Direct Cytoskeleton Forces Cause Membrane Softening in Red Blood Cells

Abstract

Erythrocytes are flexible cells specialized in the systemic transport of oxygen in vertebrates. This physiological function is connected to their outstanding ability to deform in passing through narrow capillaries. In recent years, there has been an influx of experimental evidence of enhanced cell-shape fluctuations related to metabolically driven activity of the erythroid membrane skeleton. However, no direct observation of the active cytoskeleton forces has yet been reported to our knowledge. Here, we show experimental evidence of the presence of temporally correlated forces superposed over the thermal fluctuations of the erythrocyte membrane. These forces are ATP-dependent and drive enhanced flickering motions in human erythrocytes. Theoretical analyses provide support for a direct force exerted on the membrane by the cytoskeleton nodes as pulses of well-defined average duration. In addition, such metabolically regulated active forces cause global membrane softening, a mechanical attribute related to the functional erythroid deformability.

Equilibrium physics breakdown reveals the active nature of red blood cell flickering

“…When the network is coupled to the bilayer, however, we predict the local bilayer incompressibility to systematically cancel the direct active normal force and we end up with an additional active source of noise for the stretching modes only, . This is therefore in contrast with previous phenomenological active models based on the existence of a direct active normal force 16,43 . Tangential metabolic noise nevertheless perturbs normal membrane motions via the curvaturemediated coupling between bending and stretching modes (non-diagonal components of the matrix ).…”

“…As a consequence, the observation of Gaussian fluctuation distributions 6,9,11,12 does not necessarily imply that RBC flickering is purely thermal. ATP-dependent temporal correlations, consistent with an active component, were recently identified 16 , however their interpretation is model-dependent and does not provide unequivocal evidence for active forces.…”

“…By comparing membrane fluctuations directly between healthy RBCs and cells depleted of ATP (adenosine triphosphate), recent experiments find a decrease of fluctuations upon ATP-depletion, hence suggesting that an active process may contribute to membrane undulations 6,[14][15][16] . However, the passive mechanical properties of the RBC membrane themselves depend strongly on ATP, as illustrated by the stiffening of the RBC membrane on starvation [17][18][19]6,15 , that correlates with a sudden transition from discocyte to spiculated echinocyte shapes 20,21 .…”

“…Modeling the RBC membrane mechanics remains a challenging task, as multiple length and timescales are involved. Most previous composite models are hence numerical [45][46][47][48] or limited to flat geometries 43,44,[49][50][51]16 . We derive here an active composite membrane model in spherical geometry, while considering explicitly the finite elasticity of the spectrin network and its sliding relative to the bilayer.…”

Effect of Endogenous and Exogenous Adenosine-Three Phosphate on Erythrocyte Deformability

“…Endojen ATP'nin eritrositlerin belirli hidrodinamik kuvvetlerin etkisi altında şekil değiştirmesinde özellikle hücre iskeleti bileşenlerinden spektrin proteini üzerinden rol aldığı gösterilmiştir (36). Bunun yanında hücre içinde bulunan ATP'nin bir kısmı dolaşım sisteminden geçerken karşılaştıkları hipoksi ve mekanik stres etkisiyle hücre dışına salınmaktadır (26)(27)(28)(29)(30).…”