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Expansion of the Lamella by Actin Polymerization

Page updated  27/6/03

When the locomotion of tissue cultured vertebrate cells are studied using time-lapse photomicrography, it can be appreciated that the most motile part of the cell is typically the very leading edge which has been described as "ruffling" (see Cell Locomotion).  It is now known that this is driven by cycles of actin polymerization and depolymerization and that actin "treadmills" as actin monomers add at the very leading edge, move with respect to the leading edge toward the cell centre where actin monomers are removed from the pointed ends.  This flux of F-actin from the leading edge to the cell body has been demonstrated by a number of ingenious experimental approaches.  In one of the earliest such studies the cellular actin pool was labelled with fluorescent actin by microinjection and after this has equilibrated with the endogenous actin, a laser was used to bleach a spot in the leading edge.  This spot was then observed to move rearward as the cell crawled forwards (Wang, 1985).  A similar experiment (Theriot & Mitchison, 1991), labelled cells with a fluorescent actin label which became fluorescent after activation by a brief pulse of laser light.  Again, the marked actin population was seen to move rearward. Using the very large flat lamella of the neurons (so called "bag cells") of Aplysia (a nudibranch or sea-slug), and two fungal actin-binding drugs cytochalasin and phalloidin (Forscher & Smith, 1988) it has been shown that actin tread-milling results from addition of actin monomer onto the barbed ends of filaments that abut the membrane at a high angle. This was shown by treating the cells with cytochalasin that blocks actin polymerization at the barbed end, and then fixing and staining the cells with rhodamine labelled phalloidin which binds F-actin. This showed that filaments shrank back from the leading edge as polymerization was blocked at the barbed end.

In cells the situation is very much more complex than simply of actin filaments tread-milling as the whole process is regulated by a huge number of different types of actin binding proteins such as the ADF/Cofilins and the Arp2/3 complex.

 

References:-

Forscher, P. & Smith, S. (1988) Actions of cytochalasins on the organization of actin filaments and microtubules in a neuronal growth cone, J. Cell Biol. 107, 1505-1516.

Theriot, J. A. & Mitchison, T. J. (1991) Actin microfilament dynamics in locomoting cells. Nature. 352, 126-131.

Wang, Y.-L. (1985) Exchange of actin subunits at the leading edge of living fibroblasts: Possible role of treadmilling, J.Cell Biol. 101, 597-602.

 
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