Diffusing wave spectroscopy microrheology of actin filament networks

Filamentous actin (F-actin), one of the constituents of the cytoskeleton, i s believed to be the most important participant in the motion and mechanica l integrity of eukaryotic cells. Traditionally, the viscoelastic moduli of F-actin networks have been measured by imposing a small mechanical strain a nd quantifying the resulting stress. The magnitude of the viscoelastic modu li, their concentration dependence and strain dependence, as well as the vi scoelastic nature (solid-like or liquid-like) of networks of uncross-linked F-actin, have been the subjects of debate. Although this paper helps to re solve the debate and establishes the extent of the linear regime of F-actin networks' rheology, we report novel measurements of the high-frequency beh avior of networks of F-actin, using a noninvasive light-scattering based te chnique, diffusing wave spectroscopy (DWS). Because no external strain is a pplied, our optical assay generates measurements of the mechanical properti es of F-actin networks that avoid many ambiguities inherent in mechanical m easurements. We observe that the elastic modulus has a small magnitude, no strain dependence, and a weak concentration dependence. Therefore, F-actin alone is not sufficient to generate the elastic modulus necessary to sustai n the structural rigidity of most cells or support new cellular protrusions . Unlike previous studies, our measurements show that the mechanical proper ties of F-actin are highly dependent on the frequency content of the deform ation. We show that the loss modulus unexpectedly dominates the elastic mod ulus at high frequencies, which are key for fast transitions. Finally, the measured mean square displacement of the optical probes, which is also gene rated by DWS measurements, offers new insight into the local bending fluctu ations of the individual actin filaments and shows how they generate enhanc ed dissipation at short time scales.