MORPHOLOGIES IN VESICLE-VESICLE ADHESION

摘要

A single cell system, such as red blood cell, shows a series of shape transitions, stomatocyte-discocyte-echinocyte, by applying a variety of chemical and physical stresses. This series of shapes is well described by minimization of elastic energy, i.e., area difference elasticity (ADE) model [1,2]. By adhering vesicles, the aggregates show rich morphologies due to the competition between the elastic energy and the adhesion energy, which gives physical basis of morphogenesis in cell division [3]. When two deformable spherical vesicles are adhered each other, the doublet has a flat contact area with two spherical caps (sphere doublet), where the total energy of the adhering vesicles is governed by the vesicle stretching energy and the adhesion energy. On the other hand, for the adhesion of non-spherical vesicles, the membrane bending energy starts to compete the adhesion energy. In this region, the total energy of the doublet, W, is expressed by a sum of the vesicles' bending energy, W_(b,1) and W_(b,2), and the adhesion energy assumed to be proportional to the contact area A_c, W = W_(b,1) + W_(b,2) - ΓA_C , where Γ (>0) is the adhesion strength and the bending energy is expressed by Helfrich model [4]. This theoretical model predicts that in the weak adhesion, the adhering vesicles prefer the minimum contact area morphology (flat adhesion), whereas in the strong adhesion, the doublet shows the maximum contact area morphology with curved interface [5]. This theoretical argument predicts fruitful morphology transitions of the doublets, although no systematic experiments have been reported so far. In this study we show the morphology transitions of adhering giant unilamellar vesicles (GUVs) induced by the changing the reduced volume of vesicles. The GUV is homogeneous single component vesicle composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). First we adhered two spherical GUVs with the aid of the depletion interaction. Thereafter we decreased the reduced volume of the adhering vesicles by using thermal expansion of membranes. Depending on the reduced volume, the doublet deformed its shape and showed a unique morphology transitions, sphere-oblate-prolate doublet and sphere-sigmoidal doublet. We describe the observed morphology transitions based on the competition between the bending and the adhesion energies and explain the origin of the adhesion energy from the inter-membrane interaction point of view.