amnestic mild cognitive impairment, they looked at complement and coagulation proteins in plasma, including CLU, before and after 6 months of a physical exercise protocol. In this small study, CLU was significantly increased in plasma of the patients who exercised.
What can we conclude from this mouse study?
Many studies have shown correlations between improved cognitive function in humans and physical exercise regimens. However, the molecular mechanisms that would explain the causation of such correlations (and there are probably many) are not well understood. In this study, the authors use mouse models to begin to tease out some of the molecular pathways that may be involved in exercise-induced neuroprotection, and they conduct one small clinical study in humans to see if any of the molecular players identified in mice might also be working similarly in human biology. The role of complement proteins in exercise-induced neuroprotection had not been previously identified. Thus, the study provides new pathways to investigate in neuroprotection and neuropathology and, hopefully, a host of new therapeutic targets for neurodegenerative disease.
In this paper, the Promega Trypsin-LysC Mix was used in quantitative proteomics experiments to identify up- or down-regulated proteins in mass spec analysis of plasma proteins.
- Mandolesi, L. et al. (2018) Effects of Physical Exercise on Cognitive Functioning and Wellbeing: Biological and Psychological Benefits Front. Psychol. 9, 509. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5934999/
- De Miguel, Z. et al. 2021. Exercise plasma boosts memory and dampens brain inflammation via clusterin. Nature 600, 494–499. https://www.nature.com/articles/s41586-021-04183-x
- Foster, E. M. et al. (2019) Clusterin in Alzheimer’s Disease: Mechanisms, Genetics and Lessons from Other Pathologies. Front. Neurosci. https://doi.org/10.3389/fnins.2019.00164
- Ferreria, V. P. et al. (2010) Complement Control Protein Factor H: The Good, the Bad and the Inadequate. Mol. Immunol. 47, 2197. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2921957/
- He, X. et al. (2015) PEDF and Its Roles in Physiological and Pathological Conditions: Implication in Diabetic and Hypoxia-Induced Angiogenic Diseases. Clin. Sci. (Lond) 128, 805–823. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4557399/
- Davis, S.M. and Pennypacker, K.R. (2018) The Role of the Leukemia Inhibitory Factor Receptor in Neuroprotective Signaling. Pharmacol Ther. 183, 50–57. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6265657/