Figure 6. Measurements of macroscale mechanics using AFM.

(a,b) Sketch of the principal measurement concept. (a) The AFM cantilever coming out of the piezoelectric ceramic was functionalized with a 10 μm bead (spherical tip), and the cantilever's response measured (using the deflection of a laser detected on a photodiode) when applying ∼0.5 μm indentation depths on micron scale (radius r≈2 μm) contact areas on the adherent HeLa cells (including the self-organized actin cortex with asters and stars). (b) This experimental procedure allowed us to selectively characterize actin cortex elasticity, specifically on top of stars or asters, as under these conditions the measured stiffness is dominated by the macroscopic mechanical properties of the cortex and contributions from the cellular cytosol are negligible. (c) Elastic modulus E measured for HeLa cells under control conditions (4-h incubation, that is, presence of stars) and after treatment with CK666 (induction of asters), Y27632 (inhibition of myosin-II activity) and Latrunculin (complete blocking of the cortical actin turnover) and (d) relative changes of E relative to the value E_ctrl of the control conditions, (E−E_ctrl)/E_ctrl. ***P<0.001 (Student's t-test). Error bars are s.e.m. from at least N=150 measurements.