A theoretical analysis for the effect of focal contact formation on cell-substrate attachment strength

Abstract

For many cell types, growth, differentiation, and motility are dependent on receptor-mediated adhesion to ligand-coated surfaces. Focal contacts are strong, specialized, adhesive connections between cell and substrate in which receptors aggregate and connect extracellular ligand to intracellular cytoskeletal molecules. In this paper, we present a mathematical model to examine how focal contact formation affects cellular adhesive strength. To calculate adhesive strength with and without focal contacts, we use a one-dimensional tape peeling analysis to determine the critical tension necessary to peel the membrane. Receptor-ligand bonds are modeled as adhesive springs. In the absence of focal contacts, we derive analytic expressions for the critical tension at low and high ligand densities and show how membrane morphology affects adhesion. Then, focal contacts are modeled as cytoplasmic nucleation centers which bind adhesion receptors. The extent of adhesive strengthening upon focal contact formation depends on the elastic rigidity of the cytoskeletal connections, which determines the structural integrity of the focal contact itself. We consider two limits to this elasticity, very weak and rigid. Rigid cytoskeletal connections give much greater attachment strengths. The dependence of attachment strength on measurable model parameters is quite different in these two limits, which suggests focal contact structure might be deduced from properly performed adhesion experiments. Finally, we compare our model to the adhesive strengthening response reported for glioma cell adhesion to fibronectin (Lotz et al., 1989. J. Cell Biol. 109:1795-1805). Our model successfully predicts the observed detachment forces at 4 degrees C and yields values for the number of fibronectin receptors per glioma cell and the density of cytoskeletal connection molecules (talin) involved in receptor clusters which are consistent with measurements for other cell types. Comparison of the model with data at 37 degrees C suggests that while cytoskeletal cross-linking and clustering of fibronectin receptors significantly increases adhesion strength, specific glioma cell-substratum attachment sites possess little mechanical rigidity and detach through a peeling mechanism, consistent with the view that these sites of < or = 15 nm cell-substrate separation are precursors to fully formed, elastically rigid focal contacts.

A Comparison of Type I Collagen, Fibronectin, and Vitronectin in Supporting Adhesion of Mechanically Strained Osteoblasts

“…This indicated that the strength of cell adhesion under cyclic strain is dependent on the number of integrin-ligand interactions, consistent with previously reported data using shear forces (fluid flow) to detach cells. (46,47) Concordant with this idea is our finding that at relatively low coating densities (modeled at 1000 molecules/m 2 ), strain resistance was significantly greater when cells were adherent to the protein with the greatest number of integrin-binding sites per molecule (type I collagen). (41) Cell adhesion to type I collagen occurs primarily through ␣ 1 ␤ 1 -and ␣ 2 ␤ 1 -receptors, although ␣ 10 -and ␣ 11 -integrins also may be involved.…”

Calreticulin affects cell adhesiveness through differential phosphorylation of insulin receptor substrate-1

“…Our observation that Y-27632 treatment accelerates cell movement with an accompanying loss of directionality is also in line with reports showing that RhoA/ROCK inhibition affects both the speed and directionality of cell movement by causing degradation of focal contacts and stress fibers [66][67][68]. The number and prominence of focal contacts have all been associated with an increase in the strength of cell-substratum adhesion [56,57,65,[69][70][71][72]. It thus came as a surprise that our MEFs that are devoid of focal contacts and stress fibers adhere more strongly to the substratum (Fig.…”

“…It has been proposed that cell adhesion can be accomplished by non-specific interactions of the charged surface of a cell with that of a substratum, according to the DLVO theory and via long distance van der Waals attraction forces operating at a 10-30 nm distance of separation between the surfaces [79,80]. While focal contact adhesion strength per unit area is greater than any other part of the cell undersurface [69,70,81,82], it is apparent from imaging of the ventral surface of cells that, in normal fibroblasts, its topography is very diversified, with the area of focal contacts covering only a small portion of a cell's undersurface. This contrasts with the Y-27632-treated cells shown in Figs.…”

“…5 and 6, in which dark close contact areas cover nearly the entire undersurface. Close contacts associate with the cortical meshwork of actin microfilaments [57,83,83,84], so it is plausible that locally controlled contractility modulates cell surface geometry on a nanoscale, thus further contributing to cell adhesiveness [69,70,72,79,85]. It has also been observed that coated pits filled with ruthenium red-positive material seem to concentrate in areas of close contacts [86,87] possibly providing a wealth of charged molecules dangling from the cell surface.…”

“…It has also been observed that coated pits filled with ruthenium red-positive material seem to concentrate in areas of close contacts [86,87] possibly providing a wealth of charged molecules dangling from the cell surface. While putative adhesion bonds in close contacts are more labile and weaker than those in focal contacts [69,70,81,88,89], their large numbers in extensive close contacts would account for increased adhesion strength. On the other hand, the lability of bonds in close contacts would permit fast locomotion as shown in Fig.…”