The mechanics of mechanotransduction : analyses of cell perturbation

Abstract

Cells sense mechanical stimuli and respond by changing their phenotype, e.g. shape, gene expression, motility. This process, termed mechanotransduction, was investigated using computational and theoretical approaches, as well as comparisons with experiments. As a first step, a three-dimensional viscoelastic finite element model was developed to simulate cell micromanipulation by magnetocytometry. The model provided a robust tool for analysis of detailed strain/stress fields induced within a single cell or cell monolayer produced by forcing one tethered microbead. On the assumption of structural homogeneity, stress and strain patterns were highly localized, suggesting that the effects of magnetocytometry are confined to a region extending less than 10tm from the bead. Modification of the model to represent experimental focal adhesion attachments supported a non-uniform force transmission to basal surface focal adhesion sites. Proteins in identified zones of high stresses in the cell are candidate mechanosensors and their molecular response to force was hence investigated, A generic model of protein extension under external forcing was created inspired by Kramers theory for reaction rate kinetics in liquids.

(cont.) The protein was hypothesized to have two distinct conformational states: a relaxed state, Ci, preferred in the absence of external force, and an extended state, C2, favored under force application. Appearance and persistence of C2 was assumed to lead to transduction of the mechanical signal into a chemical one. While the level of applied force and the energy difference between states largely determined equilibrium, the dominant influence on the extension time was the height of the transition state. Force-induced distortions in the energy landscape were also shown to have a significant influence on extension time, however, exhibiting a weaker force dependence than exponential. Finally, the link between membrane receptors and the extracellular matrix -- or the bead in magnetocytometry experiments -- was investigated as a primary path for force transduction to the cell interior. To shed light on the role of bonds formed by membrane receptors on measurements of cellular rheology, we modeled the process by which a forced, cell-tethered microbead translates and rotates as influenced by the stochastic formation and. rupture of adhesion bonds. We show that this process is crucial in the inference of cell mechanical properties from microbead experiments.