First described in 2012, the glymphatic system acts as a waste management system, washing out the proteins and other buildup that can hinder brain activity. The critical component of the system is the cerebrospinal fluid flow, according to Liu, which his research suggests is activated by the global BOLD signal activation. In sleep, the glymphatic system can speed up its cleaning processes significantly.
“We saw the strongest coupling in subjects who were lightly sleeping based on other ongoing research in the lab,” Liu said, noting that they were not specifically studying the coupling in sleep but called the occurrence interesting, nonetheless. “We know sleep can promote the process of the glymphatic system and help clean out brain toxins. It looks like it could be through this coupled mechanism.”
To better understand the nuances of sleep and the coupled signaling, Liu collaborated with Orfeu M. Buxton, Elizabeth Fenton Susman Professor of Biobehavioral Health in the College of Health and Human Development, who directs the Sleep, Health, and Society Collaboratory at Penn State.
“The glymphatic system is highly related to sleep — we know that from intensive research conducted in animal studies,” said Buxton, who co-authored the study. “This new work expands the limited evidence base for evaluating the importance of this system in humans. This model, based on non-invasive imaging, has the potential to apply to anyone in any population to assess the activity of their glymphatic system.”
This research supports the emerging idea that a lack of sleep, a common issue for people with Alzheimer’s disease, could contribute to this reduced coupling, Liu and Buxton said. While it is not clear if sleep problems are a cause or a symptom of Alzheimer’s disease, the suggestion supports another recent international study that found people who sleep six hours or less were about 30% more likely to be diagnosed with dementia, another common symptom in people with Alzheimer’s disease, than those who sleep seven hours or more a night.
“Sleep is complicated, but this may be an avenue to begin understanding how sleep problems and Alzheimer’s disease influence each other,” Buxton said.
In addition to identifying and understanding how the coupled mechanism works, Liu said, the connection may also be a way to begin treating the disorder.
“Potentially, we could develop a therapy to increase the brain activity, which would increase the cerebrospinal fluid flow to more efficiently and regularly clear the toxins,” Liu said. “That’s purely theoretical at this point, but there is value in investigating new approaches.”
The team used human data collected by the Alzheimer’s Disease Neuroimaging Initiative (ADNI), a longitudinal, multi-site study launched in 2004 to investigate early detection and tracking of Alzheimer’s disease and facilitating data sharing between researchers. Currently, the initiative includes scientists from 59 research centers across the United States and Canada, who have collected varied data on almost 2,000 human subjects. Fewer than 150 subjects fit Liu’s study parameters of longitudinal fMRI information on the BOLD and cerebrospinal fluid movements.
“We’re continuing to investigate this link in other cognitive disorders, as well as in other types of data, such as genetics and other imaging approaches,” Liu said.
Co-authors include Feng Han, Jing Chen, Aaron Belkin-Rosen and Yameng Gu, all with the Department of Biomedical Engineering; and Liying Luo, assistant professor of sociology and criminology and in the Population Research Institute in the College of the Liberal Arts.
The National Institutes of Health and the White House Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative supported this work. The data collection for this project was funded by the ADNI, which is funded by the National Institutes of Health and the Department of Defense.