An axon branch (left) is pulled in by turning off a RhoGAP (right).


Learning spurs axons in the brain to grow and connect with other neurons. However, scientists know little about what maintains these connections, which sometimes last a lifetime. A new study reveals that a protein known as p190 RhoGAP stabilizes the axon. The authors report that p190 works by stifling a built-in “retraction pathway” that causes axons to shrivel.

Liqun Luo of Stanford University, Palo Alto, CA, and colleagues investigated p190's function in the Drosophila mushroom body, the part of the brain responsible for olfactory learning and memory. When they inhibited p190 in neurons, the axon branches shrank or even disappeared. They got the same results by activating the RhoA pathway, which p190 normally blocks. “The idea that in mature neurons there is a pathway whose job is to destroy the axon is surprising,” says Luo.

The authors did not look for behavioral abnormalities in flies with inactivated p190. The results suggest, however, that regulation of the protein may have a role in the neural rewiring responsible for learning and memory, Luo says. p190 may also be linked to two other proteins that are necessary for memory formation or storage: Src and integrin. Since Src and integrin likely inhibit p190, their effects on memory may stem from changes in neuronal structure rather than changes in synapse function, says Luo. ▪


Billuart, P., et al.