Delivery of Single siRNA Molecules - Nanotechnology for the Delivery of Therapeutic Nucleic Acids - page 98

in the plasma membrane to induce internalization and subsequently
RNAi (Morris

1997). This non-covalent interaction strategy
continued in development over the years, and in the most recent
report, a new PTD, CADY, was introduced, which consists of aromatic
tryptophan and cationic arginine residues. Upon binding to the
plasma membrane, CADY adopts a helical conformation, with one
side exposing the charged arginine residues to bind the siRNAs and
the other side exposing the hydrophobic residues to favor
cellular uptake. Incubation of siRNAs with CADY PTDs resulted
in nanoparticles that were able to induce RNAi in mammalian
cells (Crombez

et al.,

., 2009). Another study incorporated a
trifluoromethylquinoline moiety covalently attached to a stearylated-
TP10 delivery peptide, named PepFect6, in order to increase the
release of nanoparticles trapped in endosomes (Andaloussi

et al

.,
2011). The trifluoromethylquinoline moiety functioned similarly to
chloroquine in that it buffered the endosomes to increase the release
of siRNA-PepFect6 nanoparticles from the vesicles. The PepFect6-
siRNA nanoparticles showed efficient RNAi responses both in tissue
culture cells and in kidney, lung, and liver, after systemic delivery
with no apparent toxicity.
Although there are successful PTD nanoparticle delivery
strategies, they may still pose a problem from a drug point of view
due to their enormous size that usually differs from particle to
particle and due to potential toxicity issues; it would be preferable
to deliver the siRNA as a single molecule with a defined size, rather
than a siRNA-nanoparticle with varying size. In order to deliver the
siRNA as a single molecule, the negative charges on the siRNA must
be masked to prevent neutralization of the PTD and inhibition of
its delivery properties before PTD conjugation can occur.

et al

4.2.3 RNA Binding Proteins

RNA-binding proteins (RBPs) play important roles in post-
transcriptional control and several RNA-regulated processes, such
as splicing, transport, stability, localization, and translation. Most
RBPs have a RNA-binding domain that recognizes specific mRNA
sequence motifs or secondary structures, whereas some recognize
common structural features. In general, RBPs bind regulatory
sequences that are usually located in 5 UTR of the

mRNAs to exert post-transcriptional regulation (Lee and Schedl,
2006). Approximately 500 RBPs have been identified, and one of

′

UTR and/or 3

′