Rheological characterization of the bundling transition in F-actin solutions induced by methylcellulose.

Simone Köhler,Andreas R Bausch,Oliver Lieleg,Jianren Lu

doi:10.1371/journal.pone.0002736

Abstract

In many in vitro experiments Brownian motion hampers quantitative data analysis. Therefore, additives are widely used to increase the solvent viscosity. For this purpose, methylcellulose (MC) has been proven highly effective as already small concentrations can significantly slow down diffusive processes. Beside this advantage, it has already been reported that high MC concentrations can alter the microstructure of polymer solutions such as filamentous actin. However, it remains to be shown to what extent the mechanical properties of a composite actin/MC gel depend on the MC concentration. In particular, significant alterations might occur even if the microstructure seems unaffected. Indeed, we find that the viscoelastic response of entangled F-actin solutions depends sensitively on the amount of MC added. At concentrations higher than 0.2% (w/v) MC, actin filaments are reorganized into bundles which drastically changes the viscoelastic response. At small MC concentrations the impact of MC is more subtle: the two constituents, actin and MC, contribute in an additive way to the mechanical response of the composite material. As a consequence, the effect of methylcellulose on actin solutions has to be considered very carefully when MC is used in biochemical experiments.

Highlights

The structural organization of the cytoskeleton determines the morphology and the mechanical response of eukaryotic cells
The obtained thicknesses are comparable to those reported for actin bundles that are induced by depletion forces in the presence of polyethylene glycol (PEG) [15] but are considerably larger compared to bundles that are formed by actin bundling proteins such as fascin [16]
High MC concentrations alter the mechanics of F-actin solutions by an initial decrease in the elastic response

Summary

Introduction

The structural organization of the cytoskeleton determines the morphology and the mechanical response of eukaryotic cells. The cytoskeleton consists of semiflexible polymers such as actin filaments which are organized into complex scaffolds by various associated actin binding proteins. Increasing the viscosity of the investigated solution is helpful for reducing this problem For this purpose, methylcellulose (MC), a polymer that forms highly viscous solutions, is widely used at concentrations of 0.2–0.5% [4,5,6,7,8,9,10]. At PEG concentrations below the bundling transition, depletion forces lead to increasingly effective physical cross-links giving rise to an increase in the elasticity of the actin/PEG solution [15]. A mixture of two polymer solutions might give rise to additional effects influencing the structure and mechanical response of the composite material

Methods

Results

Conclusion