An Ion Switch Regulates Fusion of Charged Membranes

Abstract

Here we identify the recruitment of solvent ions to lipid membranes as the dominant regulator of lipid phase behavior. Our data demonstrate that binding of counterions to charged lipids promotes the formation of lamellar membranes, whereas their absence can induce fusion. The mechanism applies to anionic and cationic liposomes, as well as the recently introduced amphoteric liposomes. In the latter, an additional pH-dependent lipid salt formation between anionic and cationic lipids must occur, as indicated by the depletion of membrane-bound ions in a zone around pH 5. Amphoteric liposomes fuse under these conditions but form lamellar structures at both lower and higher pH values. The integration of these observations into the classic lipid shape theory yielded a quantitative link between lipid and solvent composition and the physical state of the lipid assembly. The key parameter of the new model, κ(pH), describes the membrane phase behavior of charged membranes in response to their ion loading in a quantitative way.

Positively charged supported lipid bilayer formation on gold surfaces for neuronal cell culture

“…48 An explanation might be that the electrostatic interaction in basic conditions might be reduced due to the screening of the positive charge of DOTAP by HPO 4 2À since this divalent counter ion (the dominant anionic ion in PBS) might effectively bind to DOTAP. 53,54 Moreover, these counter ion might also generate an entropic repulsion when vesicles approach the gold surface as reported by Zhu et al 55 These effects might weaken the electrostatic adhesion force of the lipid to the surface.…”

Development and antitumor activity of a BCL-2 targeted single-stranded DNA oligonucleotide

“…Third, ether linkages often used to anchor polyethylene glycol (PEG) or pH responsive headgroups are not used. Finally, the amphoteric nature enables a transition of charge from cationic to net neutral to anionic or vice versa depending on the microenvironment [ 24 , 26 ]. For example, during transient acidic ethanolic conditions used during formulation, there is an overall positive charge to the mixture enabling the efficient encapsulation of the negatively charged PNT100.…”

“…We speculate that the unique mixture of lipids (that lack ether linkages or PEG) and particle size of the amphoteric liposomes contributes to the therapeutic activity of PNT2258 in several ways. These include imparting stability [ 24 , 26 ], surface characteristics which prolong circulation times in blood, and permit metabolic breakdown to enable repeated dosing. Toxicology findings in preclinical studies with PNT2258 showed dose-dependent toxicities that were reversible, and attributable to high doses of lipid (>300 mg/kg) rather than PNT100 [ 27 ].…”

“…We describe here the development of PNT2258, containing PNT100, a 24-base single-stranded DNA oligonucleotide specific to BCL-2 encapsulated in amphoteric liposomes. The liposomes protect the oligonucleotides from nuclease degradation, facilitate cellular uptake, and enable endosomal escape to the nucleus, where the biological effects of the oligonucleotides occur [ 24 – 26 ]. PNT2258 is currently undergoing clinical development against BCL-2-driven malignancies [ 27 ].…”

Making Sense of Antisense Oligonucleotide Therapeutics Targeting Bcl-2

“…Their anionic state averts the aggregation of serum components thereby promoting serum stability and reducing toxicities. Their cationic state mediates efficient encapsulation of oligonucleotides during production and fusion with endosomes, thereby enabling endosomal escape within the cells [97].…”