Cytoskeletal reguation in growing axons

Proposed Microtubule (MT) related mechanisms of axon elongation. Neuron extending into a zone of attractant factor (blue dots), showing cell body (S), nucleus (N), axon (A), growth cone (GC), lamellipodia (L), filopodia (F), MTs (grey tubes) and F-actin (red lines). Numbered boxed areas are shown as close-ups and illustrate the following mechanisms: 1) Polymerisation dynamics: MT plus end-binding factors (orange ellipses) regulate the polymerisation dynamics of MT elongation (black arrows; >>>); 2-4) Anterograde transport: MT fragments, generated through katanin-mediated plus end severing of MTs (scissors; black ball, centrosome), contribute to axon extension by being transported anterogradely (dashed blue arrows); their rapid but discontinuous transport is mediated by dynein/dynactin (blue Y structure) anchored either to longer MTs or the F-actin network (>>>; >>>); 5) Bundling & stabilisation: microtubule associated proteins (tau, map1b, map2; green L's) organise MTs through cross-linking, enable MT-based transport through spacing of MTs, and tau protects MTs from the activity of severing factors (>>>; >>>); 6, 7) F-actin-MT interactions: MT advance into lamellipodia and filopodia occurs either through polymerisation (black arrow) or, potentially, through dynein or myosinX activity (not shown; >>>; >>>); MT advance is antagonised by myosin II-driven F-actin backflow (red arrows; >>>), but only if MTs and F-actin are coupled (yellow stars; >>>); 8) upon engagement with extracellular attractant, F-actin clears out from the local periphery, promoting efficient invasion of MTs (>>>), which might be further influenced by bundling factors (as suggested in 5).

The key question remaining in the field is how these various mechanisms interrelate and combine into the systemic output of regulated axon advance. Mapping spectraplakins as key integrators into these functional contexts lies at the heart of this problem.

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Altered growth cone morphologies of neurons deficient for different actin-binding proteins. Images of primary Drosophila neurons stained against actin (act; green) and tubulin (tub; magenta): wildtype (wt) shows filopodia (arrow heads) and lamellipodia, in DAAMEx68/Ex1 and Sop21/Q25sd mutants filopodia numbers are reduced, in ena23/GC1 mutants numbers and lengths of filopodia are reduced, in profilin-deficient (chic221) filopodia are shorter but increased in number, in cpa69E mutant growth cones filopodia numbers are increased, and in DAAMEx68/Ex1 Sop21/Q25sd double mutants filopodia are completely eliminated. Taken from Gonçalves-Pimentel et al., 2011 (>>>).
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Model of filopodia formation in Drosophila neurons. A) Arp2/3 and the formin DAAM are the essential nucleators in Drosophila neurons; Arp2/3 is expected to require nucleation promoting factors (NPF), such as Scar, nucleation is negatively regulated by profilin (a; e.g. by competing for G-actin). B) Once nucleation occurred, barbed end polymerization becomes energetically favourable and can be promoted by DAAM; inhibition of actin filament elongation through capping proteins is antagonised by formins and Enabled; anti-capping activities of Enabled do not require profilin but can be stimulated by it (b). C) Through its tetramerising activity, Enabled clusters the barbed ends of elongating actin filaments; also DAAM might contribute to this clustering event, since it has F-actin bundling activity and can bind Enabled. D) Processive actin elongation in filopodia of Drosophila growth cones is performed by DAAM and Enabled; profilin potentially cooperates with both proteins in this context (c, d), but its cooperation with Enabled appears more important for filopodial length regulation in cultured fly neurons. Taken from Gonçalves-Pimentel et al., 2011 (>>>).