University of Groningen
Tuning the lipid bilayer: the influence of small molecules on domain formation and membrane
fusion
Bartelds, Rianne
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Publication date: 2018
Link to publication in University of Groningen/UMCG research database
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Bartelds, R. (2018). Tuning the lipid bilayer: the influence of small molecules on domain formation and membrane fusion. Rijksuniversiteit Groningen.
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Summary
The eukaryotic cell membrane is a highly regulated, heterogeneous lipid bilayer that embeds transport, receptor and channel proteins as well as membrane-bound enzymes. Saturated lip-ids and cholesterol have been shown to form distinct domains in the cell membrane, named rafts, when present at certain concentrations together with non-saturated lipids. Proteins as-sociate with rafts or are excluded, which brings proteins together or isolates them from others. Since these rafts are very small (10-200 nm) and short-lived, model membranes have been developed to study lipid-lipid interactions and protein partitioning in larger, longer-lived domains. One of these model membrane systems are giant unilamellar vesicles (GUVs), mi-crometer-size vesicles that can be studied by optical microscopy. Three component GUVs consisting of a saturated lipid, an unsaturated lipid and cholesterol can phase separate into a
dense liquid-ordered (Lo) and more fluid liquid-disordered (Ld) phase.
In this thesis, I have shown that small molecules such as some sugars, specific hydrocar-bons and certain lipids influence the phase separation of this model membrane. In chapter
2, non-reducing sugars were found to induce mixing of the Lo and Ld domain, a feature that
could explain the protective effects of these non-reducing sugars seen during anhydrobiosis in plants, invertebrates and microorganisms. In chapter 3, similar effects were observed for aromatic hydrocarbons. Here, dissipation of phase separation was seen for molecules of
sim-ilar size as pyrene, and these compounds partitioned equally over both the Lo and Ld phase.
In chapter 4, the partitioning of the model peptide WALP is described. This peptide localizes
in the Ld phase and the influence of palmitoylation (the addition of a saturated lipid tail) was
studied using a newly synthesized trifunctional linker. Addition of this palmitoyl moiety did not alter peptide partitioning, nor did elongating the peptide or altering the membrane by addition of the ganglioside GM1, which contrasts observations made by molecular dynamics observations.
The second part of this thesis focuses on small vesicles, so-called large-unilamellar vesicles (LUVs), with a diameter from 100 to 500 nm. I have explored the use of LUVs as drug delivery vehicle or membrane system for building a synthetic cell through membrane fusion. Vesicles were made from non-ionic surfactants, single-chain amphiphiles, are characterized in chapter 5. These molecules are shown to form closed, stable vesicles that may be a cost-effective alter-native for already existing liposome-based drug formulations.
To create a synthetic cell, many compartments have to be brought together. This could be achieved by fusing two or more populations of vesicles. Approaches to do so are summa-rized in chapter 6, together with new data that I generated in the course of my thesis work. I show that fusion requires a delicate balance between disturbing the membrane, needed for fusion, and minimizing content leakage. I show that non-leaky fusion with relatively high yields can be obtained using a commercially available enzyme that cuts off a fraction of lipid head groups.
Summ
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