Vesicles matrix effect

Vesicles are commonly considered models for biological cells. This is due to the bilayer spherical structure which is also present in most biological cells, and to the fact that vesicles can incorporate biopolymers and host biological reactions. Self-reproduction, an autocatalytic reaction already illustrated in the chapters on self-reproduction and autopoiesis, also belongs to the field of reactivity of vesicles. Some additional aspects of this process will be considered here, together with some particular properties of the growth of vesicles - the so-called matrix effect. 

The very peculiar molar ratio 0.4 DDAB to 0.6 oleate, which gives rise to the narrow size distribution, is really noteworthy. This molar ratio corresponds closely to electroneutrality (this is not at 50 50 molarity, due to the relatively high pK of oleate carboxylate in the bilayer) and suggests that small mixed vesicles with an approximately equal number of positive and negative charges may enjoy particular stability. More detailed studies are needed, and this indicates the richness of the unexplored in the field of vesicles. This is shown in its fullness in the next section on the matrix effect, which is also an unexpected phenomenon and one that may have implications for the origin of early cell. 

Figure 10.20 (a) Matrix effect for oleate addition to pre-formed POPC liposomes. In this case, mixed oleate/POPC vesicles are finally formed. Note the extraordinary similarity between the size distribution of the pre-formed liposomes and the final mixed ones. By contrast, the size distribution of the control (no pre-existing liposomes) is very broad, (i) Sodium oleate added to POPC liposomes, radius = 44.13, P-index = 0.06 (ii) POPC liposomes, radius = 49.63, P-index = 0.05 (iii) sodium oleate in buffer, radius = 199.43, P-index = 0.26. (b) matrix effect for the addition of fresh oleate to pre-existing extruded oleate vesicles. In this case, the average radius of the final vesicles is c. 10% greater than the pre-added ones, and again the difference with respect to the control experiment (no pre-added extruded vesicles) is striking, (i) Oleate vesicles extruded 100 nm, radius = 59.77, P-index = 0.06 (ii) oleate added to oleate vesicles, extended 100 nm, radius = 64.82, P-index 0.09 (iii) sodium oleate in buffer, radius = 285.88, P-index = 0.260. (Modified from Rasi et al, 2003.)... 

The matrix effect is thus a way to reproduce a vesicle population of a given size distribution. In an origin of life scenario, the constancy of size during selfreproduction is probably important, as it would have ensured a constancy of physicochemical and biological properties over various generations. 

In the case of the matrix effect, contrary to the autopoietic experiments described earlier, there is no need of water-insoluble precursors - it is the very addition of the same surfactant to an already existing family of vesicles that brings about the multiplication of the same size distribution. All that is needed is an initial narrow distribution of vesicles, and a continuous addition of fresh surfactant. Methods to obtain narrow size distributions in the case of spontaneous vesiculation have been described (Domazou and Luisi, 2002 Stano et al., in press). In fact a prebiotic scenario may be conceived where the fresh surfactant is continuously synthesized in situ, and thanks to the matrix effect the same sizes are propagated over and over again. Of course there is no way to demonstrate that this is what really happened in prebiotic times - it is fair, however, to claim that, given the simpUcity of the process, there is a reasonable probability that a process of this sort may have occurred (Luisi et al, 2004). 

The uptake of oleate by pre-added vesicles, and in particular the matrix effect, permits regulation of the growth of the size and the number of particles, and in this way it is possible to tackle a series of novel questions. One such question is is there a difference in the rate of uptake of fresh surfactant between two vesicle populations of different sizes ... 

In the meantime, the intense study of the simpler vesicle systems has unravelled novel, unsuspected physicochemical aspects - for example growth, fusion and fission, the matrix effect, self-reproduction, the effect of osmotic pressure, competition, encapsulation of enzymes, and complex biochemical reactions, as will be seen in the next chapter. Of course the fact that vesicles are viewed under the perspective of biological cell models renders these findings of great interest. In particular, one tends immediately to ask the question, whether and to what extent they might be relevant for the origin of life and the development of the early cells. In fact, the basic studies outlined in this chapter can be seen as the prelude to the use of vesicles as cell models, an aspect that we will considered in more detail in the next chapter. 

Berclaz, N., Blochliger, E., Muller, M., and Luisi, P. L. (2001a). Matrix effect of vesicle formation as investigated by cryotransmission electron microscopy. J. Phys. Chem. 

Blochiger, E., Blocher, M., Walde, R, and Luisi, R. L. (1998). Matrix effect in the size distribution of fatty acid vesicles. / Phys. Chem., 102,10383-90. 

Lonchin, S., Luisi, P. L., Walde, P, and Robinson, B. H. (1999). A matrix effect in mixed phospholipid/fatty acid vesicle formation. J. Phys. Chem. B, 103, 10910-16. 

Matrix effect in oleat-micelles-vesicles transformations. Orig. Life Evol. 

Reiffen, F., and Gratzl, M. (1986). Ca Binding to Chromaffin Vesicle Matrix Proteins Effect of pH, Mg, and Ionic Strength, Biochemistry 25 4402-4406. 

Lonchin et al. (27) have also studied the matrix effect of phospholipid vesicles. These authors investigated the formation of mixed phospholipid/fatty acid vesicles by using a combination of kinetic and structural methods to study the process of spontaneous formation of vesicles on addition of oleate micelles to preexisting vesicles of l-palmitoyl-2-oleoyl-.yyn-glycero-3-phosphocholine (POPC). 

Evidence was found for a POPC matrix effect, in that the pre-added vesicles influenced the size distribution of the newly produced vesicles. The matrix effect leads to rapid formation of vesicle aggregates and control over the final size distribution, such that the mixed systems are more monodisperse than in the absence of the matrix, where the oleic acid vesicles grow in an uncontrolled way. This effect was more obvious at lower concentrations of oleic acid/oleate than at higher concentrations. 

Fig. 10.8 The importance of the vesicle for the Darwinian evolution of a replicase. Compart-mentalisation ensures that related molecules tend to stay together. This permits superior mutant replicases (grey) to replicate more effectively than the parent (black) replicases. The evolutionary advantage spreads in the form of vesicles with superior replicase molecules, leading with a greater probability to vesicles with at least two replicase molecules (or a replicase and a matrix molecule). Vesicles with less than two replicase molecules are struck out their progeny cannot continue the RNA self-replication. Thus, the vesicles with better replicases form the growing fraction of vesicles which carry forward the replicase activity (Szostak et al., 2001)...

Dephosphorylated synapsin inhibits axonal transport of MBOs in isolated axoplasm, while phosphorylated synapsin at similar concentrations has no effect [21]. When a synaptic vesicle passes through a region rich in dephosphorylated synapsin, it may be cross-linked to the available MF matrix by synapsin. Such cross-linked vesicles would be removed from fast axonal transport and are effectively targeted to a synapsin- and MF-rich domain, the presynaptic terminal. 

In Chapter 11 the effects of binding of hormones to cell surface receptors have been emphasized. Equally important are the mechanisms that control the secretion of hormones. The topic of exocytosis has been considered briefly in Chapter 8, Section C,6 and aspects of the Golgi in Fig. 20-8 and associated text. Both hormones and neurotransmitters are secreted by exocytosis of vesicles. Cells have two pathways for secretion.386 387 The constitutive pathway is utilized for continuous secretion of membrane constituents, enzymes, growth factors, viral proteins, and components of the extracellular matrix. This pathway carries small vesicles that originate in the trans-Golgi network (TGN Fig. 20-8). The regulated pathway is utilized for secretion of hormones and neurotransmitters in response to chemical, electrical, or other stimuli. 

We have already seen that photoactive clusters, e.g. CdS, can be introduced into vesicles and BLMs (Sect. 5.2 and 5.3). Similar support interactions are possible with both inorganic and organic polymeric supports. Photoactive colloidal semiconductor clusters can be introduced, for example, into cellulose [164], porous Vycor [165], zeolites [166], or ion exchange resins [167]. The polymer matrix can thus influence the efficiencies of photoinduced electron transfer by controlling access to the included photocatalyst or by limiting the size of the catalytic particle in parallel to the effects observed in polymerized vesicles. As in bilayer systems,... 

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