ADF/Cofilin-Mediated Actin Retrograde Flow Directs Neurite Formation in the Developing Brain

Abstract

Neurites are the characteristic structural element of neurons that will initiate brain connectivity and elaborate information. Early in development, neurons are spherical cells but this symmetry is broken through the initial formation of neurites. This fundamental step is thought to rely on actin and microtubule dynamics. However, it is unclear which aspects of the complex actin behavior control neuritogenesis and which molecular mechanisms are involved. Here, we demonstrate that augmented actin retrograde flow and protrusion dynamics facilitate neurite formation. Our data indicate that a single family of actin regulatory proteins, ADF/Cofilin, provides the required control of actin retrograde flow and dynamics to form neurites. In particular, the F-actin severing activity of ADF/Cofilin organizes space for the protrusion and bundling of microtubules, the backbone of neurites. Our data reveal how ADF/Cofilin organizes the cytoskeleton to drive actin retrograde flow and thus break the spherical shape of neurons.

Actin Aggregations Mark the Sites of Neurite Initiation

“…First, by examining the effects of actin and microtubule stabilizing and destabilizing agents on neuritogenesis, and using each agent over multiple concentrations to assess their dosage effect, we provide a systematic study of the relative contributions of actin and microtubules to neurite formation. These results complement those of previous studies using molecular tools to investigate the role of the actin cytoskeleton in neuritogenesis [41,47,48]. Our subdivision of stage 1 into stages 1a and 1b helps to clarify the need for both actin depolymerization and actin polymerization in neurite initiation (Figs.…”

“…At all time points examined, Tuj 1 redistribution lagged that of F-actin. Thus, our results, consistent with those of recent publications [41,47,48], suggest a more critical role of the actin cytoskeleton than of microtubules, in neurite initiation.…”

“…Our results, together with those of previous studies [41,47,48], pose the next interesting question, namely, what signal initiates the formation of actin aggregations? Since actin aggregations form within the cell and at physical locations close to the cell membrane, they could either be initiated by intrinsic, membrane-associated cues, or extracellular cues that signal via transmembrane receptors.…”

“…It has been proposed that microtubule-associated proteins that can also interact with actin, such as MAP2c, promote neurite extension by facilitating microtubule assembly and extension at the sites of actin aggregates [34,38,39,41,47,78,79]. Similar effects have also been reported for the F-actin-associated protein drebrin, which has recently been shown to bind to the microtubule plusend protein EB3 and also promote neuritogenesis [80].…”

“…In mice lacking all three Ena/VASP proteins, cortical neurons lacked filopodia and failed to initiate neurites, a process that can be rescued either by over-expression of the actin nucleating protein mDia2 or by plating neurons on the filopodia-promoting laminin substrate [41]. In another recent study, the actin depolarizing factor (ADF)/Cofilin family, which depolymerizes and severs actin filaments [45,46], was shown to promote neurite formation by driving actin retrograde flow and thereby allowing microtubule protrusion [47]. Similarly, polarized actin dynamics has been shown to mark the site of first polarization in round cells [48].…”