By Rosalia Franco Fernandez
Imagine celebrating a huge party at your home. All your friends have attended and brought snacks and booze. After a fun and long night, most of your friends have left. You look around and realize that you are surrounded by empty beer bottles, crumbs of food and even some confetti that seems to have popped out of nowhere. You decide it’s better to tidy up now so that you can wake up in a clean house tomorrow. You and your remaining friends start cleaning right away. While scrubbing, you start wondering, would our brains do the same for us?
For a long time, it was uncertain how waste products were removed from the brain. In most tissues, lymphatic vessels are responsible for removing solutes and excess fluid. Besides this cleaning function, lymphatics are also involved in initiating immune responses by either transporting immune cells or immune-activating substances to where they are needed. Although brain tissue itself does not have lymphatics, the brain is surrounded by a protective envelope, the meninges, which do have lymphatics. Removal of substances has traditionally been attributed to the meninges1 and in that sense, the meninges could be compared to a city’s sewage system that flushes out the accumulated waste from the previous day.
Waste disposal by the meninges is considered a passive process, but since the brain is highly metabolically active, it creates great amounts of waste during the day. This caused scientists to ponder whether there could be a more efficient process behind cleaning the brain. It’s a controversial idea, but research suggests that the brain harbors an active cleaning system that resembles lymphatic vessels: the glymphatic system2.
The glymphatic system model
According to the model, the glymphatic system relies on certain star-shaped cells called astrocytes, which are a type of non-neuronal cells that naturally reside within the brain (Figure 1). According to the glymphatic system model, astrocytes might serve as an active cleaning crew operating in the brain tissue. These cells express a water channel called aquaporin-4 (AQP4) on their extrusions, also known as endfeet3. These channels are used to initiate a flow of cerebrospinal fluid within the brain, which allows for more efficient clearance of brain solutes and waste2. In addition, it seems that this flow begins in spaces surrounding arteries, then flushes the brain and finally exits the brain via spaces surrounding veins. Ultimately, waste is disposed of from the central nervous system via cerebrospinal fluid and the meninges.
Just like most clean-up crews, these astrocytes seem to prefer working night shifts. Normally, their AQP4 molecules are directed towards spaces surrounding blood vessels. Studies in mice showed that sleep deprivation alters the direction of AQP4 in such a way that these are less orientated towards these spaces4. As a consequence, sleep-deprived brains were less efficient to clear solutes, causing them to buildup. Moreover, studies in mice suggest that the absence of AQP4 promotes accumulation of solutes in the brain and reduces their removal2,5–7, while solutes are cleared more effectively in the brains of sleeping mice5,8. Taken together, this underscores the importance of sleep for glymphatic function.
Although the theory is intriguing, it is still a controversial topic whether astrocytes indeed have this cleaning function. While there are several studies that do assign this role to the astrocytes, there are also two studies that do not agree with this theory. Using both animal and computer models, a research group suggested that removal of solutes from the brain is based on diffusion9,10, meaning that they support the earlier theory of how waste removal occurs in the brain: that it is a passive, rather than an active process. So, what explains these discrepancies?
There are some noteworthy differences in methodologies. In their first study, the authors used a computer model to simulate solute transport dynamics in the brain9. Because the brain is such a complex organ, it could be that their model does not accurately mimics the brain. Furthermore, their model was based on data derived from primates, whereas most studies conducted research in rodent models. In their second study, the authors also used rodent models, but came to different conclusions10. An important methodological difference is the way the researchers administered tracers. Most studies injected these into the cerebrospinal fluid, whereas in this study, tracers were inserted directly into the brain. Despite these controversies, there are far more studies in favor of AQP4-dependent solute transport. Therefore, it seems more probable that astrocytes are indeed involved in cleaning the brain.
A glymphatic role for neurodegenerative brain diseases?
It looks as if the proposed glymphatic system has similar functions as lymphatic vessels, which operate in most other tissues, but with a limited role in initiating immune responses. The glymphatic system with its AQP4 channels seems only able to facilitate transport of small molecules. Most studies that examined solute dynamics used fluorescent tracers with a molecular weight between 3 and 70 kilo Dalton. When larger tracers were used, there was no evidence of solute transport2. For this reason, the glymphatic system seems unable to transport immune cells, unlike the rest of the body’s lymphatic system.
However, a glymphatic system might be able to transport small substances from the brain, which provoke an immune response outside of the central nervous system11. If this happens with particles derived from normal brain tissue, it may encourage immune cells to migrate towards the brain and to cause damage. This process of attacking healthy tissue is also called autoreactivity.
Autoreactivity is observed in several neurodegenerative diseases, such as multiple sclerosis and Alzheimer’s disease. In multiple sclerosis, immune cells infiltrate the brain and attack the fatty substance, myelin, that normally insulates the neurons. Due to the small size, myelin particles could be transported via a glymphatic system. It is therefore conceivable that the glymphatic system might contribute to the autoreactivity we observe in multiple sclerosis12.
With Alzheimer’s disease, there is an accumulation of amyloid beta peptides that lead to the formation of harmful plaques and subsequent neurodegeneration. Studies in mice show that injected amyloid beta is cleared less effectively in mice without the AQP4 and more efficiently in sleeping mice2,8. This suggests that altered sleep, leading to reduced glymphatic function, might contribute to the development of the disease. However, in both of these neurodegenerative diseases, more research is certainly needed to validate and better understand such interactions.
We still have a lot to learn about how the brain cleans up after itself to function effectively and prevent disease. This is especially the case when translating our current glymphatic model to the human brain, since most studies until now have been conducted using animal models, particularly rodents. Understanding and validating the glymphatic system model has the potential to revolutionize the way we imagine the brain’s waste disposal system, but also how we interpret future research and eventually treat the brain’s neurodegenerative diseases.
About the writer
Rosalia Franco Fernandez is a second-year Biomedical Science student who has several years of research experience in immunology and immunotherapy. Besides her fascination with the immune system, she is passionate about birds.
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