However, AC severing of actin filaments also creates new barbed ends, which can synergize with actin polymerization
factors to promote filament assembly PI3K inhibitor cancer and membrane protrusion (Kuhn et al., 2000 and Pollard et al., 2000). The opposite functions of AC on actin filaments likely depend on its local concentration of AC and the ratio of AC against actin monomers: severing and disassembly are more favorable when AC is at a lower concentration, whereas nucleating occurs at higher AC concentrations (Andrianantoandro and Pollard, 2006). The precise function of AC in nerve growth cones remains to be fully understood. AC is expressed at high levels and colocalizes with F-actin in neuronal growth cone (Bamburg and Bray, 1987). Overexpression of AC in neurons leads to increased neurite outgrowth (Meberg et al., 1998), indicating that actin turnover may promote motility (Bradke and Dotti, 1999). However, AC activation has also been associated with growth cone collapse (Aizawa et al., 2001, Hsieh et al., 2006 and Piper et al., 2006), demonstrating a negative
impact of AC on growth cone motility. In growth cone steering, asymmetric AC inhibition was shown to mediate attractive turning of the growth cone, whereas local AC activation elicited repulsion (Wen et al., 2007). These findings are consistent with the classic depolymerizing/severing functions of AC on the actin cytoskeleton. However, AC activation was shown in some cases to promote selleck screening library actin-based membrane protrusion in nonneuronal cells (DesMarais et al., 2005 and Ghosh et al., 2004) and to mediate growth cone attraction in cultured dorsal root ganglion neurons (Marsick et al., 2010). It is plausible that different types of cells exploit specific end results
of AC activity, and their unique cytosolic environment may contribute to the opposite outcomes of increased AC activity on motility. It is also possible that the same neurons may have varying levels of basal actin dynamics, upon which AC may generate different effects. For example, new growth cones from young neurons tend to be very motile and have a high level of actin turnover, whereas those from more mature neurons have relatively stable F-actin and reduced motility. AC activation could in principle impact the motility of these growth cones in an opposite manner. We propose that an optimal range of AC activity is required to generate the dynamic turnover of the actin cytoskeleton underlying high growth cone motility and that this range is dependent on the kinetic state of the actin network at that time. In this instance, modulation of AC activity in either direction could either accelerate or decrease motility (Figure 1C) or, if done assymetrically within the growth cone, cause a positive or negative turning response.