Vesicles fusion

The kinetics of vesicle fusion, followed by monitoring the position of the minimum of the SPR reflectivity curves, depends on the composition and molecular shape ofthe vesicular lipids and on the nature ofthe substrate. As a rule, bilayer formation by vesicle unrolling onto a hydrophilic surface is faster than monolayer formation by vesicle fusion onto a hydrophobic surface. This is probably due to the fact that the processes involved in forming a planar bilayer starting from a vesicular bilayer are considerably less complex than those involved in forming a planar monolayer [8, 9]. 

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In the central nervous system (CNS) the concept that localized microdomains of calcium are responsible for triggering vesicle fusion generally refers to neurons (Rizzuto and Pozzan 2006). Concerning glial cells although the existence of structural... 

Schipke CG, Kettenmann H (2004) Astrocyte responses to neuronal activity. Gha 47 226-232 Schneggenburger R, Neher E (2005) Presynaptic calcium and control of vesicle fusion. Curr Opin Neurobiol 15 266-274... 

Littleton, JT and Bellen, HJ (1995) Synaptotagmin controls and modulates synaptic-vesicle fusion in a Ca + dependent manner. Trends Neurosci. 18 177-183. 

Figure 13.1 Schematic illustrations of vesicle fusion process.

The artificial lipid bilayer is often prepared via the vesicle-fusion method [8]. In the vesicle fusion process, immersing a solid substrate in a vesicle dispersion solution induces adsorption and rupture of the vesicles on the substrate, which yields a planar and continuous lipid bilayer structure (Figure 13.1) [9]. The Langmuir-Blodgett transfer process is also a useful method [10]. These artificial lipid bilayers can support various biomolecules [11-16]. However, we have to take care because some transmembrane proteins incorporated in these artificial lipid bilayers interact directly with the substrate surface due to a lack of sufficient space between the bilayer and the substrate. This alters the native properties of the proteins and prohibits free diffusion in the lipid bilayer [17[. To avoid this undesirable situation, polymer-supported bilayers [7, 18, 19] or tethered bilayers [20, 21] are used. 

The total curvature energy of a spherical vesicle is given by 4tt(2/cc + k). As all experimental data on phospholipids indicate that kc is not small, one is inclined to conclude that the vesicles are thermodynamically unstable the reduction of the number of vesicles, e.g. by vesicle fusion or by Ostwald ripening, will reduce the overall curvature energy. However, such lines of thought overlook the possibility that k is sufficiently negative to allow the overall curvature free energy of vesicles to remain small. 

Going in the opposite direction, i.e. when we consider the membrane stability with increasing ionic strength, we notice the approach of k towards zero. Going towards this value, the tendency of the bilayers to form saddle-shaped connections (also called stalks ) between bilayers increases. Saddle-shaped membrane structures also occur in processes like vesicle fusion, endo and exocytosis. The SCF predictions thus indicate that these events will occur with more ease at high ionic strength than at very low ionic strength. 

The link from lipid properties to mechanical properties of the bilayers is now feasible within the SCF approach. The next step is to understand the phase behaviour of the lipid systems. It is likely that large-scale (3D) SCF-type calculations are needed to prove the conjectures in the field that particular values of the Helfrich parameters are needed for processes like vesicle fusion, etc. In this context, it may also be extremely interesting to see what happens with the mechanical parameters when the system is molecularly complex (i.e. when the system contains many different types of molecules). Then there will be some hope that novel and deep insights may be obtained into the very basic questions behind nature s choice for the enormous molecular complexity in membrane systems. 

Peters, J.-M., Walsh, M. J., and Franke, W. W. An abundant and ubiquitous homo-oligomeric ring-shaped ATPase particle related to the putative vesicle fusion proteins Sec 18p and NSF. EMBOJ. 1990, 9, 1757-1767. 

Rab5C, which also regulates vesicle fusion (GFP-Rab5 is also available)... 

Gruenberg J, Griffiths G, Howell KE. Characterization of the early endosome and putative endocytic carrier vesicles in vivo and with an assay of vesicle fusion in vitro. J Cell Biol 1989 108(4) 1301-1316. 

The MARTINI model effectively replaces three to four heavy atoms with a bead, parameterized to reproduce condensed-phase thermodynamic data of small molecules [23]. The MARTINI model has been used to investigate many biological processes, such as lung surfactant collapse [24], nanoparticle permeation in bilayers [25], large domain motion of integral membrane proteins [26], vesicle fusion [27,28], and lateral domain formation in membranes [29]. 

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). 

Similar rosette-structures have been detected on the P-fracture faces of membranes of cytoplasmic vesicles in Micrasterias.367 The freeze-fracture images obtained suggested that the rosette complexes are first assembled into cytoplasmic vesicles, and are subsequently incorporated into the plasma membrane by a process of vesicle fusion. This supports the evidence obtained by Kiermayer and Dobberstein,371,372 in Micrasterias, that the flat vesicles that contain the rosette complexes are formed by the Golgi apparatus. 

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