, 2002). However, the downstream effectors and precise actin mechanisms that control the directional motility of growth cones remain to be fully determined. Actin filaments are built

through a balancing act of filament assembly at the barbed ends and disassembly at the pointed ends, and these rates are influenced by a wide ABT 737 range of regulatory proteins. Moreover, an even larger number of accessory proteins are present in cells to organize actin filaments into distinct networks in specific subcellular locations (Chhabra and Higgs, 2007, Pollard et al., 2000 and Pollard and Borisy, 2003). For example, lamellipodia and filopodia, two membrane protrusions that function in growth cone movement and environmental sensing, respectively, are based on distinct F-actin structures. selleck products The former contains a meshwork of short, branched actin filaments that depends on the Arp2/3 nucleation complex, whereas the later is supported by long unbranched actin filaments involving formin family of molecules and regulated by Ena/Vasp proteins. A number of excellent reviews are available that have provided comprehensive coverage on the actin structures and dynamics of lamellipodia and filopodia in both nonneuronal cells and nerve growth cones (Dent et al.,

2011, Lowery and Van Vactor, 2009, Pollard and Cooper, 2009 and Rodriguez et al., 2003). Here, we will only discuss a few of the actin regulatory molecules whose function in growth cone motility is complex secondly and remains to be fully understood. In vertebrate cells, a large array of regulatory proteins control the actin network and its dynamics through a diverse set of actions, including filament nucleation, severing, crosslinking, and end capping, as well as monomer sequestering. Many of these proteins have not been well studied in neuronal growth cones, and whether and how they function in growth cone migration and guidance remains to be seen (Dent et al., 2011). In a minimal model proposed for the actin assembly and disassembly underlying lamellipodial protrusion, just five families of actin-binding proteins were thought to be

needed: WASp, Arp2/3, capping protein, ADF/cofilin, and profilin/β-thymosin (Pollard et al., 2000). Of them, WASp, Arp2/3, and ADF/cofilin have been investigated in nerve growth cones (Dent et al., 2011 and Lowery and Van Vactor, 2009), whereas thymosin/profilin and capping protein have received less attention. Capping barbed ends of actin filaments represents an important mechanism to regulate filament elongation (Pollard and Borisy, 2003). Capping proteins bind to free barbed ends and prevent addition or loss of actin subunits. Of the known actin-capping proteins, the predominant species in most nonmuscle cell types is CapZ (commonly abbreviated as CP). CP is an obligate heterodimer consisting of α and β subunits (Cooper and Sept, 2008 and Schafer, 2004). While both α1 and α2 isoforms are abundant in most tissues (Hart et al.