Regulation of red blood cell deformability via modulation of protein-protein interactions between the membrane and the skeleton

Rafi Korenstein,Dept. of Physiology and Pharmacology, Sackler Faculty of Medicine,
Tel Aviv University

Cell membrane dynamics is manifested in the well-known nanoscale fluctuations of the cell membrane (CMF) observed initially in red blood cells (RBCs) but later found to exist in different types of nucleated cells. So far the analysis of this dynamics was based on the assumption that the membrane is in equilibrium with the environment and is driven solely by thermal energy. Though this approach has been applied successfully to fluctuations of model lipid membranes it can not be applied to living cells where there is a constant production of energy that drives different processes. Thus, there is a challenge to explore the effect of an active driving force on membrane dynamics, in relation to the real molecular structure of the membrane of the red blood cell, the simplest cellular model system. The basic structure of the RBC consists of a lipid-bilayer associated with the underlying spectrin skeleton network by two types of protein-protein complexes band3-ankyrin-spectrin and glycophorin C-protein 4.1-actin. In the present study we aimed at exploring the role of these protein-protein interactions in the dynamics of red blood cell membrane using a combined experimental study of cell membrane fluctuations and their physical modeling.

The interaction within the two types of protein-protein complexes was modified by incubating the RBCs in media of different pH. We could demonstrate, by biochemical means, that elevating the pH in the extracellular medium of RBCs to 8.6, led to decreased association of the band 3-ankyrin-spectrin complex. When the pH was lowered to 5.6 an increased dissociation of glycophorin C- protein 4.1-actin complex occurred. Control RBCs were maintained at a physiological pH of 7.4. Measurement of cell membrane fluctuations at the three conditions of pH revealed that at high pH the half-width of the amplitude histogram distribution increased, while at the low pH it decreased. Furthermore, increasing the extracellular medium viscosity up to 2.4cP, led to an attenuation of the band–width of the amplitude histogram at the high pH, while no effect was observed at the low pH. These findings support our hypothesis that CMF are driven by a putative molecular motor associated with the glycophorin C- protein 4.1-actin complex, in addition to a thermally driven force.

An initial attempt to model driven CMF was carried out. A system consisting of an incompressible liquid and a hard solid mobile sphere was considered within the Stokes approximation. The response of the system to an external force density applied to the liquid was obtained. The velocity field generated around the sphere, which is consistent with the center of mass velocity and rigid rotation of the sphere, was obtained. The motion of the sphere was also obtained from the applied force density. This was done in three stages. In the first the hydrodynamic response to a force density applied to the liquid in the presence of a fixed sphere was obtained, using a method of images. In the next step the velocity field due to such a force density was obtained for a sphere with externally given center of mass velocity and rigid rotation. The final stage was obtaining the motion of the sphere from the force density and using it in conjunction of the results obtained by the previous step.