The Role of MicroRNAs in Natural Tissue Development and Application in Regenerative Medicine - RNA Interference from Biology to Therapeutics - page 58

Fig. 3.1 Illustrates three routes for miRNA-mediated tissue regeneration. MicroRNA modulators
can be injected directly where they will migrate and affect cells within the patient (route 1).
Alternatively, cells can be explanted from a patient and be transfected with miRNA ex vivo. This
can be done in standard cell culture (route 2) or on miRNA functionalised scaffolds (route 3);
afterwards, the cells are then reinjected (route 2) or transplanted along with the scaffold (route 3)

it is implanted into the patient. Alternatively, the scaffolds loaded with cells, miRNA
modulators and/or small molecular cues can be transplanted immediately and grown
in vivo.

3.2 RNA Interference

RNA interference (RNAi) is a cellular process, which utilises small RNAs for gene
expression regulation and innate defence [ 13 ]. RNAi is operational in most eukary-
otic species, including fission yeast, plants, flies, nematodes, mammals and humans
[ 14 ] . Figure 3.2 gives an overview of RNAi in mammals focusing on the miRNA
pathway.
The miRNA pathway is initiated in the nucleus by polymerase II transcription of
primary precursor miRNAs (pri-miRNAs) either as part of an ordinary protein-cod-
ing mRNA or as a unique non-coding RNA. These transcripts are processed by an
RNase III enzyme named Drosha, resulting in a ~70-nt-long precursor hairpin (pre-
miRNA), exported by Exportin 5 into the cytoplasm where it is cleaved by another
RNase III enzyme, named Dicer. The result is a ~19-23 base pair long double-
stranded RNA composed of the miRNA and a passenger strand [ 15 ] , these RNAs
can also be introduced into the cytoplasm as synthetic molecules in which case they