The morphology of cells is determined by passive (structural) components which can be converted into active (dynamical) elements by a complex interplay of signaling molecules, gene expression and protein-protein interaction. This induces ongoing rearrangements of the cytoskeleton and changes of the intracellular hydraulic pressure described as the morphodynamics of a cell. Obviously, such a complex behavior results from the dynamic architecture of the system rather than from specific properties of isolated components. Thus characterizing cells morphodynamics should reflect the general condition of a cell.

We used atomic force microscopy (AFM) to perform real-time monitoring of the biomechanical behavior of living bronchial epithelial cells qualitatively and quantitatively. We indented living cells under physiological conditions with a colloidal probe using a constant loading force of 1 nN and measured the force the cells exert against the indenter. Activity of myosin motor proteins change the resistance of the cell against indentation: i) a contraction increases, ii) a relaxation decreases this resistance. At constant loading force the indentation depth of the indenter will therefore increase or decrease in response to changes of myosin motor protein activity. These height changes were monitored with a subnanometer resolution in real-time. Cells show under control conditions height changes of about 200 nm. Joule (J) is defined as the energy expended in applying a force of one newton through a distance of one meter (1J = 1Nm). A displacement of the indenter over a distance of 200 nm against 1 nN is equivalent to energy of 10^-18 J. The cell exerted this energy against the indenter within 250s which corresponds to a power of 0.8 · 10^-18 W.

In conclusion, our experimental approach allowed quantifying the energy of cellular morphodynamics in real-time. This could provide deep insight into cellular dynamics in response to changes of the chemical and mechanical environment