Kenneth Yamada and the Galbraiths have an interesting paper out in last week’s Science, concerning their observations that Polymerizing Actin Fibers Position Integrins Primed to Probe for Adhesion Sites. In it, the authors demonstrate some fantastic high-contrast differential interference contrast (DIC) microscopy, along with four video clips of their fibroblast cells. The science, however:
Observations on protrusion dynamics led to the hypothesis that these protrusion variations are used to search for permissive sites to form new adhesions (Abercrombie et al., 1970; Giannone et al., 2004). However, no known direct relation between protrusion dynamics and the ability of adhesion receptors to probe the matrix has been identified.
To test if the localized increases in actin polymerization are involved in matrix probing, we compared them with the location of integrin adhesion receptors. Using a fluorescent, non-perturbing antibody, we observed clustering of ß1 integrins that was temporally and spatially synchronized with actin polymerization at the crests of ripples (Fig. 2, D to F, and movie S2).
Figure panels 1A and 2E:
This is the “sticky finger” analogy, where motile cells focus receptors sensitive to specific matrix components where they’re needed – the front of the cell. The movies really clearly show this correlation.
How cells are able to dynamically influence the movement of proteins in the “fluid mosaic” that is the lipid bilayer is, perhaps, a little counter-intuitive. The lipid membrane represents a phase interface, much like the boundry of oil and water. That cells have such subtle ways of bi-directionally transducing signals is something that, while most biologists may take it for granted, is not a trivial thing.
- Galbraith CG, Yamada KM, Galbraith JA. Polymerizing actin fibers position integrins primed to probe for adhesion sites. Science. 2007 Feb 16;315(5814):992-5. Pubmed
- Abercrombie M, Heaysman JE, Pegrum SM. The locomotion of fibroblasts in culture. 3. Movements of particles on the dorsal surface of the leading lamella. Exp Cell Res. 1970 Oct;62(2):389-98. Pubmed
- Giannone G, Dubin-Thaler BJ, Dobereiner HG, Kieffer N, Bresnick AR, Sheetz MP. Periodic lamellipodial contractions correlate with rearward actin waves. Cell. 2004 Feb 6;116(3):431-43. Pubmed