Transcriptional Characterization of Glioma Neural Stem Cells Diva ...

4.1 MicroRNA regulation Introduction
genome regulating up to two thirds of protein coding genes [150,256]. The
way that microRNAs regulate gene expression is through an enzymatic pro-
cess that starts in the nucleus and is driven by Drosha. The biogenesis of
microRNAs in animal models starts when they are transcribed by RNA poly-
merase II as primary transcripts, termed "pri-microRNAs". The initiation
step of "cropping" is mediated by the Drosha-DGCR8 complex, also known as
the Microprocessor complex. Drosha and DGCR8 are both located mainly in
the nucleus. The product of this nuclear processing step is an approximate 70
nucleotide pre-microRNA, which possesses a short stem plus an approximate
two nucleotide 3' overhang. This structure might serve as a signature mo-
tif that is recognised by the nuclear export factor exportin-5. Pre-microRNA
constitutes a transport complex together with exportin-5 and the GTP-bound
form of cofactor Ran. Following export outside of the nucleus, the cytoplasmic
RNase III Dicer participates in the second processing step termed "dicing" to
produce microRNA duplexes. The duplex is separated and usually one strand
is selected as the mature microRNA, whereas the other strand is degraded.
When the other strand is not degraded it is indicated with the "*" mark in
microRNA target prediction algorithms [232].
Transcriptional and Post-transcriptional Regulation
MicroRNA genes are encoded either in independent transcription units, in
polycistronic clusters or within introns of protein coding genes. Indepen-
dently of Drosha, pre-microRNA hairpins can also be generated from introns
through the combined action of the spliceosome and the lariat-debranching en-
zyme [239]. Animal microRNAs have been shown to function differently from
plant microRNAs, with the former class binding to their target 3'UTRs by
imperfect matching. In fact, the RNA-Induced Silencing Complex (RISC) me-
diates target mRNA cleavage by loading siRNAs and mature microRNAs with
perfect sequence complimentarily to their target mRNA. The extent of the
mismatch region varies amongst different microRNA-mRNA duplexes in ani-
mals, but it is rarely the case that sequence complementarity spans the entire
microRNA sequence, meaning that transcript cleavage would seem to be just
as rare a mechanism of repression. Because the extent of this complementarity
is low in animals, there is a region spanning two to eight nucleotides from the
5' end of the microRNA that proves to be essential for the correct recognition
of the message target. The 5' most nucleotide (t1) of the microRNA sequence
does not have a significant role in this recognition process, as even when base
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