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Micron-scale, liquid-liquid phase separation occurs in membranes of living cells, with physiological consequences. To discover which lipids might support phase separation in cell membranes and how lipids might partition between phases, miscibility phase diagrams have been mapped for model membranes. Typically, model membranes are composed of ternary mixtures of a lipid with a high melting temperature, a lipid with a low melting temperature, and cholesterol. Phospholipids in ternary mixtures are chosen primarily to favor stable membranes (phosphatidylcholines and sphingomyelins) or add charge (phosphatidylglycerols and phosphatidylserines). A major class of phospholipids missing from experimental ternary diagrams has been the phosphatidylethanolamines (PEs). PE-lipids constitute up to 20 mol% of common biological membranes, where they influence protein function and facilitate membrane fusion. These biological effects are often attributed to PE’s smaller headgroup, which leads to higher monolayer spontaneous curvatures and higher melting temperatures. Taken alone, the higher melting points of saturated PE-lipids imply that liquid-liquid phase separation should persist to higher temperatures in membranes containing PE-lipids. Here, we tested that hypothesis by substituting a saturated PE-lipid (DPPE) for its corresponding PC-lipid (DPPC) in two well-studied ternary membranes (DOPC/DPPC/cholesterol and DiphyPC/DPPC/cholesterol). We used fluorescence microscopy to map full ternary phase diagrams for giant vesicles over a range of temperatures. Surprisingly, we found no micron-scale, liquid-liquid phase separation in vesicles of the first mixture (DOPC/DPPE/cholesterol), and only a small region of liquid-liquid phase separation in the second mixture (DiphyPC/DPPE/cholesterol). Instead, coexisting solid and liquid phases were widespread, with the solid phase enriched in DPPE. An unusual feature of these ternary membranes is that solid and liquid-ordered phases can be distinguished by fluorescence microscopy, so tie-line directions can be estimated throughout the phase diagram, and transition temperatures to the 3-phase region (containing a liquid-disordered phase, a liquid-ordered phase, and a solid phase) can be accurately measured.
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Several common methods of making vesicles (except an emulsion method) capture intended lipid ratios
Researchers choose different methods of making giant unilamellar vesicles in order to satisfy different constraints of their experimental designs. A challenge that arises when researchers use a variety of methods is that each method may produce vesicles with a different average lipid ratio, even if all experiments use lipids from a common stock mixture. Here, we use mass spectrometry to investigate ratios of lipids in vesicle solutions made by five common methods: electroformation on indium tin oxide slides, electroformation on platinum wires, gentle hydration, emulsion transfer, and extrusion. We made vesicles from either 5-component or binary mixtures of lipids chosen to span a wide range of physical properties: di(18:1)PC, di(16:0)PC, di(18:1)PG, di(12:0)PE, and cholesterol. For a mixture of all five of these lipids, ITO electroformation, Pt electroformation, gentle hydration, and extrusion methods result in only minor shifts in lipid ratios (≤ 5 mol%) relative to a common stock solution. In contrast, emulsion transfer results in ~80% less cholesterol than expected from the stock solution, which is counterbalanced by a surprising overabundance of saturated PC-lipid relative to all other phospholipids. Experiments using binary mixtures of saturated and unsaturated PC-lipids and cholesterol largely support results from the 5-component mixture. In general, our results imply that experiments that increment lipid ratios in small steps will produce data that are highly sensitive to the technique used and to sample-to-sample variations. For example, sample-to-sample variations are roughly ±2 mol% for 5-component vesicles produced by a single technique. In contrast, experiments that explore larger lipid ratio increments or that seek to explain general trends and new phenomena will be less sensitive to sample-to-sample variation and the method used.
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- PAR ID:
- 10584472
- Publisher / Repository:
- Biophysical Society / Cell Press
- Date Published:
- Journal Name:
- Biophysical Journal
- Volume:
- 123
- Issue:
- 19
- ISSN:
- 0006-3495
- Page Range / eLocation ID:
- 3452 to 3462
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.bpj.2024.08.019
