Transmission of External Environmental pH Information to the Inside of Liposomes <i>via</i> Pore-Forming Proteins Embedded within the Liposomal Membrane

Abstract

Liposomes are closed-membrane vesicles comprised of lipid bilayers, in which the inside of the vesicles is isolated from the external environment. Liposomes are therefore often used as models for biomembranes and as drug delivery carriers. However, materials encapsulated within liposomes often cannot respond to changes in the external environment. The ability of enclosed materials to maintain their responsiveness to changes in the external environment following encapsulation into liposomes would greatly expand the applicability of such systems. We hypothesize that embedding pore-like "access points" into the liposomal membrane could allow for the transmission of information between the internal and external liposomal environments and thus overcome this inherent limitation of conventional liposomes. To investigate this, we evaluated whether a change in the pH of an external solution could be transmitted to the inside of liposomes through the poreforming protein, yeast voltage-dependent anion channel (VDAC). Transmission of a pH change via VDAC was evaluated using a polyglutamic acid/doxorubicin complex (PGA/Dox) as an internal pH sensor. Upon encapsulation into conventional liposomes, PGA/Dox exhibits no pH sensitivity due to isolation from the external environment. On the other hand, PGA/Dox was found to retain its pH sensitivity upon encapsulation into VDAC-reconstituted liposomes, suggesting that VDAC facilitated the transmission of information on the pH of the external environment to the inside of the liposomes. In conclusion, we successfully demonstrated the transmission of information between the external and internal liposomal environments by a stable porelike structure embedded into the liposomal membranes, which serve as access points.Key words pH-response; liposome; channel protein Liposomes are closed-membrane vesicles comprised of phospholipid bilayers, in which the aqueous phases of the vesicles are separated from the external environment.1) Owing to their structural characteristics, liposomes are often utilized as models of bio-membranes for constructing artificial cells, 2,3) and as drug delivery carriers for stable delivery of unstable drugs to target sites. 4,5) An inherent limitation of liposomes is that they cannot transmit information about the external environment (e.g. ion concentration) to the inner core. By overcoming this limitation, the utilities of liposomes would be expanded. Such improved liposomes could potentially be utilized as artificial cellular systems for analysis of intracellular response to changes in the external environment. Or additional functionalities derived from encapsulated materials could be given to liposomal drug delivery carriers modified their surface with antibodies and polyethylene glycol. 6,7)Biological cells exhibit systems for transmitting information between the internal and external environments on their membrane, such as receptors and ion channels, [8][9][10] which allows them to respond to extracellular information. Based on this fact, we hypothesized th...

Lipid vesicle-based molecular robots

“…Similar to planar lipid membrane systems, the transport of molecules in the lipid vesicle system is highly dependent on the size of the protein nanopores. In addition to conventional studies using protein nanopores of different sizes to transport fluorophores, there are reports on the use of protein nanopores to transport molecules into lipid vesicles, including: the transmission of pH information by a voltage-dependent anion channel (VDAC, 2.5-3 nm) from yeast mitochondria, 80 the light-/pH-dependent release of fluorescent molecules by a mechanosensitive channel of large conductance (MscL, 3 nm) from E. coli, 81,82 the transport of ATP 83 and UTP 84 by αHL (1.4 nm) from Staphylococcus aureus, and the translocation of T7 RNA polymerase (98 kDa) by streptolysin O (SLO, 25-30 nm) from group A, C, and G streptococci 85 (Fig. 4(b)).…”