NETWORK TOPOLOGY VERSUS INTRINSIC MOLECULAR ELASTICITY IN EXPLAINING ALTERED RIGIDITY OF BIOLOGICAL MEMBRANES
D.W.Knowles, N.Mohandas, E.A.Evans, J.A.Chasis

 
Abstract 
Recent work to understand the elasticity of large single biological polymers has indicated that molecules like DNA and titin behave as entropic springs. The mechanical character of these polymers is described by two intrinsic molecular features, contour length and inherent elasticity. In erythrocytes, spectrin is the large structural polymer which determines the mechanical properties of the membrane skeleton such as membrane rigidity. Increased interactions of spectrin with transmembrane proteins has been shown to rigidify the erythrocyte membrane but the essential question of the underlying molecular mechanism is unknown. Specifically, does erythrocyte membrane rigidification, induced by increasing network interactions, involve shortening the contour length of spectrin by restricting it's ability to extend and there by altering the topology of the network, or alternatively, by increasing spectrin's inherent stiffness and there by leaving the network topology unaltered. To answer this question, monoclonal antibody BRAC18 binding to band 3 was targeted as a model system because transmembrane protein band 3 is a major tethering point between the skeletal network and the bilayer and it regulates membrane rigidity through its interaction with the spectrin based membrane skeleton. Membrane rigidity was measured by micropipette aspiration and fluorescence imaged microdeformation (FIMD) was employed because it measures component molecular density in stretched networks and would be sensitive to topological alterations involving altered contour length. Cells were prepared by fluorescently labeling erythrocyte band 3 and actin, in situ, with fluorescein-5-maleimide (FMA) and rhodamine-phalloidin (RhPh), respectively. Actin was chosen because it is an ideal topological marker of spectrin end-to-end length and network density.  Appropriateness of the model was confirmed because with increasing incubation concentrations of BRAC18 there was a dose-dependent increase in membrane rigidity. This increased rigidity was associated with increased interaction of band 3 with spectrin as observed by FIMD, following the addition of BRAC18, which showed an increased slope of entrance density minus the cap density (Pe-Pc), of FMA-labeled band 3, versus aspiration length normalized by pipette radius (L/Rp). However, and surprisingly, BRAC18 binding had no measurable effect on the density difference for RhPh-labeled actin. These results indicated that the binding of BRAC18 antibody to band 3, which produced a marked increase in membrane rigidity, did not alter the topology of the skeletal network. These data imply that the molecular mechanism for rigidification is increased inherent stiffness of spectrin rather than topological crosslinking resulting in a shortening of the contour length of spectrin. 
Figure 1. 

As a potential model system for membrane rigidification, antibody binding to band 3 was targeted because this transmembrane protein is a major tethering point between the skeletal network and the bilayer and because it regulates membrane rigidity through its interaction with the spectrin based membrane skeleton. Appropriateness of the model was tested by determining whether binding the monoclonal antibody BRAC18 to band 3 increased membrane rigidity, as measured by the technique of micropipette aspiration of single red cells (Fig. 1A). Indeed, with increasing incubation concentrations of BRAC18 there was a dose-dependent increase in membrane rigidity (Fig. 1B and 1C). Within the incubation concentration range of 0 to 100ug/ml of BRAC18, the membrane rigidity increased 3.8 fold compared to nonliganded cells and plateaued around 10 µg/ml. 

Figure 2.

To determine whether BRAC18 binding altered the interaction of band 3 cytoplasmic domain with the network, Fluorescence Imaged Microdeformation (FIMD) was employed. In this technique, aspiration of single cells into a glass micropipette results in a density gradient of the network which drives the redistribution of other membrane components. For these experiments the difference between the densities of FMA-labeled band 3 at the pipette entrance and aspiration cap (Fig. 2A) was measured as a function of aspiration length scaled by the pipette radius (L/Rp) (Fig. 2B). In non-liganded cells, the density difference, Pe-Pc, for FMA-labeled band 3 increased monotonically with L/Rp (Fig. 2C). This increase reflected the increasing deformation and subsequent strain of the network as a function of L/Rp. With the addition of BRAC18, at 100ug/ml, there was a clear increase in the slope of Pe-Pc versus L/Rp (Fig. 2C). This showed that BRAC18 binding increased the interaction of band 3 molecules with the spectrin based skeleton thereby decreasing its ability to redistribute in response to deformation and indicating that antibody  binding had altered at least one network associated interaction. These data therefore confirmed that BRAC18 antibody binding to band 3 provides a suitable model system for determining the role of network connectivity versus spectrin's intrinsic elasticity in membrane rigidification.

Figure 3. 

Finally FIMD was used to test whether BRAC18 binding altered spectrin's crosslink density by measuring the deformation maps of RhPh-labeled actin. Actin, an oligomer at the junctional complexes within the network, was chosen because it is an ideal topological marker of spectrin end-to-end length and network density. Increased crosslink density of spectrin would be realized by an altered density map of the network under deformation. Surprisingly, BRAC18 binding had no measurable effect on the density difference between RhPh-labeled actin at the entrance and the cap (Fig. 3A). Further, both the entrance and the cap densities of RhPh-labeled actin were unaltered (Fig. 3B) implying that Brac18 binding did not alter the networks ability to condense or dilate, respectively. These results indicated that the binding of BRAC18 antibody to band 3, which produced a marked increase in membrane rigidity, did not alter the topology of the network.  

Conclusions 
 

  • In-situ density maps of erythrocyte network redistribution under large cell deformations were unaltered by binding BRAC18, a monoclonal antibody to band 3, even though the membrane was stiffened four-fold. 
  • Since FIMD would be sensitive to altered network redistribution caused by increasing the crosslink density of spectrin, and because the aspirated density gradients did not change even under extreme dilations, it appears that the ligand-induced rigidification was not due to increased crosslinking or changes in topology of the network but rather to intrinsic stiffening of the spectrin chains themselves.