Short-term binding of fibroblasts to fibronectin: optical tweezers experiments and probabilistic analysis

The biophysical properties of the interaction between fibronectin and its m embrane receptor were inferred from adhesion tests on living cells. Individ ual fibroblasts were maintained on fibronectin-coated glass for short time periods (1-16 s) using optical tweezers. After contact. the trap was remove d quickly, leading to either adhesion or detachment of the fibroblast. Thro ugh a stochastic analysis of bond kinetics, we derived equations of adhesio n probability versus time, which fit the experimental data well and were us ed to compute association and dissociation rates (k(+) = 0.3-1.4 s(-1) and k(off) = 0.05-0.25 s(-1), respectively). The bond distribution is binomial, with an average bond number less than or equal to 10 at these time scales. Increasing the fibronectin density (100-3000 molecules/mu m(2)) raised k() in a diffusion-dependent manner, leaving k(off) relatively unchanged. Inc reasing the temperature (23-37 degrees C) raised both k(+) and k(off), allo wing calculation of the activation energy of the chemical reaction (around 20 k(B)T). Increasing the compressive force on the cell during contact (up to 60 pN) raised k(+) in a logarithmic manner, probably through an increase in the contact area, whereas k(off) was unaffected. Finally, by varying th e pulling force to detach the cell. we could distinguish between two adhesi ve regimes, one corresponding to one bond, the other to at least two bonds. This transition occurred at a force around 20 pN, interpreted as the stren gth of a single bond.