Cytoskeleton Dynamics

How does nucleation of actin initiate lamellipodia formation?

In the first phase of lamellipodia formation, actin filament polymerization produces a protrusive force on the cell membrane that promotes the spreading out and enlargement of the lamellipodia. In polarized, migrating cells this is known as the leading edge.

Actin polymerization begins with nucleation of actin filaments, and in the lamellipodia, this is primarily mediated by the Arp2/3 complex.This process, which has become known as the ‘dendritic nucleation model’ [1] is widely accepted despite several papers bringing this model, which is also known as the ‘branched model’ into question in recent years. For example, one study reported a lack of branched filaments in lamellipodia of 3T3 fibroblasts and another study suggested branches are an experimental artifact [2]. Subsequent re-analysis of results reported in this study, which suggested a predominantly unbranched array, revealed the existence of over 200 branched filaments [3].

Which nucleators are involved in the nucleation of actin filaments in lamellipodia?

Current evidence suggests multiple nucleators may function alongside the Arp2/3 complex [4], although the extent to which they influence the growth of the actin filament network and its ability to exert a protrusive force on the cell membrane remains unclear. For example, members of the formin family of nucleators have been found to localize to lamellipodia and were implicated in the nucleation of unbranched filaments [5]. Similar reports suggest spire and other proteins possessing multiple WH2 domains (e.g. JMY [6]; cordon bleu [7]) are involved in unbranched actin filament nucleation within lamellipodia. WH2 (Wiskott-Aldrich homology 2) domains are highly evolutionarily conserved domains of approximately 35 amino acids in length and serve as actin binding sites [8]. A mechanism by which WH2 domain-containing proteins promote filament nucleation has been proposed [9], however recent studies have indicated a greater level of complexity surrounds the biological function of the WH2 domains. For example, it has been shown that Cordon bleu functions with similar characteristics to profilin, being a weak nucleator, but also readily severs filaments and sequesters ADP-actin [10]. Nevertheless, proteins possessing WH2 domains are likely to have an influence on actin filament nucleation in lamellipodia, albeit at a regulatory level or supplementary to the influence of Arp2/3 itself.

There is indirect evidence suggesting that actin may exist in an oligomer state, neither as F-actin nor G-actin, yet diffuses like G-actin, except with slower dynamics. Fast turnover rates for F-actin in lamellipodia are challenging to explain with existing models of actin treadmilling, and some believe that oligomeric actin fragments disassemble from the barbed ends of F-actin [11]. Indirect evidence suggests that actin oligomers could re-associate with the F-actin network, or break up into G-actin monomers, after a characteristic time [12].

References

1. Svitkina TM, Bulanova EA, Chaga OY, Vignjevic DM, Kojima S, Vasiliev JM, and Borisy GG. Mechanism of filopodia initiation by reorganization of a dendritic network. J. Cell Biol. 2003; 160(3):409-21. [PMID: 12566431]
2. Urban E, Jacob S, Nemethova M, Resch GP, and Small JV. Electron tomography reveals unbranched networks of actin filaments in lamellipodia. Nat. Cell Biol. 2010; 12(5):429-35. [PMID: 20418872]
3. Yang C, and Svitkina T. Visualizing branched actin filaments in lamellipodia by electron tomography. Nat. Cell Biol. 2011; 13(9):1012-3; author reply 1013-4. [PMID: 21892140]
4. Ridley AJ. Life at the leading edge. Cell 2011; 145(7):1012-22. [PMID: 21703446]
5. Chesarone MA, DuPage AG, and Goode BL. Unleashing formins to remodel the actin and microtubule cytoskeletons. Nat. Rev. Mol. Cell Biol. 2009; 11(1):62-74. [PMID: 19997130]
6. Zuchero JB, Coutts AS, Quinlan ME, Thangue NBL, and Mullins RD. p53-cofactor JMY is a multifunctional actin nucleation factor. Nat. Cell Biol. 2009; 11(4):451-9. [PMID: 19287377]
7. Campellone KG, and Welch MD. A nucleator arms race: cellular control of actin assembly. Nat. Rev. Mol. Cell Biol. 2010; 11(4):237-51. [PMID: 20237478]
8. Paunola E, Mattila PK, and Lappalainen P. WH2 domain: a small, versatile adapter for actin monomers. FEBS Lett. 2002; 513(1):92-7. [PMID: 11911886]
9. Ahuja R, Pinyol R, Reichenbach N, Custer L, Klingensmith J, Kessels MM, and Qualmann B. Cordon-bleu is an actin nucleation factor and controls neuronal morphology. Cell 2007; 131(2):337-50. [PMID: 17956734]
10. Husson C, Renault L, Didry D, Pantaloni D, and Carlier M. Cordon-Bleu uses WH2 domains as multifunctional dynamizers of actin filament assembly. Mol. Cell 2011; 43(3):464-77. [PMID: 21816349]
11. Miyoshi T, and Watanabe N. Can filament treadmilling alone account for the F-actin turnover in lamellipodia? Cytoskeleton (Hoboken) 2013; 70(4):179-90. [PMID: 23341338]
12. Smith MB, Kiuchi T, Watanabe N, and Vavylonis D. Distributed actin turnover in the lamellipodium and FRAP kinetics. Biophys. J. 2013; 104(1):247-57. [PMID: 23332077]