Figure 3

Drosophila growth cones and the (potential) factors regulating their cytoskeletal dynamics. (a) Growth cones of aCC (arrows) and RP2 motorneurons (double chevrons; cell bodies named) in two consecutive segments of the trunk of a Drosophila embryo, stained with a cell-specifically expressed membrane marker. (b,b') Cultured Drosophila growth cone stained for microtubules (green) and filamentous actin (magenta); some filopodia lack microtubules (curved arrows), whereas others are deeply invaded (arrow heads indicate microtubule tips). (c) Schematic representation of the cytoskeletal organisation in Drosophila growth cones as extrapolated from work on growth cones in other species (detailed in the section 'Principal structure and function of growth cones'): veil-like lamellipodia (black arrowhead) contain mesh-like networks of actin filaments (randomly oriented red lines), whereas pointed filopodia (white arrowhead) contain bundled actin filaments (parallel red lines); microtubules (blue lines) are bundled in the axon, but single splayed microtubules extend into the periphery of the growth cone (curved white arrows indicate splayed microtubule tips), reaching into filopodia, as was similarly reported for growth cones of other species or migrating cells [63,330]. (d) Details of the boxed area in (c); circled numbers correlate with the numbers in Table 1 and represent the following molecular activities: 1, actin filament nucleation by Arp2/3 (which subsequently stays with the pointed ends); 2, actin filament nucleation and elongation by formins (which stay with barbed ends); 3, actin monomer binding; 4, barbed-end capping; 5, pointed end-depolymerisation/severing; 6, actin filament bundling; 7, retrograde flow of actin cytoskeleton; 8, microtubule plus end binding; 9, microtubule stabilising; 10, actin-microtubule linkage. Black straight arrows indicate growth of actin filaments or microtubules, grey straight arrows shrinkage, black curved arrows addition of actin monomers, grey curved arrows removal of actin monomers or filamentous fragments, hatched arrows indicate direction of retrograde actin flow, and the grey dashed curved double arrow linkage of actin and microtubules. (e) Current view of the effectors downstream of the Slit receptor Robo mediating repulsion from the midline of the ventral nerve cord. Robo (top right) habours five immunoglobulin domains (half elipses) and three fibronectin type III domains (blue boxes) extracellularly, and four conserved cytoplasmic (CC) domains (light to dark green) intracellularly. Robo induces growth cone repulsion by controlling cytoskeletal dynamics via either Abelson kinase (Abl) and Enabled (Ena), or Rac activity. Ena binds at CC2 and acts most likely through Chickadee/Profilin on actin dynamics. Abl binding to Robo at CC3 influences actin dynamics via Capulet and microtubule dynamics via the +TIP protein Chromosome Bows (Chb/Orbit/MAST). Simultaneously, Abl phosphorylates CC1 to antagonise Robo function. The regulation of Rac activity through Robo occurs through CC2/3 recruitment of the SH3-SH2 adaptor molecule Dreadlocks (Dock) which, in turn, activates Rac through both Pak and the GEF Sos. In parallel, active Robo can influence Rac activity via the binding of RhoGAP93B (vilse/CrGAP) to CC2, but it remains unclear whether RhoGAP93B is positively or negatively regulated by Robo. Paradoxically, both decrease and increase of Rac activation levels can cause midline crossing, suggesting that: Rac might influence other effectors to cause repulsion; a precise Rac activation level is required to mediate Slit-induced repulsion; or a sequential modification of Rac in response to Robo activation has to occur, such as an initial role to prevent extension towards the source of the repellent and another role to encourage extension away from the Slit source. Calmodulin and GEF64C have additionally been identified as modifiers of Robo activity, although it is not clear yet how they influence Robo signalling (calmodulin possibly through Chic).