(650b) Stochastic Models Integrating Adhesion and Intracellular Signaling Dynamics Affecting Membrane Protrusion in Migrating Cells

Cell migration is regulated by adhesion to the extracellular matrix (ECM) and force generation through the acto-myosin network. Although a vast amount of information has been collected regarding the interaction networks of adhesion proteins, we are only beginning to appreciate how the dynamic interactions between proteins can shape cellular events. Here, we analyze signaling mediated by the integrin family of adhesion receptors in the form of different types of adhesion structures. Nascent adhesions, which form in the actively protruding regions of a migrating cell, can activate the small GTPase Rac by at least two distinct signaling pathways mediated by different guanine exchange factors (GEFs) that are localized to adhesions by the scaffolding function of paxillin. Active Rac promotes protrusion of the cell's leading edge by enhancing actin polymerization, and formation of nascent adhesions varies with the rate of protrusion. Protrusion is antagonized by formation of stable adhesions, which occurs by myosin-dependent maturation of nascent adhesions.

The relationships between protrusion, nascent adhesions, and stable adhesions, which we term the core protrusion cycle, are complemented by other forms of feedback that may shape the dynamics of membrane protrusion during cell migration. In addition to creating binding sites for the scaffolding proteins that recruit GEFs to nascent adhesions, paxillin provides binding sites for additional proteins that have been shown to have diverse effects on adhesion remodeling and cell migration. Here, we analyze how two possible additional feedback mechanisms that stem from the scaffolding function of paxillin may affect protrusion dynamics under various ECM conditions and levels of myosin activity. By binding directly to paxillin, the protein kinase src has been shown to attenuate myosin activity, thus creating a negative feedback between paxillin localization and adhesion maturation. Following localization to nascent adhesions, the kinase Pak is activated by Rac and further phosphorylates paxillin to enhance nascent adhesion signaling and cellular protrusion. We show that these additional feedback mechanisms, coupled with the core protrusion cycle, are capable of tuning the frequencies of stochastic protrusion and adhesion maturation events under various intracellular and extracellular conditions, thus rendering the appearance of stochastic cell protrusion events more robust to perturbations and potentially facilitating persistent cell migration under different conditions.