Exercise fends off some of the ills of aging, maybe even Alzheimer’s disease, but just how physical activity benefits the brain is unclear. In a new study in the September 7 Science, researchers led by Rudy Tanzi at Massachusetts General Hospital, Charlestown, used a combination of drugs and gene therapy to mimic the effects of exercise on memory in a mouse model of AD. When given treatments that promoted production of new neurons in the hippocampus and hiked levels of nurturing trophic factors, mice that lounged around all day maintained their cognitive prowess as if they had run on a wheel for hours.
- Drugs, gene therapy mimic exercise benefit in AD mice.
- Treated mice were as smart as mice running hours per day.
- Therapy boosted neurogenesis and boosted BDNF, but did not reduce plaques.
The gene-therapy interventions used in the study are not likely to fly in people. But the pharmacological interventions? Tara Spires-Jones and Craig Ritchie, University of Edinburgh, think they are promising. “In the best-case scenario, assuming these results are replicated in other models and are relevant to human disease, this study suggests that we could bottle the effects of exercise to prevent or treat dementia,” they wrote in an accompanying commentary in Science. Don’t cancel your gym membership just yet—such exercise mimetics are years away, at best, scientists agree.
Epidemiological data has shown that regular exercisers reduce the risk of dementia (for example, Mar 2018 news and Hamer et al., 2018). In mice, exercise reduces Aβ pathology and improves cognitive function (Mar 2005 conference news). It’s been known for some time that exercise fuels the birth of new neurons in the hippocampus in adult mice and that new neurons play a role in learning and memory (van Praag et al., 1999; Toda et al., 2018). Could stimulating neurogenesis in sedentary mice mimic the effects of exercise?
Sweat Equity. Exercise benefits the brain via multiple pathways, some of which can be mimicked with drugs and gene therapy to improve cognition in AD mice. [Image adapted from Spires-Jones and Ritchie, 2018; courtesy of Science/AAAS.]
To find out, first author Se Hoon Choi took advantage of P7C3, a small molecule discovered in a screen for drugs that enhance neurogenesis in adult mice (Jul 2010 news; Sep 2014 news). P7C3 and similar compounds promote survival of new neurons, and protect older neurons, too, in mouse models of Parkinson’s disease, ALS, and traumatic brain injury (Oct 2012 news; Yin et al, 2014). In a rat model of AD, a related compound prevented deterioration in cognition, but it did not reduce amyloid load (Vorhees et al., 2017).
In their study, Choi and colleagues injected P7C3 once daily into sedentary, two-month-old 5XFAD mice. One month later, Choi injected lentiviral vectors expressing the neuron progenitor survival factor WNT3 into their hippocampi to further drive proliferation of progenitors. The researchers gave another group of mice access to running wheels for three hours each day. After three months, either regimen enhanced hippocampal neurogenesis by an average of 50 percent. Not all the mice ramped up neurogenesis in response to exercise, but those that did also performed better in three different maze-navigation tasks. However, P7C3/WNT –treated mice did no better in the mazes than untreated animals whether they had a boost in neurogenesis or not.
As expected, the exercising mice had less amyloid plaque burden than their untreated, sedentary counterparts. P7C3/WNT treatment did not reduce amyloid.
Did exercise help the 5xFAD mice navigate because it reduced amyloid plaques, or are other factors at play beyond neurogenesis that are not evoked by the P7C3/WNT mimetic? Exercise also induces cytokines and neurotrophins, particularly brain-derived neurotrophic factor (BDNF), and indeed, Choi found that exercise, but not P7C3/WNT3, boosted hippocampal levels of BDNF and the BDNF stimulator FNDC5. Wheel running also elevated the synaptic markers PSD95 and SYP, suggesting that exercise not only enhances the birth of new neurons in the hippocampus, but also promotes their survival and plasticity.
To similarly mobilize neurotrophic support in the P7C3/WNT 5xFAD mice, the scientists turned to AICAR, an AMP kinase activator that increases BDNF expression (Guerrieri and van Praag, 2015). In 5.5-month old P7C3/WNT-treated mice, daily injections of AICAR for two weeks resulted in better memory scores, similar to the effect induced by exercise. AICAR by itself did not change neurogenesis, plaque levels, or cognition.
Even as the combination treatment helped memory, it failed to reduce amyloid plaque in the mice. This lends further support to the idea that amyloid plaques are a poor marker of disease progression, Spires-Jones and Ritchie wrote.
“The study nicely confirms that adult-generated neurons play a critical role in hippocampal resilience, and it suggests that exercise mimetics are indeed a possible route to preventive or therapeutic strategies," wrote Gerd Kempermann, Center for Regenerative Therapies, Dresden, Germany, in an email to Alzforum (full comment below). However, given the low cost and low risk of exercise, Kempermann noted in most cases, simple physical activity would be the better choice.
Shouldn’t people just get off the couch and exercise? Mimetics may benefit elderly people who have trouble staying active at the level required for brain health, said Tanzi. “Our mice run on the wheel for three hours a day. They love it, and they have nothing else to do, but it’s a lot of work,” he said. “To get the equivalent exercise in elderly AD patients is probably impossible. If we can use a drug/gene therapy to mimic the effects of exercise on cognitive ability, that gives us hope that we can translate these benefits from mice to humans.” A scientist told Alzforum that P7C3 analogs are being developed for human use by Proneurotech, Inc. of South San Francisco, California. The company declined to comment on the status of its research program.
It is important to replicate the results in different AD models, said Fernando Gomez-Pinilla, University of California Los Angeles. He also wonders how long the cognitive effects last, and how the treatments affect tau pathology, considered a better marker for cognitive decline than amyloid. Though adult neurogenesis occurs only in the hippocampus and the olfactory, Helen Scharfman of the Nathan Kline Institute in New York wondered about other regions of the brain. “The authors focus on the hippocampus and dentate gyrus, but this is not necessarily the only area to think about, and not the site where the disease necessarily starts,” she wrote.—Pat McCaffrey.
- 44-Year Study Ties Midlife Fitness to Lower Dementia Risk
- Sorrento: More Fun, Less Amyloid for Transgenic Mice
- Mouse Screen Yields Pro-neurogenesis Elixir
- Neuron-Protecting P7C3 Compounds Take Steps Toward the Clinic
- Compound to the Rescue in Parkinson’s, ALS Models
Research Models Citations
- Hamer M, Muniz Terrera G, Demakakos P. Physical activity and trajectories in cognitive function: English Longitudinal Study of Ageing. J Epidemiol Community Health. 2018 Jun;72(6):477-483. Epub 2018 Feb 6 PubMed.
- van Praag H, Kempermann G, Gage FH. Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus. Nat Neurosci. 1999 Mar;2(3):266-70. PubMed.
- Toda T, Parylak SL, Linker SB, Gage FH. The role of adult hippocampal neurogenesis in brain health and disease. Mol Psychiatry. 2018 Apr 20; PubMed.
- Yin TC, Britt JK, De Jesús-Cortés H, Lu Y, Genova RM, Khan MZ, Voorhees JR, Shao J, Katzman AC, Huntington PJ, Wassink C, McDaniel L, Newell EA, Dutca LM, Naidoo J, Cui H, Bassuk AG, Harper MM, McKnight SL, Ready JM, Pieper AA. P7C3 neuroprotective chemicals block axonal degeneration and preserve function after traumatic brain injury. Cell Rep. 2014 Sep 25;8(6):1731-40. Epub 2014 Sep 15 PubMed.
- Voorhees JR, Remy MT, Cintrón-Pérez CJ, El Rassi E, Khan MZ, Dutca LM, Yin TC, McDaniel LN, Williams NS, Brat DJ, Pieper AA. (-)-P7C3-S243 Protects a Rat Model of Alzheimer's Disease From Neuropsychiatric Deficits and Neurodegeneration Without Altering Amyloid Deposition or Reactive Glia. Biol Psychiatry. 2017 Nov 6; PubMed.
- Guerrieri D, van Praag H. Exercise-mimetic AICAR transiently benefits brain function. Oncotarget. 2015 Jul 30;6(21):18293-313. PubMed.
No Available Further Reading
- Choi SH, Bylykbashi E, Chatila ZK, Lee SW, Pulli B, Clemenson GD, Kim E, Rompala A, Oram MK, Asselin C, Aronson J, Zhang C, Miller SJ, Lesinski A, Chen JW, Kim DY, van Praag H, Spiegelman BM, Gage FH, Tanzi RE. Combined adult neurogenesis and BDNF mimic exercise effects on cognition in an Alzheimer's mouse model. Science. 2018 Sep 7;361(6406) PubMed.
- Spires-Jones TL, Ritchie CW. A brain boost to fight Alzheimer's disease. Science. 2018 Sep 7;361(6406):975-976. PubMed.