Microtubule transport in the axon: Re-thinking a potential role for the actin cytoskeleton.

Microtubules are transported down the axon as short pieces by molecular motor proteins. One popular idea is that these microtubules are transported by forces generated against the actin cytoskeleton. The motor for such transport is thought to be cytoplasmic dynein. Here, the authors review this model and discuss recent studies that sought to test it. These studies suggest that the model is valid but incomplete. Microtubule transport is bidirectional and can utilize either actin filaments or longer microtubules as a substrate in the anterograde direction but only longer microtubules in the retrograde direction. Cytoplasmic dynein is one participating motor but not the only one. The authors speculate that the category of anterograde microtubule transport that involves actin filaments may have specialized functions. The relevant forces that transport short microtubules may also be crucial for the manner by which the longer immobile microtubules interact with actin filaments during events such as axonal retraction and growth cone turning.