New culprit in early Alzheimer’s: Blood–brain barrier blocks protective proteins

A team led by neuroscientist Andrew Yang at the Gladstone Institute of Neurological Disease in San Francisco, California, catalogued 92 proteins that cross the blood-brain barrier into the brain, according to Nature. The work suggests the barrier does more than block substances: it actively transports proteins the brain needs, and a failure of that transport appears early in Alzheimer's disease.

Yang and his colleagues developed a technique to label a large array of proteins in the blood and track which ones can enter the brain. Of the 861 proteins they mapped, 92 crossed the barrier. Most were produced in distant organs, particularly the liver, Nature reported.

The team focused on apolipoprotein A1 (APOA1), a protein linked to potential protective effects against Alzheimer's disease. Once inside the brain, APOA1 stimulates brain cells called astrocytes to release cholesterol, which is important for maintaining the synapses through which neurons communicate. The researchers found two receptors for APOA1, SR-B1 and ABCA1, on the blood-brain barrier and on astrocytes, suggesting a route by which the protein moves from the blood into the specific brain cells that need it. When Yang's team blocked the ABCA1 receptor in laboratory-grown astrocytes, the protein no longer had the same cholesterol-regulating effects.

"The paradigm in the field has been that the blood-brain barrier is normally impermeable and then becomes leaky as a result of Alzheimer's disease," study co-author Andrew Yang said, according to Nature. His team's work instead suggests the barrier actively imports proteins the brain needs to function, and the loss of those proteins drives the dysfunction seen early in the disease.

Scientists have long known that the blood-brain barrier breaks down as Alzheimer's advances, letting harmful molecules leak into the brain. Yang's team found that a different problem emerges first. Using their labelling technique in a mouse model of Alzheimer's disease, the researchers found the barrier stopped importing protective proteins such as APOA1 before it began to break down. That disruption was accompanied by a loss of neuronal connections, behavioural changes and other early hallmarks of the disease.

The team then engineered mice with Alzheimer's to produce extra APOA1 in the liver. When more APOA1 crossed the blood-brain barrier and entered the brain, the mice showed fewer Alzheimer's-disease-related markers, including reduced fat build-up in astrocytes and lower levels of inflammatory molecules, Nature reported.