JNK Gives Axons a Second Chance

Stages of normal (A) and aberrant (B, C) hippocampal neuron differentiation in vitro. A, The cleavage plane of the last mitotic division determines the position of the centrosome, which primes the area of formation of the initial neurite. Shortly afterward, a number of similar neurites develop (stage 2). Although at this stage every neurite has the potential to become the axon, some proteins involved in axon development, such as active JNK, are already present in the initial neurite. In normal conditions, this is the neurite that will begin to elongate rapidly and form the axon (A). B, When some of the proteins required for axon formation are inhibited, no axon is formed and all of the neurites become dendrites. In the case of JNK, this aberrant polarization can be reverted if JNK inhibitor is removed from the medium. C, When the activity of these proteins is induced ectopically, axons develop from nonprimed neurites. Some examples of experiments that lead to aberrant polarization are shown at the right (for review, see Wiggin et al., 2005). Green neurite, Axon; red neurite, dendrite; −, inhibition; +, activation