Micromechanics of the red cell membrane skeleton: large, anisotropic strain and filament reorientation in a soft, deformed structure

The red cell membrane´s spectrin-actin skeleton is well-studied but continues to motivate deeper understanding. In the present set of studies, the skeleton is directly shown to be a thermally-soft surface structure capable of sustaining large, localized strains. Laser-photobleaching of a one micron line of fluorescent phalloidin-actin demonstrates not only that skeleton deformation is reversible, i.e. elastic, but also that the network can sustain in-plane extensions up to at least 200% and contractions down to 40%, referenced to an undistorted cell. Such kinematic quantities are, however, true thermodynamic-averages since thermal fluctuations of the F-actin nodes of the network are a significant fraction of the unstretched length of spectrin. The fluctuations, in fact, reveal the softness of the network and yield a simple estimate of the network´s local shear modulus. Fluorescence polarization studies of phalloidin-actin indicate actin protofilaments lie tangent to the membrane, and, when the cell is pulled into a micropipette, filaments tend to align with extended spectrin. Central aspects of the microstretching, fluctuation, and rotation fields for the red cell network are captured in multi-scale, statistical mechanical simulations of polymer networks in deformation