Cross-linking vesicles

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. 

Amphiphilic ABA-type triblock copolymers containing PDMS, two poly(2-methyl-oxazoline) -PMOXA- side blocks and methacrylate terminal groups have a eomplex selfassembling behavior, which allows for the synthesis of covalently cross-linked hydrogels, with applications as contact lenses [106]. Cross-linked vesicles of such polymers are nano-and microcapsules which act as stable nanoreactors with controlled permeability [107-109]. 

Hales, M., Bamer-Kowollik, C., Davis, T.P., and Stenzel, M.H. (2004) Shell-cross-linked vesicles synthesized from block copolymers of poly(D,L-lactide) and poly(N-isoprop acr amide) as thermoresponsive mnocontainers. Langmuir, 20 (25), 10809. 

Figure L Schematic representation of cross-linked vesicle formed from DLPE-DVBA, DPPE-DVBA or EGGPE-DVBA.

Differential scanning calorimetry was performed to measure the chain melting temperature for non-cross-linked phospholipids (NCP) 1-3. Non-cross-linked vesicle dispersions (NCVD) and cross-linked vesicle dispersions (CVD) of 1 and 2 were also measured along with the chain melting transition of CVD-1 with a hydrophobic dye entrapped. Parent phospholipids DPPE and DLPE were used as references for NCP 1 and 2. Parent phospholipid EGGPE was used as reference for phospholipid 3. NCPs were weighed directly into the DSC pans. Vesicle dispersions were used for the non-cross-linked vesicle samples and freeze-dried powder was used for the cross-linked vesicle dispersion samples. 

NCP is an abbreviation for non-cross-linked phospholipids, NCVD is an abbreviation for vesicle dispersion non-cross-linked, CVD is an abbreviation for cross-linked vesicle dispersion. 

Measurements were recorded on the thermotropic chain melting and cooling transitions of CVD-1 with the hydrophobic dye oil red-0 entrapped within the lipophilic membrane. A chain melting transition was found at 15.3 C and a cooling transition was seen at 13.1 C. There is a 14 C difference in the transition of the cross-linked vesicles without dye entrapped and the cross-linked vesicles with dye entrapped. The AH value was shown to be 0.055 kcal/mole at 15.3 C. 

The mechanical and chemical stability of the cross-linked vesicles were analyzed by negative staining TEM after exposure to protic and aprotic organic solvents. The solubilizing effects of organic solvents are known to destroy the structural integrity of conventional vesicles. We have shown that the cross-... 

Table II. Diameter of Cross-linked Vesicles after Treatment with Solvents...

Entrapment of Oil Red-O in Freeze-dried Cross-linked Vesicles... 

Figure 7. Percent leakage of carboxyfluorescein from DPPE-DVBA vesicles (A) non-cross-linked vesicles and (A) cross-linked vesicles.

PolymGrizabiG Block Copolymars. Unsaturated hydrocarbon polymers with a large number of polymerizable double bonds can be used to prepare fully cross-linked vesicle membranes. This has been demonstrated for polybutadiene block copolymers (23,168) and methacroyl-fimctionalized block copolymers (87) using free-radical polymerization, and for polycinnamoylethylmethacrylate block copolymers using a [2-1-2] photoaddition (68). Poly(ethylene oxide-6-3-(trimethoxysilyl)propyl methacrylate) vesicles could be efficiently cross-linked with triethylamine (79,80). PLA-PNIPAM block copolymer vesicles were cross-linked by chain extension of the PNIPAM block using hexamethylene diacrylate (77). 

As amphiphilic block copolymers tend easily to form vesicles rather than tubes, Helmerson et al. reported another strategy for the formation of long polymer nanotubes. They started with polymersomes, which are cross-linked vesicles of amphiphilic block copolymers filled with water. Pulling on the membrane of those polymersomes with a micropipette or with optical tweezers led to the formation of water containing nanotubes stretched from the vesicles (Figure 5.46). However, this process required the application of a lot of force. To overcome this problem, they incorporated a detergent-like triblock copolymer in the formation of the polymersomes, with the effect that the membranes were rendered more fluid. 

Fig. 20.8 Schematic illustration of the SC4A-TPA cross-linked vesicle...

In addition, based on the electrophysiological data, the vesicular mode itself has been questioned for the exocytosis of neurotransmitters (Tauc and Poulain, 1993). Therefore, it is very important to identify other possible targets for the neurotoxins in addition to those already identified. It is likely that the neurotoxins act on the regulatory and constitutive components of the secretory machinery simultaneously. An evidence of this possibility is a report that tetanus neurotoxin activates transglutaminase activity which could cross-link vesicle synapsin 1 to actin to block the vesicular movement (Facchiano et al., 1993). 

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