Pro­tru­sion is based on the uni­di­rec­tional growth (poly­meri­sa­tion) of actin fil­a­ments, whereas retrac­tion of the cell rear is a con­trac­tile process based on the inter­ac­tion of fil­a­ments of actin and myosin in bun­dles of actin fil­a­ments, or stress fibres (see Push­ing and Pulling). We gain an impres­sion of these two processes of pro­tru­sion and retrac­tion from the video below, which shows the actin cytoskele­ton in a melanoma cell.

Video sequence of a B16 melanoma cell express­ing GFP-actin.

Note that the breadth of the lamel­lipodium net­work at the cell front remains con­stant as the cell moves for­ward. This fea­ture is explained by a so-called “tread­milling” of actin fil­a­ments, whereby the addi­tion of actin monomer at the front of the lamel­lipodium is bal­anced by the release of monomer at the rear (Pan­taloni et al., 2001; Car­lier et al., 2003):

Schematic illus­tra­tion of the tread­milling of actin monomers dur­ing poly­meri­sa­tion and depoly­meri­sa­tion at steady state in vitro. Addi­tion of monomer occurs pref­er­en­tially at the “plus end” and release of monomer at the “minus end” (from Pan­taloni et al., 2001). 

The tread­milling of actin in the lamel­lipodium can be demon­strated in dif­fer­ent ways, two of which are shown in the fol­low­ing fig­ures:

Tread­milling of actin as illus­trated by the recov­ery of actin flu­o­res­cence after pho­to­bleach (FRAP). The fig­ure shows time-lapse images of the lamel­lipodium region of a B16 melanoma cell before (pre) and after pho­to­bleach. Num­ber at top right indi­cates time in sec­onds. From Lai et al. 2008.

The video shows a B16 melanoma cell that was trans­fected with actin-GFP. In con­trast to the régime in the fig­ure above, a scan­ning laser beam was used to pho­to­bleach the flu­o­res­cence of actin in the region behind the lamel­lipodium marked by the square. This is imme­di­ately fol­lowed by incor­po­ra­tion of the bleached mol­e­cules at the lamel­lipodium tip. Note: the changes in flu­o­res­cence observed cor­re­spond to the flu­o­res­cence from poly­merised fil­a­ments; monomeric actin dif­fuses very rapidly and adds only a weak homo­ge­neous back­ground to the image. From Lai et al., 2008.

In the method of speckle microscopy intro­duced by Clare Water­man (fig­ure below) small amounts of fluorescently-labelled actin are injected into cells so that only a frac­tion of the actin fil­a­ments are labelled, giv­ing a “speck­led sig­nal” (Com­pare with the more com­plete labelling of actin with GFP actin in the fig­ures above). The “ret­ro­grade flow” of speck­les is con­sis­tent with the con­tin­u­ous incor­po­ra­tion of new actin monomer at the front of the lamel­lipodium.

Tread­milling of actin in the lamel­lipodium of a migrat­ing cell as shown by speckle microscopy. Video was pro­duced by Clare Water­man.

Related Pub­li­ca­tions

  • Car­lier, M. F., Le Clainche, C., Wies­ner, S., Pan­taloni, D. (2003). Actin-based motil­ity: from mol­e­cules to move­ment. Bioes­says 25, 336345. NCBI PubMed
  • Lai, F. P. L., Szc­zo­drak, M., Block, J., Mannherz, H. G., Small, J. V., Stradal, T. E. B., Dunn, G. A., Rot­tner, K. (2008). Arp2/3 com­plex inter­ac­tions and actin net­work turnover in lamel­lipo­dia. EMBO J. 27, 982992. PDF
  • Pan­taloni, D., Le Clainche C., Car­lier, M. F. (2001). Mech­a­nism of actin-based motil­ity. Sci­ence. 292, 15021506. NCBI PubMed