Tether extrusion from Red Blood Cells : integral
proteins unbinding from cytoskeleton
Nicolas Borghi 1* and Françoise Brochard-Wyart 1
1 Institut Curie
* To whom correspondence should be addressed. E-mail: nborghi{at}stanford.edu.
Submitted on April 26, 2006
Revised on October 1, 2006
Accepted on 9 April 2007
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Abstract |
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We investigate the mechanical strength of adhesion and the dynamics of detachment of the membrane from the cytoskeleton of Red Blood Cells (RBCs). Using hydrodynamical flows, we extract membrane tethers from RBCs locally attached to the tip of a micro-needle. We monitor their extrusion and retraction dynamics versus flow velocity (i.e. extrusion force) over successive extrusion-retraction cycles. Membrane tether extrusion (MTE) is carried out on healthy RBCs and ATP depleted or inhibited RBCs. For healthy RBCs, extrusion is slow, constant in velocity, and reproducible through several extrusion-retraction cycles. For ATP depleted or inhibited cells, extrusion dynamics exhibit an aging phenomenon through extrusion-retraction cycles: because the extruded membrane is not able to retract properly onto the cell body, each subsequent extrusion exhibits a loss of resistance to tether growth over the tether length extruded at the previous cycle. In contrast, the additionally extruded tether length follows "healthy" dynamics. The extrusion velocity L depends on the extrusion force f according to a non-linear fashion. We interpret this result with a model that includes the dynamical feature of membrane-cytoskeleton association. Tether extrusion leads to a radial membrane flow from the cell body toward the tether. In a distal permeation regime, the flow passes through the integral proteins bound to the cytoskeleton without affecting their binding dynamics. In a proximal sliding regime, where membrane radial velocity is higher, integral proteins can be torn out, leading to the sliding of the membrane over the cytoskeleton. Extrusion dynamics are governed by the more dissipative permeation regime: this leads to an increase of the membrane tension and a narrowing of the tether, which explains the power law behavior of L(f). Our main result is that ATP is necessary for the extruded membrane to retract onto the cell body. Under ATP depletion or inhibition conditions, the aging of the RBC after extrusion is interpreted as a perturbation of membrane-cytoskeleton links dynamics.
Key Words:
cytoskeleton, integral proteins, membrane, tethers
