Vesicle division

Figure 10.13 Demonstration of the process of vesicle division upon addition of fresh oleate surfactant to ferritin-labeled pre-existing POPC liposomes. (Adapted from Berclaz et al., 2001 a, b). Comparison of the absolute number-weighted size distribution (a) of the empty and (b) filled pre-formed POPC liposomes ([POPC] = 6.1 mM ) with the vesicles obtained after addition of oleate ([POPC] = 3 mM, [oleic acid -I- oleate] = 3 mM ).

The process opposite to vesicle division is that of fusion, when two or more vesicles come together and merge with each other, yielding a larger vesicle. As outlined in the previous chapter, vesicle fusion is generally not a spontaneous process. If two populations of POPC liposomes with different average dimensions are mixed with each other, they do not fuse to produce a most stable intermediate structure - they stay in the same solution as stable, distinct species. This is connected to the notion of kinetic traps, as discussed previously, and is supported by theoretical and experimental data from the literature (for example, Hubbard etal, 1998 Olsson and Wennerstrom, 2002 Silin et al, 2002). 

Figure 20.5. Transmission electron microscopic (TEM) images of hpid-Uke glycine peptide enclosures. Glycine tail and aspartic acid head peptides formed tube and vesicle structures. Note the growth of the tube opening (A, B, C) and the presumed vesicle division (D). If these dynamic enclosures can encapsulate other biomolecules, this may be one step closer for prebiotic molecular evolution.

Certain proteins endow cells with unique capabilities for movement. Cell division, muscle contraction, and cell motility represent some of the ways in which cells execute motion. The contractile and motile proteins underlying these motions share a common property they are filamentous or polymerize to form filaments. Examples include actin and myosin, the filamentous proteins forming the contractile systems of cells, and tubulin, the major component of microtubules (the filaments involved in the mitotic spindle of cell division as well as in flagella and cilia). Another class of proteins involved in movement includes dynein and kinesin, so-called motor proteins that drive the movement of vesicles, granules, and organelles along microtubules serving as established cytoskeletal tracks. ... 

In general, the mechanism of self-reproduction of micelles and vesicles can be considered an autopoietic mechanism, since growth and eventually division comes from within the structure itself. This point will be considered again in Chapter 8, on autopoiesis, where the mechanism of the self-reproduction process will also be discussed. 

These considerations are important also in view of the processes of division and/or fusion of vesicles. In particular, when a vesicle divides up, and the total surface area remains constant, the total volume must decrease. This means that water must be eliminated in the process, so as to keep the volume to surface ratio constant. Conversely, when two vesicles fuse with each other, with a constant surface area (no fresh surfactant being added), the total volume must increase to keep the volume/surface constant and water must come in. This important, characteristic feature of vesicles is represented in Figure 9.27. 

Vesicle self-reproduction described until now can be defined as autopoietic, since growth and eventually reproduction comes from within the structure boundary. One can also induce growth and division of fatty acid vesicles by adding fresh surfactant from the outside, for example as a micellar solution at high alkaline pH. 

Figure 10.9 Addition of fresh surfactant (or a micellar solution thereof) to a solution containing pre-existing vesicles can follow two alternative (not exclusive) pathways either formation of new vesicles or binding to the existing vesicles, which may bring about growth and division phenomena (a highly idealized case is shown, with vesicles dividing in two).

Figure 10.11 The use of ferritin as a label for the mechanism of growth of vesicles (adapted from Berclaz et al, 2001a b). Schematic representation of the possible vesicle formation and transformation processes when oleate, and oleic acid, are added to pre-formed vesicles which have been labelled, (a) The situation if only de novo vesicle formation occurs, (b) Growth in size of the pre-formed and labeled vesicles which may lead to division, either yielding vesicles that all contain marker molecules (case i, a statistical redistribution of the ferritin molecules) or also yielding vesicles that do not contain markers (case ii). Compare all this with Figure 10.9.

However, this is not all as shown in the Figure 10.13, under certain conditions there is a small but significant concentration of small ferritin-containing vesicles of sizes that were not present before the addition of oleate. This clearly shows the process of division, as illustrated in Figure 10.11(b), process i. 

In conclusion then, there is direct evidence that vesicles grow in size when fresh surfactant is added and direct evidence of division processes. All this is of course... 

Figure 11.8 Replication of RNA in self-reproducing vesicles. The initial vesicles contained the enzyme Q(3 replicase and the four ribonucleotides in excess, as well as the RNA template (the MDV-1 template). The division of vesicles is induced by the addition of oleic acid anhydride and the duplication of the figure is idealized, as in reality division occurs on a statistical basis. (Adapted from Oberholzer etal, 1995b.)...

Figure 11.10 Protein expression inside the liposomes a working plan. Schematic illustration of two critical steps on the road map to the minimal cell (a) Protein expression of a simple protein (GFP) or any other simple protein and (b) protein expression of the enzymes that catalyze the formation of the vesicle boundary. For the sake of simplicity, growth and division is illustrated as an ideal duplication.

In a second series of experiments, Hanczyc et al. [52] found that the myristoleic acid vesicles could be induced to grow by addition of fatty acid to the medium, presumably by incorporating fatty acid molecules into the membrane, rather than by fusion of vesicles. If the resulting suspension of large vesicles was then filtered through a polycarbonate filter having pores 0.2 pm in diameter, the larger vesicles underwent a kind of shear-induced division to produce smaller vesicles. This process could be repeated several times (Fig. 5). 

Bozic and Svetina [36] analysed a different situation, where addition of membrane constituents happens from the external milieu, and there is no metabolism inside, but there is limited permeability. They supposed that the membrane assumes spontaneous membrane curvature. This is non-zero if the properties of the inside and outside solutions differ, or if the two layers of a bilayer membrane differ in composition, or if some membrane-embedded constituents are asymmetrically shaped. They were able to show that under these assumptions membrane division is possible provided TLkC4 > 1.85, where T is the time taken to double the membrane area, L is the hydraulic permeability of the membrane, k is the bending modulus, and C is the spontaneous membrane curvature. In this model growing vesicles first retain spherical shape, then are distorted to a dumbbell, then to a pair of asymmetric vesicles coupled by a narrow neck, and finally to a pair of spherical vesicles linked by a narrow neck. Separation of the two daughter vesicles occurs as a result of mechanical agitation in the solution. 

Fig. 9 One of the vesicle models (as depicted by the SCM). Different templates (labelled by open and closed circles) contribute to the well being of the compartments (protocells) in that they catalyse steps of metabolism, for example. During protocell growth (-- ) templates replicate at differential expected rates, but stochastically. Upon division (- ) there is chance assortment of templates into offspring compartments. Stochastic replication and reassortment generate variation among protocells, on which natural selection at the compartment level can act and oppose to (correct) internal deterioration due to within-cell competition...

An alternative to an extensive inorganic structure is an extensive organic one. Candidates for this could include any number of polymers. One approach is to make self-assembled capsules from complementary polymers that form layer by layer (LbL) vesicle-like structures [26], This has been achieved by templating the co-assembly of polymers around a removable core. The alternation of polymers with opposite charges allows the composition and thickness of the artificial cell walls to be controlled. The size of the core determines if the resulting capsule is a model for a cell or a smaller capsule like an organelle within a cell. The porous nature of the polymer allows chemical species to enter and leave the capsule but the potential for capsule growth and division, even with the presence of polymers in the external solution is very limited. 

Fig. 7 Mode of vesicle growth and division by increasing concentration of surfactants. (Reproduced from [50])...

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