Title: Universal kinetics for engagement of mechanosensing pathways in cell adhesion,
Abstract: When plated onto substrates, cell morphology and even stem cell
differentiation are influenced by the stiffness of their environment. Stiffer
substrates give strongly spread (eventually polarized) cells with strong focal
adhesions, and stress fibers; very soft substrates give a less developed
cytoskeleton, and much lower cell spreading. The kinetics of this process of
cell spreading is studied extensively, and important universal relationships
are established on how the cell area grows with time. Here we study the
population dynamics of spreading cells, investigating the characteristic
processes involved in cell response to the substrate. We show that unlike the
individual cell morphology, this population dynamics does not depend on the
substrate stiffness. Instead, a strong activation temperature dependence is
observed. Different cell lines on different substrates all have long-time
statistics controlled by the thermal activation over a single energy barrier
dG=19 kcal/mol, while the early-time kinetics follows a power law $t^5$. This
implies that the rate of spreading depends on an internal process of
adhesion-mechanosensing complex assembly and activation: the operational
complex must have 5 component proteins, and the last process in the sequence
(which we believe is the activation of focal adhesion kinase) is controlled by
the binding energy dG.