Role of Actin Filaments in the Axonal Transport of Microtubules

Abstract

Microtubules originate at the centrosome of the neuron and are then released for transport down the axon, in which they can move both anterogradely and retrogradely during axonal growth. It has been hypothesized that these movements occur by force generation against the actin cytoskeleton. To test this, we analyzed the movement, distribution, and orientation of microtubules in neurons pharmacologically depleted of actin filaments. Actin depletion reduced but did not eliminate the anterograde movements and had no effect on the frequency of retrograde movements. Consistent with the idea that microtubules might also move against neighboring microtubules, actin depletion completely inhibited the outward transport of microtubules under experimental conditions of low microtubule density. Interestingly, visualization of microtubule assembly shows that actin depletion actually enhances the tendency of microtubules to align with one another. Such microtubule-microtubule interactions are sufficient to orient microtubules in their characteristic polarity pattern in axons grown overnight in the absence of actin filaments. In fact, microtubule behaviors were only chaotic after actin depletion in peripheral regions of the neuron in which microtubules are normally sparse and hence lack neighboring microtubules with which they could interact. On the basis of these results, we conclude that microtubules are transported against either actin filaments or neighboring microtubules in the anterograde direction but only against other microtubules in the retrograde direction. Moreover, the transport of microtubules against one another provides a surprisingly effective option for the deployment and orientation of microtubules in the absence of actin filaments.

Regulation of Microtubule Severing by Katanin Subunits during Neuronal Development

“…In addition, microtubules need to be severed, albeit less avidly, throughout other compartments of the neuron to ensure that sufficient numbers of microtubules are able to undergo rapid transport. The precise relationship between microtubule length and transport remains unresolved but presumably requires a tight balance between short microtubules that move and long microtubules that act as railways for the movement (Hasaka et al, 2004). Microtubule severing also produces more free ends of microtubules, which are thought to be sites for the interaction of the microtubule with a number of other structures and proteins (Kornack and Giger, 2005).…”

Cytoplasmic Linker Proteins Regulate Neuronal Polarization through Microtubule and Growth Cone Dynamics

“…To test whether changes of the microtubule network affect the protrusion of microtubules within the growth cone, we studied the growth rates of microtubules in control as well as in DN-CLIP-and MBD-CLIP-transfected neurons. As EB3 binding to microtubules is independent of CLIP localization and is not affected by DN-CLIP expression (data not shown) (Komarova et al, 2002;Goodson et al, 2003;Bieling et al, 2008), we visualized polymerizing microtubule ends by mCherry-EB3, a well established marker to examine microtubule dynamics (Stepanova et al, 2003;Hasaka et al, 2004).…”

Generation of noncentrosomal microtubule arrays

“…Cytoplasmic dynein itself moves with slow axonal transport at a rate similar to that of actin (Dillman et al, 1996). Also, drug-induced depolymerization of actin filaments in neurons modifies the organization of MTs in growth cones and specifically reduces the rate of anterograde MT transport (Hasaka et al, 2004). Understanding how dynein interacts with actin would provide further support for this model.…”