That Shrinking Feeling

Amber Roguski,

Editor (Science)   

 

Scientists at University of Rochester may have solved the great mystery of why animals sleep so much. In their study published in the journal Science on October 18, the researchers revealed their conclusion that during sleep, cells in the brain shrink to allow increased waste clearance.

Last year it was discovered that the brain has its own ‘waste removal’ process which is made possible by the glymphatic system. In much the same way that the lymphatic system removes waste molecules from the body, the glymphatic system transports potentially neurotoxic material away from the brain and spinal cord. The waste molecules are carried through the brain in cerebrospinal fluid, which deposits them into the circulatory system and eventually these potentially damaging materials end up being broken down by the liver.

The research team at University of Rochester Medical Centre, led by Dr Lulu Xie, used two-photon microscopy to image the brains of mice, both sleeping and awake, to reveal that when the mice were asleep (or anaesthetised), the glymphatic system was ten times more active than when the mice were awake.

The brain comprises two types of cells — neurones and glial cells. While the neurones are responsible for passing information around the brain, it is the glial cells that keeps the structures of the brain glued together and in working order. It is one type of glial cell, known as astrocytes, which are responsible for the newly discovered glymphatic system. Astrocytes run alongside the blood vessels in the brain, creating a paravascular space between the two structures. It is through this space that cerebrospinal fluid flows, draining the brain of excess proteins and molecules. When asleep, the astrocytes appear to shrink away from the blood vessels, increasing the space between cells and blood vessels by 60%, allowing for increased rate of clearance of cerebrospinal fluid.

So why do we have to be asleep for this effective drainage system to be at its optimum?

“The brain only has limited energy at its disposal and it appears that it must (choose) between two different functional states — awake and aware or asleep and cleaning up. You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can’t really do both at the same time,” explains researcher and lead author of the study, Dr. Maiken Nedergaard, the scientist behind last year’s discovery of the glymphatic system.

It is essential that this clearance of molecules occurs in the brain, as build-up of proteins can lead to neurodegenerative conditions such as Alzheimer’s and Parkinson’s diseases. The study found that the glymphatic system removed significant levels of the protein beta-amyloid, which is a molecule characteristic in Alzheimer’s disease, where beta-amyloid plaques form in the brain. This finding is particularly exciting, as it could suggest that people with Alzheimer’s disease have a less effective glymphatic system. Although much more research would have to be undertaken for any conclusions to be drawn, with Alzheimer’s Research UK charity stating the findings of this study provide a “potential new avenue for investigation.”

Image Courtesy: Wikimedia Commons (Gordana Adamovic-Mladenovic, Tomwsulcer)