Biomedical Engineering Reference
In-Depth Information
payload, and can vary from maturation and intracellular trafficking to lysosome
formation. The latter is not desirable for nucleic acid payloads, as the acidic envi-
ronment in lysosome may damage the plasmid DNA. However, a moderate decrease
in pH can be used as a trigger to disrupt the endosome and allow the plasmid DNA
to be released into the cytoplasm. The escape of one important class of cationic
polymers, i.e., PEI, has been attributed to the 'proton sponge' hypothesis (Boussif
et al.
1995
); excess protons during endosome acidification are absorbed by the
amine groups on PEI, which causes an excess influx of counterions and water, lead-
ing to the endosome swelling, burst, and eventual release of complexes into the
cytoplasm. Although this mechanism has been generally accepted for PEI-mediated
delivery, exceptions to this mechanism were also noted (Hoekstra et al.
2007
).
A number of peptides have been utilized to disrupt vesicular membranes to
facilitate the necessary escape. One commonly used motif is a pH-sensitive
amphipathic helix, which has aspartic and/or glutamic residues concentrated on one
side of the helix. At neutral pH, the peptide has no secondary structure, but as the
pH decreases along the endocytic pathway and the acidic residues become proto-
nated, alpha-helix formation results that can lead to multimerization and membrane
destabilization (Wagner
1999
). The fusogenic N-terminus of hemagglutinin HA-2
from influenza virus is one example and has been used in both poly-L-lysine (PLL)
and liposomal systems to successfully increase endosomal escape (Wagner et al.
1992
; Subramanian et al.
2002
). Endosomal escape has been also increased with the
synthetic peptide H
5
WYG (GLFHAIAHFIHGGWHGLIHGWYG) which is a
derivative of the HA-2 N-terminus (GLF
G
AIA
G
FI
E
GGW
T
GMI
D
GWYG) with
five histidines residues replacing the native amino acids (Pichon et al.
2001
). The
proton sponge effect is thought to impart an endosomolytic property to H
5
WYG
due to histidine protonation. Another molecule employing membrane-destabilizing
helix formation is the VP-1 protein from rhinovirus and the synthetic peptides JTS-
1, ppTG1, ppTG20, GALA, and KALA which all mimic the same viral model of
helix-mediated endosomal escape (Zauner et al.
1995
; Gottschalk et al.
1996
;
Rittner et al.
2002
; Futaki et al.
2005
; Wyman et al.
1997
).
The arginine-rich TAT peptide from the lentivirus HIV has also been used to
facilitate vesicular escape. This peptide has been utilized in both polymer and lipo-
somal systems to increase transfection (Xiong et al.
2010
; Rudolph et al.
2003
;
Torchilin et al.
2001
). A number of linear and branched arginine-rich synthetic
peptides inspired by the TAT peptide have been developed that can also act as cell-
penetrating peptides (Futaki et al.
2001, 2002
; Nakamura et al.
2007
). While TAT
and similar synthetic peptides have been used in a variety of non-viral systems,
both their preferential endocytic route (likely a combination of direct fusion,
clathrin-mediated endocytosis and macropinocytois) and their mechanism of
traversing membranes remain ill-defined (Futaki
2006
; Futaki et al.
2007
). It
appears that their ability to bind to cell membrane plays a functional role as much
as their ability to facilitate endosomal escape.
Another notable viral peptide utilized by nanocarriers is the penton base protein
of adenovirus serotype 5 (Ad5) (Rentsendorj et al.
2004
). As mentioned before, this
peptide permits binding to alpha-V integrins mediating cellular internalization and
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