The flow of fluid through the brain was drastically reduced in a mouse model of Alzheimer’s disease, and cholesterol may be responsible. Disruption to the glymphatic system, the brain’s waste-clearance network, contributes to the build-up of harmful proteins associated with the condition. Now, cholesterol has been linked to a reduction of this circulation, perhaps by blocking crucial passages within the brain.
However, this is far from established and many questions remain. “I would regard this more as a hypothesis-generating piece,” says Roslyn Bill at Aston University in Birmingham, UK, who wasn’t involved in the research.
Alzheimer’s disease is the most common form of dementia, which affects around 50 million people worldwide. People with Alzheimer’s accumulate two types of harmful protein in their brain: beta-amyloid, which aggregates into plaques, and tau, which forms tangles.
Many studies suggest that one contributing factor in Alzheimer’s is a failure to clear waste chemicals from the brain. The brains of people without Alzheimer’s flush out harmful waste through the glymphatic system: a network of narrow channels running along the outer walls of blood vessels in the brain. Glymphatic flow is reduced in mouse models of the condition, but evidence is mounting that this could be restored via drugs, light and sound exposure or even inhaling a high dose of carbon dioxide.
To better understand this disruption, Do-Geun Kim and his colleagues at the Korea Brain Research Institute in Daegu, South Korea, studied two groups of mice. Some had been genetically engineered to express five mutations associated with Alzheimer’s in people, while the rest were healthy.
The team injected a dye into the cisterna magna, a chamber at the back of the brain that is full of cerebrospinal fluid. Different mice were then euthanised 5 minutes, 15 minutes, 30 minutes, 60 minutes or 120 minutes after the injection.
In the healthy mice, the dye spread along the outside of arteries running along the underside of the brain, and then into brain tissue, over the first 30 minutes. By 120 minutes, the dye was almost entirely gone, suggesting it had been cleared by the glymphatic system.
However, in the genetically modified mice, the dye accumulated around the outsides of the arteries, with little of it reaching brain tissue. “The passage is somewhat blocked,” says Kim.
Normal glymphatic flow occurs just outside blood vessels. Brain cells called astrocytes wrap their foot-like extensions around these vessels, creating narrow channels – called perivascular spaces – through which liquid can flow. The researchers found that these spaces were significantly narrower in the mice modelling Alzheimer’s disease than in the healthy ones.
Furthermore, they found that what little dye made it into the modified mice’s brains mostly ended up in their amyloid plaques. The researchers suggest the plaques acted like a sponge, soaking up fluid that would normally circulate through the brain.
Finally, the team looked into the potential role of cholesterol in glymphatic dysfunction. One of the most important risk factors for Alzheimer’s disease is mutations in a protein called apolipoprotein E, and one of its main functions is to produce cholesterol in the brain.
The researchers stained mice brains with a fluorescent marker that binds to cholesterol. In the healthy mice, they saw lots of fluorescence in the brain after 5 minutes, with levels then declining. Although unclear, this may be due to cholesterol being released into the brain in response to the dye and then being cleared. However, in the modified mice, the cholesterol signal increased up to 120 minutes.
Cholesterol has an array of roles in healthy brains, including regulating signalling between neurons. It is possible cholesterol also transports chemicals from perivascular spaces to neurons, says Kim, who says there may be something about Alzheimer’s disease that changes cholesterol so it blocks that flow.
“The perivascular spaces are the highway for fast fluid inflow,” says Maiken Nedergaard at the University of Rochester in New York, one of the discoverers of the glymphatic system. “If that is compromised, it’s really hard to get something into the brain.”
However, she says the study doesn’t clearly show what role cholesterol may have in glymphatic function or why it behaved differently in the modified mice. “Why is the cholesterol mobilised?” she asks. Bill wants to see the experiments repeated with live mice, so the flow of cerebrospinal fluid can be tracked in real time, rather than in microscope slides from dead mice.