Delivery of Single siRNA Molecules - Nanotechnology for the Delivery of Therapeutic Nucleic Acids - page 95

consequently, leading to a decrease of the corresponding protein.
It is a natural cellular process initiated by Dicer, a ribonuclease
III enzyme that cleaves endogenous long double-stranded RNA
into 21-23 mer RNA duplexes with 5

′

-phosphate groups and

2-nucleotide 3 overhangs. After Dicer has cleaved, the duplex is

unwound and one of the strands, the antisense strand also called
the guide strand, is loaded into the RNA-induced silencing complex
(RISC). One of the RISC proteins, the human immunodeficiency virus
(HIV)-1 transactivating response RNA-binding protein (TRBP), then
recruits another protein to the complex, Argonaute 2 (Ago2) that
subsequently cleaves the complementary passenger strand (Frank
et al.,

′

2005). A crucial part of the guide
strand responsible for recognizing the mRNA is called the seed
sequence and corresponds to nucleotides 2-8 starting from the 5

2010, Chendrimada et al.,

′

end. The RISC-guide strand complex employs the seed sequence as a
template to find and bind complementary mRNAs and directs Ago2
to cleave the mRNA between nucleotides 10-11 from the 5

′

end of

the guide strand.
Once the mRNA is cleaved, the whole process is repeated as the
RISC-guide strand complex detects and cleaves new mRNA strands,
continuously. Thus, RNAi is an extremely efficient process since one
guide strand alone can cleave a high number of complementary
mRNA, resulting in a substantial decrease of the corresponding
protein. Importantly, it has been shown that the RNAi process can be
hijacked by introducing exogenous, synthetic 21 nucleotide siRNAs,
to inhibit the production of specific target proteins in cultured
mammalian cells (Elbashir

2001). Moreover, compared to small
molecule drugs, it is a preferable treatment strategy to use against
many diseases, since it is possible to target any kind of pathological
protein. Due to the sequence-specificity of this process, a new field
of therapeutic drug design has opened up.

et al.,

4.1.2 Modification of siRNAs

Many tissues, in contrast to local targets, can only be reached by
systemic administration. Therefore, a wide range of modifications
has been applied to the siRNA molecule, in order to overcome the
obstacles that siRNA formulations face when they are administered
via the bloodstream. Before the siRNA reaches the cytoplasm of its
target cell, it needs to avoid many challenges such as phagocytic
uptake, kidney filtration, and enzymatic degradation by nucleases