Vesicle shape change

In the above tentative scenario on the vesicle shape change, the elastic interaction between the bundle of actin filaments and the lipid membrane was assumed to play a role. Therefore, it was of interest to see what kind of change occurs to the vesicle shape, if the bundle structure is altered by some means. Cytochalasin D (CD) was chosen for this purpose. CD is a fungal metabolite which enters the cell and dismpts the actin filament therein, when it is added to the medium surrounding the cell [28]. This drug has been widely used to investigate the role of actin in the cell [29]. The mechanism of the drug action is twofold one is to bind to the barbed end of actin filaments and inhibit polymerization, the other is to sever (cut up) the filaments [30]. 

The above argument assumes that an elastic interaction between the actin filaments and the vesicle membranes is an important determinant of the vesicle shape. Because manipulation of the actin bundle with CD resulted in the vesicle shape change, it would be interesting to examine if the other elastic entity, the vesicle membrane, is mechanically perturbed. 

Cortese., J.D., Schwab III, B., Frieden, C., Elson, E.L. (1989). Actin polymerization induces a shape change in actin-containing vesicles. Proc. Natl. Acad. Sci. USA 86, 5773-5777. 

Yamashita Y, Masum SM, Tanaka T, Tamba Y, Yamazaki M (2002) Shape changes of giant unilamellar vesicles of phosphatidylcholine induced by a de novo designed peptide interacting with their membrane interface. Langmuir 18 9638-9641. 

Eukaryotic cells have an internal scaffolding called the cytoskeleton or cytomatrix that maintains their cellular morphology and enables them to migrate, undergo shape changes, and transport vesicles. Microfilaments, made of actin, intermediate filaments, which are composed of laminin and other proteins, and microtubules, formed from the protein tubulin, along with many different accessory proteins, comprise the cytoskeleton. Both the microfilaments and the microtubules can assemble and disassemble rapidly in the cell, whereas disassembly of intermediate filaments may require their destruction. Although much is known about the molecular composition of the cytoskeleton, the molecular events involved in most cell movements are still unknown. 

In the presence of salt in a vesicle exterior, unusual shape changes are observed during an applied DC pulse [107]. The vesicles adopt spherocylindrical shapes (Figure 7.5) with lifetimes of the order of 1ms. These deformations occur only in the presence of salt outside the vesicles, irrespective of their inner content (note that, in the absence of salt in the external solution, the vesicles deform only into prolates see Figure 7.5a). When the solution conductivities inside and outside are identical, Ain vesicles with square cross section are observed (Figure 7.5d). For the case Ain < the vesicles adopt disc-like shapes (Figure 7.5c), while in the... 

Mathivet, L., Cribier, S., and Devaux, P.F., Shape change and physical properties of giant phosphohpid vesicles prepared in the presence of an ac electric field, Biophys. J., 70, 1112-1121, 1996. 

Fig. 19 Left) Schematic representation of the proposed mechanism for topological changes in dioleoyl phosphoethanolamine (DOPE) based liposomal membranes upon ultrasound irradiation. Right) (a) Giant DOPE-based unilamellar vesicle, before sonication, which shows an inhomogeneous membrane DOPE-rich domains of negative curvature are marked in red, embedded in zones rich in dioleoyl phosphocholine (DOPC) of zero mean curvature, (b) Illustration of shape changes upon ultrasound stimuli. Reproduced with permission from [99]. Copyright 2014 The Royal Society of Chemistry...

The small fraction of single free chains together with slow exchange kinetics essentially freezes the exchange process. A sufficiently fast exchange of amphiphiles is, however, necessary to sustain thermodynamic equilibrium. Therefore vesicles, once formed, are in a metastable, trapped, or quenched thermodynamic state. The number of amphiphiles and therefore their bilayer area is essentially constant on timescales of most experiments. An extreme case is glassy polymers with very slow lateral mobility which can even impede shape changes of polymer vesicles ( frozen vesicles). 

As another example. Figure 5 shows the morphologies for a series of PB-PEO block copolymers in aqueous solutions, where a decrease of the hydrophilic block ratio f leads to shape changes from sphere cylinder vesicle (102). Similarly,... 

The changes of vesicle shapes are due to temperature or pressure changes, the addition of various amphiphiles or adsorbents, mechanical, electrical or magnetic treatments, or to adhesion [14,22,23,25,47]. Temperature changes induce area/ volume differences due to the different expansion coefficients of lipid and water, as well as to the two- and three-dimensional response of the system to external stress. 

In these studies vesicle shape is influenced either by change in temperature or composition [22,29,47,48,54-57]. Similar topological changes of GUV under various physical and chemical stresses are extensively studied by Monger s group. They study hydration, adhesion, aggregation, fusion, fission, and disintegration of vesicles, which they refer to as cytomimetic supramolecular chemistry [58,59]. 

Shape changes of GUV were observed upon decompression of egg lecithin vesicles. Because water was pushed out during the compression (due to the fact that lecithin is more compressible than water), upon quick decompression, smaller vesicles budded off due to the excess of surface area. The compressibility of cholesterol-containing bilayers is smaller and no shape changes were observed [60]. 

As discussed at length by Evans [29], and reviewed previously [5,82], three independent shape changes are usually considered in analyses of vesicle membrane... 

For a vesicle of radius the area available for shape changes is... 

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