Effect of acyl chain mismatch on the contact mechanics of two-component phospholipid vesicle during main phase transition
Introduction
Cell membrane is a supramolecular structure composed of proteins, carbohydrates, glycocalix, etc., embedded in a two-dimensional matrix of a lipid bilayer. Numerous studies demonstrate that various physiochemical properties of cell membrane directly affect major biological functions such as cell fusion, endocytosis and protein sorting [1]. Among all membrane constituents, the phospholipid bilayer provides the structural integrity of cells against external stimulations, e.g. mechanical stress. In general, the lipid composition of the cell membrane is highly dependent on cell types and is usually made up of multi-component mixtures. Therefore, the cell membrane has a complex phase behavior which provides the physical driving forces for performing and regulating numerous biological functions [2].
Recently, Korlach et al. demonstrated that there is a two-phase coexistence on the wall of a single unilamellar vesicle (ULV) composed of a binary lipid system [3]. The group demonstrates that the interesting micron-scale patterns of gel-rich and fluid-rich regions on the vesicle wall only emerge under two prescribed conditions. Firstly, the lipid bilayer contains two types of phospholipids with a significant difference in acyl chain length, e.g. dipalmitoyl-phosphatidylcholine (DPPC) and dilauroyl-phosphatidylcholine (DLPC). Secondly, the mole ratio of the two phospholipids in the binary mixture must fall within a certain range (e.g. 0.2<DLPC/DPPC<0.6). Moreover, this group shows that the addition of cholesterol to the DLPC/DPPC mixture regulates the micro-domain formation of the vesicle wall [3]. However, there is still a missing link between the unique phase behavior of the binary phospholipid mixture and its underlying biological implications.
One critical role of cell membrane in biological functions is cell adhesion on the extracellular matrix. Recently, one group has shown that the thermal fluctuation of membrane–substrate distance within the adhesion contact for the bound vesicle composed of a highly heterogeneous lipid mixture (cholesterol, DEPC, PEG-DEPC), is different from that for vesicle composed of DEPC at constant temperature [4]. However, no quantitative information on the mechanical deformation and adhesion strength for the vesicle composed of highly miscible lipid mixtures during thermal transition has yet been provided. We have previously shown that the contact mechanics of the adherent vesicle composed of a single lipid is significantly modified during the gel to liquid crystalline transition of the bilayer [5]. Exploiting the fundamental interactions between a vesicle composed of thermodynamically well-characterized lipid mixtures and non-deformable substrates is critical for revealing the role of the multi-component lipid bilayer in cell adhesion. Recently, Muresan et al. illustrated that a change of composition in a binary lipid mixture (saturated phosphatidylcholines with different chain length) alters the rupture/fusion mechanism of small ULV on mica above the main phase transition temperature [6]. Fundamentally, the adhesion and deformation of vesicles on mica must occur before the fusion of the binary lipid bilayer on the substrate.
In this study, we demonstrate that the contact mechanics and adhesion strength of model vesicle composed of binary lipid mixture during the main phase transition are modulated on fused silica substrate compared to that in single-component vesicle. In the process, we measure the degree of vesicle deformation with confocal-reflectance interference contrast microcopy (C-RICM) in conjunction with cross-polarized light microscopy on both fused silica and amino-modified substrates, examine the effect of surface functionality, and determine the adhesion energy with a proven contact mechanics model at both gel and liquid crystalline phase co-existences.
Section snippets
Materials
Dibasic sodium phosphate (Na2HPO4); monobasic potassium phosphate (KH2PO4); dibasic potassium phosphate (K2HPO4); sodium chloride (NaCl); monobasic sodium phosphate (NaH2PO4); potassium chloride (KCl); 1 N hydrochloric acid (HCl); 3-amino-propyl-triethoxy-silane (APTES); acetic acid methanol and chloroform were obtained from Fisher Chemicals Inc. (USA) and used as received. Dimyristoyl-phosphatidylcholine (DMPC) and dipalmitoylphosphocholine (DPPC) in powder form were obtained from Matryea Inc.
Result and discussions
Confocal-reflectance interference contrast microscopy (C-RICM) has been proven as an effective biophysical probe for adherent vesicle composed of a single phospholipid on non-deformable substrates [5]. Fig. 1 shows a cross-polarized light image (a) and a C-RICM image (b) of a typical unilamellar vesicle (ULV) composed of a binary DMPC/DPPC (ratio=9:1) mixture on a fused silica substrate at 20 °C. DMPC/DPPC binary lipid bilayer is a well-characterized two-component biological membrane [20]. From
Conclusion
In summary, this study provides new evidence that acyl chain mismatch and lateral heterogeneity directly modulates the contact mechanics of two-component vesicles. First, our contact mechanics model has been validated for application to adhering vesicle composed of binary phospholipid mixtures on non-deformable substrate. Our C-RICM results demonstrate the lack of large domain formation (micron-scale) on the wall of two-component vesicle upon adhesion on fused silica substrate. The introduction
Acknowledgements
NF, ACL and VC were supported by NTU AcRF fund (RG 15/00). VC would like to thank Professors Kin Liao and Kuo-Kang Liu for helpful suggestions.
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Focused characteristics and effects of light reflected from spherical lipid membrane of giant unilamellar vesicles
2020, Colloids and Surfaces B: BiointerfacesCitation Excerpt :Technology utilizing light reflected from GUV membranes has continuously evolved. It is used to detect GUV adhesion [11–13] and measure the tension [14], deformation [15,16], and fluctuation [17,18] of GUV membranes. Light reflected from cellular membranes was also reported in previous research on interactions between cells and substrate surfaces [19,20] and improving optical microscopic technology for cell observation [21,22].