In the orchestra of the aging brain, the instruments go out of tune one after another. The vessels that deliver oxygen thin out. The insulation around nerve fibers, like the coating on old wires, loses its density. The workshop that turns out new neurons slows its assembly line. The conductor of this orchestra, it turns out, sits not in the head but in the belly.
The density of blood vessels rose in the cortex. New cells appeared in the corpus callosum, rewrapping nerve fibers in fresh insulation. In the hippocampus, which handles memory, stem cells woke up and produced new neurons. The brain's immune cells calmed down. Among the inflammatory molecules, one protein stood out: eotaxin-1. Its level dropped, and when the protein was given back to the mice, the rejuvenation was cancelled. Blocking eotaxin-1 without antibiotics had the same effect: the mice were less anxious and remembered new objects better.
Until now it was unclear whether removing the microbiome from an old animal could reverse brain aging, and which molecules do the work. Now it is clear: it can, and the key mediator has been found. Blocking eotaxin-1 or changing the composition of gut microbes could become the basis for treating age-related memory decline. What remains is to find out which bacteria trigger production of this protein, and whether diet can achieve the same effect without resorting to antibiotics.
Eotaxin-1 was first tied to brain aging in 2011, when its blood level correlated with memory decline in people, and giving the protein to young mice impaired their neurogenesis. Now it turns out its source may be not only the brain but also the gut, where eosinophils accumulate with age.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.