Doublecortin associates with microtubules preferentially in regions of the axon displaying actin-rich protrusive structures.

Here we studied doublecortin (DCX) in cultured hippocampal and sympathetic neurons during axonal development. In both types of neurons, DCX is abundant in the growth cone, in which it primarily localizes with microtubules. Its abundance is lowest on microtubules in the neck region of the growth cone and highest on microtubules extending into the actin-rich lamellar regions. Interestingly, the microtubule polymer richest in DCX is also deficient in tau. In hippocampal neurons but not sympathetic neurons, discrete focal patches of microtubules rich in DCX and deficient in tau are present along the axonal shaft. Invariably, these patches have actin-rich protrusions resembling those of growth cones. Many of the DCX/actin filament patches exhibit vigorous protrusive activity and also undergo a proximal-to-distal redistribution within the axon at average rates approximately 2 microm/min and thus closely resemble the growth-cone-like waves described by previous authors. Depletion of DCX using small interfering RNA had little effect on the appearance of the growth cone or on axonal growth in either type of neuron. However, DCX depletion significantly delayed collateral branching in hippocampal neurons and also significantly lowered the frequency of actin-rich patches along hippocampal axons. Branching by sympathetic neurons, which occurs by growth cone splitting, was not impaired by DCX depletion. These findings reveal a functional relationship between the DCX/actin filament patches and collateral branching. Based on the striking resemblance of these patches to growth cones, we discuss the possibility that they reflect a mechanism for locally boosting morphogenetic activity to facilitate axonal growth and collateral branching.