UCL preprint maps 2-micrometre channels that carried tau and amyloid toward the mouse brain surface

"The pathways by which toxic proteins are removed from the brain are poorly understood," David Attwell, a neuroscientist on the UCL team, told ScienceAlert ^[13]. In Alzheimer's disease, amyloid beta and abnormal tau accumulate, so explaining why they build up means studying how they are removed as well as how they are made ^[19].

Two exits had already been proposed. One runs along the walls of arterioles. The other runs through the spaces around venules and features in explanations of the glymphatic system ^[16]. The UCL group reports a physical conduit in both places. "We have now shown that, in both these locations, proteins are in fact removed through tiny tubes that appear to be similar to the tubes present in lymph nodes outside the brain (e.g. in the armpit or neck)," Attwell said ^[14]. The tubes are about 2 micrometres across. They link into a mesh that runs along blood vessels into the membranes covering the brain, and along nerves leaving it ^[4].

The design choice I like best is the check against artefact. A tracer goes where the experimenter puts it. The amyloid found in the Alzheimer's-model mice was different, because the animals made it themselves ^[8]. Even so, that finding does not establish how much protein ultimately leaves through the network ^[8]. Other routes operate alongside it. Some proteins are broken down inside brain tissue, and some cross blood-vessel cells into the bloodstream ^[17].

The 7.5-minute figure is an arrival time ^[6]. The thing it doesn't tell you is how much fluid moves, or what moves it. How proteins enter the channels, and how fluid moves along them that fast, is still an open question for the researchers. The beating of blood vessels may play a part, but what drives the flow has not been identified ^[12].

The human sample is small, with tissue from five tumour operations ^[9]. It shows that channels resembling the mouse ones are present in human brain tissue and take up these proteins once the tissue is outside the body ^[10]. Tissue kept outside the body cannot show the whole clearance process in a living person ^[11]. Attwell drew the same line. "In the future it will be important to demonstrate that these tubes function in the same way in living humans," he said ^[15].

I think the mouse anatomy is the solid part of this work. The human result shows the structure exists in people but not that it clears anything from a living brain. The study is a preprint and has not been peer reviewed ^[3].