Plasticity of polarization:: Changing dendrites into axons in neurons integrated in neuronal circuits

Developing neurons can change axonal and dendritic fate upon axonal lesion [1-6], but it is unclear whether neurons retain such plasticity when they are synaptically interconnected [7, 8]. To address whether polarity is reversible in mature neurons, we cut the axon of GFP-labeled hippocampal neurons in dissociated and organotypic cultures and found that a new axon arose from a mature dendrite. The regenerative response correlated with the length of the remaining stump: proximal axotomies (<35 mu m) led to the transformation of a dendrite into an axon (identity change), whereas distal cuts (>35 mu m) induced axon regrowth, similar to what is seen in young neurons (3]. Searching for a putative landmark in the distal axon that could determine axon identity, we focused on the stability of microtubules, which regulate initial neuronal polarization during early development [9]. We found that functionally polarized neurons contain a distinctively high proportion of stable microtubules in the distal axon. Moreover, pharmacological stabilization of microtubules was sufficient to induce the formation of multiple axons out of differentiated dendrites. Our data argue that mature neurons integrated in functional networks remain flexible in their polarity and that mechanisms acting during initial axon selection can be reactivated to induce axon growth out of functionally mature dendrites.