Mechanics of living cells measured by laser tracking microrheology

To establish laser-tracking microrheology (LTM) as a new technique for quan tifying cytoskeletal mechanics, we measure viscoelastic moduli with wide ba ndwidth (5 decades) within living cells. With the first subcellular measure ments of viscoelastic phase angles, LTM provides estimates of solid versus liquid behavior at different frequencies. In LTM, the viscoelastic shear mo duli are inferred from the Brownian motion of particles embedded in the cyt oskeletal network. Custom laser optoelectronics provide sub-nanometer and n ear-microsecond resolution of particle trajectories. The kidney epithelial cell line, COS7, has numerous spherical lipid-storage granules that are ide al probes for noninvasive LTM. Although most granules are percolating throu gh perinuclear spaces, a subset of perinuclear granules is embedded in dens e viscoelastic cytoplasm. Over all time scales embedded particles exhibit s ubdiffusive behavior and are not merely tethered by molecular motors. At lo w frequencies, lamellar regions (820 +/-: 520 dyne/cm(2)) are more rigid th an viscoelastic perinuclear regions (330 +/- 250 dyne/cm(2), p < 0.0001), b ut spectra converge at high frequencies. Although the actin-disrupting agen t, latrunculin A, softens and liquefies lamellae, physiological levels of F -actin, alone (II +/- 1.2 dyne/cm(2)) are similar to 70-fold softer than la mellae. Therefore, F-actin is necessary for lamellae mechanics, but not suf ficient. Furthermore, in time-lapse of apparently quiescent cells, individu al lamellar granules can show similar to 4-fold changes in moduli that last >10 a. Over a broad range of frequencies (0.1-30,000 rad/s), LTM provides a unique ability to noninvasively quantify dynamic, local changes in cell v iscoelasticity.