In this view, microtubules may grow along radial F-actin bundles and filopodia because they offer the path of least resistance GW786034 datasheet in the retrogradely flowing actin network. Since there is less interweaving of actin fibers in radial actin bundles compared to the actin network in regions of lamellipodia and proximal actin arcs, microtubules can grow in an unimpeded fashion. Indeed, analysis of microinjected fluorescent particles of different molecular weights showed that the actin network in cells can be dense enough

to prevent the movement of structures having the size of microtubules ( Luby-Phelps and Taylor, 1988). Consistent with this idea, the pharmacological destabilization of actin filaments in AC KO neurons restored neuritogenesis. Furthermore, it also allowed the proper orientation and growth of microtubules, enabling them to protrude

through the cell rim to induce a neurite. Thus, our data suggest that during neuritogenesis, AC proteins enable microtubule protrusion both by dismantling dense actin structures to free intracellular space and by helping organize parallel F-actin bundles that facilitate radial microtubule growth and bundling ( Figure 8F). In the absence of AC proteins, the congestion and lack of “permissive” F-actin bundles obstructs directed microtubule protrusion, ultimately leading to a failure of neuritogenesis ( Figure 8G). Our study shows that the effects of AC-mediated actin dynamics on early brain development are of paramount p38 MAPK inhibitor these importance and relevant for human brain development. For example, the neurocognitive disorder Smith-Lemli-Optiz syndrome has recently been linked to neurite growth defects rooted, perhaps, in a misregulation of Cofilin activity (Jiang et al.,

2010). Furthermore, the cortical ectopias we observed in embryonic AC KO brains resemble the cobblestone cortex of mouse models of type II lissencephaly ( Bielas et al., 2004). Our study, along with others, highlights the importance of exploring the role of cytoskeleton-mediated mechanisms in human brain disorders ( Heng et al., 2010). We therefore see the further elucidation of the mechanism of microtubule-actin interactions and the involved players during neurite growth as essential to future studies in understanding brain development and pathology. Conditional ablation of ADF/Cofilin proteins in the nervous system was achieved by crossing mice with genomic ADF ablation and expressing Cofilin floxed alleles (ADF−/−, cofilinflox/flox) ( Bellenchi et al., 2007) with mice lines expressing Cre recombinase (Cre) from the nervous system-specific promoters (see Supplemental Experimental Procedures for details). The heads of E17 mouse embryos were fixed in 4% paraformaldehyde, 4% sucrose in PBS or PHEM buffer and prepared for cryosectioning using standard procedures (Tahirovic et al., 2010).