pH-Induced destabilization of phosphatidylethanolamine-containing liposomes: role of bilayer contact

Abstract

The mechanism of pH-induced destabilization of liposomes composed of phosphatidylethanolamine and a charged cholesteryl ester was studied by following the release of encapsulated aqueous contents. The kinetics of release were measured continuously by using the water-soluble fluorophore 8-aminonaphthalene-1,3,6-trisulfonic acid in combination with the water-soluble quencher p- xylylenebis (pyridinium) bromide. With this fluorescence assay, release of contents from liposomes composed of phosphatidylethanolamine and cholesteryl hemisuccinate was shown to be a function of pH, ratio of phosphatidylethanolamine to cholesteryl hemisuccinate, and acyl chain composition of the phosphatidylethanolamine. Leakage was very slow at pH 5.5 and increased dramatically with decreasing pH down to 4.0. Replacing phosphatidylethanolamine by phosphatidylcholine eliminated the effect of pH on leakage. Analysis of the kinetics of release by a mass action model demonstrated that bilayer destabilization and leakage occur subsequent to aggregation. The requirement of bilayer contact for destabilization has been found previously for acidic phospholipid bilayers in the presence of divalent cation and for saturated phosphatidylcholine bilayers below the isothermal phase transition temperature. The phosphatidylethanolamine-containing bilayers examined here satisfy the same requirement.

pH‐sensitive liposomes mediate cytoplasmic delivery of encapsulated macromolecules

“…When incubated with cells, such liposomes deliver their contents to the cytoplasm by a mechanism which requires metabolic energy as well as mildly acidic pH in some intracellular compartment, Here, oleic acid and phosphatidyleth~olamine were found to be obligatory constituents of pH-sensitive liposomes, as substitution of PS for OA, or PC for PE drastically decreases pH-sensitivity and cytoplasmic delivery. Other negatively charged ~phiphiles, such as cholesterol hemisuccinate [29] or palmitoylhomocysteine [30] can be substituted for oleic acid in making PE-rich liposomes that lose their contents in response to mildly acidic pH. However, it is as yet unknown whether liposomes of those components will be useful for cytoplasmic delivery of macromolecules.…”

Interactions between human defensins and lipid bilayers: Evidence for formation of multimeric pores

“…However, we could detect no mixing of contents (data not shown) within a limit of approximately 3% fusion. "Leaky" vesicles that are also undergoing fusion will generally show at least a transient decrease in fluorescence during the initial stages of fusion (Ellens et al, 1984), but no such transients were observed. The apparent lack of contents mixing and the limited mixing of bilayer lipids detected with the NBDhhodamine assay indicates that the defensin-induced fusion of the inherently stable POPG LUV is not a significant issue.…”

“…This indicates that lipid mixing can only occur when the HNP-2 is in its folded, compact state. The extent to which the contents of the vesicles were mixed during the lipid-mixing interactions, indicative of complete fusion, was assayed by the method of Ellens et al (1984). HNP-2 was added to a solution of POPG vesicles containing either the fluorescent dye ANTS or the quencher DPX.…”

“…The ANTS-DPX fluorescence quenching system (Ellens et al, 1984) was also used to assess the leakage of vesicle contents induced by HNP-2. In this case, the fluorescent ANTS and the quencher DPX were coencapsulated within POPG LUV so that leakage would be signaled by an increase in ANTS fluorescence due to dilution into the extravesicular space.…”

Carboxylated hyperbranched poly(glycidol)s for preparation of pH-sensitive liposomes

“…Various methods have been applied to produce pH-sensitive liposomes. For example, pH-sensitive amphiphiles, such as oleic acid and cholesteryl hemisuccinate, have been mixed with non-bilayer-forming phospholipid dioleoylphosphatidylethanolamine (DOPE) to yield pH-sensitive liposomes [1,2]. Another efficient method for pH-sensitization of liposome is modification of stable liposomes with pH-sensitive membrane active molecules such as fusion peptides derived from viral fusogenic proteins, or synthetic polymers with carboxyl groups such as poly(alkyl acrylic acid)s [3,4].…”