DNA-mediated fusion of lipid vesicles [electronic resource]
Keywords: Chemistry

Vesicle fusion is a central process in transport and communication in biology. In neuronal transmission, synaptic vesicles carrying neurotransmitters dock and fuse to the plasma membrane of the neuron, a process mediated by a combination of several membrane anchored and soluble proteins. Fusion results in the merger of the two apposing lipid bilayers, leading to the exchange of both the lipidic and aqueous components. The fusion reaction is thought to proceed through several stages: first, the membranes are brought into close proximity (docking), second, the outer leaflets mix, but the inner leaflets and contents remain separate (hemi-fusion), and finally, the inner leaflet and contents exchange (full fusion). Due to the complex nature of the fusion reaction and the multitude of proteins involved, the mechanism of the fusion reaction is not well understood. Simplified model systems for vesicle fusion can bring insight into the mechanism by studying the fusion reaction in a more defined and controllable system. This thesis describes a DNA-based model for the protein fusion machinery. Previously, DNA-lipids were used to tether lipid vesicles to glass-supported lipid bilayers. These vesicles could be observed by fluorescence microscopy, and are laterally mobile along the plane parallel to the supported bilayer. DNA-mediated docking between vesicles was characterized, but fusion was not observed due to the fact that the DNA partners were both coupled at the 5' end, so antiparallel hybridization holds the membranes apart. In this work, a new synthesis of DNA-lipid conjugates is described which allows coupling at both the 3' and 5' end of the DNA. Incorporation of complementary DNA-lipids coupled at opposite ends mediates fusion between lipid vesicles. Vesicle fusion was measured in bulk fluorescence assays (Chapter 2 and 3), by both lipid mixing and content mixing assays. The rate of vesicle fusion showed a strong dependence on the number of DNA per vesicle, as well as the sequence of the DNA. Consistent with previous results measured for the docking reaction, fusion was faster for a repeating DNA sequence than for a non-repeating sequence that required full overlap of the strands for hybridization. The role of membrane proximity on the rate of vesicle fusion was investigated in Chapter 3 by insertion of a short spacer sequence at the membrane-proximal end of fusion sequences. The length of the spacer sequence was varied between two and 24 bases, corresponding to length scales of approximately 1-12 nm. Fusion, as measured in bulk assays by lipid and content mixing, decreases systematically as the membranes are held progressively further apart, demonstrating a clear dependence of the rate of the fusion reaction on membrane proximity. While the bulk vesicle fusion assays showed that DNA-lipids can mediate vesicle fusion, these ensemble measurements convolve the multiple steps (docking, hemifusion, and full fusion) of the fusion reaction, complicating any kinetic analysis. In order to image individual vesicle fusion events between tethered vesicles, a new tethering strategy was developed (Chapter 4). This strategy exploits the dependence of DNA hybridization on salt by covalently attaching lipid vesicles to a glass-supported lipid bilayer, then triggering DNA-mediated docking and fusion by spiking the salt concentration. The kinetics of individual vesicle fusion events were subsequently measured using a FRET-based lipid mixing assay for many vesicles (Chapter 6). An analysis of the distribution of waiting times from docking to fusion indicated that this transition occurs in a single step. A second model membrane architecture was used to study individual fusion events between vesicles and a planar bilayer (Chapters 5 and 6). This architecture uses a DNA-tethered planar free-standing bilayer as the target membrane. The kinetics of individual vesicle fusion events to this membrane patch were also consistent with a single step process, as for vesicle to vesicle fusion. In this system, it was also possible to observe content transfer of vesicles containing a self-quenched aqueous dye (Chapter 5). By analyzing the diffusion profile of the dye, it was shown that the dye indeed is transferred into the region below the planar membrane patch, and is not released into the solution above the patch due to vesicle rupture or leakage.