Cytoskeletal Mechanics
One key challenge in the field of ‘Cell Biophysics’ is that it is difficult to perform measurements inside living cells, a lot of what is known is from experiments with cell extracts or experiments with purified proteins. We have used a new technique- femtosecond laser ablation to quantify the mechanical properties of the sarcomere- a tensile force generating unit containing actin filaments that associate with myosin motor proteins. The technique uses a focused laser to generate high energies in a localized spot to ablate material (see figure below). On severing the fiber, the tension becomes zero allowing the fiber to retract under zero tension. Based on the measurements, we have proposed a new mechanical model for sarcomere contraction (in collaboration with Prof. Richard Dickinson). The model allows us to explain many relevant findings currently available on endothelial sarcomere contraction. Read the paper by Russell et al, 2009 to learn more.
|
|
Sarcomeres contract in a severed fiber. (A) Schematic showing structure of a sarcomere with: dense bodies of actin bundling proteins such as a-actinin marking the ends and polymerization competent ends pointing inwards joined by bipolar myosin filaments. (B) An epi-fluorescence micrograph illustrating the punctate staining of GFP-a-actinin in dense bodies of bovine capillary endothelial cell stress fibers (Scale bar, 20 µm). (C) A kymograph showing the results for one half of a severed fiber cut at the thin arrow. Notice that sarcomeres contract and that the contraction is not uniform in every sarcomere (thick arrow) (Scale bar, 1.4 µm and time between frames is 840 ms). (D).The distance the severed edge moves follows an exponential form in agreement with the results of others |
Russell R, Xia S-L, Dickinson RB and Lele TP. (2009) “Sarcomere mechanics in capillary endothelial cells", Biophysical Journal, 97 (6), 1578-1585
Robert J. Russell, Alexandria Y. Grubbs, Sunil Mangroo, Sandra Nakasone, Richard B. Dickinson and Tanmay P. Lele (2010). Sarcomere Length Fluctuations and Flow in Capillary Endothelial Cells, Cytoskeleton (Accepted).
Lele TP, Sero JE, Matthews BD, Kumar S, Xia S, Montoya-Zavala M, Polte T, Overby D, Wang N, Ingber DE. "Tools to study cell mechanics and mechanotransduction", Methods in Cell Biology 2007;83:441-72.
Kumar S, Maxwell IZ, Heisterkamp A, Polte TR, Lele TP, Salanga M, Mazur E, Ingber DE, “Viscoelastic retraction of single living stress fibers and its impact on cell shape, cytoskeletal organization and extracellular matrix mechanics”, Biophysical Journal, 90: 3762-73 (2006).
Lele TP and Kumar SK, “Brushes, cables and ratchets: Recent insights into multiscale assembly and mechanics of cellular structural networks”, Cell Biochemistry and Biophysics, 47: 348-360 (2007).