N. Wang, Z. Suo," Long-distance propagation of forces in a cell." Biochemical and Biophysical Research Communications 328, 1133-1138 (2005). Submitted for publication on 12 January 2005.Accepted for publication on 18 January 2005

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Wiki Entry by Robin Kirkpatrick, AP 225, Fall 2011

Introduction

There is a lot of experimental evidence to suggest that mechanical force plays a role in gene expression, protein synthesis, and cell proliferation. Localized force on the surface of a cell has been shown to propagate long distances, which is in contract to simple mechanics models that treat cells a homogenous bodies, which predict that force decays over short distances. Thus the authors propose that long-distance propagation of force is a result of the inhomogeneities in a cell. There are two limits that are of interest. IF the prestress of the actin bundle is much less than that of the CSK, then the bundle moves like a violin string when subjected to transverse force. If the modulus of the bundle is larger than that of the CSK, then the force can propagate a long distance. Figure 1 below shows the physiological.

File:ROBINMT1.jpg

Methods

Model

Italic text Trasnverse motion was analyzed by modeling actin as a a string anchored on its ends surround by a compliant mdeium (the CSK). Longitudal motion is modeled by a shear lag model.


Experimental Results

The models presented here predict that locally applied stress should propogate long distances on an actin bundle. When the authors used magnetic twisting cytometry to apply local stress to the cells, the actin bundles beahved like strings, which is consistent with the proposed model. Their model also predicts thatm without prestress actin, cell movement with be localized around the magnetic bead. This was confirmed by comparing the displacement fields generated after applygin force to cells with and without long actin bindles. As shown in Figure 4, cells with long actin bundle showed larger displancement fields than that containing shorrt actin bundle, which diplsayed short acitn bundles.

To further test the validity of (2), the authors wanted to demonstrate that decrease in signma_b relative to G_m would result in a decrease int he charactistic length (L1). The authors added an actomyosin inhibitor and demonstrated that the distplacement field was localised around the bead (cinsstent with 2). The number of actin bindles reamined costbnt, suggesting that the prestress was reduced via addition of inhibitor, anbd theus the localization of stress was a result of decreas in prestress.


Discussion

Here, the authors assume that the stiff actin bundles are responsible for force proprogation, while presence of the CSK results in an elastic restoring force.


Keywords

Cytoskeleton, Microfilaments, Microtubules, Intermediate filaments, Mechanotransduction