Mass Transport via Cellular Barriers and Endocytosis - Intracellular Delivery: Fundamentals and Applications - page 32

Fig. 3 The figures display the two principles behind the four endosomal and lysosomal escape
methods discussed. (I) is a simplified version depicting how pH sensitive fusogenic peptides and
fusogenic lipoplexes escape, and (II) is a simplified depiction of how dynamic polyconjugates and
nanoparticles which act as proton sponges escape from an endosome or lysosome. In the case of
pH sensitive fusogenic peptides and fusogenic lipoplexes, the particles fuse with the endosomal
or lysosomal membrane and release their contents into the cytoplasm. Nanoparticles that act as
proton sponges create an osmotic gradient that eventually causes the endosome or lysosome to
burst releasing the contents. Meanwhile, dynamic polyconjugates disrupt and destabilize the endo-
somal or lysosomal membranes leading to the release of the nanoparticle's payload
oligonucleotide payload from the degradative enzymes of the extracellular and
lysosomal compartments, but when the particle comes into contact with the pre-
dominantly zwitterionic interior surface of the endosomal membrane, it attracts
the anionic phospholipid particles on the outer surface causing some of them to
flip. Once flipped the attraction between the two phospholipids creates an ionic
pair, which then leads to membrane destabilization and causes the two bilayers to
merge. With the bilayers merged, the oligonucleotide complex is delivered to the
cytoplasm unharmed (Fig. 3 ) (Zelphati and Szoka 1996 ).
Another two methods focus on protonation of the nanoparticle or of a complex
bound to its surface. The first and simplest of these concepts was originally explored
with polyethylenimine (PEI) in the application of DNA transfection. First described
by Bousiff et al. in 1995 (Boussif et al. 1995 ) and verified a decade later by Akinc
et al. ( 2005 ), this concept explores molecules capable of buffering the internal lyso-
somal environment. In this proton sponge mechanism, PEI, a molecule with a high
cationic charge-density potential, absorbs the protons pumped into the lysosome.
The internal environment of the lysosome is buffered, preventing lysosomal enzymes
from degrading its payload. As the internal environment's pH remains elevated, the
V+ATPase continues to pump protons into the lumen, which is followed by anions
such as Cl − (Sonawane et al. 2002 ). Eventually, the ions within the lysosome create
an ionic gradient that causes the lysosome to swell and eventually burst, releasing its
contents into the cell (Fig. 3 -II). This application, while applied originally to PEI,
has also been applied to PEI-conjugated particles and nanoparticles composed of