How quickly can filopodia extend?2018-02-06T10:33:57+00:00

How quickly can filopodia extend?

The extension rate of a filopodium will differ depending on the cell type (table 1). In each case however, this rate is controlled by the availability of G-actin-ATP, associated structural components and the energetics of membrane bending. The growth of long filopodia (>10 μm in length) requires the rapid transport of key materials towards the growing end [1] and this process is facilitated by the myosin motor proteins such as myosin-X or myosin V using an ATP-dependent ‘walking’ mechanism. Although the extension of filopodia is often described in a highly ordered manner, and does rely on the defined movements of Myosin-X for component delivery to the filopodia tip, the contribution of random diffusion of components must also be considered. Stochastic simulation models were recently presented that describe such phenomena, where ‘molecular noise’ may influence concentration gradients of G-actin as well as efficacy of the machinery responsible for filopodia growth.

One such study indicates that although the spatial distribution of static Myosin-X is universally consistent, and not altered by organelle length, the concentration of walking motors may vary. “Traffic jams” of myosin-X may occur for example at the base of the filopodia. Although logically this will impede progression of the proteins and their cargo down the filament, it was calculated that following a build up of G-actin at the blockage site, a concentration gradient is generated that enables its diffusion down the filopodia, (so long as the G-actin is not sequestered by the blocked motors) which subsequently sustains filopodia extension [2]. Similarly, the constant association and dissociation of capping protein at the barbed ends of actin filaments has been shown, also using stochastic simulations, to influence the dynamics of filopodia extension. In this case amplification of these regulatory proteins from initially low concentrations may trigger the fast retrograde flow of actin and induce repeated extension-retraction cycles that occur on a micro-meter scale. Compared to actin-only models, these dynamic cycles enable the growth of substantially longer filopodia [3].

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References

  1. Schmidt CE, Dai J, Lauffenburger DA, Sheetz MP, and Horwitz AF. Integrin-cytoskeletal interactions in neuronal growth cones. J. Neurosci. 1995; 15(5 Pt 1):3400-7. [PMID: 7751919]
  2. Zhuravlev PI, Lan Y, Minakova MS, and Papoian GA. Theory of active transport in filopodia and stereocilia. Proc. Natl. Acad. Sci. U.S.A. 2012; 109(27):10849-54. [PMID: 22711803]
  3. Zhuravlev PI, and Papoian GA. Molecular noise of capping protein binding induces macroscopic instability in filopodial dynamics. Proc. Natl. Acad. Sci. U.S.A. 2009; 106(28):11570-5. [PMID: 19556544]