On the uptake of cationic liposomes by cells: from changes in elasticity to internalization

In this study, we assessed the capacity of a previously reported engineered liposomal formulation, which had been tested against model membranes mimicking the lipid composition of the HeLa plasma membrane, to fuse and function as a nanocarrier in cells. We used atomic force microscopy to observe physicochemical changes on the cell surface and confocal microscopy to determine how the liposomes interact with cell membranes and released their load. In addition, we performed viability assays using methotrexate as an active drug to obtain proof of concept of the formulation´s capacity to function as a drug delivery-system. The interaction of engineered liposomes with living cells corroborates the information obtained using model membranes and supports the capacity of the engineered liposomal formulation to serve as a potential nanocarrier

Atomic force microscopy and force spectroscopy of Langmuir-Blodgett films formed by heteroacid phospholipids of biological interest

AbstractLangmuir–Blodgett (LB) films of two heteroacid phospholipids of biological interest 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), as well as a mixed monolayer with χPOPC=0.4, were transferred onto mica in order to investigate by a combination of atomic force microscopy (AFM) and force spectroscopy (FS) their height, and particularly, their nanomechanical properties. AFM images of such monolayers extracted at 30 mN m−1 revealed a smooth and defect-free topography except for the POPE monolayer. Since scratching such soft monolayers in contact mode was proved unsuccessful, their molecular height was measured by means of the width of the jump present in the respective force–extension curves. While for pure POPC a small jump occurs near zero force, for the mixed monolayer with χPOPC=0.4 the jump occurs at ∼800 pN. Widths of ∼2 nm could be established for POPC and χPOPC=0.4, but not for POPE monolayer at this extracting pressure. Such different mechanical stability allowed us to directly measure the threshold area/lipid range value needed to induce mechanical stability to the monolayers. AFM imaging and FS were next applied to get further structural and mechanical insight into the POPE phase transition (LC–LC′) occurring at pressures >36.5 mN m−1. This phase transition was intimately related to a sudden decrease in the area/molecule value, resulting in a jump in the force curve occurring at high force (∼1.72 nN). FS reveals to be the unique experimental technique able to unveil structural and nanomechanical properties for such soft phospholipid monolayers. The biological implications of the nanomechanical properties of the systems under investigation are discussed considering that the annular phospholipids region of some transmembrane proteins is enriched in POPE