To rid itself of extracellular proteins, the brain relies on water transport by cells called astrocytes and on the flow of cerebrospinal fluid. This fluid was thought mainly to provide mechanical and immunological protection to the brain.
Jeffrey Iliff and Maiken Nedergaard at the University of Rochester Medical Center in New York and their teams tracked fluorescently labelled molecules as they moved through the brain. The proteins — including amyloid beta, thought to be pathogenic in Alzheimer's disease — were transferred into cerebrospinal fluid. This, the authors report, flows into the brain through the spaces surrounding arteries, and out by means of the spaces around veins. The blood vessels were surrounded by projections from astrocytes. In mice lacking a protein that transports water across the astrocyte cell membrane, 70% fewer molecules were cleared from the brain than in normal animals.
Understanding this clearance system could help to explain how some neurodegenerative diseases develop, the researchers suggest.