Mechanically stimulated cytoskeleton rearrangement and cortical contraction in human neutrophils

A mechanical test with micropipets is used to characterize cytoskeleton rearrangement and contraction induced by mechanical stresses in human neutrophils. The yield shear resultant of the cell cortex is on the order of 0.06 to 0.09 mN.m-1. The measured yield shear resultant suggests that the neutrophil cortex is a weakly cross-linked structure. When a tether is pulled out from the cell surface, a polymer structure starts to fill it and spreads out from the cell body. The rate of advancement of the polymerization front is almost constant and, therefore, is not diffusion limited. The measured rate is much smaller than the one of spontaneous actin polymerization, suggesting that the limiting process is either the dissociation of actin monomers from their dimers with the capping proteins or the rate of formation of new nucleation sites or both. Polymerization is also observed after applying sufficient mechanical stresses on a small portion of the cell surface. The polymerization is followed by mass transfer from the cell into the prestressed region and later on by contraction of the main cell body. The pressure generating the flow is located in the prestressed region and most probably is a result of its "swelling" and contraction. The contraction of the main cell body is very similar (in its time dependence and magnitude) to the contraction during phagocytosis. The measured maximum cortical tension is on the order of 0.5 mN.m-1, which for a 3.5-microns diameter pipet corresponds to a maximum contraction force of 11 nN.