SIMULATION OF INTEGRIN-CYTOSKELETAL INTERACTIONS IN MIGRATING FIBROBLASTS

Cell migration is a dynamic phenomenon requiring a physical interactio n between the internal cell motile machinery and the external substrat um in which adhesion receptors, such as integrins, serve as the transm embrane link. To analyze quantitatively this interaction, we apply a m odified Brownian dynamics algorithm to simulate cytoskeleton-mediated transport of integrin on the dorsal surfaces of migrating fibroblasts. Previously, we experimentally demonstrated that integrin is transport ed in an intermittent fashion, with directed excursions interspersed b y diffusive periods, preferentially toward the cell edge where the int egrin is likely used in the formation of nascent adhesions. Integrins containing mutations in the cytoskeleton-binding region of the cytopla smic domain display statistically different degrees of directed transp ort, indicating that this phenomenon is dependent on cytoskeletal asso ciations. In the present work, we develop a computer algorithm generat ing simulated integrin transport trajectories, given estimates for the rate constants defining coupling (k(c)) and uncoupling (k(u)) of inte grin with cytoskeletal components. Other parameters supplied to the pr ogram, the diffusion coefficient (D) for integrin in the membrane and the instantaneous velocity (V-i) of the integrin/cytoskeleton complex, have been measured independently in our experimental. system. By comp aring the simulated trajectories with those obtained experimentally, w e are able to estimate the coupling and uncoupling rate constants for the interaction of integrin with cytoskeletal elements in vivo. We fin d that integrin couples with cytoskeletal elements at a rate approxima tely 10 times slower than its rate of uncoupling (k(c) = 0.3 s(-1), k( u) = 3 s(-1)). Comparison of these rate constants with an equivalent r ate constant for diffusion, k(+) = 0.4 s(-1), indicates that the coupl ing interaction is likely a diffusion-limited process, as is typically expected for membrane processes. We further show by calculation that directed transport is necessary for integrin to traverse the length of an extending lamellipod to its leading edge; diffusion alone is not s ufficiently fast to supply adhesion receptors to points of new cell/su bstratum contact.