Cytoskeleton in Axon Growth


Axons and dendrites are the neuronal processes that transmit nerve signals throughout the body. These processes are formed and maintained by the neuronal cytoskeleton, particularly microtubules and actin filaments. Microtubules are the supportive framework of neurites, axons and dendrites, as well as the rails for transporting organelles and cytoplasmic components. Actin filaments support the neuron's outer cortex, and the dynamic protrusion of actin‐filled filopodia and lamellipodia drives neuronal morphogenesis, including axon guidance, branching and regeneration. The initiation of axon formation requires advance of microtubules into filopodial protrusions adherent to the substratum. Polymerisation of microtubule plus ends and dynein‐driven transport of short microtubules provides ‘push’ for continued axonal growth. Actin‐filled protrusions at the axonal terminal, the growth cone, explore the environment for cues that bind receptors and trigger either protrusion and microtubule advance or cessation of protrusion and myosin II contractility that retracts protrusions and limits microtubule advance.

Key Concepts

  • The neuronal cytoskeleton generates and supports neuronal shape.
  • Axonal elongation involves accumulation of microtubules through polymerisation, stabilisation and transport.
  • The motile growth cone at the tip of an elongating axon is where the axon is steered to its synaptic target.
  • Dynamic actin filaments drive the growth cone's exploration of its environment.
  • Growth cone adhesive interactions allow traction that advances the growth cone.
  • Neuronal polarisation involves regulation of cytoskeletal dynamics.
  • Extrinsic guidance cues locally regulate actin dynamics and microtubule advance to steer the growth cone.
  • Axonal branching involves cytoskeletal dynamics to produce new streams of microtubule advance.
  • Success in axonal regeneration will require recapitulation of the cytoskeletal dynamics of developing neurons.

Keywords: neurite; axon; actin; microtubule; myosin II ; dynein; growth cone; guidance; branching; regeneration

Figure 1. Stages of axon formation. A hippocampal neuron in vitro passes through several stages of axon formation. (a) The neuron is initially a sphere on the substratum. Microtubules (red) circle the neuronal soma, and actin filaments (green) form a sub‐plasmalemmal cortex. (b) In stage one, dynamic actin filaments protrude filopodia from the neuronal perimeter, while microtubules remain parallel to the periphery. (c) To begin stage two, myosin II‐mediated tension exerted in an adherent filopodium directs a microtubule plus end into the base of a filopodium. (d) In stage two, further microtubule advance and actin‐based protrusion leads to formation of a neurite with growth cone. (e) In stage three one, neurite has become the axon, which contains microtubules that are stable, and with a growth cone rich in dynamic actin filaments and microtubules that probe into the actin‐filled P‐domain. Microtubule advance is limited in the growth cones of neurites.
Figure 2. Dynamics of actin filaments. Actin filaments grow by adding ATP‐actin (red) to the barbed end, which faces the leading edge of the growth cone. ADP‐actin (green) is disassembled from the filament at the opposite pointed end. Polymerisation at the distal membrane and depolymerisation proximally lead to treadmilling of F‐actin. ADP‐actin becomes recycled to ATP‐actin and can be used again for polymerisation. Actin‐binding proteins (ABPs) regulate these dynamics by either promoting polymerisation like Ena/Vasp proteins or by severing and depolymerising F‐actin like ADF/cofilin. Furthermore, ABPs can organise the actin filaments into higher order structures, for example by bundling through proteins like fascin.
Figure 3. Dynamics of microtubules. Microtubules are dynamic polymers of tubulin dimers. The polymers rapidly switch from a growing mode to a shrinking mode (catastrophe) or from shrinking to growing (rescue). Microtubule‐associated proteins (MAPs) regulate microtubule dynamics and organisation. For instance, CRMP‐2 and plus‐end tracking proteins (+TIPs) promote microtubule assembly. In contrast, SCG10 and stathmin destabilise microtubules. Furthermore, proteins such as katanin and spastin sever microtubules and are important in axon growth and branching.
Figure 4. The growth cone. (a) Growth cone of a hippocampal neuron. The actin cytoskeleton is labelled by fluorescent phalloidin (red). Microtubules are stained with antibodies against tubulin (green). The peripheral domain (P‐domain) of the growth cone contains dynamic actin filaments, while microtubules dominate the central domain (C‐domain) of the growth cone and the axon shaft. Scale bar, 10 µm. (b) Schematic diagram of a growth cone. The growth cone can be divided into two domains: The P‐domain is composed of finger‐like filopodia that are separated by veils and lamellipodia. Filopodia are formed by F‐actin bundles while lamellipodia are based on an F‐actin meshwork. The central domain (C‐domain) is filled with microtubules that enter the growth cone bundled from the axon shaft. Single, dynamic microtubules grow forward from the C‐domain to explore the P‐domain.
Figure 5. A schematic model of growth cone navigation. Actin polymerisation pushes the leading margin of the growth cone forward. Forces generated by myosin II pull actin filaments backward, where filaments are disassembled. When growth cone receptors make adhesive contacts, a molecular ‘clutch’ links the adhesive contact to actin filaments, the retrograde flow of actin filaments slows and traction forces pull the growth cone forward. This permits the advance of microtubules and organelles and promotes axonal elongation. Intracellular signals generated by attractive and repulsive axonal guidance cues interact with the mechanisms of actin polymerisation, myosin II force generation, adhesive contacts and microtubule advance to regulate the paths of growth cone migration. Reproduced with permission from Gomez and Letourneau (2013) © John Wiley and Sons.


Blackmore MG , Wang Z , Lerch JK , et al. (2012) Kruppel‐like factor 7 engineered for transcriptional activation promotes axon regeneration in the adult corticospinal tract. Proceedings of the National Academy of Sciences, USA 109 (19): 7517–7522.

Bradke F and Dotti CG (1999) The role of actin instability in axon formation. Science 283 (5409): 1931–1934.

Brown J and Bridgman PC (2003) Role of myosin II in axon outgrowth. Journal of Histochemistry & Cytochemistry 51 (4): 421–428.

Carlstrom LP , Hines JH , Henle SJ and Henley JR (2011) Bidirectional remodeling of ß1‐integrin adhesions during chemotropic regulation of nerve growth. BMC Biology 9: 82.

Chadborn NH , Ahmed AI , Holt MR , et al. (2006) PTEN couples Sema3A signaling to growth cone collapse. Journal of Cell Science 119 (8): 951–957.

Challacombe JF , Snow DM and Letourneau PC (1997) Dynamic microtubule ends are required for growth cone turning to avoid and inhibitory guidance cue. Journal of Neuroscience 17: 3085–3095.

Coles CH and Bradke F (2015) Coordinating neuronal actin‐microtubule dynamics. Current Biology 25 (15): R677–691.

Dehmelt L , Nalbant P , Steffen W and Halpain S (2006) A microtubule‐based, dynein‐dependent force induces local cell protrusions: Implications for neurite initiation. Brain Cell Biology 35: 39–56.

Dent EW , Gupton SL and Gertler FB (2010) The growth cone cytoskeleton in axon outgrowth and guidance. Cold Spring Harbor Perspectives in Biology. DOI: 10.1101/cshperspect.a001800.

Dotti CG , Sullivan CA and Banker GA (1988) The establishment of polarity by hippocampal neurons in culture. Journal of Neuroscience 8 (4): 1454–1468.

Edson KJ , Lim SS , Borisy GG and Letourneau PC (1993) A FRAP analysis of the stability of the microtubule population along the neurites of chick sensory neurons. Cell Motility and Cytoskeleton 25: 59–72.

Gallo G , Yee HF and Letourneau PC (2002) Actin turnover is required to prevent axon retraction driven by endogenous actomyosin contractility. Journal of Cell Biology 158: 1219–1228.

Gallo G and Letourneau PC (1998) Localized sources of neurotrophins initiate axon collateral sprouting. Journal of Neuroscience 18: 5403–5414.

Gallo G and Letourneau PC (1999) Different contributions of microtubule dynamics and transport to the growth of axons and collateral sprouts. Journal of Neuroscience 19: 3860–3873.

Gallo G and Letourneau PC (2000) Neurotrophins and the dynamic regulation of the neuronal cytoskeleton. Journal of Neurobiology 44: 159–173.

Gallo G (2011) The cytoskeleton and signaling mechanisms of axon collateral branching. Developmental Neurobiology 71: 201–220.

Geraldo S and Gordon‐Weeks PR (2009) Cytoskeletal dynamics in growth cone steering. Journal of Cell Science 122: 3595–3604.

Gomez TM and Letourneau PC (2013) Actin dynamics in growth cone motility and navigation. Journal of Neurochemistry 129: 221–234.

Gomez TM and Letourneau PC (2014) Actin dynamics and growth cone motility. Journal of Neurochemistry 129: 221–234.

Hammarback JA , Palm SL , Furcht LT and Letourneau PC (1985) Guidance of neurite outgrowth by pathways of substratum‐bound laminin. Journal of Neuroscience Research 13: 213–220.

Heidemann SR and Bray D (2015) Tension‐driven axon assembly: a possible mechanism. Frontiers in Cellular Neuroscience 9: 316. DOI: 10.3389/fncel.2015.00316.

Hellal F , Hurtado A , Ruschel J , et al. (2012) Microtubule stabilization reduces scarring and enables axon regeneration after spinal cord injury. Science 331 (6019): 928–931.

Henle SJ , Carlstrom LP , Cheever TR and Henley JR (2013) Differential role of PTEN phosphatase in chemotactic growth cone guidance. Journal of Biological Chemistry 288 (29): 20837–20842.

Hu J , Bai X , Bowen JR , et al. (2012) Septin‐driven coordination and microtubule remodeling regulates the collateral branching of axons. Current Biology 22: 1109–1115.

Hur EM , Saijilafu and Zhou FQ (2012) Growing the growth cone: remodeling the cytoskeleton to promote axon regeneration. Trends in Neuroscience 35 (3): 164–174.

Jurney WM , Gallo G , Letourneau PC and McLoon SC (2002) Rac1 mediated endocytosis during ephrin‐A2 and semaphorin 3A induced growth cone collapse. Journal of Neuroscience 22: 6019–6028.

Kalil K and Dent EW (2014) Branch management: mechanisms of axon branching in the developing vertebrate CNS. Nature Reviews Neuroscience 15: 7–18.

Kanno H , Pearse DD , Ozawa H , Itoi E and Bunge MB (2015) Schwann cell transplantation for spinal cord injury repair: its significant therapeutic potential and prospectus. Reviews in Neuroscience 26 (2): 121–128.

Kerstein PC , Nichol RH IV and Gomez TM (2015) Mechanochemical regulation of growth cone motility. Frontiers in Cellular Neuroscience 9: 244. DOI: 10.3389/fncel.2015.00244.

Lee AC and Suter DM (2008) Quantitative analysis of microtubule dynamics during adhesion‐mediated growth cone guidance. Developmental Neurobiology 68 (12): 1363–1377.

Letourneau PC (1982) Analysis of microtubule number and length in cytoskeletons of cultured chick sensory neurons. Journal of Neuroscience 2: 806–814.

Letourneau PC (1983a) Axonal growth and guidance. Trends in Neuroscience 6: 451–455.

Letourneau PC (1983b) Differences in the distribution of actin filaments between the growth cones and the neurites of cultured chick sensory neurons. Journal of Cell Biology 97: 963–973.

Letourneau PC and Ressler AH (1984) Inhibition of neurite initiation and growth by taxol. Journal of Cell Biology 98: 1355–1362.

Letourneau PC , Shattuck TA and Ressler AH (1986) Branching of sensory and sympathetic neurites in vitro is inhibited by treatment with taxol. Journal of Neuroscience 6: 1912–1917.

Letourneau PC , Shattuck TA and Ressler AH (1987) “Push” and “pull” in neurite elongation: observations on the effects of different concentrations of cytochalasin B and taxol. Cell Motility and Cytoskeleton 8: 193–207.

Lewis TL , Courchet J and Polleux F (2013) Cellular and molecular mechanisms underlying axon formation, growth and branching. Journal of Cell Biology 202 (6): 837–848.

Lim SS , Edson KJ , Letourneau PC and Borisy GG (1990) A test of microtubule translocation during neurite elongation. Journal of Cell Biology 111: 123–130.

Loudon RP , Silver LD , Yee HF and Gallo G (2006) RhoA‐kinase and myosin II are required for the maintenance of growth cone polarity and guidance by nerve growth factor. Journal of Neurobiology 66 (8): 847–867.

Marsick BM , Roche FK and Letourneau PC (2012a) Repulsive axon guidance cues ephrin‐A2 and slit3 stop protrusion of the growth cone leading margin concurrently with inhibition of ADF/cofilin and ERM proteins. Cytoskeleton 69: 496–505.

Marsick BM , San Miguel‐Ruiz J and Letourneau PC (2012b) Activation of Ezrin/radixin/moesin mediates attractive growth cone guidance through regulation of growth cone actin and adhesion receptors. Journal of Neuroscience 32: 282–296.

McKerracher L and Guertin P (2013) Rho as a target to promote repair: translation to clinical studies with cethrin. Current Pharmaceutical Design 19 (4): 4400–4410.

Namba T , Funahashi Y , Nakamuta S , et al. (2015) Extracellular and intracellular signaling for neuronal polarity. Physiological Reviews 95 (3): 995–1024.

Polleux F and Snider W (2010) Initiating and growing an axon. Cold Spring Harbor Perspectives in Biology 2: a001925.

Prokop A (2013) The intricate relationship between microtubules and their associated motor proteins during axon growth and maintenance. Neural Development 8: 17–27.

Rhee J , Mahfooz NS , Arregui C , et al. (2002) Activation of the repulsive receptor Roundabout inhibits N‐cadherin‐mediated cell adhesion. Nature Cell Biology 4: 798–806.

Roossien DH , Lamoureux P and Miller KE (2014) Cytoplasmic dynein pushes the cytoskeletal meshwork forward during axonal elongation. Journal of Cell Science 127: 3593–3602.

Siddiq MM and Hannila SS (2015) Looking downstream: the role of cyclicAMP‐regulated genes in axonal regeneration. Frontiers of Molecular Neuroscience 18 (8): 26. DOI: 10.3389/fnmol.2015.00026.

Song Y , Kirkpatrick LL , Schilling AB , et al. (2013) Transglutaminase and polyamination of tubulin: posttranslational modification for stabilizing axonal microtubules. Neuron 78 (1): 109–123.

Spillane M , Ketschek A , Donnelly CJ , et al. (2012) Nerve growth factor‐induced formation of axonal filopodia and collateral branches involves the intra‐axonal synthesis of regulators of the actin‐nucleating Arp2/3 complex. Journal of Neuroscience 32 (49): 17671–17689.

Tahirovic S and Bradke F (2009) Neuronal polarity. Cold Spring Harbor Perspectives in Biology 1: a001644.

Toriyama M , Kozawa S , Sakumura Y and Inagaki N (2013) Conversion of a signal into forces for axon outgrowth through Pak1‐mediated shootin phosphorylation. Current Biology 23: 529–534.

Witte H , Neukirchen D and Bradke F (2008) Microtubule stabilization specifies initial neuronal polarization. Journal of Cell Biology 180 (3): 619–632.

Further Reading

Barnes AP and Polleux F (2009) Establishment of axon‐dendrite polarity in developing neurons. Annual Review of Neuroscience 32: 347–381.

Bearce EA , Erdogan B and Lowrey LA (2015) TIPsy tour guides: how microtubule plus‐end tracking proteins (+TIPs) facilitate axon guidance. Frontiers in Cellular Neuroscience 9: 241. DOI: 10.3389/fncel.2015.00241.

Berry M , Ahmed Z , Morgan‐Warren P , Fulton D and Logan A (2015) Prospects for mTOR‐mediated functional repair after central nervous system trauma. Neurobiology of Disease 85: 99–110.

Conde C and Caceres A (2009) Microtubule assembly, organization and dynamics in axons and dendrites. Nature Reviews. Neuroscience 10: 319–332.

Hall A and Lalli G (2010) Rho and Ras GTPases in axon growth, guidance and branching. Cold Spring Harbor Perspectives in Biology 2 (2): a001818. DOI: 10.1101/cshperspect.a001818.

Kevenaar JT and Hoogenraad CC (2015) The axonal cytoskeleton: from organization to function. Frontiers in Molecular Neuroscience 8: 44. DOI: 10.3389/fnmol.2015.00044.

Lowery LA and Van Vactor D (2009) The trip of the tip: understanding the growth cone machinery. Nature Reviews . Molecular Cell Biology 10: 332–343.

Pak CW , Flynn KC and Bamburg JR (2008) Actin‐binding proteins take the reins in growth cones. Nature Reviews. Neuroscience 9: 136–147.

Tamariz E , Valera‐Echavarria A (2015) The discovery of the growth cone and its influence on the study of axon guidance. Frontiers in Neuroanatomy. DOI: 10.3389/fnana.2015.00051.

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Letourneau, Paul C(Mar 2016) Cytoskeleton in Axon Growth. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0021855.pub2]