ATP-dependent mechanics of red blood cells

TLDR

Red blood cells are highly deformable, recovering their shape after large deformations due to the composition of the membrane and its interaction with the cytoskeleton. The study investigates RBC mechanics and dynamics using weak optical tweezers to measure membrane fluctuations with microsecond temporal and sub‑nanometer spatial resolution. An enhanced edge‑detection method enables frequency coverage over more than four orders of magnitude. The measurements yield a bending modulus of 2.8 × 10⁻¹⁹ J, tension of 6.5 × 10⁻⁷ N/m, and effective viscosity of 8.1 × 10⁻² Pa s, revealing unknown dissipative processes, and show that membrane mechanics depend on spectrin‑4.1R phosphorylation—with inhibition or activation altering tension and viscosity; equilibrium holds only for <100 ms, while longer times exhibit 40 % higher fluctuations, implying a nonthermal effective energy up to 1.4 kBT.

Abstract

Red blood cells are amazingly deformable structures able to recover their initial shape even after large deformations as when passing through tight blood capillaries. The reason for this exceptional property is found in the composition of the membrane and the membrane-cytoskeleton interaction. We investigate the mechanics and the dynamics of RBCs by a unique noninvasive technique, using weak optical tweezers to measure membrane fluctuation amplitudes with mus temporal and sub nm spatial resolution. This enhanced edge detection method allows to span over >4 orders of magnitude in frequency. Hence, we can simultaneously measure red blood cell membrane mechanical properties such as bending modulus kappa = 2.8 +/- 0.3 x 10(-19)J = 67.6 +/- 7.2 k(B)T, tension sigma = 6.5 +/- 2.1 x 10(-7)N/m, and an effective viscosity eta(eff) = 81 +/- 3.7 x 10(-3) Pa s that suggests unknown dissipative processes. We furthermore show that cell mechanics highly depends on the membrane-spectrin interaction mediated by the phosphorylation of the interconnection protein 4.1R. Inhibition and activation of this phosphorylation significantly affects tension and effective viscosity. Our results show that on short time scales (slower than 100 ms) the membrane fluctuates as in thermodynamic equilibrium. At time scales longer than 100 ms, the equilibrium description breaks down and fluctuation amplitudes are higher by 40% than predicted by the membrane equilibrium theory. Possible explanations for this discrepancy are influences of the spectrin that is not included in the membrane theory or nonequilibrium fluctuations that can be accounted for by defining a nonthermal effective energy of up to E(eff) = 1.4 +/- 0.1 k(B)T, that corresponds to an actively increased effective temperature.