Capabilities of liposomes for topological transformation - Surfactants

Capabilities of liposomes for topological transformation
approved December 19, 2000 (received for review August 31, 2000)
Fumimasa Nomura, Miki Nagata, Takehiko Inaba, Haruka Hiramatsu, Hirokazu Hotani, and Kingo Takiguchi*
PNAS | February 27, 2001
Department of Molecular Biology, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
Edited by Donald L. D. Caspar, Florida State University, Tallahassee, FL
Abstract
Dynamic behaviors of liposomes caused by interactions between liposomal membranes and surfactant were studied by direct real-time observation by using high-intensity dark-field microscopy. Solubilization of liposomes by surfactants is thought to be a catastrophic event akin to the explosion of soap bubbles in the air; however, the actual process has not been clarified. We studied this process experimentally and found that liposomes exposed to various surfactants exhibited unusual behavior, namely continuous shrinkage accompanied by intermittent quakes, release of encapsulated liposomes, opening up, and inside-out topological inversion.
Introduction
Liposomes (which are closed membrane vesicles) have been well studied as simplified models of biological membranes (1-5) and are now used in a number of applications (5, 6), for example, as carriers of drug or DNA delivery or as artificial membranes for reconstructing membranous enzyme activities (7-9). Recently, many important phenomena affecting lipid bilayers, including their detergent solubilization, have been explored by using liposomes; such studies promote a better understanding of the biophysical properties of bilayer membranes and moreover will improve the handling of membrane proteins when they are isolated from or reconstructed into lipid bilayers (10-13). However, studies of intermediate stages in the detergent solubilization of liposomes are only now in progress (14-17), and the interaction mechanism between membranes and surfactants has remained unclear. Therefore, real-time approaches by using optical microscopy to study the dynamic behavior of liposomes are very important.
High-intensity dark-field microscopy has enabled us to obtain real-time high-contrast images of giant unilamellar liposomes in aqueous solutions (18-22). In this study, we used such techniques to characterize the interactions between liposomal membranes and surfactants. Eight kinds of liposomes and various types of surfactants (Fig. 1) were mixed in all possible combinations in a mixing chamber to generate a concentration gradient of each surfactant for microscope specimens, and morphological changes of liposomes exposed to those surfactants were monitored (18, 23). In the absence of surfactant, liposomal membranes were spherical, and thermal fluctuations of their spherical shape were largely suppressed by the surface tension of their membranes. Hereafter, this morphological state of liposomes will be called tense. In this study, we found several unusual behaviors of liposomes (which are published as supplemental data on the PNAS web site, www.pnas.org).
Fig. 1. Summary of the solubilization processes. C, continuous-stepwise or -smooth shrinkage; I, inside-out inversion; O, opening up; B, burst. Lipids: PC, phosphatidylcholine; DMPC, dimyristoyl phosphatidylcholine; PG, phosphatidylglycerol; DMPG, dimyristoyl phosphatidylglycerol; PA, phosphatidic acid; DMPA, dimyristoyl phosphatidic acid; DMDAP, 1,2-dimyristoyl 3-dimethylammonium propane; DMTAP, 1,2-dimyristoyl 3-trimethylammonium propane. Among eight kinds of lipids, PC, PG, and PA are isolated from egg yolk or other natural sources, so that they had nonuniform lengths of acyl chains, and others had uniform tail lengths (14 C). PC, DMPC, PG, and DMPG had large head groups, and PA, DMPA, DMDAP and DMTAP had small head groups. Among the eight kinds of liposomes used, seven had binary lipid compositions (1:1 mol/mol). The charge carried by liposomes at neutral pH is shown. The surfactants shown here were added to each liposome solution. Abbreviations for surfactants: C12E8, polyoxyethylene 8 lauryl ether; CHAPS, 3-[(3-cholamidopropyl) dimethylammonio]-1-propane sulfonate; SB 3-14, N-tetradecyl-N, N-dimethyl-3-ammonio-1-propane sulfonate; HPC, hexadecyl pyridinium chloride; HTAB, hexadecyl trimethyl ammonium bromide. The charge carried by each surfactant at neutral pH and its critical micelle concentration (mM) is shown. We usually selected giant liposomes whose diameters exceeded 5 ?m for observations to make them and later analysis easy. The experimental results shown here were essentially not altered even when the concentration of surfactant was changed severalfold; however, the rate of liposomal solubilization in each process was changed. It is noted that, by monitoring the changing of solution turbidity, we confirmed that liposomal solubilization started when the concentrations of each added surfactant were higher than its critical micelle concentration.
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