Stretching molecules and deforming cells using optical tweezers with high-precision force sensing

Abstract

High-precision embedded probe fluctuation measurements are now widely used for characterizing viscoelastic properties of biological samples. However, despite the simplicity of this approach, novel adaptations for molecular applications and cell deformation measurements are still lacking. Furthermore, it has been hypothesized that the unfolding of deformed molecules in mechanosensitive cells, triggers the onset of mechanotransduction by exposing cryptic sites that may bind to support a functional cascade. Therefore, we adapted a theoretical approach characterizing winding probabilities in stretched polymers for probing elastic properties. Winding probabilities characterize folding preference for molecules under tension. Thus, we modelled counting unfolding events in single fibrous biopolymers under tensile stretching based on approximating the observed probe fluctuation as originating from a damped harmonic oscillator with forcing. In addition, we derived an adapted Worm-like chain model expressing force-displacement relations using an expansion in strain, which extends its applicability to force ranges beyond thermal regimes. Here, the expansion index is characteristic of tensile stiffness. Also, surface interactions between a living cell and a deforming spherical probe has been modelled by incorporating an adapted Lenard-Jones potential as the source of force contact generation as a first approach. The surface interaction predicts a deviation from the simple Hertz-model deformation, enabling an approximation of adhesion force in-
teraction, which in turn provides avenues for understanding trans-membrane forces triggering the onset of mechanosensing.