The Brain-Blood Filter?

A new study is changing the way we think about the so-called brain-blood barrier. The study conducted by Stanford researchers and recently published in the journal Nature demonstrates that hundreds of proteins that occur naturally in healthy young mice's blood routinely enter into the mice's brains. While previous research had shown almost no such permeability, these earlier studies traced the passage into the brain of injected proteins that are naturally absent or rare in the blood while the new research added fluorescent labels to all of the hundreds of different proteins naturally occurring in mouse plasma. The plasma proteins were then injected back into mice, and the researchers looked for the proteins in the mice's brains. In young mice, about 2-3% of those labeled proteins ended up in the brain.  The proteins' entry was enabled by specialized receptors found on the cells lining cerebral blood vessels while a small amount of proteins leaked into the brain in a non-selective manner.  Interestingly, the selective receptor-driven form of physiological uptake by the brain dwindled as the mice aged as did production of the receptors required for their uptake.  This previously unrecognized age-associated change in how proteins are transported into the brain could have clinical significance.  For example, changes in protein trafficking to the brain may result in mood and behavior changes and potentially underly age-associated pathologies.  Rather than thinking of it as the brain-blood barrier it might be more helpful to think of it as a filter, one that can get leaky or clogged as we age.