Proton-induced permeability and fusion of large unilamellar vesicles by covalently conjugated poly(2-ethylacrylic acid)

Proton sensitive large unilamellar vesicles (LUV) were constructed by immobilization of the pH sensitive synthetic polymer poly(2-ethylacrylic acid) onto the outer monolayer. Thiolated poly(2-ethylacrylic acid) (PEAA-SH) was covalently conjugated to the surface of LUVs composed of egg phosphatidylcholine (EPC) and cholesterol (Chol) through the thiol-reactive maleimide lipid MPB-DSPE (N-(4-(p-maleimidophenyl)butyryl)-1,2-distearoyl-sn-glycero-3-phosphoethanolamine). The resulting PEAA-LUVs were shown to be stable at neutral pH (pH 7.0 to 8.0). Under acidic conditions, however, protonation of PEAA resulted in interaction with both the membrane it was linked to and the membrane of target vesicles, causing membrane destabilization and release of vesicle contents. Moreover, conjugated PEAA is shown to mediate fusion with target membranes in a pH dependent manner. PEAA-mediated permeabilization and vesicle-vesicle fusion occurred only when the polymer was covalently linked to the LUV surface. Proton dependent fusion of PEAA-LUVs was also observed with erythrocyte ghosts. This pH-dependent release of vesicle contents and fusion of PEAA-LUVs occurred below pH 6.8, which is well within the pH range expected to be encountered inside the endosomes in the endocytic pathway, indicating the potential of PEAA-LUVs as a drug carrier system for intracellular drug delivery.